Heredity

HEREDITY

HEREDITY

By J. ARTHUR THOMSON, M.A.

Regius Professor of Natural History in the University
of Aberdeen

AUTHOR OF "THE STUDY OF ANIMAL LIFE," "THE
SCIENCE OF LIFE," "OUTLINES OF ZOOLOGY," " HER-

BERT SPENCER,

DARWINISM AND HUMAN LIFE,

"THE BIOLOGY OF THE SEASONS," "INTRODUCTION

TO SCIENCE," ETC.; JOINT-AUTHOR OF "THE

EVOLUTION OF SEX," "EVOLUTION," ETC.

SECOND EDITION

3-

O

LONDON ^ b

JOHN MURRAY, ALBEMARLE STREET, W.

1912

f-3

nix

All Rights Reserved

(1907)

I DEDICATE THIS BOOK
WITH THEIR KIND PERMISSION
TO

FRANCIS GALTON and AUGUST WEISMANN

WHOSE

MAGISTRAL STUDIES ON HEREDITY

HAVE MADE US ALL THEIR DEBTORS

PREFACE TO FIRST EDITION

Tnis book is intended as an introduction to the study of
heredity, which every one admits to be a subject of fascinating
interest and of great practical importance. In recent years
much progress has been made in the scientific study of heredity,
and, as the literature is widely scattered, and often very technical,
there may be utility in an exposition which aims at being com-
prehensive and accurate, without being exhaustive or mathe-
matical. Simple the exposition cannot be, if one has any
ambition for thoroughness, but it is probably simple enough
for those who have got beyond the pottering, platitudinarian
stage, which deals in heredity with a capital H. My stacks of
unused manuscript remind me sadly of how much I have had
to leave out, to keep the volume approximately within the
limits of the series to which it belongs; but the bibliography
will enable serious students to fill in details, and follow up the
clues I have given. It is arranged with a subject-index, so that
the literature dealing with particular points can be seen at a
glance. I have tried to avoid partisan handling of any theme,
though I have been at no pains to conceal my general adherence
to what is called Weismannism, or — to take a particular case —
my conviction that we do not know of any instance of the trans-
mission of an acquired character. I have also tried throughout
to keep the practical side of the study in view, but I have re-
frained from making many suggestions, in the belief that the

vii

viii PREFACE

inquiry is not ripe for more than a general recommendation to
take thought for the morrow by considering the ideal of Eu-
genics.

A glance at the book will show that much prominence has
been given to three kinds of conclusions — those reached by
microscopic study of the germ-cells, those reached by the appli-
cation of statistical methods, and those reached through ex-
periment. I have equal sympathy with all these ways of
attacking the mysterious problems, and since I have not, to
my lasting regret, found any opportunity, amid the continuous
claims of professional duties, of working along any one of them,
I can, without seeming to recommend my own wares, press a
consideration of the results which have been achieved on the
attention of all thoughtful men and women. The new facts
are of especial interest to medical practitioners, to educationists,
including clergymen, to social reformers, and to actual or
prospective parents.

I have, throughout, acknowledged my indebtedness to autho-
rities, and the bibliography (which is merely representative)
shows how many fields there are from which to glean. In
particular, I have been indebted to the works of Galton, Weis-
mann, Pearson, Bateson, and De Vries.

I have to thank my friends Mr. E. S. Russell and Dr. John
Rennie for going over the proofs, and saving the pages from
many mistakes. Dr. Leslie Mackenzie was kind enough to
read the chapter on Heredity and Disease, and some of his
helpful suggestions have been incorporated. I have to thank
Professor C. Correns and Professor H. E. Ziegler for generously
allowing me to copy four admirable diagrams ; also Mr. Young
Pentland and the Walter Scott Publishing Company for allowing

PREFACE ix

me the use of a number of figures which have done duty in other
books of mine. My thanks are also due to Mr. Murray, who
has encouraged me in a work which I was often tempted to
abandon, whose good-humoured patience over many delays I
should long since have exhausted had he been as many men
are.

J. A. T.

The University of Aberdeen,
August 1907.

PREFACE TO SECOND EDITION

The demand for a second edition has given me the opportunity
of correcting some errors — for a knowledge of which I am in
part indebted to my critics — and of inserting references to
some of the new discoveries that have been made in the last
five years in this rapidly progressive department of Biology.

J. A. T.

The University of Aberdeen,
May 1912.

CONTENTS

CHAPTER I

PAGE

HEREDITY AND INHERITANCE : DEFINED AND ILLUS-
TRATED I

§ i . Importance of the Study of Heredity. § 2. What the Terms
Mean. § 3. Heredity and Inheritance in Relation to other Bio-
logical Concepts. § 4. A Question of Words. § 5. The Problems
Illustrated. § 6. Denials of Inheritance.

CHAPTER II
THE PHYSICAL BASIS OF INHERITANCE .... 26

§ 1. What is true in the Great Majority of Cases. § 2. Diverse
Modes of Reproduction. § 3. The Hereditary Relation in Uni-
cellular Organisms. § 4. The Hereditary Relation in the Asexual
Multiplication of Multicellular Organisms. § 5. Nature and
Origin of the Germ-cells. § 6. Maturation of the Germ-cells.
§ 7. Amphimixis and the Dual Nature of Inheritance in Sexual
Reproduction. § 8. Inheritance in Parthenogenesis. § 9. Wherein
the Physical Basis precisely consists.

CHAPTER III

HEREDITY AND VARIATION 66

§ 1. Persistence and Novelty. § 2. The Tendency to Breed
True. § 3. Different Kinds of Organic Change. § 4. Classifica-
tion and Illustration of Variations. § 5. Fluctuating Variations.
§ 6. Discontinuous Variations. § 7. De Vries on Fluctuations and
Mutations. § 8. Causes of Variation.

xi

xii CONTENTS

CHAPTER IV

PAGE

COMMON MODES OF INHERITANCE ..... 106

§ i. Though Prediction in Individual Cases is insecure, there are
some Common Modes of Inheritance. § 2. Certain Necessary Saving
Clauses. § 3. Blended Inheritance. § 4. Exclusive Inheritance
(Unilateral, Absolutely Prepotent, or Preponderant). § 5. Parti-
culate Inheritance. § 6. Alternative Inheritance. § 7. Summary
of Possibilities.

CHAPTER V

REVERSION AND ALLIED PHENOMENA .... II9

§ 1. What is meant by Reversion. § 2. Suggested Definitions.
§ 3. Theoretical Implications. § 4. Phenomena sometimes con-
fused with Reversion. § 5. " Skipping a Generation." § 6. Men-
delian Interpretation of Reversion. § 7. Reversion in Crosses.
§ 8. Reversion of Retrogressive Varieties. § 9. Interpretations in
Terms of Reversion. § 10. Further Examples of Reversion.

CHAPTER VI
TELEGONY AND OTHER DISPUTED QUESTIONS . . . I43

§ 1. What is meant by Telegony. § 2. The Classic Case of
Lord Morton's Mare. § 3. Representative Alleged Cases of Tele-
gony. § 4. Ewart's Penycuik Experiments. § 5. Suggestions
which explain away Telegony. § 6. Suggestions as to how Telegonic
Influence might be effected. § 7. A Statistical Suggestion. § 8.
The Widespread Belief in the Occurrence of Telegony. § 9. An
Instructive Family History. § 10. A Note on Xenia. § 11.
Maternal Impressions,

CHAPTER VII
THE TRANSMISSION OF ACQUIRED CHARACTERS . . 164

§ 1. Importance of the Question. § 2. Historical Note. § 3.
Definition of the Problem. § 4. Many Misunderstandings as to
the Question at Issue. § 5. Various Degrees in which Parental
Modifications might affect the Offspring. § 6. Widespread Opinion
in favour of Affirmative Answer. § 7. General Argument against
the Transmissibility of Modifications. § 8. General Argument

CONTENTS xiii

PAGB

for the Transmissibility of Modifications. § 9. Particular Evi-
dence in support of the Affirmative Answer. § 10. As regards
Mutilations and the Like. § n. Brown-Sequard's Experiments
on Guinea-pigs. §12. Negative Evidence in favour of the Affirma-
tive Answer. § 13. The Logical Position of the Argument. § 14.
Indirect Importance of Modifications. § 15. Practical Con-
siderations.

CHAPTER VIII

HEREDITY AND DISEASE • . . 250

§ 1. Health and Disease. § 2. Misunderstandings in regard
to the " Inheritance " of Disease. § 3. Are Acquired Diseases
transmissible ? § 4. Can a Disease be transmitted ? § 5. Pre-
dispositions to Disease. § 6. Particular Cases. § 7. Defects,
Multiplicities, Malformations, and other Abnormalities. § 8.
Some Provisional Propositions. § 9. Immunity. § 10. Note on
Chromosomes in Man. § 11. Anticipation and Intensification of
Disease. § 12. Practical Considerations.

CHAPTER IX

STATISTICAL STUDY OF INHERITANCE . ', 309

§ 1. Statistical and Physiological Inquiries. § 2. Historical
Note. § 3. A Hint of the Statistical Mode of Procedure. § 4.
Filial Regression. § 5. Law of Ancestral Inheritance. § 6. Criti-
cisms of Galton's Law. § 7. Illustration of Results reached by
Statistical Study.

CHAPTER X

EXPERIMENTAL STUDY OF INHERITANCE .... 336

§ 1. Mendel's Discoveries. § 2. Theoretical Interpretation.
§ 3. Corroborations. § 4. Illustrations 0) Mendelian Inheritance.
§ 5. Mendel's Discovery in Relation to Other Conclusions. § 6.
Practical Importance of Mendel's Discovery. § 7. Other Experi-
ments on Heredity. § 8. Consanguinity.

CHAPTER XI

HISTORY OF THEORIES OF HEREDITY AND INHERITANCE . 39I

§ 1. What is required of Theories of Heredity and Inheritance.
§ 2. The Old Theories of Heredity. § 3. Theories of Pangenesis.
§ 4. Theory of Genetic or Germinal Continuity.

xiv CONTENTS

CHAPTER XII

PAGE

HEREDITY AND DEVELOPMENT . 412

§ 1. Theories of Development. § 2. Weismann's Theory of the
Germ-Plasm. § 3. Note on Rival Theories. § 4. Weismann's
Theory of Germinal Selection.

CHAPTER XIII

HEREDITY AND SEX 472

§ 1. Relations between Sex and Inheritance. § 2. The Deter-
mination of Sex. § 3. Different Ways of Attacking the Problem.
§ 4. Classification of Theories. § 5. First Theory : Environment
affects Offspring. § 6. Second Theory : Fertilisation is Decisive.
§ 7. Third Theory : Two Kinds of Germ-Cells. § 8. Fourth
Theory : Maleness and Femaleness are Mendelian Characters.
§ 9. Fifth Theory : Nurtural Influences operate on the Germ-Cells
through the Parents. § 10. Another Way of looking at the Facts.
§ 11. Conclusion.

CHAPTER XIV

SOCIAL ASPECTS OF BIOLOGICAL RESULTS .... 510

§ 1. Relations of Biology and Sociology. § 2. The Chief Value

of the Sociological Appeal to Biology. § 3. Originative Factors

in Evolution. § 4. Social Aspects of Heredity. § 5. Directive
Factors in Evolution.

BIBLIOGRAPHY .... ..... 543

SUBJECT-INDEX TO BIBLIOGRAPHY 605

INDEX . . ... • : ■ « ■ 619 I

LIST OF ILLUSTRATIONS

FIG. PAGE

i. Ovum of a threadworm (from Carnoy) .... 5

2. Diagram of cell division (after Boveri) .... 31

3. Diagram of cell structure (after Wilson) 33

4. " Comet-form " of starfish (after Haeckel) 35

5. Asexual reproduction of a sea-worm (after McIntosh) , 36

6. Diagram of ovum and somatic cell (after carnoy) facing 38

7. volvox globator ......... 39

8. Forms of spermatozoa . . . . . . . 41

9. Diagram of germinal continuity (after Wilson) . . 43

10. Parallelism of spermatogenesis and oogenesis (after

Boveri) 47

11. Diagram of reduction and amphimixis (after Ziegler) facing 48

12. Fertilised ovum of ascaris (after Boveri) .... 49

13. Diagram of maturation and fertilisation (from Ziegler).

facing 51

14. Chromatin elements of nuclei (after Pfitzner) . . 59

15. Diagram of fertilisation in ascaris (after Boveri) . 60, 61

16. a pollen grain with its nuclei (from carnoy) ... 63

17. Diagram to illustrate the difference between modifica-

tions AND VARIATIONS 71

18. Varieties of wall-lizard (after Eimer) .... 74

19. Variations in wasp (after Kellogg and Bell) . facing 76

20. Variations in beetle (after Tower) ... .,76

21. Mutation in medusoids . 89

22. Mutations of hart's tongue fern (after Lowe) . facing 98

23. Karyokinesis (after Flemming) . . ,, 102

24. Leaves of willow (after Wiesner) . . . . .111

25. Devonshire pony with stripes (from Darwin) . . .121

26. Varieties of domestic pigeon (after Darwin) . .139

27. Brine-shrimp, Artemia salina ...... 213

27a. Tail-lobes of Artemia salina (after Schmankewitsch) . 213
38. Half-lop rabbit (from Darwin) ...... 289

xv

XVI

LIST OF ILLUSTRATIONS

FIG. PAGE

29. Diagram illustrating Galton's law of ancestral inherit-

ance (after Galton and Meston) .... 325

30. Diagram to illustrate the difference between statis-

tical AND PHYSIOLOGICAL FORMULATION (AFTER DaRBISHIRE) 330

31. Peas showing Mendel's law .... facing 339

32. Diagram of Mendel's law . . . . . 340

33. Diagram of Mendelian inheritance in Mirabilis jalapa

facing 343

34. Diagram illustrating Mendel's law. . . . 347

35. Diagram illustrating segregation of germ-cells . . 344

36. Combs of Fowls ...... .facing 353

17. Hybridisation in Mirabilis jalapa (from Correns) . ,, 355

38. Mendelian phenomena in nettles (from Correns) . 357

39. Mendelian phenomena in wheat (after R. H. Biffen) facing 358

40. Mendelian phenomena in Helix hortensis (after Lang)

facing 360

40a. Pure lines in Paramecium (from Jennings) . . . 378

41. Varieties of wheat (after R. H. Biffen) . . . 384

42. Modes of segmentation .... facing 438

43. Relation between reproductive cells and the " body " . 430

44. Diagram of maturation and fertilisation . . . 436

45. Sexual dimorphism in humming-birds (from Darwin, after

Brehm) ......... 479

46. Sexual dimorphism in grasshoppers (from Darwin) . .481

47. Diagram illustrating the relation between reproduction

and individuation . . ..... 539

HEREDITY

CHAPTER I

HEREDITY AND INHERITANCE : DEFINED AND ILLUSTRATED

§ i. Importance of the Study of Heredity.

§ 2. What the Terms Mean.

§ 3. Heredity and Inheritance in Relation to other Bio

logical Concepts.
§ 4. A Question of Words.
§ 5. The Problems Illustrated.
§ 6. Denials of Inheritance.

§ 1. Importance of the Study of Heredity

Heredity determines the Individual Life.— There are no

scientific problems of greater human interest than those of
Heredity — that is to say, the genetic relation between successive
generations. Since the issues of the individual life are in great
part determined by what the living creature is or has to start
with, in virtue of its hereditary relation to parents and ancestors,
we cannot disregard the facts of heredity in our interpretation
of the past, our conduct in the present, or our forecasting of the
future. Great importance undoubtedly attaches to Environ-

2 HEREDITY AND INHERITANCE

ment in the widest sense, — food, climate, housing, scenery, and
the animate milieu ; and to Function in the widest sense, —
exercise, education, occupation, or the lack of these ; but all
these potent influences act upon an organism whose fundamental
nature is determined, though not rigidly fixed, by its Heredity —
that is, we repeat, by its genetic relation to its forebears. As
Herbert Spencer said, " Inherited constitution must ever be the
chief factor in determining character " ; as Disraeli said, more
epigrammatically and less correctly, " Race is everything."

Heredity is a Condition of all Organic Evolution. — In the
same way, when we consider the race rather than the individual,
we must admit that in so far as evolution depends on inborn
organic changes, on what is bred in the bone and imbued in the
blood, as distinguished from individual efforts and acquirements,
external institutions and traditional culture, it is conditioned
by the hereditary relation which binds one generation to another.
Heredity is a condition of all organic evolution. Innate changes
or variations, which form the raw material of constitutional
progress or degeneracy, have direct racial importance because
they are certainly transmissible ; while, on the other hand,
bodily modifications or acquired characters, due to changes in
environment or in function, probably have no direct racial
importance, since there is little or no evidence that they are
ever hereditarily entailed. They are individually important,
and in human society they are of much moment, but if they
are not transmissible they do not take organic grip, and they
cannot afford material for selection to work with. For the
human race, the external heritage of tradition, institutions, and
law, the permanent products of literature and art, the registrated
results of science, and so on, are of paramount importance, but
they are outside the immediate problem of organic or natural
inheritance. As far as the slow, sure process of constitutional
or organic evolution is concerned, everything depends on the
heritable resemblances and the heritable variations which form

A CONDITION OF EVOLUTION 3

the material on which the many diverse forms of selection and
isolation operate.

In olden days thoughtful men seemed to see the threads of
life within the hands of three sister Fates, — of one who held the
distaff, of another who offered flowers, and of a third who bore
the abhorred shears of death. So, in Scandinavia, the young
child was visited by three sister Norns, who brought characteristic
gifts of the past, the present, and the future, which ruled the
life to be as surely as did the hands of the three Fates. So, too,
in days of scientific enlightenment, we still think of Fates and
Norns, though our conceptions and terms are very different.
What the living creature is or has to start with in virtue of its
hereditary relation ; what it does in the course of its activity ;
what surrounding influences play upon it, — these are the three
determining factors of life. Heredity, function, and environ-
ment— famille, travail, lieu — are the three sides of the bio-
logical prism, by which, scientifically, we seek to analyse the light
of life, never forgetting that there may be other components
which we cannot deal with scientifically, just as there are rays of
light which our eyes can never see.

In novels like Zola's Dr. Pascal, in plays like Ibsen's Ghosts,
in sermons and newspaper articles, in large books and health
lectures, in season and out of season, we have all heard in the
last few years much about the importance of heredity ; and
though it is to be feared that many widespread impressions on
the subject are misleading, the awakening of keen interest is
in itself a symptom of progress. What is now required is a
serious study of what has been securely established. Otherwise
we shall continue to think in platitudes and act on guesses.

Practical Importance to Breeders and Cultivators. — And
what is important in regard to Man's heredity is even more
demonstrably important in regard to his domesticated animals
and cultivated plants. What has been achieved in the past m
regard to horses and cattle, pigeons and poultry, cereals and

4 HEREDITY AND INHERITANCE

chrysanthemums, by experimental cleverness and infinite
patience, may be surpassed in the future if breeders and cultiva-
tors can attain to a better understanding of the more or less
obscure laws of inheritance on which all their results depend.
Importance in Biological Theory. — The study of heredity
is also of fundamental importance in the domain of pure science,
in the biologist's attempt to interpret the process of evolution
by which the complexities of our present-day fauna and flora
have gradually arisen from simpler antecedents. For heredity
is obviously one of the conditions of evolution, — of continuance
as well as of progress. There would have been heredity even
if there had been a monotonous world of Protists without any
evolution at all, but there could not have been any evolution
in the animate world without heredity as one of its conditions.
The study of heredity is inextricably bound up with the problems
of development, reproduction, fertilisation, variation, and so
on ; in short, it is one of the central themes of Biology.

§ 2. What the Terms Mean

The Terms are tinged with Metaphor. — In the popular, if
not also in the biological mind, there often lurks the idea of a
hypothetical agent possessing the organism and uniting the
congeries of its characters. Expressed in diverse ways, there
is a prevalent conception of an organismal unity which gives
coherence to the sum of qualities (see Sandeman, 1896). Espe-
cially in reference to higher animals with a rich mental life, many
find it impossible not to think of a " soul " or " self " to which
the body belongs. Naturally enough, therefore, the reappear-
ance in the offspring of qualities which characterised its parents
or its ancestors has been persistently likened to the inheritance
of a legacy. But this is to some extent a metaphorical expres-
sion, and not without its dangers.

At first the Organism and the Inheritance are Identical. — A

WHAT THE TERMS MEAN 5

moment's consideration suffices to show that ideas and phrases
borrowed from the inheritance of property — something quite
apart from the individual who inherits— are apt to cause ob-
scurity and fallacy when applied to the inheritance of characters
which literally constitute the organism and are inseparable
from it. Therefore, as the biological conception of inheritance
seems still to suffer from the irrelevancy of the analogy to which

Fig. 1. — Ovum of a threadworm (Ascaris), showing (a) the chromosomes
of the nucleus, and the reserve products in the surrounding cell-
substance. — From Carnoy.

the term owes its origin, let us dwell for a little on the fact that,
at the start of an individual life, the inheritance and the organism
are identical. In other words, the idea of organic inheritance
is merely a convenient scientific abstraction, by which we seek
to distinguish what the organism is, in virtue of its germinal
origin, from what it is as the result of the influence of ensuing
circumstances. If we may use Galton's and Shakespeare's
terms, the idea of organic inheritance is an abstraction by which

6 HEREDITY AND INHERITANCE

we seek to distinguish what is due to " Nature " from what is
due to " Nurture."

Heredity and Inheritance defined. — In regard to property
there is a clear distinction between the heir and the estate which
he inherits, but at the beginning of an individual life we cannot
biologically draw any such distinction. The organism and its
inheritance are, to begin with, one and the same. It is easy to
make this clear. Every living creature arises from a parent
or from parents more or less like itself ; this reproductive or
genetic relation has a visible material basis in the germinal
matter (usually egg-cell and sperm-cell) liberated from the
parental body or bodies ; by inheritance we mean all the qualities
or characters which have their initial seat, their physical basis,
in the fertilised egg-cell ; the expression of this inheritance in
development results in the organism. Thus, heredity is no
entity, no force, no principle, but a convenient term for the
genetic relation between successive generations, and inheritance
includes all that the organism is or has to start with in virtue of its
hereditary relation.

Nature and Nurture.— The fertilised egg-cell implicitly con-
tains, in some way which we cannot image, the potentiality
of a living creature, — a tree, a daisy, a horse, a man. If this
rudiment is to be realised there must be an appropriate
environment, supplying food and oxygen and liberating-stimuli
of many kinds. Surrounding influences — maternal or external —
begin to play upon the developing germ, and without these
influences the inheritance could not be expressed, the potentiali-
ties could not be realised. Thus, the organic inheritance implies
an environment, apart from which it means nothing and can
achieve nothing. Indeed, it is only by an abstraction that we
can separate any living creature from an environment in which
it can live. Life implies persistent action and reaction between
1 organism and environment.

But while the inherited nature and its possibilities of action

NATURE AND NURTURE 7

and reaction must be regarded as rigorously determined by the
parental and ancestral contributions, the nurture — the en-
vironmental influences — must not be thought of as pre-deter-
mined. In fact, the surrounding influences are very variable,
and the nature of the young organism may be profoundly
changed by them. Thus, we soon find it possible to distinguish
between the main features, which are the normal realisations
of the inheritance in a normal environment, and peculiarities
which are due to peculiarities in nurture. The characters of a
newly-hatched chick stepping out of the imprisoning egg-shell
are in the main strictly hereditary ; but they need not be alto-
gether so, for during the three weeks before hatching there has
been some opportunity for peculiarities in the environment to
leave their mark on the developing creature. Still more is
this the case with the typical mammalian embryo, which develops
often for many months as a sort of internal parasite within the
mother — in a complex and variable environment. And as life
goes on, peculiarities due to nurture continue to be superimposed
on the hereditary qualities.

William of Occam's Razor. — Our preliminary attempt to get
rid of capitals, to make the terms heredity and inheritance quite
objective, is in line with what has occurred in other departments
of science. For one of the distinctive features of the nineteenth
century has been a reduction in the number of supposed separate
powers or entities— the use of William of Occam's razor, in fact.
" Entia non sunt mnltiplicanda prceter necessitatem." " Caloric "
was one of the first to be eliminated, yielding to the modern
interpretation of heat " as a mode of motion " ; " Light " had
to follow, when the undulatory or the electro-magnetic theory
of its nature was accepted ; a specific " Vital Force " is disowned
even by the Neo-vitalists ; " Force " itself has become a mere
measure of motion ; and even " Matter " tends to be resolved
into units of negative electricity, carrying with them a bound
portion of the ether in which they are bathed; and so on. In

8 HEREDITY AND INHERITANCE

view of this progress towards greater precision and simplification
of phraseology, it cannot be a matter for surprise that a biologist
should affirm that to speak of the " Principle of Heredity " in
organisms is like speaking of the " Principle of Horologity " in
clocks. The sooner we get rid of such verbiage the better for
clear thinking, since heredity is certainly no power, or force,
or principle, but a convenient term for the relation of organic
or genetic continuity which binds generation to generation,
Ancestors, grandparents, parents are real enough ; children
and children's children are also very real ; heredity is a term
for the relation of genetic continuity which binds them together.
We study it as a relation of resemblances and differences which
can be measured or weighed, or in some way computed ; as a
relation which is sustained by a more or less visible material basis
—namely, the germinal matter

§ 3. Heredity and Inheritance in Relation to other Biological

Concepts

Development. — All living creatures arise from parents more
or less like themselves. The reproduction may be asexual, — by
fission, fragmentation- budding, and similar processes ; or
sexual, — by special germ-cells or gametes, which usually unite
in pairs (fertilisation or amphimixis) to start a new individual
body. Whatever the mode of reproduction may be — and that is
a long story by itself — there is a hereditary relation, a genetic
continuity. It is the business of the theory of heredity to inquire
into the precise nature of this genetic relation in the diverse
modes of reproduction. In what relation, for instance, does a
liberated germ-cell or gamete stand to the body which liberates
it ? In what relation does a fertilised ovum stand to the germ-
cells of the body into which it develops ? What contribution
does each parent make to the inheritance ? Do ancestors also
make contributions, and if so, how ? To answer this kind of
question is the business of the theory of heredity.

RELATION TO OTHER BIOLOGICAL CONCEPTS 9

The separated fragment or the liberated germ-cell has in it
the possibility of becoming, in an appropriate environment, a
fully-developed organism. Is it possible to form any conception
— verifiable or speculative — of the manner in which the in-
heritance is thus condensed into a fragment or into a germ-cell ?
Is it possible to picture in any way how the potentialities come
to be realised in development ; how the obviously complex
grows out of the apparently simple ? To answer these and
similar questions is the business of the theory of development.

The facts of inheritance are those which rise into prominence
when we compare the characters of an organism with those of
its parents and its offspring, or when we compare the characters
of one generation with those of its predecessors and successors.
This is a thoroughly concrete study, for the facts observed are
quite independent of any theory of the precise organic relation
which binds generation to generation {the theory of heredity),
and are also quite independent of any theory as to the way in
which the germ grows into the adult (the theory of development).
It is, in the main, an experimental and statistical study.

Before the middle of the nineteenth century considerable
attention was given to what may be called the demonstration
of the general fact of inheritance — that like tends to beget like.
This had, indeed, always been the general opinion of physicians
and naturalists, as well as of the laity, but it was a useful task
to collect documentary evidence showing that all the inborn
characteristics of an organism, whether physical or psychical,
normal or abnormal, important or trivial, were transmissible
to the offspring, if the possibility of having offspring had not
been excluded. This task of demonstrating inheritance was
well finished by Prosper Lucas, whose large treatise, published
in 1847, gave ample evidence for what we now take for granted, —
that the present is the child of the past ; that our start in life is
no haphazard affair, but is rigorously determined by our paren-
tage and ancestry ; that all kinds of inborn characteristics may

io HEREDITY AND INHERITANCE

be transmitted from generation to generation. In short, the
fundamental importance of inheritance was long ago demon-
strated up to the hilt.

It remains, however, (i) to make the evidence of transmissibility
more precise and systematic ; (2) to inquire into the trans-
missibility of subtle characters such as longevity and fecundity ;
(3) to discover the different degrees of transmissibility, for some
characters are much more heritable than others ; and (4) to
classify different modes of hereditary resemblance — e.g. blending
of the characters of the two parents, taking after the father in
one feature and after the mother in another, apparently resem-
bling one parent only, rehabilitating a grandsire's features,
harking back to a remoter ancestor, and so on. What happens
when there is close in-breeding or pairing within a narrow radius
of relationship ? What happens when two hybrids are paired ?
In what sense, if any, is a disease heritable ? These and many
similar questions will be discussed in our inquiry into the facts
of inheritance.

Variation — Whenever we begin to compare the characters
of an organism with those of its parents, we discover that the
familiar saying, " Like begets like," must be modified into, " Like
tends to beget like." On the one hand, the child is like its
parents, " a chip of the old block," a literal reproduction ; on
the other hand, the child is something original, a new pattern,
a fresh start — leading the race. We do not gather grapes of
thorns, or figs of thistles ; yet two brothers may be very unlike
one another or either of their parents, and even the peas in one
pod may be different. On the one hand, there is a tendency
towards continuity, towards persistence of characters, towards
complete hereditary resemblance — in short, a kind of organic
inertia in a family or stock or species. On the other hand, there
is a tendency towards variation, towards new departures, to-
wards incomplete hereditary resemblance, or much more than
that. It is necessary to hold the balance between these two

VARIATIONS AND MODIFICATIONS n

sets of facts, both expressions of the hereditary relation, —
inertia, persistence, continuity, resemblances, on the one hand ;
deviation, novelty, differences, on the other.

Can we hope to discriminate an apparent difference between
parent and offspring, which is really due to an incompleteness
in the expression of the inheritance, from a real difference, which
is due to the dropping out of an old hereditary item or the
addition of a new one ? Can we distinguish between inborn
peculiarities — germinal variations — and acquired, nurtural pecu-
liarities ? Can we distinguish between variations which seem
to be simply a little less or a little more of some hereditary
character, and variations which involve something new ?
These and similar questions must be faced in the study of
variation.

Modifications. — Furthermore, whenever the study of the facts
of inheritance becomes critical, it is necessary to try to dis-
criminate between inborn changes, which must have a germinal
origin, and are therefore in the strict sense inherited, and are
liable to be transmitted, and those theoretically quite different
changes which are acquired by the body of the individual off-
spring as the result of peculiarities in function and environment.
This is the contrast between germinal variations and bodily
modifications, a contrast which is of fundamental importance
in several ways. It is important to try to distinguish resem-
blances and differences due to inherited nature from resemblances
and differences due to nurture. A collier may have his collier
father's red hair, and he may also resemble him in having " col-
lier's lung." But while the first resemblance is a fact of in-
heritance, the second is due to the similarity in their life-con-
ditions. This distinction remains important whatever conclusion
be reached in regard to the transmissibility of modifications,
but its importance is enhanced when we discover that practically
all variations (except sterility) are transmissible, though not
always transmitted, and that the evidence of any modification

12 HEREDITY AND INHERITANCE

being transmissible, among multicellular organisms reproducing
sexually, is extremely doubtful.

Evolution. — Briefly and concretely stated, the general doc-
trine of organic evolution suggests, as we all know, that the
plants and animals now around us are the results of natural
processes of growth and change working throughout unthinkably
long ages ; that the forms we see are the lineal descendants of
ancestors on the whole somewhat simpler ; that these are de-
scended from yet simpler forms, and so on, backwards, till we
lose our clue in the unknown, but doubtless momentous, vital
events of pre-Cambrian ages, or, in other words, in the thick
mist of life's beginnings. The essentially simple idea is that
the present is the child of the past, and the parent of the future.
It is a way of looking at organic history, a genetic description,
a modal formulation. A process of Becoming leads to a new
phase of Being ; the study of evolution is a study of Werden
mid Vergehen mid Wetter- werden.

But we have to pass from a modal interpretation to a causal
one. We have to try to discover the factors in the age-long
process, and this leads us into a region where at present uncer-
tainties abound. As biologists we start with the postulate of
simple living organisms — feeding, working, growing, wasting,
reproducing in an appropriate environment. And we try to
discover the possible factors in the long evolution-process, the
outcome of which is the present-day world of life. Amid all
the uncertainties, this is certain, that the fundamental condition
of evolution is that genetic relation which we call heredity, — a
relation such that it admits, on the one hand, of a continuity
of hereditary resemblance from generation to generation ; and,
on the other hand, of an organic changefulness which we call
variability. Without the hereditary relation there could have
been no succession of generations at all. Without hereditary
resemblance on the one hand, and hereditary variation on the
other, there could have been no evolution. Any discussion of

A QUESTION OF WORDS 13

the secondary or directive factors which operate upon the raw
materials of progress which variability supplies — notably
Selection and Isolation — is not relevant at present.

§4-/1 Question of Words

In every discussion with a serious purpose it is important that
there should be clearness as to the terms used. We must,
therefore, ask the reader to notice our definition of the chief
terms. Thus by " heredity " we do not mean the general fact
of observation that like tends to beget like, nor a power making
for continuity or persistence of characters — to be opposed to the
power of varying — nor anything but the organic or genetic relation
between successive generations ; and by " inheritance " we mean
" organic inheritance " — all that the organism is or has to start
with in virtue of its hereditary relation to parents and ancestors.
We do not forget that for man in particular there is an external
heritage — a social inheritance — which counts for much. By
innate or inborn we mean all that is potentially implied in the
fertilised egg-cell ; by the expression of the inheritance we
mean the realisation of inborn potentialities in the course of
development in an appropriate environment ; by a congenital
character {pace many medical writers) we mean one demonstrable
at birth, which is not necessarily germinal, being often due to
peculiarities — e.g. infection or poisoning or mechanical injury
during pre-natal development. Thus, tubercle may be con-
genital, but it is never inherited. By modifications or acquired
characters we mean structural changes in the body induced
by changes in the environment or in the function, and such that
they transcend the limit of organic elasticity, and therefore
persist after the inducing conditions have ceased to operate.
By a variation we mean not any observed difference between
offspring and parent, between an individual and the mean of

i4 hEREDlTY AND INHERITANCE

the stock in respect of a given character ; we mean observed
differences minus all bodily modifications, we mean changes
which have a germinal origin.

These definitions will become clearer in the course of our
exposition. Our present point is to warn the reader against
starting on his journey without reading the conditions on the
ticket, and to protest against the slackness with which the terms
are so often used. A large part of the energy expended on the
long-drawn-out controversy as to the transmission of acquired
characters or modifications has been wasted through inattention
to the precise significance of the technical terms employed.*

To speak of a man " fighting against his heredity " may
express a real fact, but it is verbally erroneous. The American's
question, " Is my grandfather's environment my heredity ? "
is an offence against ordinary English as well as against scientific
phrasing ; it should probably read, " Have the structural
changes induced by environmental influences on my grand-
father's body had any effect on my inheritance ? " Nor can
we pardon from an expert such a sentence as this, " I look upon
Heredity as an acquired character, the same as form or colour,
or sensation is, and not as an original endowment of matter "
(Bailey, 1896, p. 23). When the moralist writes : " The only
limitations imposed on a man are those which his own nature
makes," the biologist asks, " But what is his own nature ? Is

* It may be noted that Galton's work on Natural Inheritance is rightly
so entitled, for it deals mainly with a statistical comparison of the char-
acters of successive generations. Inheritance is also the chief subject of
the works of Lucas and Ribot, although these have heredity for their
title. Or, to take another example, Weismann's work entitled The Germ-
Plasm, a Theory of Heredity, is in great part a theory of heredity, but,
naturally enough, it is also in great part a theory of development. The
German language has the same word, Vererbung, for both Heredity and
Inheritance. As the English language is rich in related terms, laxity
of expression is less excusable. Besides " heredity " and " inheritance"
we have " heritage," " transmission," and so on. It may be convenient
to speak of the parent as transmitting and of the offspring as inheriting.

DEFINITIONS 15

it not the expression of a predetermined inheritance in a more
or less predetermined environment ? "

Definitions of " Heredity." — It may be of interest to give a
few samples of definitions :

" The word ' Heritage ' has a more limited meaning than ' Nature,'
or the sum of inborn qualities. Heritage is confined to that which
is inherited, while Nature also includes those individual variations
that are due to other causes than heredit}', and which act before
birth." — Francis Galton, Natural Inheritance, 1898, p. 293.

" Heredity is the law which accounts for the change of type
between parent and offspring, i.e. the progression from the racial
towards the parental type." — Karl Pearson, The Grammar of
Science, 1900, p. 474.

" Under heredity we understand the transference to the offspring
of qualities of the parent or parents." — T. H. Montgomery, Jr.,
Proc. American Phil. Soc. xliii. 1904, p. 5. [But the line of descent
is from germ-cell to germ-ceil. The parent is the custodian or
trustee of the germ-cells rather than their producer. It is too
metaphorical to speak of the " parent transferring qualities to the
offspring." The hereditary relation includes the occurrence of
variations as well as the reproduction of likenesses. And what
are the offspring apart from their inheritance ?]

" ' Heredity ' is most usually defined by biologists as referring
generally to all phenomena covered by the aphorism ' like begets
like.' In this sense it denotes, inter alia, the phenomenon of the
constancy of specific or racial types and of sexual characters ; a
character may be said to be inherited when it always, in one genera-
tion after another, is one of the characters of the species, of the
race, or of the one sex of the race, as distinct from the other. The
species, race, or sex, so to speak, ' begets its like ' as a whole. But
then a further question remains ; even if the type of the race is
constant, do individual types within the race beget their like ?
In so far as any individual diverges in character from the mean of
the race, do his offspring tend to diverge in the same direction, or
not ? It is to this question that statisticians have confined them-
selves, and they speak of a character being ' inherited ' or not
according as the answer to the question is yes or no — they deal
solely with what we may term ' individual heredity.' " — G. Udney
Yule, 1902, p. 196. [Biologists are as much concerned with individual

1 6 HEREDITY AND INHERITANCE

heredity as statisticians are, indeed more so ; statistical results are
based on individual data, but they do not admit of individual
application.]

" Living matter has the special property of adding to its bulk
by taking up the chemical elements which it requires and building
up the food so taken as additional living matter. It further has the
power of separating from itself minute particles or germs which
feed and grow independently and thus multiply their kind. It is
a fundamental character of this process of reproduction that the
detached or pullulated germ inherits or carries with it from its
parents the peculiarities of form and structure of its parent. This
is the property known as Heredity. It is most essentially modified
by another property — namely, that though eventually growing to
be closely like the parent, the germ (especially when it is formed,
as is usual, by the fusion of two germs from two separate parents)
is never identical in all respects with the parent. It shows Variation.
In virtue of Heredity, the new congenital variations shown by a
new generation are transmitted to their offspring when in due time
they pullulate or produce germs." — E. Ray Lankester, Kingdom
of Man, 1907, p. 10.

" By inheritance we mean those methods and processes by which
the constitution and characteristics of an animal or plant are handed
on to its offspring, this transmission of characters being, of course,
associated with the fact that the offspring is developed by the
processes of growth out of a small fragment detached from the
parent organism." — R. H. Lock, Recent Progress in the Study of
Variation, Heredity, and Evolution, 1906, p. 1.

" Heredity. — The transference of similar characters from one
generation of organisms to another, a process effected by means of
the germ-cells or gametes." — Lock, op. cit. p. 292.

§ 5. The Problems Illustrated

Even in ancient times men pondered over the resemblances
and differences between children and their parents, and wondered
as to the nature of the bond which links generation to generation.
But although the problems are old, the precise study of them is
altogether modern. The foundations of embryology had to be
laid, the nature and origin of the physical basis of inheritance

THE PROBLEMS ILLUSTRATED 17

— the germ-cells — had to be elucidated, the general idea of evolu-
tion had to be realised, before the problems of heredity and
inheritance could even be stated with precision. Moreover,
it seems to have required the experience of many years of
" fumbling " before the main body of biologists became con-
vinced that the problems could not be satisfactorily studied
in the armchair, nor settled by a priori argument. Now, however,
it is unanimously agreed that a satisfactory study of heredity
and inheritance demands a minute inquiry into the history of the
germ-cells, a statistical study of the characters of successive
generations, a careful criticism of the older data and of popular
impressions, and a testing of hypotheses by experimental
breeding. Let us give a few random illustrations in order
to show what some of the problems are :

The race-horse Eclipse was the sire of many foals : it is a
problem in heredity to compare them with him, and to inquire
into the vital arrangements, in virtue of which many of them
reproduced his remarkable quality of swiftness. He had also
a peculiar, quite useless spot of colour, which reappeared even
in the sixth generation of his progeny.

In the ancestry of Kaiser Wilhelm II. there have been four
grandparents, eight great-grandparents, fourteen (not 16) great-
great-grandparents, twenty- four (not 32) great-great-great-grand-
parents : it is a problem in heredity to compare the qualities of
these successive generations of ancestors, and to inquire if they
render more intelligible the illustrious personality whose doings
and sayings are familiar to us all.

The assassin of the Empress of Austria is said to have been
the child of a dissolute mother and a dipsomaniac father : it
is a problem in heredity to inquire whether this parentage
may render more intelligible an outrage which made Europe
shudder.

A white man of considerable intellectual ability marries a
negro woman of great physical beauty and strength ; the result

2

18 HEREDITY AND INHERITANCE

may be — has been — a mulatto who inherits some of his father's
intellectual virtue and some of his mother's physical strength,
including, for instance, a peculiar insusceptibility to yellow-
fever. Here are complex problems of inheritance. How is
it that certain characteristics of the son are almost wholly
of paternal origin, while in other respects he takes after his
mother ?

An English sheep-dog may show a paternal eye on one side of
the head, a maternal eye on the other. A piebald foal may
have its mother's hair on some patches, its father's hair on
others. Such cases raise the problem of the different modes of
hereditary resemblance, of the mosaic-like constitution of an
inheritance, and of the various ways in which this may find
expression in development.

Given in our British population a thousand fathers six feet
high, we can predict with great accuracy the average height of
their sons. Though we cannot make any prediction as to an
individual family, we know that the average height of all the
sons of these tall men will be nearer the average height of the
total male population than the height of six feet is. We know,
however, that the tall do not always beget the tall, nor the
small the small ; that stature in mankind is a character that
blends ; and that even among the sons of the thousand fathers
we have spoken of, there will be every gradation between the
tallest and the smallest. How different this is from stature in
pure-bred peas, for if a tall variety of pea be crossed with a
dwarf, all the offspring are tall, and among their offspring
in turn three-fourths are tall and one-fourth dwarf, but none
between the two.

White fowls crossed with black ones often have white off-
spring ; black guinea-pigs crossed with white ones have black
offspring ; black-eyed white guinea-pigs crossed with albinos
have black offspring. It seems at first sight arbitrary, but a
rational interpretation of earh of these results has been given.

THE PROBLEMS ILLUSTRATED 19

A pair of blue Andalusian fowls of selected breed have chickens.
But only about half of these are " blue," the rest are blacks or
splashed whites. Why is this ? The blacks inbred produce
only blacks, the splashed whites produce splashed whites or
whites, but if the blacks and splashed whites are paired the
progeny is altogether " blue." Why is this ?

We read of a mare which, after bearing a foal to a quagga,
bore a zebra-striped foal to a horse. Breeders of dogs say that
a thoroughbred bitch is spoilt for true breeding if she has once
been crossed by a mongrel. Is it possible that a father can
influence the subsequent offspring of the same mother by a
different father ? This is a problem partly in scientific criticism
of evidence, but it raises interesting questions regarding the
physiology of reproduction and regarding the hereditary relation.

In the sixteenth century Montaigne was puzzled by the fact
that, at the age of forty-five, he developed, just like his father,
a stone in the bladder. The puzzle of the supposed legacy had
its fine point in the fact that his father did not develop his stone
till he was sixty-seven years of age, or twenty-five years after
Montaigne was born ! It is possible that there was here an
interesting problem in inheritance ; but the likelihood is that it
merely illustrated the commonest of phenomena, the inheritance
of a constitutional tendency and the repetition of more or less
similar habits of life.

Far too much has been made of homochronous heredity ! —
i.e. of the fact that some item in the inheritance may be ex-
pressed in the offspring at the same age as in the parents. Thus
two brothers, their father, and their maternal grandfather be-
came deaf at the age of forty ; blindness occurred in a father
and in his four children at the age of twenty-one. But if the
constitutions are similar and if the conditions of life are similar,
it is not surprising that the expression of an item in the con-
stitution should reach its climax at the same age.

A case is recorded of abnormalities' in the fingers traceable

20 HEREDITY AND INHERITANCE

through six generations, and the pathologist Bouchut (cited by
Ziegler) refers the origin of the evil to the rage of an ancestor,
who terrified his wife during her pregnancy with the wish that
the fingers with which she had plucked an apple against his
orders might be cut off ! Apart from the story's quaint sugges-
tion of a much older episode, it requires but an elementary know-
ledge of the facts of heredity and inheritance to convince us that
the alleged cause was inadequate to account for the effects.

In two hundred families tainted with a predisposition to
haemophilia — an excessive and chronic liability to immoderate
haemorrhage — Grandidier * found six hundred and nine male
" bleeders." It is a problem of inheritance (and partly perhaps
of sexual physiology) to discover why the disease should be
restricted to males ; and the interest of the problem is enhanced
by the fact that the disease rarely passes from father to son, but
usually from a male parent, through an apparently unaffected
daughter, to a grandson. In short, the female offspring of
bleeders hand on the taint to male offspring, without themselves
showing the disease, f

De Candolle I reported from American statistics that thirty
per cent, of the children of congenitally deaf-mute parents were
deaf-mute, but that the percentage was fifteen when only one
parent was congenitally deaf-mute. It is a problem of heredity
to interpret the greater frequency of inheritance when both
parents were affected.

While there is much and justifiable uncertainty in regard to
the origin of what are called instincts, there is no doubt that an
organism's inheritance often includes the power of carrying out
a complex series of operations without experience and without
education when the appropriate stimuli occur.

* Grandidier, Die Hemophilic (1876).

| Bulloch and Fildes, Htsmophilia. Treasury of Inheritance, Pt. xiva.
(1911).

% De Candolle, Arch. Sci. Phys. Nat. xv. p. 25, cited by Ziegler (1886).

THE PROBLEMS ILLUSTRATED 21

Simple illustrations are afforded by instinctive likes and dis-
likes, attractions and repulsions. " So old is the feud between
the cat and the dog," says Spalding, " that the kitten knows its
enemy before it is able to see him, and when its fear can in no
way serve it. One day, after fondling my dog, I put my hand
into a basket containing four blind kittens three days old. The
smell that my hand carried with it set them puffing and spitting
in a most comical fashion."

Experiments with young birds hatched from artificially in-
cubated eggs and kept away from all contact with their kind
show conclusively that certain capacities are truly part of the
inheritance, and require no experience or suggestion, while
others not more complex require to be learnt. Thus the power
of uttering the characteristic call-note is inborn, but chicks
require to learn what is good for eating and what is deleterious.
Thus the power of executing the proper swimming and diving
movements is inherited, but chicks do not instinctively know that
water is drinkable. It is one of the problems of inheritance to
distinguish between inborn capacities and those which require
education.

An even more difficult problem, which Prof. Pearson has
successfully tackled by an ingenious indirect method, relates
to the inheritance of man's mental and moral qualities. Though
very plastic, there is no doubt that they are inherited in rudi-
ment, just like physical characters. Just as the Romans dis-
tinguished physically the long-nosed Nasones, the thick-lipped
Labeones, the swollen-cheeked Buccones, and the big-headed
Capitones, so, as Voltaire points out, " the Appii were ever proud
and inflexible, and the Catos always austere."

The literature of inheritance is crowded with examples of the
transmissibility of what we cannot but call trivial peculiarities,
though the probability is that they are often the correlates of
what is important. A few illustrations may be selected :

" A gentleman had a peculiar formation of the right eyebrow.

22 HEREDITY AND INHERITANCE

It was strongly arched, and some of the hairs in the centre grew
upwards. Three of his sons have the same peculiarity ; one of
his grandsons has it also ; so has his great-granddaughter, and,
if we are to believe the artists, this gentleman's grandfather
and great-grandfather had the same peculiarity " (R. W.
Felkin).

" There was a family in France, of whom the leading repre-
sentative could when a youth pitch several books from his head
by the movement of the scalp alone. His father, uncle, grand-
father, and his three children possessed the same power to the
same unusual degree. This family became divided eight genera-
tions ago into two branches, so that the head of the above-
named branch was cousin in the seventh degree to the head of
the other branch. This distant cousin resided in another part
of France, and on being asked whether he possessed the same
faculty, immediately exhibited his power."

A woman with blonde hair, a birth-mark under the left eye,
and a lisp, married a man with dark hair and normal utterance.
There were nineteen children, none of whom showed any of the
mother's characters. Nor among the numerous grandchildren
was there any trace. In the third generation, however, there
was a girl with blonde hair, a mark below the left eye, and a lisp.

Girou tells of a man who had the peculiar habit of always
sleeping on his back with his right leg crossed over his left. His
daughter showed the same habit almost from infancy, and per-
sisted in it in spite of efforts made to make her sleep in an ortho-
dox position. Darwin gives an even better case where a very
peculiar gesture reappeared ; and there seems no doubt that
trivial peculiarities, e.g. playing with a lock of hair and idio-
syncrasies of handwriting, may reappear even in cases where
imitation was out of the question (Biichner, 1882, p. 42).

And thus the list may be followed till we end with evidence
of the inheritance of minutiae often of a most trivial character.
Thus : " Schook relates the case of a family nearly all the mem-

DENIALS OF INHERITANCE 23

bers of which could not endure the smell of cheese, and some of
them were thrown into convulsions by it " (R. W. Felkin).
Here again we are forced back to the general thesis that the
germinal organisation is a coherent individualised unity, which
may find similar expression in the most detailed peculiarities
of the body.

§ 6. Denials of Inheritance

The resemblance between offspring and their parents, both
in general and in particular, as to abnormal as well as normal
characteristics, cannot be denied as a fact, but it has often been
denied as the result of transmission. Although the denials,
which have varied greatly in degree and motive, are for the
most part due to misunderstanding, they may deserve brief
consideration, since even to-day we sometimes hear cultured
men declaring that " they do not believe in heredity."

The extreme position may be represented by Wollaston, a
scientific philosopher of the end of the eighteenth century, who
sought to conserve the integrity and sanctity of the human spirit
by altogether denying transmission. Each new life was to his
mind a fresh start, unrelated in any real sense to parents or
ancestors.

The speculative naturalist Bonnet and many others admitted
the inheritance of generic and specific characters, but denied
that of individual characteristics.

Buckle is the most illustrious example of those who, while
admitting the inheritance of bodily characters, firmly deny
that the same is true in regard to the mind. Buckle maintained
that the ordinary method of demonstrating the inheritance of
talents by collecting examples of similar mental peculiarities
in father and son is in the highest degree illogical ; it neglects,
for instance, the frequency of coincidence, and yet more the
results of similar upbringing and environment.

24 HEREDITY AND INHERITANCE

A consideration of these denials, which have ceased to appeal
to many, may be of use as affording opportunity for emphasising
two facts.

I. Reappearance of a character from generation to generation
does not of itself prove the inheritance of that character, if it
be originally interpretable as the result of nurture (influences
of activity and surroundings operative on the body), and if
there be from generation to generation a persistence of the
conditions which were originally instrumental in evoking the
character. It is plain that the reappearance may be the result
of similar effects hammered on each successive generation.

Alpine plants brought to a lowland garden have been known
to become much changed, and their descendants likewise. But
there is good reason to believe, as we shall afterwards see, that
the novel conditions directly impressed their effects on each
successive crop.

What impressed Buckle was the power of the environment
in the widest sense ; it holds the organism in its grip, and hammers
it into shape. This no one will gainsay, but we know that
similar nurture has different results on different natures ; the
duckling is not known to be less a duckling because hatched
and brought up by a hen. Moreover, we know of the reappear-
ance from generation to generation of many characteristics
which cannot be interpreted as due to nurture — which often
emerge, indeed, in the very teeth of nurture.

At the same time, it is of great importance to bear in mind that
an organism cannot be separated from its environment except at
the risk of some fallacy. We may say that along with the organic
heritage contained in the germ-cells every organism has what
may be called an external heritage of appropriate environmental
influences, which supply the stimuli for normal development.

Appropriate food is part of the normal environment, and the
supply of oxygen and water may be grouped in the same set ; other
factors, like the osmotic pressure or the presence of calcium salts in

DENIALS OF INHERITANCE 25

the water, are conditions of embryonic coherence ; others, like light
and heat, serve to accelerate or to inhibit. It seems, also, that par-
ticular combinations of factors are required as the " liberating
stimuli" of particular characters in the developing organism. De-
velopment is the expression of the inheritance, and the fullness of the
expression depends on there being a normal environment. What is
called a hereditary defect may be simply a defect in expression due
to inadequate environment.

How fundamental the germinal nature is may be realised if we think
of Heape's experiment of transferring the fertilised ovum of a long-
haired white angora rabbit into another variety of rabbit — a short-
coated gray Belgian hare. The young were not less long-haired or
less white because of the transplantation of the ova. Similarly
Castle and Phillips removed the ovaries from a white albino guinea-
pig, inserted those of a young black individual, and had the grafted
animal mated with a male albino. Normal albinos mated together
always have albino young, but the animal experimented on had to the
albino male three litters (six young) all black. The foster-body did
not count.

2. Beneath the misunderstanding which has led some to deny
the facts of inheritance there is, as we have seen, a reasonable
though exaggerated recognition of the potency of similar function
and environment in producing resemblance ; and there is, per-
haps, the recognition of another fact — that of variation. For
several reasons — for instance, because the new life usually springs
from a fertilised ovum which combines maternal and paternal
contributions — the child is never quite like its parents. In other
words, we suppose that the germinal material from which a
child develops is not quite the same as that from which the parents
developed, or not quite the same as that from which its brothers
and sisters developed, and the result is variation in the true
sense. Each offspring has its individuality and is a new creation.
Even within a family no two are alike, especially to the care-
ful parent's eye, though the impartial onlooker may be struck
by the monotony. On the one hand, " Alle Gestalten sind
ahnlich " ; on the other, " Keine gleichet der andem."

CHAPTER II

THE PHYSICAL BASIS OF INHERITANCE

" Gebt mir Materie, und ich will daraus eine Welt schaffen." — Kant.

" We may regard the nucleus of the cell as the principal organ of
inheritance " (a prophecy proved true). — Haeckel, Generelle Morpho-
logic 1866, vol. i. p. 288.

" The cell is not only the seat of vital activity, but is also the vehicle
of hereditary transmission ; and the life of successive generations of
living beings shows no breach of continuity, but forms a continuous vital
stream in which, as Virchow said, rules an ' eternal law of continuity.' " —
Wilson, 1900, p. 76.

§ i. What is true in the Great Majority of Cases.

§ 2. Diverse Modes of Reproduction.

§ 3. The Hereditary Relation in Unicellular Organisms.

§ 4. The Hereditary Relation in the Asexual Multiplica-
tion of Multicellular Organisms.

§ 5. Nature and Origin of the Germ-cells.

§ 6. Maturation of the Germ-cells.

§ 7. Amphimixis and the Dual Nature of Inheritance
in Sexual Reproduction.

§ 8. Inheritance in Parthenogenesis.

§ 9. Wherein the Physical Basis precisely consists.

§ 1. What is true in the Great Majority of Cases

The Inheritance is usually carried by the Germ-cells.—

What was for so long quite hidden from inquiring minds, or but
dimly discerned by a few, is now one of the most marvellous of
biological commonplaces — that the individual life of the great
majority of plants and animals begins in the union of two minute
elements — the sperm-cell and the egg-cell. These microscopic
individualities unite to form a new individuality, a potential
offspring, which will by-and-by develop into a creature like to,
and yet different from its parents. If we mean by inheritance

26

INHERITANCE CARRIED BY GERM-CELLS 27

to include all that the living creature is or has to start with in
virtue of its genetic relation to its parents and ancestors, then
it is plain that the physical basis of inheritance is in the fertilised
ovum. The fertilised egg-cell is the inheritance, and at the
same time the potential inheritor. What might be compared to
an inheritance of property as apart from the organism itself is
the store of food which may be inside the egg, or round about it.

To the general fact stated in the preceding paragraph, a few
exceptions must be made — e.g. for bananas which have no longer
any seeds, for potatoes which are multiplied by cutting, for the
drone-bees and summer green-flies who have mothers but no
fathers, and for simple unicellular organisms in which there is
no sexual reproduction ; but the exceptions are trivial compared
with the vast majority of living creatures, in regard to which
it is certain that each life begins in a fertilised egg-cell.

An organic inheritance means so much, even when we use the
comfortable word potentiality, that, although we are quite sure
that the germ-cells constitute the physical basis of inheritance,
we may consider for a moment the difficulty which rises in the
minds of many when they are told that the egg-cell is often
microscopic, and the sperm-cell often only jooVtroth of the
ovum's size. Can there be room, so to speak, in these minute
elements for the complexity of organisation supposed to be
requisite ? And the difficulty will be increased if the current
opinion be accepted that only the nuclei within these minute
germ-cells are the true bearers of the hereditary qualities.
Darwin spoke of the pinhead-like brain of the ant as the most
marvellous little piece of matter in the world, but must we not
rank as a greater marvel the microscopic germ-cells which
contained potentially all the inherited qualities of that ant ?

From one microscopic egg of a sea-urchin cut into three,
Delage reared three larvae. In another case he reared an embryo
from ^th of an egg. Twin animals are often developed from
one egg. Wilson obtained quadruplets by shaking apart the

28 THE PHYSICAL BASIS OF INHERITANCE

four-cell stage in the development of the lancelet. Marchal
describes a " legion of embryos " developing from a single ovum
of a peculiar Hymenopterous insect Encyrtus. In development,
indeed, a half may be as good as a whole.

In reference to the difficulty raised in some minds by the
minuteness of the physical basis, it may be recalled that the
students of physics, who make theories regarding the sizes of
the atoms and molecules which they have invented, tell us that
the image of an ocean liner filled with framework as intricate
as that of the daintiest watches does not exaggerate the possi-
bilities of molecular complexity in a spermatozoon, whose actual
size is usually very much less than the smallest dot on the
watch's face. Secondly, as we learn from embryology that one
step conditions the next, and that one structure grows out of
another, there is no need to think of the microscopic germ-cells
as stocked with more than initiatives. Thirdly, we must re-
member that every development implies an interaction between
the growing organism and a complex environment without which
the inheritance would remain unexpressed, and that the full-
grown organism includes much that was not inherited at all, but
has been acquired as the result of nurture or external influence.
The fact is that size does not count for much in these matters,
and the difficulty that some beginners feel in believing that the
inheritance of the whale is packed into a pinhead-like egg is
mainly due to ignorance of what may be called the fine com-
plexity, or from another point of view the " coarse-grainedness,"
which must form part of our conception of every speck of matter.
Nowhere more than in biology are we made to feel that " a
little may go a long way."

It should be noted that the degree of visible complexity, even
in the microscopic nucleus of a germ-cell, is often very consider-
able. Thus Eisen observed in the nucleus of a species of sala-
mander twelve chromosomes, each of six parts, and in each part
six granules — altogether 432 visible units.

DIVERSE MODES OF REPRODUCTION

29

§ 2. Diverse Modes of Reproduction

In the preceding paragraph we have given prominence to
what is true of the great majority of living creatures, — that a
new life begins as a fertilised egg-cell. It is necessary, however,
to refer to the other ways in which a new organism may arise,
for some of them help us to understand what the hereditary
relation means. The following scheme will probably serve to
recall the familiar facts :

/ In unicellular
organisms.

Multiplication

By division into two.

By budding, a modified form of division.

By sporulation, or division into many units.
The reproduction may be wholly asexual : (1) in the sense
that there is nothing corresponding to fertilisation or amphimixis ;
and (2) in the sense that there are no special germ-cells. But in
many unicellular organisms there are elaborate processes of am-
phimixis, and in colonial forms, like Volvox, there is a definite
beginning of egg-cells and sperm-cells. Among the parasitic
Sporozoa or Gregarines in the wide sense there is also a close
approximation to the mode of sexual reproduction seen in most
multicellular organisms. No hard-and-fast line can be drawn.

In multicelhtlai
organisms.

I. Without special germ-cells — e.g. by division
of the body, by giving off buds (and as the
result of artificial cutting).
II. With special germ-cells :
(a) Eggs from one parent are fertilised by sperms from another

parent — heterogamy, the commonest mode ;
(6) Eggs from one parent are fertilised by sperms from the same

^hermaphrodite) parent — autogamy, a very rare mode.
(c) Eggs may develop without fertilisation — parthenogenesis.

[A multicellular organism may also multiply by spore-
cells — specialised germ-cells, yet hardly equivalent to eggs
— which do not require fertilisation.] *

* If we lay emphasis on the presence or absence of special reproductive elements, the classifi-
cation of the modes of multiplication would read as follows :

I. Without special repro- / Division, budding, etc., in most unicellulars.

ductive elements. (Division, budding, etc., in some multicellulars.

(More or less distinct specialisation of reproductive elements in
some unicellulars.
Specialised ova and spermatozoa in most multicellulars.
Formation of spore-cells in some multicellulars.

If we lay emphasis on the occurrence or non-occurrence of amphimixis (= fertilisation) the classi-
fication of the modes of reproduction would read as follows :

'Without special reproductive-cells: (a) division, budding, etc.,
in many unicellulars ; and (b) division, budding, etc., in
some multicellulars.
With special reproductive-cells : (a) formation of spores in some

L multicellulars ; (b) parthenogenetic ova.

{Without specialised reproductive elements, amphimixis occurs
in most unicellulars.
With specialised reproductive elements, amphimixis occurs in
a few unicellulars and in most multicellulars.

I. Without any form ofj
amphimixis.

II. With some form
amphimixis.

30 THE PHYSICAL BASIS OF INHERITANCE

The reasons for lingering over the modes of reproduction —
which it is confessedly difficult to arrange in a perfectly clear
scheme — are (i) that our general view of the hereditary relation
must be one which is applicable to all cases and not merely to
the most frequent, and (2) that some of the simplest cases shed
light upon the more complex. It is also important that we
should make clear that the common phrases, " asexual repro-
duction " and " sexual reproduction," are somewhat ambiguous,
since attention has to be directed to two distinct points —

(a) whether there are specialised reproductive elements, and

(b) whether there is any form of amphimixis.

§ 3. The Hereditary Relation in Unicellular Organisms

At what is called " the limit of growth," when the cell has
attained to as much volume as its surface can adequately supply
with food and oxygen, and so on, a unicellular organism normally
divides into two, obviating the difficulties which would ensue
if volume increased out of proportion to surface. The halves
separate and grow. Two more or less exact replicas of the
original unit result. It has been demonstrated that the division
is often preceded by that intricate and orderly process of nuclear
division, known as karyokinesis, which results in an equal
partition of the nuclear constituents between the two daughter-
cells. As each of the halves is in the strictest sense half of the
organisation of the parent unit, we are not surprised that each
should in appropriate environment grow into an almost exact
image of the original whole. In most cases we have no methods
subtle enough to detect any difference. There is complete here-
ditary resemblance, and it would be puzzling if it were otherwise.
Even when the unit divides into many units (as in spore-forma-
tion), there is no puzzle in the fact that each reproduces the
likeness of the original whole, except the puzzle of growth — of

HEREDITARY RELATION IN UNICELLULARS 31

life, which is at present insoluble. Analogies may be found in
methods of treating chemical molecules so that one gets at the
end of the operation twice as many molecules as one had to
start with ; or in the multiplication of crystals by breaking
them into fragments and placing them in solutions of the same
substance ; but, at the present time, these analogies are of no
particular service, since we do not understand the nature of
living matter. That a fragment of a unicellular's organisation
may, in an appropriate environment, reproduce an apparently
perfect replica of the original unit, is not in any way explained
by pointing out that there may be reproduction of like by like
in the case of crystals or chemical molecules.

chr*

Fig. 2. — Diagram of cell division (after Boveri).
chr. chromosomes, forming an equatorial plate; cs. centrosome.

In slightly more complex cases there is a difference between
the two units into which the unicellular organism divides.
Thus, in the oblique division of the slipper animalcule {Parame-
cium), the one half goes off with the " mouth," the other has
none. In a short time, however, the mouthless half forms a
" mouth," and each half grows into a replica of the original.
But as the organisation of each half is essentially the same as,
and directly continuous with the organisation of the original
cell, the development of the halves into similar wholes presents
no special difficulty. Similar organisation and similar surround-
ings yield similar results. That an injured infusorian should
by re-growth repair its loss is an analogous phenomenon. Thus

32 THE PHYSICAL BASIS OF INHERITANCE

we are led to see the force of Haeckel's definition of reproduction
as discontinuous growth.

But in many unicellular elements, what is liberated to begin
a new life is not a half of the original nor anything like it, but a
minute unit often called a " spore." It also grows into a com-
plete reproduction of the original. In such cases, we again try
to make the matter more intelligible, by saying that each spore
is a representative fragment of the organisation of the original
unit, and will therefore, in appropriate surroundings, grow and
differentiate as the original did. Exactly the same often occurs
when the unicellular organism is artificially divided into several
parts ; and the results of these microscopic vivisection experi-
ments, to which no one can on any grounds object, show that,
if the excised fragment is to survive and develop, it must have
a portion of the nuclear substance as well as of the general
cell-substance. Without the nuclear constituent it may live
for a time, as in Stenlor, moving and responding to stimuli,
but it cannot assimilate. Therefore, if we are asked what we
mean by " organisation," we may say, at this stage, a certain
protoplasmic architecture which implies essential relations
between nucleoplasm and cj'toplasm. The protoplasmic unit
is like a firm with many partners of different kinds, each kind
having many representatives ; and the retention of vitality, the
possibility of regeneration on the part of the fragments, has
this for its essential condition, that the integrity of the firm — in
which lies its secret — is maintained by each fragment having
at least one representative of the different kinds of partners.

The reader who is not familiar with the subject should linger
over the fact that a fragment or a minute spore, separated from
a unicellular organism, may grow into (literally, reproduce) a
unit, which to our senses is exactly like the original. This is
(within the limits of our senses) complete hereditary resemblance,
and we interpret it as due to the fact that the fragment or spore
has to start with the essential organisation of the original. This

MULTIPLICATION OF UN1CELLULARS

33

is, without complications, the fundamental fact in regard to
inheritance.

It should also be borne in mind that many of the unicellular
organisms (Protozoa, at the base of the animal series ; Proto-
phyta, at the base of the plant series) are highly differentiated—
i.e. with great complexity of structure even within the narrow
limits of size (where a diameter of X\^W\ of an inch is considered
large) — and that many have very definite and interesting modes

Gr

Fig. 3. — Diagram of cell structure. (After Wilson.)

PI. Plastids in cytoplasm or cell-substance ; cc. centrosome ; n. nucleolus ; Chr. chromo-
somes ; N. nucleus ; ct. general cytoplasm ; V. vacuole ; Gr. granules.

of behaviour, such as swimming in a spiral, seeking light or
avoiding it, approaching certain substances and retreating from
others, trying one kind of behaviour after another, — functional
peculiarities — some of which cannot be described without using
psychical terms — which are also included in the inheritance.

The case of a fragment of crystal growing into a complete
crystal is interesting enough, but that a fragment or spore of
apparent simplicity should reproduce the obvious complexity
of the unit from which it was separated is relatively more mar-
vellous.

34 THE PHYSICAL BASIS OF INHERITANCE

A note is needed in regard to the misunderstanding which
has led many to cite cases of inheritance in unicellulars as
relevant to the discussion on the transmission of " acquired
characters." Although we can no longer say that unicellular
organisms are without sexual reproduction, since many exhibit
the liberation of special reproductive units and the occurrence
of amphimixis, we may still say that, apart from transitional
forms (like Volvox, which form colonies or " bodies " of one
thousand to ten thousand cells), there is among the unicellulars
only the beginning of the important distinction between somatic
or bodily and germinal or reproductive material which distin-
guishes multicellular organisms. This makes a notable differ-
ence.

§ 4. The Hereditary Relation in the Asexual Multiplication
of Multicellular Organisms

In many of the simpler, but multicellular, plants and animals,
a portion of the parent is separated off to form the beginning of
a new life. The freshwater sponge multiplies in part by minute
gemmules, which float away from the corpse of the parent and
develop into new sponges ; many polypes produce buds which
may be set adrift, as in the freshwater Hydra, or may remain
attached and help to form the great colonies that we see in
zoophytes and Anthozoa ; not a few worms also multiply by
dividing or by budding, and the examples highest in the scale
are found among the Tunicates, which are really vertebrate
animals. Moreover, in some cases where asexual multiplication
does not normally occur, it may still be a possibility, as is shown
by the fact that cut-off portions may, in appropriate conditions,
grow into entire individuals. Thus, two earthworms may
occasionally be produced by cutting one ; a sponge whicn
does not normally liberate buds may be cut into pieces
and bedded out successfully ; the arms of the starfish, whicn

ASEX UAL MUL TI PLICA TION

35

the fisherman tears asunder, may give rise to several new in-
dividuals. From nine excised fragments of a single Planarian
worm, Voigt reared nine individuals (see Weismann, 1904,
vol. ii. p. 25).

Similarly, in regard to plants, many of the simpler multi-
cellular forms produce detachable buds, familiar in the case of
the liverworts ; and even in the flowering plants the same may
occur, as in the bulbils of the tiger-lily. As in animals, great
colonies may be formed, consisting of many individuals materially
continuous, well seen in strawberries, whose creeping stems root
here and there and give rise to independent plants. It is also a

Fig. 4. — " Comet-form " of Starfish, showing how one arm regenerates
the other four. (After Haeckel.)

familiar fact that cut-off portions of a plant may readily give
rise to entire individuals ; a little piece of moss, a Begonia leaf,
a corner of a potato tuber — and hundreds of instances might be
given— will suffice to start a new plant. In many ways the
whole vegetable kingdom seems comparable to the sedentary
sections of the class Ccelentera among animals (zoophytes,
sea-anemones, corals, etc.), e.g. in the various forms of alternation
of generations which occur, and in the readiness with which
representative fragments will regrow the whole. This capacity
of regenerating the whole from a small piece is the more striking
when there is considerable differentiation of tissues and organs,
as there is in flowering plants and the higher animals. The

36 THE PHYSICAL BASIS OF INHERITANCE

fact being that the leaf of a plant, or a quarter of a zoophyte,
or an eighth of a sea-anemone, may grow into an entire organism
with reproductive cells, we must infer that the characteristic
heritable material, usually segregated in the reproductive cells,
is present in the cells of the body in these organisms.

The feature common to the ordinary forms of asexual multi-
plication is, that the reproduction is independent of eggs or
sperms, or of any process comparable to fertilisation. What
starts the new life, and lorms in this case the material basis of

\ij/.j

Fig. 5. — Asexual reproduction. A sea-worm (Syllis ramosa), in which
budding has produced a branched temporary colony. (After Mcintosh.)

inheritance, is a liberated portion of the parent. The heredity-
relation is one of obvious material continuity.

As regards inheritance, the feature characteristic of asexual
multiplication is that the resemblance between parent and
offspring tends to be complete. As Sedgwick (1899) expresses
it : " The offspring do not merely present resemblances to the
parent — they are identical with it ; and this fact does not appear
to be astonishing when we consider the real nature of the process.
Asexual reproduction consists in the separation of a portion of

NATURE AND ORIGIN OF THE GERM-CELIS 37

the parent, which, like the parent, is endowed with the power
of growth. In virtue of this property it will assume, if it does
not already possess it, and if the conditions are approximately
similar, the exact form of the parent. It is a portion of the
parent ; it is endowed with the same property of growth ; the
wonder would be if it assumed any other form than that of the
parent."

In asexual reproduction the resemblance of the offspring to the
parent tends to be very complete, and the reason for like producing
like is no puzzle, when the separated off -portion is a representative
sample of the whole organism.

§ 5. Nature and Origin of the Germ-cells

Re-statement of the Central Problem of Heredity. — The

central problem of inheritance is to measure the resemblances and
differences in the hereditary characters of successive generations,
and to arrive, if possible, at formulae which will sum up the facts,
such as Galton's Law of Ancestral Inheritance and Mendel's
Law. The central problem of heredity is to form some con-
ception of what is essential in the relation of genetic continuity,
which binds generation to generation. Weismann's theory
of the continuity of the germ-plasm is, in the first instance,
a theory of heredity, and as important as Galton's law of
inheritance.

We know that almost every multicellular plant or animal has
the beginning of its individual life in the union of two germ-cells
(ovum and spermatozoon), and what must be found if the prob-
lem of heredity is to be illumined at all is some reason why
the germ-cells should have this power of developing, and of
developing into organisms which are on the whole like the
parents. In what respects are the germ-cells peculiar, and

38 THE PHYSICAL BASIS OF INHERITANCE

different from the ordinary cells of the body ? Let us, then,
concentrate our attention for a little on the nature and origin of
the germ-cells.

It is inexpedient to lay on the shoulders of the student of
heredity the burden of problems which are not in any special
sense his business. It is no doubt interesting to ask how an
organisation, supposed to be very complex, may be imagined to :
find physical basis in a microscopic germ-cell, but the same sort
of question may be raised in regard to a ganglion-cell. It is not
distinctively a problem of heredity. It is interesting to inquire
into the orderly and correlated succession of processes by which
the fertilised egg-cell gives rise to an embryo, but this is the
unsolved problem of physiological embryology. It raises
questions distinct from those of heredity and inheritance, and
apparently much less soluble.

We shall return in the historical chapter to the various theories
of heredity which have been suggested ; in the meantime, we
require to refer to them only in outline.

The Typical OYum. — The germ-cell produced by the maternal
parent is usually a relatively large sphere of living matter (cyto-
plasm), and various not-living included substances, such as
nutritive yolk, pigment, oil-globules, and so forth. In the
cytoplasm there lies a central kernel surrounded by a delicate
membrane, the nucleus — a microcosm in itself. It contains
a network or coil or some arrangement of delicate (linin) threads,
carrying minute masses of a readily stainable material, the
chromatin. Under high magnification the chromatin is seen to
be built up of small corpuscles, sometimes like beads on a string,
the microsomes. In certain phases of activity the chromatin
forms a definite number of separate masses. They are then
called chromosomes or idants, and the same number is always
present in all the cells of the body of any particular species.
In the nuclear sap which fills the nucleus there is often a
rounded body or vesicle — the nucleolus ; or there may be

chr.

Fig. 6a. — Diagram of ovum, showing
diffuse yolk-granules, g.v. germinal
vesicle or nucleus ; chr. chromosomes.

Fig. 6b.

-Diagram of body-cell, show-

ing the nucleus with coil of chromatin
filaments and the surrounding cyto-

plasm. (After Carnoy.)

[Facing p. 38.

NATURE AND ORIGIN OF THE GERM-CELLS 39

several nucleoli. As they are very variable and often tran-
sient, the nucleoli are not regarded as very important. Often
they seem to be aggregations of reserve material or of waste-
products.

The Typical Spermatozoon. — The germ-cell produced by the

Fig. 7. — Volvox globator, an Infusorian forming a colony of cells,
showing the ordinary cells (c) that make up the colony or incipient
" body " ; a and b, the special reproductive cells, both male and
female — the beginning of the distinction between germ-cells and
somatic cells.

male parent, the spermatozoon, is very different from the ovum
in appearance and structure, and is also very much smaller.
When the egg is swollen with yolk, which does not count as living
material, the spermatozoon may be less than a millionth of its
volume. Most of the cytoplasm of the spermatozoon forms a
locomotor flagellum or tail, often of intricate structure, which
drives the " head" or nucleus before it, always working against

4o THE PHYSICAL BASIS OF INHERITANCE

a current if there is one. It is obviously a specialised adaptation
which helps the spermatozoon to find the ovum, and it may be
absent in cases where no journey or search is required. The
so-called head of the spermatozoon contains the stainable
material or chromatin, and in many cases it has been shown that
the ripe spermatozoon has the same number of chromosomes as
the ripe ovum. At the junction of the " head " and the " tail "
there is a short " middle piece " or " neck," in which there is
often seen a minute " centrosome."

There is in animals in most cases a great superficial contrast
between the two kinds of "germ-cells when fully mature. The
typical ovum is relatively large, often laden with yolk, usually
passive, and surrounded by some sort of membrane. The
typical spermatozoon is relatively very minute, with no
reserve material, and adapted to active locomotion. It is
significant, however, that both contain the same number of
chromosomes.

Old Attempts to interpret the Uniqueness of the Germ-
cells. — In the preformationist theories, which held sway in the
seventeenth and eighteenth centuries — theories which asserted
the pre-existence of the organism and all its parts, in miniature,
within the germ — there was a kernel of truth well concealed
within a thick husk of error. For we may still say, as the
preformationists did, that the future organism is implicit in the
germ, and that the germ contains not only the rudiment of the
adult organism, but the potentiality of successive generations
as well. But what baffled the earlier investigators was the
question, How the germ-cell comes to have this ready-made
organisation, this marvellous potentiality. Discovering no
natural way of accounting for this, the majority fell back upon
a hypothesis of hyperphysical agencies — that is to say, they
abandoned the scientific method, and drew cheques upon
that bank where credit is unlimited as long as credulity
endures.

THE THEORY OF PANGENESIS

4i

An attempt to solve the difficulty which confronted the
preformationists — the difficulty of accounting for the complex
organisation presumed to exist in the germ-cell — is expressed
in a theory which seems to have occurred at intervals in the long
period between Democritus and Darwin, the theory of pangenesis.
On this theory the cells of the body are supposed to give off
characteristic and representative gemmules ; these are supposed
to find their way to the reproductive elements, which thus come
to contain, as it were, concentrated samples of the different
components of the body, and are therefore able to develop into

Fig. 8. — Forms of spermatozoa, enormously magnified, not drawn to

scale.

1 and 2, Immature and mature spermatozoa of snail ; 3, of bird ; 4, of man — h. head, m,
middle portion, t, tail ; 5, of salamander, with vibratile fringe (/) ; 6. of Ascaris, slightly
amoeboid, with cap (c) ; 7, of crayfish.

an offspring like the parent. The theory is avowedly unverifiable
in direct sense-experience, but the same may be said of many
other hypotheses, and is not in itself a serious objection. It is
more to the point to notice that it involves many hypotheses,
some of them difficult to accept even provisionally. Galton long
ago tried, by experiments on the transfusion of blood, to test one
of these hypotheses, and found no confirmation. But it is
still more to the point to notice that there is another theory of

42 THE PHYSICAL BASIS OF INHERITANCE

heredity which is, on the whole, simpler — which seems, on the
whole, to fit the facts better, for instance the fact that our
experience does not warrant the conclusion that the modifica-
tions or acquired characters of the body of the parent affect
in any specific and representative way the inheritance of the
offspring.

The Idea of Germinal Continuity. — As is well known, the view
which many, if not most, biologists now take of the uniqueness
of the germ-cells is rather different from that of pangenesis. It
is expressed in the phrase " germinal continuity," and has been
independently suggested by several biologists, though Weismann
has the credit of working it out into a theory. Let us state its
purport. There is a sense, as Galton says, in which the child is
as old as the parent, for when the parent's body is developing
from the fertilised ovum, a residue of unaltered germinal material
is kept apart to form the future reproductive cells, one of which
may become the starting-point of a child. In many cases,
scattered through the animal kingdom, from worms to fishes, the
beginning of the lineage of germ-cells is demonstrable in very
early stages before the differentiation of the body-cells has more
than begun. In the development of the threadworm of the horse,
according to Boveri, the very first cleavage divides the fertilised
ovum into two cells, one of which is the ancestor of all the body-
cells, and the other the ancestor of all the germ-cells. In other
cases, particularly among plants, the segregation of germ-cells
is not demonstrable until a relatively late stage. Weismann,
generalising from cases where it seems to be visibly demonstrable,
maintains that in all cases the germinal material which starts
an offspring owes its virtue to being materially continuous with
the germinal material from which the parent or parents arose.
But it is not on a continuous lineage of recognisable germ-cells
that Weismann insists, for this is often unrecognisable, but on
the continuity of the germ-plasm — that is, of a specific substance
of definite chemical and molecular structure which is the bearer

THE IDEA OF GERMINAL CONTINUITY 43

of the hereditary qualities. In development a part of the germ-
plasm, " contained in the parent egg-cell, is not used up in the
construction of the body of the offspring, but is reserved un-
changed for the formation of the germ-cells of the following
generation." Thus the parent is rather the trustee of the
germ-plasm than the producer of the child. In a new
sense, the child is " a chip of the old block." As Sir Michael
Foster put it, " The animal body is in reality a vehicle for
ova ; and after the life of the parent has become potentially

Fig. 9. — Diagram illustrating idea of germinal continuity.
(After E. B. Wilson.)

G', fertilised ovum dividing into lineage of body-cells (B) and lineage of germ-cells— tht
base line ; B', B", the bodies of two successive generations ; G1, G% G3, G*, Gs, the chain
of germ-cells.

renewed in the offspring, the body remains as a cast-off
envelope whose future is but to die." To use another
metaphor, the germ-plasm is the lighted torch handed on
from one runner to another. " Et quasi cursores vital lampada
tradunt."

Early segregation of the germ-cells is in many cases an ob-
servable fact — and doubtless the list of such cases will be added
to ; but the conception of a germ-plasm is hypothetical, just as
the conception of a specific living stuff or protoplasm is hypo-
thetical. In the complex microcosm of the cell we cannot point
to any one stuff and say, " This is protoplasm " ; it may well be

44 THE PHYSICAL BASIS OF INHERITANCE

that vital activity depends upon several complex stuffs which,
like the members of a carefully constituted firm, are character-
istically powerful only in their inter-relations. In the same
way, it must be clearly understood that we cannot demonstrate
the germ-plasm, even if we may assume that it has its physical
basis in the stainable nuclear bodies or chromosomes. The
theory has to be judged, like all conceptual formulae, by its
adequacy in fitting facts.

Let us suppose that the fertilised ovum has certain qualities,
a, 6, c . . . x, y, z ; it divides and re-divides, and a body is built
up ; the cells of this body exhibit division of labour and dif-
ferentiation, losing their likeness to the ovum and to the first
results of its cleavage. In some of the body-cells the qualities
a, b, find predominant expression, in others the qualities y, z,
and so on. But if, meanwhile, there be certain germ-cells
which do not differentiate, which retain the qualities a, b, c . . .
x, y, z, unaltered, which keep up, as one may say figuratively,
" the protoplasmic tradition," these will be in a position by-and-
bye to develop into an organism like that which bears them.
Similar material to start with, similar conditions in which to
develop — therefore, like tends to beget like.

May we think for a moment of a baker who has a very precious
kind of leaven ; he uses much of this in baking a large loaf ;
but he so arranges matters by a clever contrivance that part of
the original leaven is always carried on unaltered, carefully
preserved for the next baking. Nature is the baker, the loaf
is a body, the leaven is the germ-plasm, and each bakiig is a
generation.

MATURATION OF THE GERM-CELLS 4$

§ 6. Maturation of the Germ-cells

We have seen that the germ-cells owe their capacity of develop-
ed to the fact that they are the unspecialised descendants of
he parental fertilised ovum — the custodians of the characteristic
erm-plasm. In some cases the lineage of germ-cells is from
he first distinct and apart from the lineage of body-forming
ells, and we argue from these clear cases of germinal con-
tinuity to the more numerous and less obvious cases where the
germ-cells are not recognisable as such until later stages
in development.

There is no need for our present purpose to follow the genera-
tions of the germ-cells within the body, or to trace the stages
of growth and differentiation between primitive germ-cells
: and the fully formed ripe ova and spermatozoa. It is
' necessary, however, to allude to the process of maturation,
which has a direct bearing on the problems of heredity and
inheritance.

Maturation. — 1. It is an elementary fact of histology that
the nucleus of each cell in the body of an organism contains a
number of readily stainable bodies or chromosomes. In many
cases it has been possible to count these, and it has been found
that (with a few explicable exceptions) the number is constant
for each species.

As Prof. E. B. Wilson says (1900, p. 67) : " The remarkable
fact has now been established with high probability that every
species of plant or animal has a fixed and characteristic number
of chromosomes, which regularly recurs in the division of all of
its cells, and in all forms arising by sexual reproduction the
number is even* Thus, in some of the sharks the number is 36 ;
in certain Gasteropods it is 32 ; in the mouse, the salamander,
the trout, the lily, 24 ; in the worm Sagitta, 18 ; in the ox, guinea-

* In a few insects the females have in their body-cells one chromo-
some in addition to the number possessed by the males.

46 THE PHYSICAL BASIS OF INHERITANCE

pig, and in man * the number is said to be 16, and the same
number is characteristic of the onion. In the grasshopper it is
12 ; in the hepatic Pallavicinia and some of the nematodes, 8 ;
and in Ascaris, another thread-worm, 4 or 2. In the crustacean
Artemia it is 168. Under certain circumstances, it is true, the
number of chromosomes may be less than the normal in a given
species ; but these variations are only apparent exceptions
[p. 87, Wilson]. The even number of chromosomes is a most
interesting fact, which, as will appear hereafter [p. 205, Wilson],
is due to the derivation of one-half the number from each of the
parents."

2. About 1883, Van Beneden made the important discovery
that the nuclei of the ovum and of the spermatozoon which
unite in fertilisation contain each one-half of the number of
chromosomes characteristic of the body-cells. This has been
confirmed in regard to so many plants and animals that it may
now be regarded as a general fact. The student should refer
to the partial list given by Wilson (1900, pp. 206-7), where
it will be seen that if the somatic nuclei have 12, 16, 18, or 24
chromosomes, the germ-nuclei have 6, 8, 9, or 12 respectively.
A striking case is found in the large thread- worm (Ascaris megalo'-
cephala) of the horse, which occurs in two varieties,— the one,
var. univalens, with two chromosomes in its body-cells has one
chromosome in its germ-nuclei ; the other, var. bivahns, with
four chromosomes in its body-cells, has two chromosomes in its
germ-nuclei.

3. If each of the nuclei which unite in fertilisation has only
half as many chromosomes as are characteristic of the species,
it follows that a reduction of the number must take place in the
history of the germ-cells, and this is the outstanding fact in
the process of maturation. Alike in the history of the egg
(oogenesis) and in the history of the sperm (spermatogenesis),

* " Flemming believed the number in man to be considerably greater
than 16." It is now generally stated to be 24.

MATURATION OF THE GERM-CELLS

47

i there is a parallel reduction in the number of chromosomes to
II one-half.

A B

P.G.C. «
OG (

OC

A l\

* » • •

A A A. !Y~.

•w

P.G.r. •

5G*

,U

SC

• • c •

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O.r

Ob

Oc

Sa

Sb-
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P.L'

Fig. io. — Parallelism of oogenesis (A) and spermatogenesis (after Boveri).

P.G.C. in both series (A and B), one of the primitive germ cells.

Following the oogenesis (A), there is first of all a period of multiplication (M), included
within the first bracket.

The primitive germ-cell gives rise to oogonia {OG).

These oogonia give rise to oocytes (OC).

Then follows a period of growth (G), included within the second bracket.

Then follows the process of ripening or maturation (R), included within the third bracket.

Od, the immature ovum, with the normal number of chromosomes.

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lessens the number of chromosomes to one half the normal.

Ob, the ovum after giving off the first polar body, with half the normal number of chromo-
somes.

P.B", the second polar body, formed by an ordinary equation division.

Oc, the ripe ovum.

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Following the Fpermatogenesis (B), there are successive periods (or zones in the testis)
of multiplication (M) , growth (G), and reduction (K).

The primitive germ-cell gives rise to spermatogonia (SG).

These spermatogonia give rise to spermatocytes (SC).

Immature spermatocytes of the first order [Sa) have the normal number of chromosomes.
In many cases by a reduction or meiotic division they give rise to spermatocytes of the
second order (Sb), with half the normal number of chromosomes.

These give rise by an eq nation division to spermatozoa [Sc).

'The one fact of maturation that stands out with perfect
clearness and certainty amid all the controversies surrounding it
is a reduction of the number of chromosomes in the ultimate germ-
cells to one-half the number characteristic of the somatic cells. It
is equally clear that this reduction is a preparation of the germ-

48 THE PHYSICAL BASIS OF INHERITANCE

cells for their subsequent union and a means by which the
number of chromosomes is held constant in the species. With
a few exceptions the first indication of the numerical reduction
appears through the segmentation of the spireme-thread, or the
resolution of the nuclear reticulum, into a number of masses
one-half that of the somatic chromosomes. In nearly all higher
animals this process first takes place two cell-generations before
the formation of the definitive germ-cells, and the process of
reduction is completed by two rapidly succeeding ' maturation-
divisions,' giving rise to four cells, all of which become functional
in the male, while in the female only one becomes the egg, and
the other three — the polar bodies or their analogues — are cast
aside. During these two divisions each of the original chromatin
masses gives rise to four chromosomes, of which each of the
four daughter-cells receives one ; hence, each of the latter
receives one-half the somatic number of chromosomes. In the
higher plants, however, the two maturation-divisions are fol-
lowed by a number of others, in which the reduced number of
chromosomes persists, a process most strikingly shown in the
pteridophytes, where a separate sexual generation (prothallium)
thus arises, all the cells of which show the reduced number "
(Wilson, 1900, p. 285).

The asexual spore-bearing fern-plant has in its cells twice as many
chromosomes (2 n) as the sexual prothallus has (n). The spores
produced by the fern-plant have n chromosomes ; they develop into
a prothallus with n chromosomes ; the prothallus produces sex-cells
with n chromosomes ; these undergo no reduction and by their
union they restore the number 2 n, which characterises the resulting
embryo and the subsequent fern-plant.

As Boveri has said : " Thus at some stage or other in the gene-
ration-series of the germ-cell there occurs a reduction of the
number of chromosomes originally present to one-half, and this
numerical reduction is therefore to be regarded, not as a mere theo-
retical postulate, but as a fact " (ZellenStudien, iii. 1890, p. 62).

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AMPHIMIXIS 49

§ 7. Amphimixis and the Dual Nature of Inheritance in
Sexual Reproduction

Apart from exceptional cases, the inheritance of a multi-
cellular animal or plant is dual — part of it comes from the mother
and part of it from the father ; in other words, the material
basis of inheritance is a fertilised egg-cell. The new individuality
has its origin in the fusion of two potential individuals, for as
such the ovum and spermatozoon must be regarded. The
exceptions referred to are cases of asexual multiplication by buds
or otherwise, as in the freshwater Hydra ; cases of partheno-
genesis, as in the case of the unfertilised eggs which develop

chr

Fig. 12.— Fertilised ovum of Ascaris. (After Boveri.)

chr. chromosomes, two from ovum-nucleus and two from sperm-nucleus ; cs. centrosome,
from which " archoplasmic " threads radiate, partly to the chromosomes.

into green flies (Aphides) in the summer ; and cases like liver-
flukes, where an animal is both mother and father to its offspring.
Apart from these exceptions the inheritance does at the start
consist of maternal and paternal contributions in intimate and
orderly union.

When a spermatozoon, outstripping its fellows (for there are

usually very large numbers), reaches an ovum and bores its way

; into it, the cytoplasmic flagellum is left behind, having performed

its function, and the sperm-nucleus and the ovum-nucleus move

I towards one another. By a rapid change in the periphery of
the ovum, the enveloping membrane becomes firmer, and the
ovum becomes non-receptive to other spermatozoa. When

4

50 THE PHYSICAL BASIS OF INHERITANCE

several effect entrance at once, abnormalities usually result.
In the mature ovum there is no centrosome ; if it was originally
present, it disappears. The spermatozoon, however, intro-
duces, along with its nucleus, its centrosome, and this divides
into two. The two centrosomes appear to take an active part
in the approximation and intimate apposition of the maternal
and paternal chromosomes, and in their subsequent partition
between the first two daughter-cells.

Prof. E. B. Wilson states the general opinion of experts some-
what as follows. As the ovum is much the larger, it is believed
to furnish the initial capital — including, it may be, a legacy of
food-yolk — for the early development of the embryo. From
both parents alike comes the inherited organisation which has
its seat (according to most biologists) in the readily stainable
(chromatin) rods of the nuclei. From the father comes a little
body (the centrosome) which organises the machinery of division
by which the egg splits up, and distributes the dual inheritance
equally between the daughter-cells.

Let us now proceed to expound four important theorems.

i. In Ordinary Sexual Reproduction the Inheritance is
very precisely Dual or Biparental. — Recent discoveries have
shown that the paternal and maternal contributions which come
together in fertilisation are, for several divisions at least, exactly
divided among the daughter-cells, thus confirming a prophecy
which Huxley made in 1878 : " It is conceivable, and indeed
probable, that every part of the adult contains molecules derived
both from the male and from the female parent ; and that,
regarded as a mass of molecules, the entire organism may be
compared to a web of which the warp is derived from the female
and the woof from the male." " What has since been gained,"
Prof. Wilson says, " is the knowledge that this web is to be
sought in the chromatic substance of the nuclei, and that the
centrosome is the weaver at the loom."

After the paternal and maternal chromosomes have united,

l.PB

Fig. 13. — Diagram of maturation and fertilisation and first stages of
cleavage. (From Prof. H. E. Ziegler, with his kind permission.) The
colours have been added.

1. —The immature ovum, with four double chromosomes, longitudinally cleft; c, centro-
some ; ch, chromosomes ; ul, nucleolus.

2.— First maturation division ; the nuclear spindle has at its equator four groups of
tetrads, three of which are visible.

3 and 4. — Formation of the first polar body (P.B.). In fig. 4 a spermatozoon (sp) is
entering. The paternal chromatin is shown throughout in red, the maternal in blue, thi
centrosome which is brought in by the spermatozoon is shown in yellow. The ovum-centro-
some disappears.

5.— The formation of the second polar body and the division of the first (1 P.B.). The
head of the spermatozoon has formed the male pronucleus {sp). The centrosome introduced
by the spermatozoon is surrounded by a clear area and rays.

6.— The second polar body (2 P.B.) has been set adrift. The first has divided into two.
The three polar bodies and the now mature ovum have in their nuclei half the normal number
of chromosomes. Thus four are seen in the female pronucleus (f.pn). The centrosome has
divided into two.

7. — The male and female pronuclei (sp and f.pn) have become like one another, and are
near together. The centrosomes (c) have become the centres of two large systems of rays.

8. — The two pronuclei are in contact and are coalescing.

9. — The nuclei have lost their membrane, and the first segmentation-spindle or cleavage-
spindle has been formed, a centrosome lying at each pole. The spindle has the normal number
of chromosomes, but each has divided, so that eight pairs are present.

10. — The egg-cell is dividing. The chromosomes are separated into two groups, each
group with eight chromosomes. The centrosome at each pole has divided into two.

11. — The division or cleavage is complete. The rays have disappeared. The chromosomes
are represented by minute vesicles or karyomeres.

12. — The new nuclei have been constituted by union of the vesicles. The centrosomes lie
closely apposed, but will occupy the poles of the spindle at the next division.

Facing p. 51.

f.pn.

{Facing p. 51

FOUR IMPORTANT THEOREMS 51

but never fused, to form one nucleus — the segmentation-nucleus
— the cleavage or segmentation of the fertilised ovum begins.

There is a centrosome, derived from the sperm-centrosome,
at each pole of the nucleus, and a system of fine rays radiates
from each, some of these rays entering into close association with
the chromosomes.

Each chromosome is halved longitudinally, as a piece of stick
might be split up the middle, and after a very complex routine
the halves of each split chromosome migrate, either actively or
passively, to opposite poles. Thus, near each centrosome there
comes to be a group of chromosomes, half of each group being
of paternal origin and half of maternal origin. Each group in
an orderly fashion rounds itself off into a unified nucleus, the body
of the cell (the cytoplasm) constricts across the equatorial plane,
and two cells are formed.

The gist and import of the whole process is the precisely equal
partition of the maternal and paternal contributions, so that
each of the daughter-cells has a nucleus half maternal and half
paternal. For many successive divisions (e.g. in Cyclops) the
duality has been demonstrated,* so that we may fairly say that
the maternal and paternal contributions form the warp and
woof of the growing orgaaism.

2. Inheritance, though Dual, is strictly Multiple. —
Although the whole inheritance which constitutes an offspring
usually comes from two parents, and may therefore be called
dual, it is obvious that the heritable material of each parent
was also dual, being derived from the grandparents, and so on
backwards ; so that inheritance is strictly not merely dual, but
in an even deeper sense multiple. Amphimixis or fertilisation
implies the subtle mingling of two minute organisations so that
they become physiologically one, but each of them was already

* According to Haecker's careful observations on the water-flea Cyclops,
the paternal and maternal contributions, i.e. chromosomes, are traceable
as distinct individualised items throughout the whole of development.

52 THE PHYSICAL BASIS OF INHERITANCE

the complex product of ancestral lineage. We shall return to
the subject when we come to consider Galton's Law of Ancestral
Inheritance.

Though a comparison with the inheritance of property is apt
to mislead, it may be of use to think for a moment of a youth
inheriting an estate, of which one might accurately say that it
had belonged in half to his father and in half to his mother.
Yet a genealogist with a full knowledge of the family might be
able to go further back, and might show, with even greater
accuracy, how this corner was due to a grandmother and that
to a great-grandfather.

This conception is so fundamentally important that I cannot
refrain from quoting an illustration from Mr. Galton's Natural
Inheritance, which puts the matter very clearly. " Many of the
modern buildings in Italy are historically known to have been
built out of the pillaged structures of older days. Here we may
observe a column or a lintel serving the same purpose for a
second time, and perhaps bearing an inscription that testifies
to its origin ; while as to the other stones, though the mason may
have chipped them here and there and altered their shape a little,

few if any came direct from the quarry This simile gives

a rude though true idea of the exact meaning of Particulate
Inheritance — namely, that each piece of the new structure is
derived from a corresponding piece of some older one, as a lintel
was derived from a lintel, a column from a column, a piece of
wall from a piece of wall. . . . We appear to be severally built
up out of a host of minute particles of whose nature we know
nothing, any one of which may be derived from any one
progenitor, but which are usually transmitted in aggregates, con-
siderable groups being derived from the same progenitor. It
would seem that while the embryo is developing itself, the
particles more or less qualified for each post wait, as it were, in
competition to obtain it. Also that the particle that succeeds
must owe its success partly to accident of position and partly to

TOUR IMPORTANT THEOREMS 53

being better qualified than any equally well-placed competitor
to gain a lodgment. Thus the step-by-step development of the
embryo cannot fail to be influenced by an incalculable number
of small and mostly unknown circumstances." (Natural Inherit-
ance, p. 9.)

3. Duality of Inheritance may be real, though it is not ex-
pressed.— It must be carefully observed that the demonstration
of the dual nature of inheritance afforded by the facts of amphi-
mixis does not necessarily imply that the dual nature of the
inheritance will be patent in the full-grown offspring. The
offspring is often like both its parents, often particularly like
one, often not very like either. The parent of children, the
breeder of animals, or the cultivator of plants, has often occasion
to remark in the offspring what looks like an entire absence of
the characteristics of one of the parents. The foal may seem
to take entirely after the sire, as if the maternal inheritance
counted for nothing. It is likely that this so-called " exclusive "
or "unilateral" inheritance is often more apparent than real,
our observation being arrested and preoccupied by a few out-
standing features. The certain fact that the resemblance,
apparently absent, often reappears in the next generation,
shows that the incompleteness was not in these cases in the
inheritance, but simply in its expression. We shall return to
this subject in connection with the different modes of inheritance.

4. Each Germ-cell has a Complete Equipment of Heredi-
tary Qualities. — It is usually assumed that each of the two
sex-cells which unite in fertilisation has in it the potentiality of
an organism with a full equipment of the essential characters of
the species ; but since the spermatozoon always dies unless it
enters the ovum, it is difficult to give experimental proof of the
assumption. Some recent daring experiments, which demand
confirmation, are very suggestive in this connection.

Prof. Yves Delage (1898) divided the minute egg of the sea-
urchin under the microscope into two parts, one containing the

54 THE PHYSICAL BASIS OF INHERITANCE

nucleus and its companion- body the. centrosome, the other
being necessarily simply half of the living matter of the egg
without any nucleus. Beside them he placed an intact ovum, and
then let the spermatozoa in. All the three objects showed equal
" sexual attraction " in respect to the spermatozoa ; all three
were fertilised ; all three segmented, the intact ovum most
rapidly, the nucleated fragment more slowly, the non-nucleated
fragment more slowly still. In one case the development
proceeded for three days ; the intact ovum had become a typical
gastrula (two-layered embryo), the nucleated fragment a smaller
gastrula, and the non-nucleated fragment also a gastrula but
with a very much reduced cavity. All the cells of these embryos
showed nuclei. Thus the experimenter was led to the conclusion
that fertilisation and some measure of development may occur in
a fragment of ovum without nucleus or centrosome. The nucleus
of the spermatozoon must have been in this case sufficient in
itself, though it will be noticed that in the experiment cited the
fragment did not develop far. Delage makes the important
suggestion that in fertilisation two things must be distinguished :
(a) the stimulus given to the ovum by some specially energetic
substance brought in by the spermatozoon, perhaps in its centro-
some ; and (b) the mingling of heritable characteristics, Weis-
mann's " amphimixis."

In subsequent experiments Prof. Delage (1899) reached even
more extraordinary results. Non-nucleated fragments of the
ovum of Echinus (sea-urchin), Dentalium (elephant's-tooth sheii),
and Lanice conchilega (a seashore worm), were effectively fer-
tilised and gave rise to the characteristic larval forms — pluteus,
veliger, and trochophore respectively. Three larvae were
reared from one ovum of a sea-urchin ; a normal blastula
embryo (a hollow ball of cells) was reared from ^-th of a
sea-urchin ovum ; a non-nucleated fragment of a sea-urchin
ovum, after fertilisation by a spermatozoon with nine chromo-
somes (nuclear rods), gave rise to a larva whose cells had the

FOUR IMPORTANT THEOREMS 55

normal number of eighteen chromosomes : such are some of the
extraordinary results reached by this clever experimenter. It
seems, then, as if fertilisation may, in many cases, be effective
without there being any ovum-nucleus present, as if the
essential fact were the union of a sperm with a mass of egg-
cytoplasm.

Delage's experiments cited above seem to prove that the
nucleus and centrosome of the ovum are not essential to ferti-
lisation. Professor Loeb (1899), of Chicago, has made experi-
ments which seem to show that the spermatozoon may be
dispensed with. In other words, he has been able to induce
parthenogenetic development artificially in cases where it does
not normally occur. He has been led to believe that the only
reason why the eggs of many marine animals do not develop
parthenogenetically is that something in the constitution of
the sea-water prevents it. This something is the presence or
absence of ions of sodium, calcium, potassium, and magnesium,
the two former requiring to be reduced, the two latter to be
increased. " The mixture of about 50 per cent, --g-n MgCl2
(magnesium chloride) with about 50 per cent, of sea-water was
able to bring about the same effect as the entrance of a sperma-
tozoon. The unfertilised eggs [of the sea-urchin Arbacia] were
left in such a solution for about two hours. When brought back
into normal sea-water they began to segment and form blastuke,
gastrulae, and plutei, which were normal in every respect. The
only difference was that fewer eggs developed, and that their
development was slower than in the case of the normal develop-
ment of fertilised eggs. With each experiment a series of control
experiments was made to guard against the possible presence
of spermatozoa in the sea- water. . . . From these experiments
it follows that the unfertilised egg of the sea-urchin contains all
the essential elements for the production of a perfect pluteus. The
only reason that prevents the sea-urchin from developing par-
thenogenetically under normal conditions is the constitution of

56 THE PHYSICAL BASIS OF INHERITANCE

the sea-water. The latter either lacks the presence of a sufficient
amount of the ions that are necessary for the mechanics of cell
division (Mg, K, HO, or others), or it contains too large a quantity
of ions that are unfavourable to this process (Ca, Na, or others),
or both. All the spermatozoon needs to carry into the egg for
the process of fertilisation are ions to supplement the lack of
the one or counteract the effects of the other class of ions in the
sea-water, or both. The spermatozoon may, however, carry
in addition a number of enzymes or other material. The ions
and not the nucleins in the spermatozoon are essential to the
process of fertilisation."

These remarkable experiments are confirmatory of the general
assumption that spermatozoon and ovum are completely
equipped potential organisms. Further confirmation may be
found in cases of partial parthenogenesis — e.g. the development
of drone-bees from unfertilised eggs ; from the close similarity
in the history of ovum and spermatozoon respectively ; from
the exactly equal way in which the paternal and maternal nuclear
contributions are distributed to each cell, during the early stages
of cleavage at least.

Or take the simple experiment of crossing a black guinea-pig
with a typical albino. All the offspring are black, although only
one of the parents — it does not matter which — has the quality of
blackness. It is evident that the germ-cells of either parent are
able to carry a complete equipment of blackness.

When we consider the ovum and spermatozoon as two fully
equipped potential individualities which unite to form the
beginning of a new individuality, we see more clearly how, on
the one hand, there is a double likelihood of the essential specific
characters being sustained, and how, on the other hand, there
is every likelihood that the intermingling will lead indirectly, if
not directly, to something new.

INHERITANCE IN PARTHENOGENESIS 57

§ 8. Inheritance in Cases of Parthenogenesis

It would be interesting to know with precision what the facts
; of inheritance are in cases where development proceeds from
an unfertilised ovum, particularly in those cases where the
parthenogenesis continues uninterruptedly for many generations.
On general grounds, from the absence of fertilisation, one would
expect to find few new departures or progressive variations ;
but rather, on the other hand, hints of degeneracy. The ob-
served facts are still very few.

Experiments which Prof. Weismann (1893, p. 344) made on

a small crustacean (Cypris reptans) showed a very high degree

I of uniformity between parent and offspring, with occasional

exceptions, which he regarded as exhibiting reversions to an

ancestral form many generations removed.

Dr. Warren's (1899) measurements of successive partheno-
genetic generations of Daphnia magna also gave evidence of
slight variability {i.e. of incompleteness of hereditary resem-
blance). They seemed to favour the view that " inheritance
in parthenogenetic generations resembles that from mid-grand-
parent to grandchildren."

§ 9. Wherein the Physical Basis precisely consists

The fertilised egg-cell divides into many cells ; these arrange
themselves in various ways ; they grow and multiply ; they
exhibit division of labour and the structural side of this — which
we call differentiation ; they form tissues and organs ; they
become integrated into a body ; they reproduce the likeness of
the parental type with variations. Meanwhile, some of the
cells remain apart from body-making or differentiation, and
form the beginnings of the reproductive organs, whence their
descendants— the mature germ-cells— are by-and-bye liberated
to start another generation. That this next generation is also
after the parental type is due to the continuous lineage of cells

58 THE PHYSICAL BASIS OF INHERITANCE

containing unspecialised germinal material. In similar con-
ditions similar material produces similar results.

But, if this has become clear, we have now to inquire into
the precise nature of the physical basis which conserves the
heritable qualities. Is it the germ-cell as a whole that is
essential, or is the cytoplasm most important, or is it the
nucleus only ?

Importance of the Chromosomes of the Germ-nuclei. — Many observa-
tions go to show that the nucleus of a cell plays an important part
in nutritive and constructive processes, and it is certain that a cell
artificially bereft of its nucleus will soon die if left to itself. The
nuclear material (karyoplasm or nucleoplasm) is an essential part of
the vital organisation. The view has gained ground that the
chromatin bodies or chromosomes are the chief, if not the exclusive,
vehicles of the hereditary qualities.

Let us consider some of the arguments in support of this
view.

i. Argument from cell-division. — Roux, Hcrtwig, Kolliker, Stras-
burger, and many others, have emphasised the fact that, in the
ordinary (mitotic) form of cell-division, the chromatin or readily
stainable material of the nucleus is divided " with the most scrupu-
lous equality " to form the basis of the nuclei of the daughter-cells,
while the cytoplasm or general cell-substance " undergoes on the
whole a mass-division — a most remarkable contrast." As Prof.
Wilson says (1900, p. 351) : " This holds true with such wonderful
constancy throughout the series of living forms, from the lowest to
the highest, that it must have a deep significance. And while we
are not yet in a position to grasp its full meaning, this contrast
[between nuclear and cytoplasmic behaviour in division] points
unmistakably to the conclusion that the most essential material
handed on by the mother-cell to its progeny is the chromatin, and
that this substance, therefore, has a special significance in in-
heritance."

2. Argument from maturation. — In the changes which lead up to
the ripe egg and the fully-formed spermatozoon, there is, as we have
seen, an elaborate preparation whereby the germ-nuclei which unite
in fertilisation arc rendered precisely equal as regards the number of
their chromosomes. On the other hand, the cytoplasm of the
relatively large, passive, often food-laden and ensheathed ripe ovum

BEARERS OF THE HEREDITARY QUALITIES 59

is typically as different as possible from that of the very minute,
■ actively mobile, usually short-lived spermatozoon. The constancy
iand frequent complexity of the reduction-processes which secure the
equivalence of chromosomes suggest that these bodies are of para-
mount importance in inheritance.

3. Argument from fertilisation. — In typical cases of fertilisa-
tion in animals, and in many plants as well, a spermatozoon
enters an ovum, sometimes a hundred thousand times larger

Fig. 14. — The chromatin elements of the nuclei in coil (a), double star (b),
and almost divided stages (c). (After Pfitzner.)

than itself. As it enters it may leave behind it the locomotor
" tail," which has discharged its function, thus further reducing
its infinitely small stock of cytoplasmic material. The " head "
of the spermatozoon, which is mostly nucleus, and the little
" middle piece " which carries the centrosome, are apparently
the important parts, and it is the ovum which furnishes the
cytoplasmic basis of further operations. The very gist of
fertilisation, so far as we can see it, is the intimate and orderly

6o THE PHYSICAL BASIS OF INHERITANCE

combination of the paternal and maternal chromosomes to
form one nucleus — the segmentation-nucleus. Moreover, the
maternal and paternal contributions are, as we have noted,
distributed with scrupulous equality, certainly to the first
two cells of the embryo, and probably to all later-formed
cells.

" The latter conclusion, which long remained a mere surmise,
has been rendered nearly a certainty by the remarkable ob-
servations of Ruckert, Zoja, and Haecker. We must, therefore,
accept the high probability of the conclusion that the specific
character of the cell is in the last analysis determined by that
of the nucleus — that is, by the chromatin ; and that in the
equal distribution of paternal and maternal chromatin to all
the cells of the offspring, we find the physiological explanation of

BEARERS OF THE HEREDITARY QUALITIES 61

V

«[

Fig. 15. — Diagram of the process of fertilisation in Ascaris. (After Boveri.)

a, female pronucleus ; b, polar bodies ; c, sperm pronucleus ; d, sperm-cap ; ac, chromosomes
of united female and male pronuclei (a and c) ; e, centrosomes ; fine (archoplasraic) threads
radiating from the centrosomes. I-V show union of paternal and maternal chromosomes ;
VI shows equatorial plate of segmentation nucleus ; VII-X show the division into the two
first cleavage-cells or blastomeres.

62 THE PHYSICAL BASIS OF INHERITANCE

the fact that every part of the latter may show the character-
istics of either or both parents " (Wilson, 1900, p. 352).

4. Argument from Boveri's ingenious experiment. — Taking a hint
from the experiments of the brothers Hertwig, who showed that non-
nucleated fragments of unfertilised sea-urchin ova (broken by-
shaking) might be successfully fertilised and might segment, Boveri
(1889, 1895) showed that such fertilised fragments developed into
dwarf, but normal, larvae. In these, as T. H. Morgan (1895) after-
wards showed, the nuclei contain only half the normal number of
chromosomes, having had only a sperm-nucleus to start with.

Interesting as this was, Boveri's further experiment was yet more
striking. He fertilised the enucleated egg-fragments of one species
of sea-urchin [Sphcerechinus granulans) with spermatozoa of another
species [Echinus microluberculatus) , and obtained in a few cases dwarf
larvae (plutei), which showed, except as regards size, the paternal
characters only. Therefore he concluded that the nucleus is the
exclusive bearer of the hereditary qualities, for it seemed from the
experiment that the enucleated maternal cytoplasm had remained
without specific influence.

It is admitted by Boveri himself that further experiments are
necessary, and it must be granted also, as has been pointed out by
Seeliger, Morgan, and Driesch, that in cases of hybridism, as in
Boveri's experiment, there may be a marked illustration of what is
called unilateral or preponderant inheritance. Most hybrid Echino-
derm larvaa show maternal characters only, some show paternal
characters only, some show both. There is also much individual
variability. Thus Boveri's famous experiment affords no secure
basis for argument.

In further support of the importance of the chromosomes
reference may be made to the fact that the number of chro-
mosomes in any given organism is always the same, except
in the reduced gametes which have half the normal number.

Another argument may be found in the fact that in some
insects the sex of the offspring seems to depend on whether the
egg is fertilised by a spermatozoon with an extra " accessory
chromosome " or by a spermatozoon without this.

Generally accepted Conclusion. — The general conclusion

I

BEARERS OF THE HEREDITARY QUALITIES 63

rom the foregoing and other arguments may be illustrated by

wo or three quotations from recognised authorities. Prof. O.

ertwig says : " The female nuclear material transmits the

characters of the mother, the male nucleus those of the father,

|to the offspring." Prof. Strasburger says for higher plants :
' The process of fertilisation depends upon the union of the

sperm-nucleus with the nucleus of the egg-cell ; the cell-substance
cytoplasm) does not share in the process; the cell-substance
}f the pollen-grain is only the vehicle to conduct the generative

micleus to its destination." Prof. Weismann says : " We can
hardly ascribe to the body of the ovum a higher import than

•"ig. 16. — A pollen grain, a, the two nuclei, with their chromosomes ;
b, the general protoplasm ; c, the outer wall. (From Carnoy.)

chat of being the common nutritive basis for the two conjugating
luclei."

Criticism. — 1. "The life of a complex multicellular organism
:ertainly depends upon the inter-relations and interactions of
nany parts ; the life of a cell apparently depends upon the
nter-relations and interactions of different parts of the cellular
rganisation, especially on the give-and-take between nucleo-
plasm and cytoplasm ; and it is not unlikely that life itself —
e. vital activity or function — may depend upon the inter-
elations and inter-actions of a number of complex substances,
one of which could by itself be called alive. Just as the secret

64 THE PHYSICAL BASIS OF INHERITANCE

of a firm's success may depend upon a particularly fortunate
association of partners, so it may be with vitality." * " We
are compelled by the most stringent evidence to admit that
the ultimate basis of living matter is not a single chemical
substance, but a mixture of many substances that are self-
propagating without loss of their specific character." f Holding
firmly to this view, which we have elsewhere expressed, that
life is a function of inter-relations, we confess to hesitation in
accepting without saving clauses any attempt to call this or
that part of the germinal matter the exclusive vehicle of the
hereditary qualities.

2. The sperm-nucleus brings with it into the ovum a little
cytoplasm, and it is also accompanied by the minute central-
corpuscle or centrosome, which seems to play an important part
in regulating the mechanism of cleavage. It may be that the
minimal quantity of cytoplasm is also important, though we
cannot trace its behaviour as we do that of the centrosome.
Strasburger says that if it were important there would be more
of it, but in these matters size and mass seem of small moment ;
the little cytoplasm there is may act like the little leaven which
leavens the whole lump. It seems in this connection very
desirable that the experiments which have been begun (Pieri and
Winkler) of extracting a ferment (" ovulase ") from seminal
matter and using it as a fertilising agent, should be confirmed
or confuted.

3. In Loeb's experiments unfertilised sea-urchin's eggs
developed into complete and normal larvae ; the sperm- nucleus
was dispensed with. In Delage's experiments non-nucleated
fragments of the ova of sea-urchin, worm, and mollusc were
fertilised and developed into normal larvae ; the ovum-nucleus
was dispensed with. But it must be noted carefully that in
both cases there was a nucleus present.

* J. Arthur Thomson, Science of Life, p. 115 (London, 1899).

t E. B. Wilson, The Cell in Development and Inheritance (1st ed., 1896).

CRITICISM 65

4. Hickson (1907) has argued forcibly in support of the view that
" for the present at any rate we can only say that the germ-cells as a
whole, and not any special part, are responsible for the transmission of
heritable characters from, generation to generation." He suggests
speculatively that the more plastic characters may be transmitted
mainly by the cytoplasm and the rigid characters by the nucleus.
In his criticism he refers to cases where chromosomes are quite
indistinct in the gametes, to the importance of cytoplasm-fusion in
the conjugation of some Protozoa, to the experiments of Herbst and
Fischel on hybridisation in Echinoderms, which indicate the im-
portance of the cytoplasm of the ovum in transmitting characters,
and to other sets of facts which indicate the danger of exaggerating
the importance of the chromosomes. The observations of Godlewski
are also strongly suggestive of the importance of the cytoplasm, as
well as the nucleus, in inheritance.

5. Batcson (1907) has pointed out that if the chromosomes were
the bearers of hereditary characters, we should expect some degree of
correspondence between the differences distinguishing the types and
the visible differences of number or shape distinguishing the chro-
mosomes. Moreover, if the chromosomes were the chief governors
of structure we should expect to find greater differences between
them in different tissues of the same body.

6. No one has protested more clearly and vigorously than Guyer
(1909, 1911) against "the inordinate importance which has been
attributed to the chromosomes as vehicles of heredity." He points
out, for instance, that there is definite experimental evidence of the
great importance of the ovum-cytoplasm, and argues that "the
number and arrangement of the chromosomes in a given species are
the effects of the fundamental constitution of a given kind of living
matter, rather than that they stand in a specifically causal relation
to such constitution." " Heredity is the problem of the handing-on
of metabolic energies already established, rather than of the transmis-
sion of a series of determinative units which create a wholly new
organism." " This much is certain : no chemical, physiological, or
morphological evidence is yet extant which places the hereditary
factors wholly within the chromosomes." It seems highly probable
that the chromosomes " control the velocities in cell-chemistry " by
supplying the proper amounts and kinds of ferments which act on a
series of fundamental cell-constituents that are largely common to
both egg and sperm.

Perhaps then the safest conclusion at present is that the chromo-
somes, along with other germ-cell constituents, " stand in some definite
causal relation to adult characters."

CHAPTER III

HEREDITY AND VARIATION

" The organic world as a whole is a perpetual flux of changing types." —

Francis Galton.

" Inheritance and variation are not two things, but two imperfect views
of a single process." — W. K. Brooks.

" Variation and inheritance are, at present, one fundamental mystery
of the vital unit." — Karl Pearson.

§ i. Persistence and Novelty.

§ 2. The Tendency to Breed True.

§ 3. Different Kinds of Organic Change.

§ 4. Classification and Illustration of Variations.

§ 5. Fluctuating Variations.

§ 6. Discontinuous Variations.

§ 7. De Vries on Fluctuations and Mutations.

§ 8. Causes of Variation.

§ 1. Persistence and Novelty

Close observers of the relation between successive generations in
mankind, or among plants and animals, are at one in record-
ing two distinct impressions, — on the one hand, of persistent
hereditary resemblance, on the other hand, of variability.
Oftenest we are first impressed by the remarkable homogeneity
which obtains from generation to generation, but as we get to
know the organisms better we become aware of individual
traits standing out against the background of general similarity.
Or it may be that, with the partiality of parents, our first

6t»

LIKE TENDS TO BEGET LIKE 67

impression is of the novelty and individuality of our children,
and only later do we recognise in those, who seemed so original,
a re-incarnation of our average selves. Oftener, perhaps, it
will be discovered that the resemblance in .habits of mind and
body is purely mimetic, and that the idiosyncrasies which were
really present, as buds at least, have been pruned off both for
good and for ill by the hook of criticism, or driven into latency
— like " sleeping-buds " — by mis-education or lack of appro-
priate stimulus.

Like Tends to Beget Like. — The hereditary relation is such
that offspring are on the whole like their parents, but the degree
of this likeness varies within wide limits. Indeed, the discre-
pancies are often very conspicuous, and we can understand how
Prosper Lucas, one of the early students of inheritance (1847) — ■
careful and scholarly according to his lights — imagined a meta-
physical entity, which he called " I'inneite " and opposed to
" I'heredite," the former originating what is new, the latter con-
serving what is old. In modern phraseology, the occurrence
of variations is a fact of life so general that we must replace
the adage " Like begets like " with the more cautious statement
" Like tends to beget like."

The popular adage " Like begets like " is often true as a
general statement. Offspring are often so like their parents
that even the scientific observer cannot tell one from the other.
In other words, the species " breeds true." But the more
intimate our acquaintance with organisms becomes, the more
plainly do we detect individual peculiarities, and we have to
change the adage to " Like tends to beget like." On the whole
it is true that average parents have average offspring, that
exceptional parents have exceptional offspring. Like tends to
beget like. Yet it is well known that, for instance as regards
stature, the tall do not always beget the tall, or the small the
small, so that we have to broaden the most general "fact of
inheritance" still further, and say that the average character

68 HEREDITY AND VARIATION

attained by the individuals of one generation tends to be
very nearly the same as the average character of the preceding
generation. This is the broad fact of specific inertia.

A False Antithesis between Heredity and Variation.—
Much obscurity of thought has been due to the false antithesis
between heredity and variation. When we say that like tends
to beget like, that offspring tend to resemble their parents
and ancestors, we are stating a fact of life. But when we
speak of an opposition between a force or principle of heredity,
securing resemblance between offspring and their parents, and
a tendency to variability which makes offspring different from
their parents, we are indulging in verbiage. Heredity, as we
have repeatedly said, is the relation of genetic continuity between
successive generations, and it is such that while many characters
seen in parents persist in their offspring, there is also in most
cases a distinct individuality in these offspring. Heredity is
a condition of evolution, a condition of inborn variations ; it
is just a name for the reproductive or genetic relation between
parents and offspring. The inheritance which was expressed
in the development of the parent may be almost identical with
the inheritance which is expressed in the development of the
offspring, but in most cases the inheritance does not persist
in this intact way from generation to generation, and then we
speak of variation. The contrast is not between heredity and
variation, but between inertia and change, between continuity
or persistence and novelty or mutation, between completeness
of hereditary resemblance and incompleteness of hereditary
resemblance.

As Prof. W. K. Brooks says (1906, p. 71) : " Living beings do
not exhibit unity and diversity, but unity in diversity. These
are not two facts, but one. The fact is the individuality in
kinship of living beings. Inheritance and variation are not
two things, but two imperfect views of a single process."

TENDENCY TO BREED TRUE 69

§ 2. The Tendency to Breed True

Relative Stability of Specific Characters.— Belonging as
we do to a race which seems to have varied very slowly within
historic times, we have not far to seek for good examples of
what is the biggest fact of inheritance — the stability of specific
characters throughout a long series of generations. If we
exclude monstrosities due to arrested development and the
like, if we set aside the numerous malformations and deforma-
tions induced on the bodies of individuals by peculiarities of
function and environment, the stability of the essential human
characteristics for many millennia is obvious. This racial
inertia, which holds in some measure at least for mental charac-
teristics, is at once the hope and the despair of the social
reformer.

If we pass from general specific characters to those of par-
ticular races, we read the same story. Not only do the salient
characteristics of the skull persist within a narrow radius of
variability, but the same is true of minor features : the oblique
eyes of the Japanese, the oval face of the Esquimaux, the
woolly hair of the Negro and the Jewish nose.

Conservative Types of Organisation.— But the persist-
ence of structural and mental characters as illustrated in man-
kind is but a tale of yesterday when compared with the persist-
ence of type exhibited by many animals which have lived on
apparently unchanged for many millions of years. Whatever
may be true in regard to the soft parts, of which no record
remains, there seem to be no differences in hard parts dis-
tinguishing the Lingula of to-day from those of the Silurian
ages ; and there are other instances of what are sometimes
called " living fossils." The reasons for such remarkable per-
sistence do not now concern us, but the fact that structural
characters established millions of years ago are reproduced with
exactness at the present moment does.

70 HEREDITY AND VARIATION

Persistent Peculiarities in Families. — Not less striking than
the long persistence of specific and stock characters is the fact that
offspring frequently reproduce the individual peculiarities — both
normal and abnormal — of their parents or ancestors. A slight
structural peculiarity, such as a lock of white hair or an extra
digit, may persist for several generations. A slight functional
peculiarity, such as left-handedness, has been recorded for at
least four generations, and colour-blindness for five. The strong
under-lip of the Hapsburgs persisted for six centuries. There are
endless illustrations of the fact that a pathological diathesis —
rheumatic, gout}', neurotic, or the like— may persist and express
itself similarly, even in spite of altered conditions of life, through-
out many generations. And what is true of bodily characteristics
is not less true of mental peculiarities : as to this, popular im-
pressions and the careful investigations of Galton and others are
in agreement. We think at once of cases like the Bachs, the
Bernouillis, the Darwins !

§ 3. Different Kinds of Organic Change

It may conduce to clearness if we think over the different
kinds of changes which occur in organisms.

1. Metabolism. — All living creatures are, as it were, whirl-
pools in the universal ocean of matter and energy. They are
continual!}' changing as they live. Streams of matter and energy
pass in and out. Organisms are animate systems which transform
matter and energy in a characteristic way which we call living.
Their physical basis is continually undergoing disruption and
reconstruction ; it breaks down and is built up again, it wastes
and is repaired, it runs down and is ever being wound up again —
until the arrears of imperfect recuperation become so serious that
the organism dies, or until some fatal accident occurs. The
chemical and physical changes involved in living are summed up

ORGANIC CHANGES

7i

in the term metabolism, the two aspects of which — constructive
and disruptive — are called anabolism and katabolism.

2. Cyclic Changes. — An equally familiar fact is that organisms
pass through a series of changes. The fertilised egg undergoes
cleavage, the resulting cells grow and differentiate, an embryo

ii is formed, and gradually — often by circuitous paths — a minia-
I ture form of the adult creature is attained. Out of apparent
simplicity an obvious complexity results. Growth still con-
: tinues, often punctuated by resting periods, often rhythmic and

A

C.

Fig. 17. — Diagram illustrative of variation and modification.

S, the soma or body ; G, the germinal material ; E, an environmental change.

A, an environmental change acting on the body directly evokes a modification (M).

B, an environmental change, without modifying the body directly, acts as a stimulus on
the germ-plasm, and is followed by a variation (V).

C, a variation (V) arises from some germinal change which cannot be causally connected
with any particular environmental change.

expressible in complex curves, often interrupted by peculiar
crises. Quickly or slowly the organism passes from youth
through adolescence to maturity, to its limit of growth and its
reproductive maturity. Quickly or slowly thereafter it sinks on
a down-grade towards death. As the old naturalists said, from
one period of vita minima the creature rises to a period of vita
maxima, and sinks back again into a vita minima which

72 HEREDITY AND VARIATION

dwindles to a vanishing point. It is characteristic of organisms
to pass through a series of cyclic changes.

3. Changes involved in Functioning. — As contrasted with
inanimate systems, organisms are characterised by their power
of effective response to environmental stimuli. A living creature's
responses tend towards self-preservation or species-preservation.
Though they may fail, the reactions are primarily and funda-
mentally effective. And these functionings or effective responses
necessarily involve changes in the system. They involve wear
and tear, and leave more or less discernible results. Normally,
however, the results, known as fatigue-effects and the like, are
obliterated by nutrition, rest, and other forms of recuperation.
In the study of an intricate structure, like a bee's brain, it is
possible to arrange on an inclined plane the changes which
are normally obliterated by a night's rest, the changes which
require prolonged recuperation before they disappear, and the
changes which cannot be recovered from — which accumulate
until the bee dies a natural death.

4. Temporary and Individual Adjustments. — In addition
to the inherent primary power of effective response, organisms
have different degrees of plasticity. They can adjust their
reactions to novel conditions. They can " try " first one mode
of reaction and then another, finally persisting in that which
is most effective. Even the unicellular Infusorians do this.
How much of this plasticity is primary, or inherent in the very
nature of living matter, how much of it is secondary and wrought
out by Natural Selection in the course of ages, must remain in
great measure a matter of uncertainty. Each case must be judged
on its own merits. It is certain that many unicellular organisms
are very plastic, and it seems reasonable to suppose that as
differentiation increased, restrictions were placed on the primary
plasticity, while a more specialised secondary plasticity was
gained in many cases, where the organisms lived in environments
liable to frequent vicissitudes. It is convenient to use the

MODIFICATIONS AND INBORN VARIATIONS 73

term " accommodations " for the frequently occurring indi-
: vidual adjustments which many organisms are able to make
1 to new conditions.

5. Modifications. — Besides being plastic, organisms are
. modifiable: that is to say, in the course of their individual life

they are liable to be so impressed by changes in surrounding
influences and by consequent changes in function that, as a
direct result, modifications of bodily structure or habit are
acquired. Modifiability is the capacity of registering the
direct results of changed function or of changed environment.
" Modifications " may be defined as structural changes in the
body of an individual organism, directly induced by changes
in function or in environment, which transcend the limit of
organic elasticity and persist after the inducing conditions
have ceased to operate. They are often inconveniently called
" acquired characters." They are not proved to be trans-
missible as such or in any representative degree, but they are
often adaptive and individually very valuable. They are dis-
tinguishable from temporary adjustments or accommoda-
tions on the one hand, and from inborn variations on the
other.

6. Inborn Variations. — Finally, when we subtract from a
total of " observed differences " between members of the same
species all that can be described as accommodations and modi-
fications, we find a large remainder which we must sharply
define off as variations. We cannot causally relate them to
peculiarities in habit or in surroundings ; they are often distinct
at birth or hinted at before birth; and they are rarely alike
even among forms whose conditions of life seem absolutely
uniform. They may be large or small in amount, fluctuations
or freaks, progressive or retrogressive — that is a matter for
further analysis — but they agree in having a germinal origin.
They are endogenous, not exogenous ; they are born, not made ;
and they are more or less transmissible, though they aie not

CLASSIFICATION OF VARIATIONS 75

always transmitted. They form — at least some of them form—
the raw material of organic evolution.

§ 4. Classification and Illustration of Variations.

" Variation. " — It is a common confession of naturalists
that a label is a necessary evil. A collection without labels is
a contradiction in terms, and yet the label is often a full-stop
to investigation. This is true in regard to the concrete ; it is
more lamentably true in regard to the abstract. Thus the
label " Variation " has been a great hindrance to progress.

As Mr. Bateson says (1905, p. 575) : " The indiscriminate
confounding of all divergences from type into one heterogeneous
heap under the name ' Variation ' effectually concealed those
features of order which the phenomena severally present,
creating an enduring obstacle to the progress of evolutionary
science. Specific normality and distinctness being regarded
as an accidental product of exigency, it was thought safe to
treat departures from such normality as comparable differences :
all were ' variations ' alike."

All organic changes imply some incompleteness in the heredi
tary resemblance — a little more of one character, a little less of
another, or the occurrence of some feature which deserves to
be called distinctly " new." Both variations and modifications
may cause this incompleteness in the hereditary resemblance ;
an apparently similar condition may result from two different
processes of change. But the variation has a germinal origin,
is blastogenic, is not directly dependent on the external con-
ditions of life, is endogenous, and is transmissible ; while the
modification has a somatic origin, is the direct result of functional
or environmental influence, is exogenous, and, so far as we
know at present, is not as such transmissible.

Classification.— There are many different ways of classifying
these variations which form the raw material of evolutionary
change.

76 HEREDITY AND VARIATION

a. If we attend to the nature of the change, we may
distinguish " meristic " variations — e.g. in the number and pro-
portions of parts, from "substantive" variations of a qualitative
sort — e.g. change in colour.

[3. If we attend to the direction of the change in successive
generations, we may distinguish " definite " variations, which
occur along one line (like stages in normal development), from
"indefinite" variations, which "fluctuate hither and thither
with no uniformity in the course of generations."

Many evolutionists have maintained that there is good
reason for believing in definite or determinate variation along
particular lines, as if certain organisms had an inherent bias to
change in certain parts and not in others, in certain directions
and not in others, just as certain inorganic substances can
crystallise in different forms but only within strict limits. It
is possible to arrange a series of species A, B, C, D, E, F, in such
a way that they suggest progressive definite variation along
a particular line, and it seems not unlikely that this kind of
evolution may sometimes occur. Moreover, along quite different
lines of evolution we find evidence that the same kind of step
has been taken independently, over and over again. This
suggests that the possibilities of variations may be limited and
defined by deep-rooted constitutional conditions or physio-
logical alternatives. But the weakness of the argument lies
in the almost insuperable difficulty of deciding whether the
apparent definiteness is not the result of the primary action of
selection which eliminates divergent variants at early stages —
nipping idiosyncrasies in the bud — or which may have estab-
lished a bias in previous generations. In conditions of rigid
elimination the lines of variation will naturally tend to become
more and more restricted.

y. If we attend to the amount of the change from one generation
to the next, we may distinguish minute fluctuations about
a mean, which are connected by intergradcs, from sudden

2 3

VA R I AT I O N

Fig. 19

Fig. 19.— Some of the numerous variations in the pattern of the abdomen in the yellow
jacket Wasp. [After Kellog and Bell. J

!

VAR I ATION
Fig.20.

FIG. 20.— " Mutations " or rapidly developing large inheritable variations in Leptinotarsa
mullitceniala. The type of the species (2) and its extreme mutants rubicunda (1) and
melanothorax (3). [After W. L. Tower.]

[ Facing p. 76

VARIATIONS 77

"sports " which reach a new position of organic equilibrium as
if by a leap. This is the contrast between " continuous "
variations small in amount, and " discontinuous " or " tran-
silient " variations in which a considerable step is taken with
apparent suddenness, without the occurrence of intermediates.

The term variation, used concretely to denote an organic pecu-
liarity or idiosyncrasy, is obviously a relative term, implying some
standard of comparison. It is a deviation from the parental type,
a divergence from the mean of the stock. Thus there are different
degrees, or perhaps even different kinds of discontinuity.

In many cases, a variation may be described as simply an in-
completeness in the inheritance or in the expression of the inherit-
:ance. The divergence from the norm is due to the suppression or
inhibition of some character. This may be illustrated by a per-
fectly white (albino) baby, born to almost coal-black parents.*
If such a form became the founder of an albino race, as in the
case of rats and mice, we should be justified in concluding that the
particular material organisation which eventually leads to the
deposition of pigment in the body had somehow dropped out of
the inheritance. If the albinism was in no respect transmitted to
the next generation, we should be justified in concluding that the
structural arrangements which lead on to pigmentation had simply
been hindered from finding their normal expression in develop-
ment.

A minus variation like albinism may be described as due to
an incompleteness in the inheritance or in the expression of the
inheritance, but there are other variations which must, so *to
speak, bear the plus sign, for they involve the augmentation or
exaggeration of a character. Plus variations of this sort have

* "Its father and mother were horrified ; their Mends and relations,
in fact all the villagers, were called to examine and criticise it. Why such
surprise ? Why such commotion ? The answer is self-evident : the law
of heredity had been broken." — R. W. Felkin. The vulgar mind is always
impressed by size and quantity ; big deviations strike the imagination,
and the normal occurrence of small deviations is forgotten.

78 HEREDITY AND VARIATION

been taken advantage of in breeding sheep with long fleece,
Japanese cocks with tails ten feet long, " wonder horses " with
manes reaching the ground, and so on.

But the offspring is sometimes so different from the parent that
we cannot describe its peculiarity as an incompleteness in the
expression of the normal inheritance, or as an exaggeration of
parental or ancestral traits. It is sometimes a new pattern, a
fresh departure, with what one might call organic originality.
It is more than a discontinuous variation. It seems to have
passed suddenly into a new position of organic equilibrium,
where it has not only individuality, but a distinctively novel
individuality. These distinctive novelties, which arise brusquely,
are often included in the conception of " mutations."

§ 5. Fluctuating Variations

When we examine a number of individuals of the same species
we usually find that they differ from one another in detail.
Some of the observed differences may be modificational or due
to differences of nurture, but it is often possible to abstract
these from differences due to hereditary nature. Thus, when
we collect a large number of specimens of the same age from the
same place at the same time, we often find that no two are
exactly alike. They have peculiarities of germinal origin — or, in
other words, they show fluctuating variations. The characteristic
feature of these fluctuations is that they are continuous, i.e. con-
nected by intergrades, and that they can be arranged in a gradual
series (a curve of frequency) on each side of a mode.

To construct such a curve (let us say of variation in stature),
take a base line, and divide it into equal parts, each to represent
a unit of measurement, say an inch. From a middle division of
this base line erect an ordinate to represent by its length the
number of those individuals whose stature is found to be the
most frequent. On each side of this, from their appropriate

FLUCTUATING VARIATIONS 79

divisions on the base line, erect ordinates representing by their
length the number of individuals of each stature, the lower
statures to the left, the greater to the right. Now a line joining
the tops of the ordinates will form a polygon or (if the divisions in
the base line be quarters of an inch) a curve, which will show
graphically the distribution of variation in stature in the population
measured. If the curve is symmetrical on each side of the highest
ordinate, the mode, it is called the " normal curve " ; the average
or mean coinciding with the " mode." If there are more varia-
tions on one side of the mode, the curve is " skew " ; if there are
two maxima or modes, the curve is " dimorphic " ; and so on.
In various ways, which are of great practical convenience, a
measure of variability can be deduced from the steepness or flat-
ness of the curve, and thus we can readily compare the variability
of different characters, or of the same character in different
groups and at different times. The curves, especially if made
year after year, may show the direction in which the species is
moving, perhaps the way in which selection is working, perhaps
even that the species is splitting up into two subspecies.

One of the results of measuring large numbers of variations is
to show that there is a relation between the amount of a deviation
and the frequency of its occurrence. The greater the divergence
from the average, the fewer instances are there. Measurements
of a large number of soldiers gave Quetelet the following result)
in which the upper line indicates the heights in inches, and the
lower line the number of soldiers of each of these heights.

60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75)
2, 2, 20, 48, 75, 117, 134, 157, 140, 121, 80, 57, 26, 13, 5, 3.

The general symmetry is plain, on each side of the most
frequent condition, 67 inches, which is called the " mode."

Registration of Variations. — " The modern methods of statistics
deal comprehensively with entire species, and with entire groups
of influences, just as if they were single entities, and express the

*

£

80 HEREDITY AND VARIATION

relations between them in an equally compendious manner. They
commence by marshalling the values in order of magnitude from
the smallest up to the largest, thereby converting a mob into an
orderly array, which, like a regiment, thenceforth becomes a
tactical unit. Conceive each value to be represented by an ex-
tremely slender rod of proportionate length, and the rods to be
erected side by side, touching one another, upon a horizontal base.
The array of closely-packed rods will then form a plane area,
bounded by straight lines at its sides and along its base, but by
a flowing curve above, which takes note of every one of the values
on which it is founded, however immense their multitude may be.
The shape of the curve is characteristic of the particular group of
values to which it refers, but all arrays have a family resemblance
due to similarity of origin ; they all drop steeply at one end, rise
steeply at the other, and have a sloping back. An array that has
been drilled into some such formation as this, is the tactical unit
of the new statistics" (Biometrika, vol. i., 1901, p. 7).

Theory of Evolution by Selection of Fluctuating Varia-
tions.— It is certain that most offspring differ from their parents
in many quantitative details. It is certain that when measure-
ments are taken of a large number of individuals of the same
species in reference to a particular character, the results, when
plotted out, conform approximately to the normal curve of
frequency. If measurements be taken in a subsequent genera-
tion there is a similar result, but the curve need not be precisely
the same. The mode of the curve — i.e. the most frequent!}'
occurring dimension of the measured character, may change
from one generation to another. It is usually believed that one
of the ways in which this change can be effected is by natural
selection. But to think of new species arising by slow changes
of this sort is in many ways difficult, apart altogether from the
fact that definite demonstration of the operation of selection
has been rarely attempted.

(1) Such a character as a Roman nose is certainly heritable,
though it is not always inherited. But we cannot speak so

FLUCTUATING VARIATIONS 81

definitely in regard to small quantitative variations. A tall
father does not necessarily have tall children. Where the
characters in which the two parents differ are such as
readily blend, regression towards the mean of the stock will
occur.

(2) Even with very thorough isolation — segregation of like
individuals — and very consistent selection, it is doubtful whether
a new race could be evolved from the cumulative increase of
small quantitative variations, e.g. in stature or colour of hair.
It is doubtful whether any domestic races have so arisen. It is
not in this way that dwarf-races and giant-races have been
formed. They arise from sudden discontinuous variations or
mutations, which are often peculiarly heritable, which are any-
thing but liable to be swamped by inter-crossing, and which
sometimes exhibit Mendelian inheritance.

(3) The result of the gradual accumulation of small
quantitative variations may be very important in a long time,
just as a small sum may become large from interest accumulated
for centuries ; but it is difficult to believe that minute fluctuations
in quantity would always have sufficient selective value to ensure
their persistence.

There are several reasons why selectionists have restricted
themselves so much to continuous variations as the raw material
of evolution. (1) Until lately we have known comparatively
little in regard to discontinuous variations or mutations. (2) It
was hastily concluded that these changes were not likely to
be transmitted — a generalisation in part due to preoccupation
with teratological non- viable freaks. (3) In many cases related
species can be arranged in a gradual series with intermediate
forms linking the extremes.

Now, there is no need to hamper the Evolution Theory by
restricting selection to minute variations. We know that sports,
mutations, or discontinuous variations are frequent, and that
they are remarkably stable in their hereditary transmission.

6

82 HEREDITY AND VARIATION

We know also that many domestic races have, as a matter of
fact, arisen by sudden mutation.

As to the series of related species which may be often arranged
as if on an inclined plane, two points should be noted :
(i) that it is likely enough that some kinds of species, e.g. vege-
tative forms like Alcyonarians and Corals, may have evolved by
minute steps, and (2) that although species are often connected
by intermediate links it does not follow that these links are
stages in the evolution. They may have been formed after the
species to which they are theoretically supposed to give rise.
We should remember Galton's warning, " If all the variations of
any machine that had ever been invented were selected and
arranged in a museum, each would differ so little from its neigh-
bours as to suggest the fallacious inference that the successive
inventions of that machine had progressed by means of a very
large number of hardly discernible steps." Many facts now
lead us to conclude that the Proteus leaps as well as creeps.

§ 6. Discontinuous Variations

One of the steps of progress in Evolution- lore since Darwin's
day is the recognition of the frequency and importance of dis-
continuous variations — i.e. of organic changes which arise
abruptly and not by a gradual series of steps. If dwarfs arise
suddenly in a tall race, and are not mere modifications, they
illustrate discontinuous variation of a quantitative sort. A
hornless calf, a tail-less kitten, a short-legged lamb, a thornless
rose, illustrate discontinuous quantitative variations of a negative
kind. Giants, " wonder-horses," long-tailed Japanese cocks,
merino-fleeced sheep, spine-covered holly leaves illustrate dis-
continuous quantitative variations of a positive kind. Sometimes
the novelty cannot be readily expressed in quantitative terms —
an entirely new colour turns up, the variant is immune to certain
diseases to which the stock is susceptible, leaves become fasciated,

DISCONTINUOUS VARIATIONS 83

a tree becomes "weeping," a genius is born. When a new
pattern of organisation or a new constitutional property turns
up, we may speak of a discontinuous qualitative variation.

Historical Note. — The idea that organic changes might come
about by leaps and bounds is not novel, though the evidence
substantiating it is quite modern.

Some of the older evolutionists, such as Etienne Geoffroy
St. Hilaire, believed in saltatory evolution, and were far from
agreeing with Lamarck that Nature is never brusque.

Darwin also recognised that big steps may be taken suddenly —
e.g. in the origin of large-crested Polish fowls, black-shouldered
peacocks, short-legged Ancon sheep, but he thought that these
discontinuous variations occurred rarely, and would be liable to
be swamped by intercrossing. He relied rather on the action of
natural selection on the small, continuous variations which are
always forthcoming.

But the modern appreciation of the importance and frequency
of discontinuous variations is mainly due to Bateson, who, in his
Materials for the Study of Variation (1894), gave many instances of
the sudden appearance of offspring which in some particular
diverge widely and abruptly from their parents ; and to De Vries,
who has observed the occurrence of " mutations " in many plants,
and has also followed them through generations, showing that they
tend to breed true ; and to Johannsen, who recognised the import-
ance of individual new departures in starting stable " pure lines."

A Change of Yiew. — Darwin and orthodox Darwinians relied
in the main on the operation of selection on small individual
variations — many of which are nothing more than quantitative
fluctuations. If new adaptations and new discontinuous species
arise in this way, the small variations must be heritable, the new
character must be capable of cumulative increase by the per-
sistent outcrop of similar variations generation after generation,
the selection must be persistent and consistent, and a long time
must be allowed.

84 HEREDITY AND VARIATION

Even when this theory is strengthened by subsidiary theories,
e.g. as to the efficacy of isolation and germinal selection, it is
more theoretically than practically convincing. It places such a
heavy burden on the shoulders of Natural Selection that the idea
of a leaping instead of a creeping Proteus has always been
welcome.

But why are evolutionists now entertaining an idea — the
importance of discontinuous variations — which Darwin con-
sidered and then rejected ? The answer is that we now know
of many instances of discontinuous variation in animals,
and even more among plants, that we have some good
evidence of these discontinuous variations or mutations
" breeding true," and that we have in the theory of Mendelian
inheritance a reason why a mutation which has once arrived
should persist.

Some modern authorities go the length of saying that
"mutations" form the sole raw material of evolution, and that
" individual fluctuations " do not count at all. This seems an
illustration of the common tendency to take up an extreme
position in the enthusiasm of a new discovery. Because dis-
continuous variations are common and important it does not
follow that continuous fluctuations are of no moment. Those
" whose humour is nothing but mutation " confess that it is
very difficult to distinguish between a small mutation and a
large fluctuation. If the large fluctuation be heritable — which
we may assume until it has been disproved — we confess that we
do not see what is gained by trying to distinguish it from a small
mutation.

The New View. — Dominated by the idea that " organisms
are mere conglomerates of adaptative devices," and that these
patents cannot but be the outcome of slow accumulation of
minute fluctuations under the directive agency of selection,
naturalists have paid little heed to the open secret that the
living creature is inherently a Proteus suddenly and discon-

DISCONTINUOUS VARIATIONS 85

tinuously passing from one guise to another by transilient
variation.

Mr. Bateson (1905, p. 577) notes that Marchant in 1719 was
the earliest to comment on the suggestiveness of sudden changes,
such as he saw in plants of Mercurialis with laciniated and hair-
like leaves which for a time established themselves in his garden.
He suggested that species may arise in like manner. " Though
the same conclusion has appeared inevitable to many, including
authorities of very diverse experience, such as Huxley, Virchow,
F. Galton, it has been strenuously resisted by the bulk of scientific
opinion, especially in England.

" Upon whatever character the attention be fixed, whether
size, number, form of the whole or of the parts, proportion,
distribution of differentiation, sexual characters, fertility, pre-
cocity or lateness, colour, susceptibility to cold or to disease —
in short, all the kinds of characters which we think of as best
exemplifying specific difference, we are certain to find illustrations
of the occurrence of departures from normality, presenting ex-
actly the same definiteness elsewhere characteristic of normality
itself. Again and again the circumstances of their occurrence
render it impossible to suppose that these striking differences
are the product of continued selection, or, indeed, that they
represent the results of a gradual transformation of any kind.
Whenever by any collocation of favouring circumstances such
definite novelties possess a superior viability, supplanting
their ' normal ' relatives, it is obvious that new types will be
created."

Heredity and Evolution.— Mr. Bateson has done good
service in exposing to ridicule the prevalent misconception that
domesticated races are " so many incarnations of the breeder's
prophetic fancy." " Except in recombinations of pre-existing
characters — now a comprehensible process — and in such intensi-
fications and such finishing touches as involve variations which
analogy makes probable, the part played by prophecy is small

86 HEREDITY AND VARIATION

Variation leads ; the breeder follows. The breeder's method is to
notice a desirable novelty, and to work up a stock of it, picking
up other novelties in his course — for these genetic disturbances
often spread — and we may rest assured the method of nature is
not very different " (1905, p. 578).

This is obviously a very important change of view, though
it is also in a way a return to what Darwin himself taught.
' Variation leads ; the breeder follows." But more than that :
Variation leads by leaps and bounds. As Mr. Bateson says, let
the believer in the efficacy of selection operating on continuous
fluctuations try to breed a white or a black rat from a pure
strain of black-and-white rats by choosing for breeding the
whitest or the blackest ; or to raise a dwarf (" Cupid ") sweet
pea from a tall race by choosing the shortest. It will not work.
Variation leads and selection follows.

Illustrations of Discontinuous Variation

Wonder Horses. — The so-called wonder-horse " Linus I."
had a mane eighteen feet long and a tail twenty-one feet long.
The parents and grandparents had unusually long hair. This
seems a good illustration of a " sport " or discontinuous variation
which not only persisted for several generations, but increased
very rapidly.

Shirley Poppies. — The well-known Shirley Poppies arose
from a single discontinuous variation, which may have occurred
often before Mr. Wilks saved it from elimination and made it
the ancestor of a prolific and distinctive stock.

Star Primrose. — The graceful star primrose {Primula stellata)
arose as a sport from the conventional Chinese primrose, and
was raised by Messrs. Sutton into a favourite stock. It had
been thrown off before as a sporadic variety over and over
again, but was " promptly extirpated because repugnant to
mid-Victorian primness."

DISCONTINUOUS VARIATIONS 87

The Moth Amphidasys. — Some sixty years ago in the urban
conditions of Manchester the black variety doubledayaria of
the moth Amphidasys betularia found its chance, and soon
practically superseded the type in its place of origin, extended
over England, and appeared even in Belgium and Germany
(Bateson, 1905, p. 577).

The Common Jelly Fish. — A good case of abundant discon-
tinuity in variation is furnished by the common jelly-fish Aurelia
aurita, whose sports have been studied by eight or more ob-
servers, from Ehrenberg (1835) onwards. Its parts are normally
in multiples of 4 (4 equal areas in the radially symmetrical disc,
4 oral lips, 4 genital organs, 16 radial canals, 8 marginal sense-
organs or tentaculocysts) ; but numerical sports are very
common. These are sometimes irregular, e.g. when the radial
symmetry of the disc is lost ; but they are oftener quite sym-
metrical, e.g. when the animal has 2 genital organs, 2 oral
lobes, 8 radial canals, and 2 marginal sense-organs.

In studying Aurelia aurita at Plymouth, Browne (1895)
found that out of 1515 young forms (ephyrae) 2i-4 per cent, had
more or fewer than 8 marginal sense-organs, and that out of 383
adults 22 "8 per cent, were similarly affected. The figures seem
to show that the abnormal forms survive quite as well as the
normal forms, yet there is no evidence that the sports were more
numerous in 1895 than when Ehrenberg studied them sixty
years before. In other words, although a plentiful crop of
brusque variations is being continually supplied by this plastic
form, there is no hint of the origin of a new race. (Bateson,
1894, p. 428.)

The Case of Pseudoclytia. — Although the numerous discon-
tinuous variations of Aurelia aurita do not suggest that any new
race is at present arising, it is possible to find an analogous case
where it does seem that we have to do with a species newly
arisen, or still in process of being established. A. G. Mayer
found at the Tortugas, Florida, large numbers of a medusoid

88 HEREDITY AND VARIATION

or swimming bell — Pseudoclytia pentata — a leptomedusan
belonging to the family Eucopidae. " It differs from all other
Hydromedusae in that it normally possesses 5 radial canals,
5 lips, and 5 gonads, all 720 apart, instead of 4 of these
various organs 900 apart, as in other Eucopidae." In the
structure of its tentacles, otocysts, gonads, and manubrium,
in the general shape of its bell, and the arrangement of
its tentacles and otocysts, it is so closely similar to Epenthesis
folleata, that it seems safe to conclude that the former
has been derived from the latter or from some closely allied
species. The two forms are somewhat different in colour and
slightly different as to the position of the gonads, but the
resemblance is exceedingly close, and no one can suppose that
a medusoid with 5 radial canals is a primitive form. As there
are pentamerous variants of Epenthesis folleata and tetramerous
variants of Pseudoclytia pentata, we are not aware of any case
which more cogently suggests the evolutionary interpretation.
As Mayer says, " P. pentata may be called ' a new race ' in the
sense that it is evidently derived from Epenthesis, and departs
from the quadratic arrangement of organs, which is almost uni-
versal among Hydromedusae. It is remarkably variable, and
its great commonness attests to its successfulness in the struggle
for existence" (Mayer, 1901, p. 20).

To obviate misunderstanding, it may be observed that by the
term " newly arisen " which Mayer uses in reference to Pseudoclytia
pentata, he means simply that " it has departed widely from the
fundamental type of all other Hydromedusae, and that it is appar-
ently derived from a genus (Epenthesis) which is itself quite highly
differentiated. It is, therefore, ' new ' in the sense that it cannot
be a primitive form, although we have no means of determining
how long a time it may have been in existence " (Mayer, 1901,
p. 8).

While we cannot exactly demonstrate that Pseudoclytia pentata
arose by discontinuous variation from Epenthesis folleata, or some
closely allied form, the evidence in favour of that interpretation

DISCONTINUO US VARIA TIONS

89

is exceedingly strong. It is interesting further to notice that " the
newly-arisen species " is very successful as regards numbers, and
that its variations have a strong family resemblance to those of its
supposed ancestor, and are yet more abundant. In regard to its
more abnormal variants, Mayer observes that they are handicapped
by their loss of symmetry, for some are neither radial nor bilateral,
and by a reduction of fertility even in cases where the number of
gonads has been increased to six or seven.

The evidence from Medusae and Medusoids is sufficient to show

Fig. 21.— Mutation in Medusoids (after A. G. Mayer). The figure to the
left is an oral view of Epenthesis folleata. The figure to the right is
an oral view of Pseudoclytia penlata.

that discontinuous variations may occur in large numbers, that
similar brusque changes may occur year after year, that there is
sometimes a strong family resemblance in the variations of related
forms. In some cases (e.g. in regard to Aurelia aurita) we are not
in a position to say that anything has come of the abundant crop
of discontinuous variations ; in other cases (e.g. the very abnormal
forms of Pseudoclytia pentata) the discontinuity has gone too far,
as shown by the reduction of fertility and the entire loss of sym-
metry ; while, thirdly, from the relationship of Pseudoclytia pentata
to Epenthesis folleata, we are led to conclude that one species may
arise from the discontinuous variation of another.

go HEREDITY AND VARIATION

§ 7. De Vries on Fluctuations and Mutations.

Professor Hugo de Vries is one of the foremost of Darwin's
intellectual heirs, with a rich endowment of his insight and
patience. Long-continued and carefully controlled observations
and experiments with generations of plants have led him to
conclusions which have given the Evolution Theory a fresh
start. His " Mutation Theory" is certainly one of the greatest
advances since Darwin's day.

The General Idea. — The origin of species and varieties is an
object for experimental inquiry. " Comparative studies have
contributed all the evidence hitherto adduced for the support of
the Darwinian theory of descent, and given us some general ideas
about the main lines of the pedigree of the vegetable kingdom,
but the way in which one species originates from another has
not been adequately explained. The current belief assumes that
species are slowly changed into new types. In contradiction to this
conception the theory of mutation assumes that new species and
varieties are produced from existing forms by sudden leaps. The
parent-type itself remains unchanged throughout this process, and
may repeatedly give birth to new forms. These may arise simul-
taneously and in groups, or separately at more or less widely distant

periods My work claims to be in full accord with the

principles laid down by Darwin, and to give a thorough and sharp
analysis of some of the ideas of variability, inheritance, selection,
and mutation, which were necessarily vague at his time " (From
preface to Species and Varieties, their Origin by Mutation"
Chicago and London, 1905).

A Theoretical Implication. — De Vries's Mutation Theory
involves the theoretical conception that " the characters of the
organism are made up of elements that are sharply separated
from each other. These elements can be combined in groups,
and in related species the same combinations of elements recur.
Transitional forms like those that are so common in the external

MUTATION THEORY OF DE VRIES 91

features of animals and plants do not exist between the elements
themselves, any more than they do between the elements of the

I chemist."

The Case of the EYening Primrose. — In 1886, De Vries began
hunting about around Amsterdam for a plant which would show
hints of being in what we may call a changeful mood. He tried

[' over a hundred species, bringing them under cultivation, but
almost all were disappointingly conservative. It seemed as if

i1 most of the species around Amsterdam were in a non-mutable

I state. It is possible, as Weismann suggested in one of his first
evolutionary essays (1872), that in the life of species periods of
constancy alternate with periods of changefulness. The human
historian has often made a similar remark.

In the course of his wanderings around Amsterdam, De Vries
came across a deserted potato-field at Hilversum — a field of
treasure for him. For there he found his long-looked-for mutable
plant, an evening primrose (Oenothera lamarckiana). Like its
nearest relatives, Oenothera biennis and Oenothera muricata, which
it excels in size and beauty of flowers, it probably came from
America, where it is a native. It had probably " escaped " at
Hilversum about 1875, and in the following ten years it had
spread in hundreds over the field. It had been extremely prolific
in its freedom, but that was not its chief interest.

Its chief interest was its changefulness. It had, so to speak,
frolicked in its freedom. Almost all its organs were varying — as
if swayed by a restless tide of life. It showed minute fluctuations
from generation to generation ; it showed extraordinary freaks like
fasciation and pitcher-forming ; it showed hesitancy as to how
long it meant to live, for while the majority were biennial, many
were annual, and a few were triennial ; it showed what can
hardly be otherwise described than as new species in the making.

It is possible that the prolific multiplication in a new environ-
Iment may have had something to do with the awakening of the
impulsive mutability.

92 HEREDITY AND VARIATION

s

In 1887, a year after his discovery of the potato-field, De Vries
found two well-defined new forms — a short-styled 0. brevistylis
and a beautiful smooth-leaved 0. Icvvifolia — distinguishable from
the parent 0. lamarckiana in many details. He hailed these as
two new " elementary species," * and he applied one of the crucial
tests of specific or subspecific rank : Did they breed true ? He
found that this was so ; from their self-fertilised seeds similar
forms arose. Neither of the two new forms was represented in the
herbaria at Leyden, Paris, or Kew ; neither had been described in
the literature of Onagraceae. They seemed to be distinctively
new. It is interesting to note that in 1887 there were few ex-
amples of these two new elementary species, and that each
occurred on a single plot in the field. The impression conveyed
was that each had arisen — by a sudden mutation — from the seed
of an individual parent.

The next chapter in the famous investigation began with a
transference of samples of the new forms and the parent stock —
partly as plants and partly as seeds— from the potato-field at
Hilversum to the botanic garden at Amsterdam.

The three stocks gave rise under cultivation to many thousands
of individuals, which bred true along certain lines, and yet gave
rise to other new forms. In short, De Vries had found a plant in
process of evolution. The predisposition to mutability — which
remains a mystery — was present, De Vries gave it scope, and
like the primeval gardener he had the pleasure of giving names to
a crop of new creations which emerged before him. From each
of his three samples there arose several distinctive groups — which
if they had been found in nature would have been reckoned as
distinct species of evening primrose. But the most interesting
feature was the apparent abruptness in the origin of the new

* By an " elementary species " is meant simply a group of individuals
which agree with one another and differ from other groups in a certain
number of characters, normally constant through successive generations.

MUTATION THEORY OF DE VRIES 93

forms. They seemed to arise by leaps and bounds, by organic
jerks ; they illustrated what De Vries has called " Mutation."

Besides the smooth-leaved O. Icevifolia and the short-styled
i O. brevistylis, both of which appeared in the potato-field, the cultiva-
tion of O. lamarckiana resulted in the emergence of seven constant
; elementary species — O. gigas (rare), O. rubrinervis, O. oblongata,
O. albida, O. leptocarpa, O. lata, and a dwarf O. nanella. Besides these
there were a few inconstant variants and a few which were sterile.
One form, 0. scintillans, that only appeared eight times, was not
I constant like the others. When self-fertilised it produced O. ob-
longata, O. lamarckiana, and others like itself.

It is interesting to notice that some of the forms — e.g. 0. oblongata

— were produced over and over again ; that five of the new forms

! appeared afterwards in the field or from seeds collected in the field,

i which shows that the cause of their origin was not to be found in the

i cultivation.

As De Vries says, the new elementary species arise suddenly
I without transitional links ; for the most part they are quite con-
stant ; within the limits of their essential constancy they exhibit
similar minor fluctuations ; they are usually represented by nu-
merous individuals within the same period of time ; the observed
changes affect many organs and parts, and in no definite direction ;
and the mutability seems to be periodic, not continuous.

If cases like that of 0. lamarckiana are indicative of what often
occurs and has occurred in nature, then our view of the evolution-
process must be in several respects modified.

It will be necessary to distinguish more sharply between fluc-
tuating variations and discontinuous mutations. If a new ele-
mentary species may arise as it were ready-made, " at a single
advance," it is not necessary to hold to the formula that species
have arisen by the gradual accumulation (under selection) of
minute individual variations. As mutations occur in large
numbers and occur repeatedly and are very constant, the familiar
difficulties in regard to the swamping of novelties, the inappre-
ciable value of incipient stages, the apparent non-utilitarian
character of some specific differences, and so on, will be greatly

94 HEREDITY AND VARIATION

lessened. The reader may be referred to Prof. T. H. Morgan's
Evolution and Adaptation (1903) for a valuable discussion of the
advantages of the Mutation Theory.

De Yries's Analysis of Variation. — In order to appreciate more
thoroughly the importance of the changes which De Vries has
necessitated in our evolutionary conceptions, we must briefly
refer to his analysis of the distinct phenomena which have been
too often unfortunately slumped under the title " Variations."

" Elementary Species." — In many groups of organisms which
are usually called Linnsean species, there are several or numerous
" subspecies," or " varieties." They remain more or less constant
in their characters from generation to generation, they breed true
in artificial conditions, they are not local races with similar modi-
fications ; De Vries calls them " elementary species." Thus
there are about two hundred " elementary species " of the com-
mon Crucifer, Draba verna, and a few " elementary species " of
the common European heartsease (Viola tricolor), and so on.

" The systematic species," De Vries says, " are the practical
units of the systematists and florists, and all friends of wild nature
should do their utmost to preserve them as Linnaeus has proposed
them. These units, however, are not really existing entities ;
they have as little claim to be regarded as such as the genera and
families have. The real units are the elementary species ; their
limits often apparently overlap, and can only in rare cases be
determined on the sole ground of field-observations. Pedigree-
culture is the method required, and any form which remains
constant and distinct from its allies in the garden is to be con-
sidered as an elementary species " (1905, p. 12).

Elementary species are considered to have originated from
their parent form in a progressive way ; they have succeeded in
attaining something quite new for themselves.

ReTrograde Varieties. — De Vries applies this term to those
numerous forms which have thrown off some peculiarity charac-
teristic of their ancestors. Like elementary species they may arise

I

DE VRIES'S ANALYSIS OF VARIATION 95

suddenly, but while " progressive steps are the marks of ele-
mentary species, retrograde varieties are distinguished by appar-
ent losses." Retrograde varieties usually differ from their parent
species by a single sharp character only, — they have lost pigment,
or hairs, or spines, and so on ; while elementary species are dis-
tinguished from their nearest allies in almost all organs. More-
over, the same kind of retrograde variety occurs repeatedly in
different series of species, hence the long lists of unrelated varieties
called by the same varietal title — e.g. alba, inermis, canescens, or
glabra.

" Varieties differ from elementary species in that they do not
possess anything really new. They originate for the greater
part in a negative way, by the apparent loss of some quality,
and rarely in a positive manner by acquiring a character already
seen in allied species " (1905, p. 152).

Ever-sporting Yarieties— De Vries uses this term to describe
cases like the striped larkspur, which for centuries has gone on
producing unstriped as well as striped flowers. " Its changes are
limited to a rather narrow circle, and this circle is as constant as
the peculiarities of any other constant species or variety. But
within this circle it is always changing, from small stripes to
broad streaks, and from them to pure colours. Here the vari-
ability is a thing of absolute constancy, while the constancy con-
sists in eternal changes ! " Plants with variegated leaves, with
double flowers, with fasciated branches, with peloric flowers, and
so on, often illustrate the " ever-sporting " tendency. The
common snapdragon (Antirrhinum ma jus) is a very good case, — -
the striped variety, for instance, cannot be fixed. There is some
inherent instability in the combination of unit-characters in
these ever-sporting varieties.

Fluctuations. — De Vries applies this term to the continually
occurring individual variations. " It is normal for organisms
to fluctuate to and fro, oscillating around an average type.
Fluctuations are linear, amplifying or lessening the existing

96 HEREDITY AND VARIATION

qualities, but not really changing their nature. They are not
observed to produce anything quite new ; they always oscillate
around an average, and if removed from this for a time, they
show a tendency to return to it." They are inadequate ever to
make a single step along the great lines of evolution, whether
progressively or retrogressively. They do not form the raw
material of evolution, as has often been supposed. But, we
submit, it is difficult with our present knowledge to discriminate
between a fairly large fluctuation and a small mutation.

Mutations. — " In contrast to the ever-recurring variability,
never absent in any large group of individuals, and determining
the differences which are always to be seen between parents and
their children, or between the children themselves, we have to
rank the so-called sports or single varieties, not rarely denomin-
ated spontaneous variations, for which I propose to use the term
' mutations.' They are of very rare occurrence, and are to be
considered as sudden and definite steps " (1905, pp. 190-1).

" De Vries recalls Galton's apt comparison between variability
and a polyhedron which can roll from one face to another. When
it comes to rest on any particular face, it is in stable equilibrium.
Small vibrations or disturbances may make it oscillate, but it
returns always to the same face. These oscillations are like the
fluctuating variations. A greater disturbance may cause the
polyhedron to roll over on to a new face, where it comes to rest
again, only showing the ever-present fluctuations around its new
centre. The new position corresponds to a mutation " (T. H.
Morgan, 1903, p. 289).

According to De Vries, mutations have furnished the material
for the process of evolution.

The Oldest Known Mutation. — A few years before the close
of the sixteenth century (1590), Fprenger, an apothecary of
Heidelberg, found in his garden a peculiar form of Chelidonium
majus or greater celandine. It was marked by having its leaves
cut into narrow lobes with almost linear tips, and by having the

THE OLDEST KNOWN MUTATION 97

petals also cut up. This sharply defined new form suddenly
appeared among the plants of Chelidonium majas which the
apothecary had cultivated for many years. It was recognised
by botanists as something quite new, and eventually it got the
name Chelidonium laciniatum ; it was not to be found wild,
or anywhere except in the Heidelberg garden. But from the
first this new cut-leaved celandine proved constant from seed.
It has been naturalised in England and other countries, and is
sometimes now found as an "escape." Its origin by mutation
seems as certain as its constancy. It is further of interest to
note that in crosses with C. majus it follows the law of Mendel.

Summary. — De Vries has done great service in analysing the
complex concept of variation ; in sharply contrasting individual
fluctuations and mutations; in defining " elementary, species,"
"retrograde varieties," and "ever-sporting varieties" ; in ob-
serving the actual origin by mutation of stable new varieties or
subspecies of CEnothera lamarckiana and some other plants ; in
showing by historical research combined with experiment that
many stable stocks of cultivated plants have arisen by mutation ;
and by corroborating throughout the fundamental idea that " the
characters of organisms are composed of units sharply distin-
guished from one another."

The contrast between fluctuations and mutations is so impor-
tant that we may state it once more. (1) Fluctuations are
continually occurring generation after generation : mutations are
rare and occur intermittently. (2) Fluctuations give rise to a
series of minute differences which may be arranged on a frequency
curve, according to the laws of chance : mutations may be large
or small, and their occurrences do not illustrate any ascertained
law of frequency. (3) Fluctuations do not lead to a permanent
hange in the mean of the species unless there be very rigorous
election, and even then, if the selection be slackened, there is
egression to the old mean : mutations lead per saltum to a new
pecific position, and there is no regression to the old mean.

7

98 HEREDITY AND VARIATION

(4) Fluctuations do not yield anything really new, they imply a
little more or a little less of characters already present : mutations
are novelties, they imply some new pattern, some new position of
organic equilibrium. According to De Vries's theory, no new
species can be established without mutation. "When a muta-
tion has occurred a new species is already in existence, and will
remain in existence, unless all the progeny of the mutation are
destroyed." . . . The phrase " survival of the fittest," as de-
scribing a process of evolution, ought to be replaced by " survival
of the fittest species." According to De Vries, species originate
by mutation instead of by the continuous selection of fluctuations.
" Natural Selection may explain the survival of the fittest, but it
cannot explain the arrival of the fittest."

In regard to these far-reaching conclusions it should be noted
that while De Vries has given much convincing evidence in regard
to plants, we have as yet very slight evidence of the origin of
species of animals by mutation. We know of many discontinuous
variations among animals, but the subsequent history of these is
not known except in a few cases. It must be remembered that,
morphologically regarded, the whole vegetable kingdom does not
correspond to more than the first three or four phyla in the animal
kingdom — to the Protozoa, Porifera, and Coelentera, where, as in
plants, the contrast between germ-plasm and somatoplasm has not
been accentuated, as it is in higher animals. It is quite conceiv-
able that a mode of evolution common among plants may be rare
among animals. It is difficult at present to apply the mutation
concept with security to the animal kingdom.

The idea of mutation is very welcome because it lessens the
burden which it has been found theoretically necessary to lay on
the shoulders of the selection hypothesis, and because it fits in well
with the a priori convictions which some naturalists have as to
the autonomy of the organism, that it is as much a self -changing
insurgent Proteus as a pawn in a game which the Environment
plays. But because it is so welcome, it is to be entertained

VARIATION IN HARTS TONGUE FERN

TlG. 22. — Mutations of Hart's Tongue Fern ( Scolopendrium milgare) After Lowe.
Typical ; 2, variety sagittato-cristatum : 8, reniforme ; 4, cristatum ; 5, contractum ; C, stansfieldii.

I Facing p. 98

FLUCTUATIONS AND MUTATIONS 99

the more cautiously. An authority on domesticated animals,
Prof. Keller of Zurich, finds but little evidence of it in the history
of the well-known stocks.

It seems to us that in emphasising the importance of mutations
De Vries has swung to the extreme of greatly depreciating the
importance of fluctuations. Until we know more about animal
mutations, it does not seem to us legitimate to deny that fluc-
tuations may form, as Darwin believed, an important part of
the raw material on which selection operates.

We cannot but regard with suspicion the distinction between
large fluctuations and small mutations. It seems to us a verbal
distinction.

Finally, it must be remembered that, as De Vries frankly
points out, we are ignorant in regard to the conditions in which
mutations occur. The Mutation Theory does not as yet give
us a theory of mutations.

" Pure Lines." — The position held by De Vries has been strength-
ened by the work of Johannsen and Jennings on " pure lines." If we
succeed in starting a " pure line " — " the progeny of a single self-
fertilised homozygous plant" — say an innately exceptional bean-plant
with very large seeds, we shall find slight individual differences in the
size of the beans from generation to generation ; if we take the biggest
and the smallest of these and start afresh, we find that their progeny
are neither larger nor smaller than the average. The original bigness
was a fixed mutation ; the other differences were probably mere
modifications and non-transmissible. If we take a considerable
number of the largest beans and the smallest beans from a field and
sow them, we are likely to get in the progeny of the former a larger
average size than in the progeny of the latter, for we are almost sure
to have started with a number of beans which are innately (not modi-
ficationally) large-sized and small-sized. What Johannsen did for
the bean and some other plants, Jennings has done for the slipper-
animalcule, Paramcecium. He isolated eight pure lines differing in
average size, and found that he made no progress by selecting the
largest in an established large pure line, the exceptional largeness
being probably the accidental result of peculiar nurture. Selection
from a mixed population, however, resulted, as in the case of the
beans, in a distinctly altered average size.

•

ioo HEREDITY AND VARIATION

The experiments were made with consummate carefulness, but it is
difficult to accept the idea of the rigid fixity of the hereditary char-
acters in a pure line. It may be that in some cases, such as beans,
the viable limit of size has been reached. It may also be that the
variational steps that count do not occur often. Perhaps some time
must elapse before the organism takes another step.

Prof. Castle asks : " Is it not possible that along with the striking
size differences due to nutrition there may occur also slight size
differences due to germinal variation within the pure line, that is
owing to variations in the potency of the same unit-character or com-
bination of unit-characters ? " And he points to Woltereck's success-
ful selecting-out of a variation in a parthenogenetic pure line of the
water-flea, Hyalodaphnia. He selected forms which showed the ex-
ceptional occurrence of a rudimentary eye, and definitely increased
the degree of development of that organ and the frequency of its
occurrence (up to 90 per cent.).

In short, it is premature to abandon belief in the efficacy of
selection even in pure lines.

§ 8. Causes of Variation

In regard to the causes of variation it is too soon to speak,
except in tentative whispers. What Darwin said must still
be said : " Our ignorance of the laws of variation is profound.
Not in one case out of a hundred can we pretend to assign any
reason why this or that part has varied."

Variability. — The difficulty which every naturalist has felt in
trying to define the concepts of variability and variation is due to
the fact that living creatures are individualities — in some degree,
personalities. In the ocean of matter and energy organisms are,
as it were, whirlpools, each one with a particular character of its
own. They are animate systems, each with a unity or individu-
ality which we cannot fully interpret. They have the power —
again an ultimate prerogative — of giving rise to other whirlpools,
to other animate systems, which tend to be like themselves. But
because each organism is a very complex whirlpool in a very com-
plex environment, and because a living individuality cannot
reproduce others without subtle molecular manoeuvres which we

CAUSES OF VARIATION 101

know only in a far-off sort of way, one individuality is very un-
likely to reproduce an absolute facsimile of itself. It is of the very
essence of a living thing to change, and an individuality cannot be
halved. From this point of view, variation is a primarily normal
occurrence, and breeding true has secondarily come about as the
result of restriction. In short, variability is a primeval character
of organisms. We cannot explain variability ; it is a datum in
the world of life. We may, however, try to show in certain cases
how it operates and what conditions help or hinder it.

The unending problem of life is to establish some sort of modus
vivendi between an extremely complex and changeful animate
system and the extremely complex and changeful environment in
which it lives and moves and has its being. In all viable organ-
isms this equilibration has been established, and it is plain that
those organisms which could secure an entailment of this equili-
bration would be the organisms to survive. The producers of
survivable descendants survive in them — an obvious economy of
successful experiment, if such a point of view can be entertained.

We have seen that during the early stages of development there
is often a visible segregation of a lineage of germ-cells which do not
share in body-making, but continue like the fertilised ovum.
This distinction between somatic cells which undergo differentia-
tion and germ-cells which retain the heritable qualities intact is
obviously an advantageous method of entailing on successive
generations that valuable asset which we have called organic
equilibration. It also economises and facilitates the process of
reproduction.

But in spite of this almost universal device, the general tend-
ency of which is to secure persistence, continuity, and complete
hereditary resemblance, there is abundant opportunity left for
the assertion of that variability which we believe to be a primary
quality of vital units. Thus an inquiry into the causes of varia-
tion seems to us to be in the main an inquiry into the oppor-
tunities for the reassertion of a pristine tendency which the

102 HEREDITY AND VARIATION

continuity of the germ-plasm has to some extent restricted. The
stream of life passing through a continuous lineage of germ-cells
is, so to speak, hemmed in, but it continually tends to deviate
from this course, and there are not a few opportunities — some
normally recurrent, some more accidental — which allow of this
or even prompt it. In some cases, as we have said, it is impossible
to distinguish offspring from parent, or brother from brother,
or cousin from cousin. On what does this completeness of heredi-
tary resemblance {i.e. the absence of variation) depend ?

It means, in the case of unicellular organisms, that the sepa-
rated parts are identical in substance and carry on the complete
organisation of the parent cell in absolute integrity. In the case
of multicellular organisms it depends on the same thing. The cell
which in the embryo begins the germ-cell lineage may be identical
with the fertilised ovum, and the complete heritage may be con-
tinued intact through successive cell-divisions until the next
generation is started, and the process begins anew. The com-
pleteness of hereditary resemblances depends, in Bateson's
phrase, on " that qualitative symmetry characteristic of all non-
differentiating cell-divisions."

It seems, therefore, useful to say that variation is " the expres-
sion of a qualitative asymmetry beginning in gametogenesis.
Variation is a novel cell-division." But to tell what specific cause
induces this novelty is still beyond our power. Yet we can point
to certain conditions which may induce novelty or qualitative
asymmetry in gametogenesis. Thus, there is the complex change-
ful environment of the developing germ-cells, there is the possible
struggle of analogous hereditary units or determinants for sus-
tenance, there is the complex process of reduction which occurs
during the maturation of the germ-cells, and there are the chances
of new combinations and permutations in fertilisation.

Results of Amphimixis. — That amphimixis is one of the provocatives
of variations is strongly suggested by what results when two breeds

c c

e-c

'%&$-

Fig. 23. — Karyokinesis. (After Flemming.)

1. Coil stage of nucleus ; cc centrosome ; 2, Division of chromatin into U-shaped loops,
nd longitudinal splitting of these (astroid stage) ; 3, 4, Recession of chromosomes from
he equator of the cell (diastroid) ; 5, nuclear spindle with chromosomes at each pole, and
achromatin threads between ; b, Division of the cell completed.

[Facing p. 102.

CAUSES OF VARIATIOX 103

are interbred. As Prof. Cossar Ewart says * : " Domestic animals
reproduce themselves with great uniformity if kept apart ; but the
moment one mixed up two different races, strains, or breeds, one did
something that was difficult to put in words, but the result was what
has been best described as an ' epidemic ' of variations."

On the other hand, Hatschek and others have pointed out that
amphimixis acts as a check on variability, obviating heterogeneous
idiosyncrasies. This was suggested even by Lamarck : "In repro-
ductive unions the crossings between the individuals which have
different qualities or forms are necessarily opposed to the continuous
propagation of these qualities and these forms." Similarly Darwin
said : " When species are rendered highly variable by changed con-
ditions of life, the free intercrossing of the varying individuals tends
to keep each form fitted for its proper place in nature."

Combinations of Chromosomes. — Prof. H. E. Ziegler has given much
attention to the number of possible combinations of parental chro-
mosomes in the offspring, supposing the distribution to be fortuitous.
If the normal number of chromosomes in a species is n, the number

n
of tetrad groups is -, the number of possible combinations in the

11
mature germ-cells is 1- 1, and the number of possible combinations

(n V
in the fertilised egg-cell is I - — lJ = — — n + 1.

If the normal number of chromosomes be 8 (as in the fluke often
found parasitic in frogs, Polystomum integerrimum), the number of
tetrad groups is 4, the number of possible combinations in the mature
germ-cells is 5, and the number of theoretically different offspring is
25, i.e. on the assumption that the chromosomes are heterogeneous.
But according to the laws of chance certain combinations are much
more frequent than others ; the larger the number of tetrad groups
the more frequent is the occurrence of an approximately equal
number of paternal and maternal chromosomes in the germ-cell.

Sutton puts the matter as follows. An individual receives from
his father 4 chromosomes, A, B, C, D, and from his mother (an equal
number) a, b, c, d. The immature germ-cell has A, B, C, D ; a, b, c, d.
These group themselves in four tetrads, each composed of two
double chromosomes, two maternal and two paternal, Aa, Bb, Cc,
Dd. The mature germ-cell receives one chromosome from each

* Discussion on Heredity in Disease, Scottish Med. and Surg. J:ut tut, vi
1900, p. 308.

104 HEREDITY AND VARIATION

tetrad, and there are 16 possible combinations — viz. a, B, C, D ;
A, b, C, D ; A, B, c, D ; A, B, C, d ; a, b, C, D ; a, B, c, D ; a, B, C, d ;

a, b, c, d ; and eight others which may be got by replacing small
letters by capital letters and vice zersd. The number of possibly
different offspring would be 162.

Sutton gives the following table, which is of some interest as
suggesting the possibilities of variation.

Normal

number of

chromosomes

Number
of Tetrad-
groups

Number of combina-
tions in the mature
germ-cells

Number of possi

bilities in the

offspring

8 ..

■ ■ 4 * *

16

256

12

6 . .

64

4,096

l6

8 ..

256 ..

65,536

24

. . 12

4096

. 16,777,216

Summary. — In certain moods biologists are accustomed to say
that they do not know anything in regard to the causes of varia-
tion. They imply that it is of the essence of living creatures to
vary, that variability is a primary property of organisms. The
sequence of generations is a life stream, changing as it flows.

In other moods, however, biologists often point out how natural
it is that organisms should vary. When the body of the parent
is a-making, a lineage of germ-cells is started and the unspecialised
descendants of these develop into offspring, which are on the
whole like the parent because they are made of the same stuff.
" True " twins developed from one ovum are usually almost
facsimiles of one another. Why should not the offspring be a
facsimile of the parent ? Sometimes, to our eyes, it is quite con-
fusable with the parent, but this is not common. Why not ?

1. It is common to point out that the germ-cell which is liber-
ated to become an offspring is not likely to be identical with the
germ-cell which developed into the parent. It has been sojourn-
ing in the parent's body, exposed to a variable food stream and
often to a variable complex environment, partly somatic and
partly external. Is it likely to be exactly the same as the original
germ-cell from which it is descended by continuous cell-division ?

SUMMARY OF CAUSES 105

The experiments of Prof. W. L. Tower, in particular, suggest that
important external changes may provoke changes in the germ-cells
without necessarily affecting the parental body. He subjected full-
grown potato-beetles (Leptinotarsa) to peculiar conditions of tem-
perature and humidity during the time when the eggs were maturing,
and found that " mutations " occurred in a certain proportion of the
offspring. The parents were not affected, having passed the plastic
stage; and some of 1h^ eggs were not affected at all. Moreover,
the same environmental peculiarity did not always produce the
same mutation in the offspring. But what Tower's experiments
forcibly suggest is this : that deeply saturating environmental changes
may serve to pull the trigger of germinal variability.

2. It is also to be remembered that if the chromosomes stand
I in some definite causal relation to heritable qualities, as seems

practically certain, then the maturation reduction of the chromo-
j somes to one half their original number offers an opportunity
|, for variation.

3. It is likely that fertilisation or amphimixis — the intimate
I and orderly union of two sets of hereditary contributions which

have often had very different histories — will promote variation.
j • It is difficult to believe that it does not bring about new permuta-
|! tions and combinations.

4. It is possible that variations may also arise in a less con-
ceivable fashion — " bathmically," as the phrase goes — for un-
known internal reasons. It is not absurd to suppose that the

I germ-plasm grows from generation to generation, and, in growing,
changes — because it is its nature so to do.

Apart from variation of internal origin and positive modifica-
tion of external origin, we must remember that the offspring may
differ from its parents through non-expression of certain items
lof its inheritance, the non-expression being due to the absence
of the appropriate liberating stimulus. This kind of deviation
may of course be obliterated next generation, when the full en-
vironment allows the latent character to re-express itself.

CHAPTER IV

COMMON MODES OF INHERITANCE

" Lord, I find the genealogy of my Saviour strangely checkered with
four remarkable changes in four immediate generations.

i. Roboam begat Abia ; that is, a bad father begat a bad son.

2. Abia begat Asa ; that is, a bad father a good son.

3. Asa begat Josaphat ; that is, a good father a good son.

4. Josaphat begat Joram ; that is, a good father a bad son.

I see, Lord, from hence, that my father's piety cannot be entailed ;
that is bad news for me. But I see also, that actual impiety is not always
hereditary ; that is good news for my son." — Thomas Fuller, Scripture
Observations, No. viii.

§ i. Though Prediction in Individual Cases is insecure,
there are some Common Modes of Inheritance.

§ 2. Certain Necessary Saving Clauses.

§ 3. Blended Inheritance.

§ 4. Exclusive Inheritance (Unilateral, Absolutely Pre-
potent, or Preponderant).

§ 5. Particulate Inheritance.

§ 6. Alternative Inheritance.

§ 7. Summary of Possibilities.

Especially among the lower animals, the offspring sometimes
appear to us as if they were perfect reproductions of the parents,
and we venture to speak of complete hereditary resemblance.
Thus, in a crowd of Myriapods collected from one place at the
same time, no individual peculiarities could be detected. A
daughter-Hydra may be easily obtained which seems identical
with the parent. A series of generations of green-flies or Aphides
may be studied and no individual peculiarities discovered.

106

MATERIALS FOR STUDY 107

In other words, there seem to be cases in which generation
succeeds generation without any variation.

But there is every reason to suspect that in most cases the
apparent absence of variation is illusory, and due to a lack of
sufficiently intimate acquaintance with the individual organisms.
The sheep which seem " all the same " to the careless eye are
often known individually by the shepherd, and it is easy to
demonstrate that the peas in one pod are often far from being
alike. Similarly, the members of a group of individuals may
seem " all the same " even to the naturalist's eye, but minute
differences are soon detected by the expert who has devoted
years to becoming intimately acquainted with that particular
type. There are observable differences between sister-bees or
ants, between the rooks from one clutch or the pigs from one
litter. Even when there is only one parent — e.g. a self-fertilising
liver-fluke or a parthenogenetic water-flea — there may be
variations among the descendants. There is no doubt, however,
that the range of variability differs greatly in different types,
and it is obviously in cases where individual peculiarities are
frequent and well marked that we can most hopefully study
the relations of resemblance and difference between parents and
offspring, or between the members of a series of generations.
In horses and dogs, in sheep and cattle, in rats and mice, in
rabbits and guinea-pigs, in pigeons and fowls, in butterflies and
small, rapidly breeding crustaceans, in wheat and barley and
maize, in peas and stocks, and in- man himself, there is ample
opportunity for studying the modes of inheritance.

§ I. Though Prediction in Individual Cases is insecure, there are
some Common Modes of Inheritance

When we are dealing with the generations of an animal or
plant in regard to which previous observation has shown us
that the members of the species are strikingly uniform in their
characters, we may venture with some security to predict that

108 COMMON MODES OF INHERITANCE

the offspring of a pair will as usual exhibit more or less complete
hereditary resemblance to their parents and ancestors. And
yet this prediction may be falsified, for variations may suddenly
crop up without known cause.

Similarly, when we are dealing with the generations of a
so-called " pure-bred " race of animals or plants, we may
venture with some security to predict that the offspring of a
pair will exhibit, as regards their more essential features, a large
measure of complete hereditary resemblance to their parents and
ancestors. And yet in individual cases this prediction also
may be falsified; for no known reason a " freak " or " sport "
may unexpectedly appear.

When we consider the variable nutritive conditions of the
germ-cells, the subtle processes of maturation and fertilisation,
and the intricate nature of the environment appropriate to
each development, we cannot be surprised that the result
may often belie individual prediction. The possibly anecdotal
instance, cited by Lucas, of the twin children of an Antillean
negress — one white with long hair, the other black with woolly
hair — may serve as a diagrammatic illustration.

On the other hand, experience shows that, in spite of uncer-
tainty in regard to individual cases, there is often perfect
certainty as to the average results where we have to do with
large numbers ; that the degree of resemblance to parents and
ancestors is sometimes capable of precise prediction ; that in
particular sets of cases (Mendelian phenomena, see Chapter X.)
we can definitely predict how many of the offspring will be like
the parents, how many like one grandparent, how many like
another ; and that, apart from such statistical generalisations,
there are what we may call alternatives of expectation with varying
degrees of probability. In other words, there are certain more
or less well-defined modes of hereditary resemblance which occur
very frequently. To explain and illustrate three of these is the
object of this chapter,

AN EXUBERANT TERMINOLOGY I69

A discussion of the different modes of hereditary resemblance
is somewhat hampered by an exuberant terminology, and by
the fact that different authors have sometimes used the same
term in different ways. We read of inheritance being unilateral
and bilateral, unisexual and bisexual, blended and conspired,
neutralised and combined, direct and collateral, atavistic and
progressive, and so on. We have tried to reduce this complex
terminology to a minimum. This is the more justifiable since
we cannot doubt that all the ordinary phenomena are of a piece,
that many of the ordinary modes will be embraced eventually
in one general formula — probably some modification of Galton's
Law of Ancestral Inheritance, and that others will be embraced
in Mendelian formulae.

We propose, then, to restrict attention to three frequently
occurring modes of hereditary resemblance, which are called
.blended, exclusive, and particulate.

§ 2. Certain Necessary Saving Clauses

Before we define and illustrate the three commonest modes of
inheritance, we must, at the risk of reiteration, notice certain
saving clauses.

We have seen that cases of apparently very complete hereditary
resemblance may be illusions due to our inability to appreciate
the differences that really exist ; but on the other hand, we must
guard against the error of supposing that the frequently con-
picuous differences between offspring and their parents neces-
arily means an incompleteness in the inheritance itself. The
act that the resemblance often reappears in the third generation
hows that the incompleteness is often not in the inheritance,
ut simply in its expression. The characters were probably
here in posse in the germinal matter, but they were neutralised,
ept latent, silenced — we can only use metaphors — by other
haracters, or else they never met with the stimulus necessary

no COMMON MODES OF INHERITANCE

for their expression in development. We can imagine the son
of a lavish millionaire reacting to plain living, the superficial
inference that the money had been lost, and the contradiction
of this in the third generation.

Similarly, when a male offspring is compared with the mother,
a female offspring with the father, it is important to bear in mind
that the difference in sex may account for some of the apparent
differences in detailed characters. Through functional cor-
relation, the differentiation of sex may bring about the non-
expression or the modified expression of a peculiarity which was
none the less transmitted in its entirety as the third generation
may demonstrate.

Another fact that must be borne in mind is the difficulty of
distinguishing even with probability between hereditary and
acquired resemblances. The Alpine plants which Nageli trans-
planted to a southern garden were changed by their new sur-
roundings ; their descendants were likewise changed, and the
new characters reappeared with constancy generation after
generation. But this was acquired or modificational, not
hereditary or innate resemblance, as was shown by the fact that
removal from the garden to poor gravelly soil was followed by
a reappearance of the original Alpine characteristics. Some
interesting cases have been alleged where the reappearance of
the Alpine characters was not immediate, but gradual. We
require, however, more circumstantial details in regard to these
cases.

§ 3. Blended Inheritance

In this mode the special characters of the two parents are
intimately mingled in the offspring. The colour of the hair
may be an almost precise average between that of the blonde
mother and that of the black-haired father. In repose the boy's
face may seem markedly paternal ; it is moved with emotion,
and he is his mother's image. This blending is particularly well

BLENDED INHERITANCE

in

seen in some plant hybrids, where the offspring shows in leat-
venation, in size of epidermic cells, in number of stomata, in
length of style, in degree of hairiness, and so on, what seems
like an accurate mean of the two parents. Prof. J. M. Macfarlane
has given some beautifully precise data regarding the blending
of characters in plant hybrids.
When in any given character of the offspring we can detect

A. <B. 'C

Fig. 24. — Leaves cf Willow : A, of one parent ; C, of the other parent ;
B, of the hybrid intermediate between them. (After Wiesner.)

both maternal and paternal peculiarities, we call the inheritance
blended ; but there may be quantitatively more of the maternal
quality or of the paternal quality expressed, and then we say
that in the blended inheritance or in its expression one of the
two parents is prepotent. An increase in the predominance of
the characteristics of one parent leads to the second common
mode of inheritance, which we call exclusive.

U2 COMMON MODES OF INHERITANCE

§ 4. Exclusive Inheritance (Unilateral, Absolutely Prepotent, or

Preponderant)

When in the expression of the biparental inheritance there
is, as regards a given character, an absolute prepotency on one
side or the other, or, conversely, an apparent reduction of the
maternal or paternal peculiarities to zero, the inheritance is
called exclusive. The terms " unilateral," " absolutely pre-
potent," or " preponderant " are also used. This mode of
inheritance is oftenest seen in regard to particular characters,
but it is sometimes consistently illustrated in so many parts
of the organism that observers say of the whole offspring that
it favours its sire, or that it takes after its mother.

In reference to a few characters a general statement may
sometimes be made with security to the effect that, on the average,
the father is prepotent in certain respects and the mother in
others. Thus, in regard to stature of human beings (in Britain),
it seems certain that the father is usually prepotent ; that is
to say, on the average children attain to a stature which is
nearer that of the father than that of the mother. But every
statement of this sort must be based on carefully collected
statistics, and not on the " impressions " — however strong —
which breeders have often formulated as laws.

There are many popular generalisations which ascribe to each
of the parents the power of transmitting particular characteristics.
Thus, the father is supposed to have to do with external form,
the mother with temperament and the organs of vegetative
life. While particular statements in respect to this are interesting
and should be accumulated in as large numbers as possible,
almost all the generalisations, including the one instanced, are
mere guesses. At present, we can only say that in some cases
the expression of the inheritance as a whole, or in regard to par-
ticular characters, may resemble one parent more or less exclu-
sively. In other words, the father sometimes seems absolutely

EXCLUSIVE INHERITANCE 113

prepotent, the mother sometimes seems absolutely prepotent,
but the characters in regard to which the prepotency is exhibited
usually vary from case to case. Goethe may have been quite
accurate in saying :

Vom Vater hab' ich die Statur,
Des Lebens ernstes Fiihren ;
Vom Miitterchen die Frohnatur
Und Lust zu fabuliren.

But this cannot be generalised as a law of inheritance !

There seems to be a widespread belief among breeders that
external form depends upon the father, while temperament and j
visceral organs depend upon the mother. But this does not,
stand examination. Nor can we rely with security on the
opinion of many horse-breeders — e.g. Stephens — that the sire
almost always counts for most all round ; for it has to be re-
membered that a sire is mated with very few dams as good as
himself. Buffon hazarded the conclusion that the mule resembles
the father ass more than the mother mare, and the hinny re-
sembles the father horse more than the mother ass ; but he dealt
nly with superficialities. Crossings between humped zebu cattle
nd those without humps show that the hump is inherited in some
egree, whether it was possessed by the ox or the cow ; and the
me is true in regard to camels with one or two humps, and in
egard to crossings of wild boar and sow or vice versa.

There is no doubt that what looks like well-marked " unilateral

nheritance " is not infrequent, where the son is, as they say,

he very image of his father, or the daughter the reflection of

er mother ; or, even more frequently, where the inheritance is,

s they say, " crossed," the son taking after the mother, and

he daughter after the father. But to generalise the latter into

formula, as some dog-breeders have done, " Chien de chienne,

t chienne de chien," is quite illegitimate. The result will depend

which of the parents has the mysterious quality of " prepo-

8

114 COMMON MODES OF INHERITANCE

tency " ; and it may be that the father is " prepotent " in regard
to some of the characters and the mother in regard to others.
A negro in Berlin had by a white woman seven daughters who
were markedly mulatto, and four sons who were white ; the
inheritance was " crossed," but other cases forbid us from
making any generalisation.

It must be carefully kept in view that where the expression of the
inheritance markedly follows one parent, it does not in the least
follow that the corresponding contributions from the other parent
have been lost. It may be that the latter will reappear in the next
generation, having simply remained latent in the custody of the
germ-cells. And again, there are cases on record where the young
boy resembled the mother and the young girl the father ; but as they
grew up, the likeness was reversed — i.e. resemblances formerly ob-
scure became conspicuous. Such cases seem to warrant our insist-
ence on the distinction between the inheritance and the expression
of the inheritance.

It must be carefully borne in mind that what we describe as a case
of exclusive inheritance may be the first step in Mendelian inherit-
ance. When a parent with a dominant unit character mates with one
having a corresponding unit character recessive the offspring all show
the dominant character.

§ 5. Particulate Inheritance

In many cases it may be seen that the peculiarities of the two
parents do not blend, but are separately expressed in different
parts of the same organ or system. The combination is, as it
were, too coarse-grained to be called a mixture or a blend. This
is termed particulate inheritance.

A familiar instance is a piebald foal — the progeny of a dark-
coloured sire and a light-coloured mare. The paternal hair is
seen in some parts, the maternal hair in other parts. " Eye-
colour is generally exclusive, but we get one or two cases per
thousand in man, in which either the two irises differ in colour, '<

PARTICULATE INHERITANCE 115

or the one iris shows different patches of colour " (Pearson, 1900,
p. 452). The case of an English sheep-dog with a paternal eye
on the one side of its head and a maternal eye on the other is
vivid enough.

When there is a marked difference in the pedigree, the vigour,
the age — in short, in the constitution — of the two parents, the
same mode of inheritance may be illustrated in a succession of
offspring. Thus a fine sire, paired with a commonplace mate,
may be prepotent birth after birth ; or a young mother mated
with a worn-out male may have it all her own way in regard to
inheritance, as well as in much else. On the other hand, when1
there are no such marked differences between the parents, the
inheritance may be a blend in one offspring, exclusive in another,
particulate in a third. Moreover, in the same offspring, different
sets of characters may illustrate different modes of inheritance.
Thus we see that these modes of inheritance are merely useful
descriptive terms, helping us to keep our facts in order, but not
directly aiding us in their interpretation. They point to the
need of some unifying conception, which shall enable us to
understand how all these alternatives are possible.

In large families there is sometimes observable an interesting
change in the direction of preponderance in the successive
children. With a virile middle-aged father and a much younger
mother, the older children may be markedly paternal in the
sxpression of their inheritance, the younger children as markedly
of the maternal type. The Benjamin is the mother's very
image, and after the father's own heart.

Similarly, it has been observed that the first fertilised, almost
mmature ova of a rabbit, liberated by an ovulation subsequent
:o the first pairing, resulted in offspring which took after the male.
U , on the other hand, the doe was served, not at the right time,
>ut a week or ten days later, the young were all exactly like the
nother.
Such cases suggest the conclusion that the expression of

n6 COMMON MODES OF INHERITANCE

inheritance follows the parent whose germ-cells are the riper at
the time of fertilisation — an inference to which we shall return
in discussing germinal selection.

The inference is further supported by Vernon's experiments in
the hybridisation of sea-urchins, for he showed that the characters
of the offspring incline to be those of the species whose gametes
were relatively the more mature when fertilisation occurred.

§ 6. Alternative Inheritance

Since the re-discovery of Mendel's Law — to which we shall
afterwards refer in detail — there has been a rapid accumulation of
instances of what is called alternative or Mendelian inheritance,
and some of the leading experimenters of to-day believe that this
mode of inheritance will be found to include other modes like
blended and particulate which seem at first sight distinct.

Let us follow one of these authorities, Dr. C. B. Davenport, in
stating the fundamental ideas.

Experimental work has driven home the conception of unit-
characters. That is to say, the characteristics of an organism
may be analysed in some cases into distinct units that are in-
herited independently. About a dozen of these, for instance,
have been demonstrated in the sweet-pea.

The theory, supported by experimental results, is that these
unit-characters are represented in the germ-cells by what may
be called representative particles, or anticipatory units, or
primary constituents, or determiners, and that these cannot
blend or make any compromise with other determiners of con-
trasted unit characters. They are either there or not there. If
two parents have the same unit-character (x), the offspring get a
corresponding determiner x from both sides, and when the germ-
cells are formed in that offspring they will all have a double
determiner x, and will be like their parents as regards the unit-
character in question. If one parent has a unit-character (x)

ALTERNATIVE INHERITANCE 117

which the other lacks, the offspring get a corresponding deter-
miner x from one side only, and when the germ-cells are formed
in that offspring half of them are supposed to have the determiner
x and half not. This hypothetical process is called the segrega-
tion of determiners, and experimental results suggest its reality.

"The characteristic in the offspring that is due to a single
(instead of the normal double) determiner is called a simplex
characteristic. Such a characteristic is frequently distinguish-
able from one that is due to the double determiner by its imper-
fect development. Thus the offspring of a pure black-eyed and
a blue-eyed parent will have brown eyes.

" It is a corollary of the foregoing that if the individual with a
simplex character be mated to one lacking the character, half of
the offspring will lack the determiner and half will be simplex,
again, in respect to the character. If in both cases the character
be simplex, the two like determiners will meet in one fourth
of the unions of egg and sperm, the two will both be absent in
one-fourth of the unions, and one only will occur in half of the
unions — such will be simplex again. If one parent have the
characteristic simplex and the other duplex, then half of the
offspring will have it simplex and half duplex.

" Starting with the principles just enunciated, we reach at once
the most important generalisation of the modern science of
heredity : " When a determiner of a characteristic is absent from the
germ-plasm of both parents (as proved * by its absence from their
bodies) it will be absent in all of their offspring " (Eugenics, 1910,
pp. 8-9).

To illustrate the precision f with which the characteristics of
offspring may be predicted in the best-studied cases, Davenport
refers to eye-colour.

* Perhaps " inferred " would be a more accurate word than " proved.''
What is inherited in the germ-plasm is not necessarily expressed in de-
velopment.

j See, however, Galloway (1912).

n8

COMMON MODES OF INHERITANCE

" Blue eyes are due to the absence of brown pigment. If
there is a determiner for brown pigment in the germ-plasm, it
will produce such pigment in the body that arises from that
germ-plasm. The absence of iris pigment is proof of the absence
of the pigment determiner from the germ plasm. If both parents
lack brown pigment, their offspring, being devoid of the deter-
miner for brown pigment, will all lack brown pigment. As a
matter of experience two parents with pure blue eyes will have
only blue-eyed offspring."

CHAPTER V

REVERSION AND ALLIED PHENOMENA

" A man can never deny his ancestry." — Laws of Manu.

" Evolution ever climbing after some ideal good,

And Reversion ever dragging Evolution in the mud." — Tennyson.

§ i. What is meant by Reversion.

§ 2. Suggested Definitions.

§ 3. Theoretical Implications.

§ 4. Phenomena sometimes confused with Reversion.

§ 5. " Skipping a Generation."

§ 6. Mewdelian Interpretation of Reversion.

§ 7. Reversion in Crosses.

§ 8. Reversion of Retrogressive Varieties.

§ 9. Interpretations in Terms of Reversion.

§ 10. Further Examples of Reversion.

§ 1. What is meant by Reversion

Most evolutionists — indeed, most naturalists — have ranked
reversion as one of the facts of inheritance. Thus Darwin
said (1881) : " Any character of an ancient, generalised, or
intermediate form may, and often does, reappear in its descend-
ants after countless generations." Wallace, Spencer, Galton,
and Weismann have all used the concept " reversion " as a
convenient way of summing up a universally admitted series of
cases, where organisms exhibit ancestral traits which their
parents did not possess. As a descriptive term for summing up

these cases, the word " reversion " is useful, convenient and, it

119

120 REVERSION AND ALLIED PHENOMENA

seems to us, entirely legitimate. When we go beyond the use
of the word as a descriptive term, and use it as implying that
the ancestral characters reappear because they are parts of
the inheritance, which have been lying latent for generations,
and have suddenly been allowed by some liberating stimulus to
express themselves in development, we pass from fact to inter-
pretation, from description to theory, and great care is necessary.
For the fact that an organism exhibits some peculiarity character-
istic of an ancestor does not necessarily imply that this is due to
the rehabilitation of latent items in its inheritance.

Darwin believed that " an inherent tendency to reversion is
evolved through some disturbance in the organisation caused by
the act of crossing," and he gave such instances as the following.
A goldfinch crossed with a plain yellow canary had offspring with
stripes on the back and flanks. Darwin concluded that " this
streaking must be derived from the original wild canary." He
crossed a White Silkie hen with a black Spanish cock and got
among the progeny a cock which looked like a rehabilitation of
the original wild Gallus bankiva type. These facts have been
confirmed, but it is now generally agreed that the Darwinian in-
terpretation of them must give place to theMendelian — that " in
hybridising we restore the factor that is missing from one strain
by introducing it from another strain ; or we remove the added
factor that veils the ancestral condition " (Davenport, 1910, p. 293,
" The New Views about Reversion ").

Illustrations. — A recognition of reversionary or atavistic phenomena
is ancient. Plutarch gives the case of a Greek married woman who,
having given birth to a black child, was brought to justice as an
adulteress, and had science enough to allege in her defence that
she was descended from an Ethiopian four generations back. This is
paralleled by a case reported by De Quatrefages of two Virginian
slaves, to whom a perfectly white child was born. " En voyant la
couleur de son enfant, elle fut saisie de terreur, . . . mais son mari
la rassura, en lui declarant que son propre frere etait blanc."

We do not mean that the instances just mentioned should bet

ILLUSTRATIONS

121

taken as serious pieces of evidence in favour of the reversion theory,
but they may serve to hint at the readiness with which the hypo-
thesis of characters lying latent has been adopted. As we shall
see, reversions in the strict sense are apparently few and far be-
tween.

A foal is sometimes born with a few stripes on its fore-legs,
as if reminding us of striped wild horses. A dovecot with
carefully bred pigeons was left to itself for some years, after
which it was found to contain numerous blue pigeons, resem-
bling in many ways the wild rock-dove (Columba livia). A

***U

Fig. 25. — Devonshire pony, showing the occurrence of stripes. (From
Darwin.)

dark-coloured child may be born in a family where there has
been some Eurasian mixture. Cultivated flowers and vegetables,
such as pansies and cabbages, sometimes produce forms hardly
distinguishable from their wild progenitors. The nectarine derived
from a peach may produce what is practically a peach again. The
white-flowering currant — derived from the common red form — may
have branches with red flowers. These are preliminary illustrations
of what are usually called reversions — the hypothesis implied being
that they are returns, or " throw-backs," to an ancestral type.

§ 2. Suggested Definitions

Darwin's introductory exposition (1868, vol. ii. p. 28) was as
follows : " When the child resembles either grandparent more

122 REVERSION AND ALLIED PHENOMENA

closely than its immediate parents, our attention is not much
arrested, though in truth the fact is highly remarkable ; but when
the child resembles some remote ancestor, or some distant member
in a collateral line — and we must attribute the latter case to the
descent of all the members from a common progenitor — we feel a
just degree of astonishment. When one parent alone displays
some newly acquired and generally inheritable character, and the
offspring do not inherit it, the cause may lie in the other parent
having the power of prepotent transmission. But when both
parents are similarly characterised, and the child does not, whatever
the cause may be, inherit the character in question, but resembles
its grandparents, we have one of the simplest cases of reversion."

"The most simple case of reversion — namely, of a hybrid or mongrel
to its grandparents — is connected by an almost perfect series with
the extreme case of a purely bred race recovering characters which
had been lost during many ages ; and we are thus led to infer that
all the cases must be related by some common bond " {ibid. p. 49).

" By the term reversion," Weismann says, " is meant the appear-
ance of characteristics which existed in the more remote ancestors,
but were absent in the immediate ancestors — i.e. the parents " (1893,
p. 299).

Prof. Karl Pearson defines a reversion as " the full reappearance
in an individual of a character which is recorded to have oc-
curred in a definite ancestor of the same race," and atavism as " a
return of an individual to a character not typical of the race at all,
but found in allied races supposed to be related to the evolutionary
ancestry of the given race. ... In reversion we are considering a
variation, normal or abnormal, from the standpoint of heredity in
the individual ; in atavism we are considering an abnormal variation
from the standpoint of the ancestry of the race." But as the two
words have been used by some authors in the converse way, and as
it is surely difficult to define the field of abnormal variation, we
adhere to Darwin's wider usage, and drop the term " atavism " as
an unnecessary synonym.

" Reversion," De Vries says, " means the falling back or returning
to another type, and the word itself expresses the idea that this latter
type is the form from which the variety has arisen. . . . Atavism
or reversion is the falling back to a prototype " — i.e. " those an-
cestors from which a form is known to have been derived." But
De Vries distinguishes sharply between true reversion due to a

THEORETICAL IMPLICATIONS 123

sudden reassertion of latent ancestral characters in a pure-bred
stock, and false reversion or vicinism due to crossing. Descriptively
both may be called " reversions," but they differ in their nature
and their causes. He also distinguishes reversion to a known
ancestor from " systematic atavism " to ancestors which are only
reputed to be such on taxonomic grounds.

" Reversion," Prof. Bateson points out, " occurs when the sum-
total of the factors returns to that which it has been in some original
type. Such a return may be brought about by the omission of an
element or elements, or by the addition of some missing element
needed to complete the original type. Reversion on crossing is thus
the particular case in which one or more missing factors are brought
in by the parents of the cross-bred." This is the Mendelian inter-
pretation of reversion, and Mr. Bateson does not believe that there
are any reversionary phenomena which do not admit of this inter-
pretation.

We would use the term " reversion " to include all cases where,
through inheritance, there reappears in an individual some character
or combination of characters which was not expressed in his immedi-
ate lineage, but which had occurred in a remoter but not hypothetical
ancestor. We say " through inheritance " in order to exclude those
cases where the reappearance can be accounted for in some other
way. There is no reason for complicating the idea by calling the re-
versionary character " abnormal," for abnormality is often difficult
to define.

If we can arrange a series of related types on an inclined plane
in order of their evolution, with the most recent highest up, we
can imagine the offspring of one of the highest slipping back (as
regards one or several of its characters) to a lower level — slipping
back beyond the grade represented by its own family or stock,
slipping back out of its species-grade altogether, and so forth.
These "throw-backs" might be described as family-reversions,
stock-reversions, species-reversions, and so on.

§ 3. Theoretical Implications

The general idea behind the term " reversion " is that particular
features characteristic of an ancestor may lie dormant — i.e. un-
expressed in development — for generations, and may suddenly
reassert themselves.

124 REVERSION AND ALLIED PHENOMENA

In the mosaic which composes an inheritance there may be
included items of ancient origin which can lie latent generation
after generation, remaining unexpressed in development for
lack of the appropriate liberating stimulus, or for other reasons.
Certain potentialities or initiatives, which really form part of
the inheritance and are really transmitted from generation to
generation, may be kept under by other components of the
inheritance, or in some way prevented from asserting them-
selves. At length, in the reconstitution which is associated with
the maturation and fertilisation of the germ-cells, or in the inti-
mate germinal struggle which is possibly always going on amongst
the diverse hereditary items, the long-latent items find their
opportunity and the result is a reversion due to the reassertion of
long-latent characters.

The garden of a shepherd's cottage swallowed up in a deer-
forest lost all trace of its previous cultivation and became a
weed-ground. After many years it was delved, and soon there
appeared many different kinds of old-fashioned flowers whose
seeds had lain dormant for several generations. So may ancient
flowers and weeds now and again reappear out of latency in
that garden which we call our inheritance.

So far the old view — a hypothetical interpretation which may
hold good in certain cases, such, perhaps, as the appearance of a
fourth toe on a guinea-pig's hind foot, or of horns in a hornless
race of cattle. What is the new view, which rests on a definite
experimental basis ? It is briefly as follows. In establishing
domesticated or cultivated varieties, man seems to have been for
the most part assisting in the " unpacking " of the extremely
complex inheritance of the wild type. Thus the colour-varieties
of the domestic rabbit are the results of analysing out in varying
measure and mixture that beautiful synthesis of hues which we
see in the wild rabbit. When certain colour-varieties are crossed
and the offspring are of the wild type, this is due to " repacking."
Colour-factors which have been separated out by anatysis come

ARRESTS OF DEVELOPMENT 125

together again and restore the wild form. There has been no
mysterious re-awakening of long-latent characters. We may
still call what occurs a " reversion," but in cases like the above
our interpretation is no longer Darwinian.

§ 4. Phenomena sometimes confused with Reversion

It is impossible to read the fairly abundant literature without
recognising that many phenomena are labelled " reversions " on
the flimsiest of evidence. Let us try to make the conception
more definite by criticism and elimination of alleged instances.
In this criticism we have especially to bear in mind that the term
" reversion " is not merely descriptive of the direction which the
variation takes ; it implies that this direction — ancestor-wards
— is due to something that occurs in the early history of the germ-
cells.

Arrests of Development. — Though popular travesties have
reduced a luminous idea to an absurdity, it remains in a general
way true that the individual development, especially in the stages
of organ-forming, is in some measure a recapitulation of the racial
history. Although it is more picturesque than accurate to speak
of " every animal climbing up its own genealogical tree," there
is a suggestive general resemblance between the stages in the
individual development of organs, such as heart, brain, and
kidneys, and the stages in the supposed racial evolution of
the same.

Now, it not infrequently happens that the recapitulation is
notably incomplete, that the development of an organ stops
before the normal " finished form " has been attained.

Through defective nutrition or other untoward conditions of
nurture, the expression of the inheritance is inhibited. The
organism is not able to perfect itself in all its parts ; not, we
suppose, through any germinal defect (as subsequent generations
may show), but simply because it was not sufficiently fed, or

126 REVERSION AND ALLIED PHENOMENA

because it was poisoned, and so forth. The results may be
congenital, but they are not germinal ; they are due to defects
not in nature, but in nurture. Thus children born in times of
famine are sometimes far below the normal human standard,
but it is an assumption to ascribe their deficiencies to their
inheritance. In short, all cases of arrested development which
can be referred to peculiarities of pre-natal or post-natal nurture
should be eliminated from the category of " true inborn rever-
sion." For practical purposes, in rough-and-ready description,
they may be called reversions, but they are modificational results ;
they do not require the hypothesis of the reawakening of latent
ancestral characters. It is reducing scientific terminology to
an absurdity to describe as a reversion what may be simply due
to premature birth or deficient nutrition.

There is a stage in the development of the human fcetus when the
openings of the nostrils communicate down the lip with the corners
of the mouth-opening ; when this communication, which is normally
closed up, persists, we have (in part) the abnormality known as
" hare-lip," normal in rabbit and hare. But there is no reason
to interpret the abnormality in man as a reversion ; it is an arrest
at a stage which is normally passed through ; it is probably due
simply to a lack of developmental vigour, or more simply still to
a lack of adequate nutrition. Dr. Joseph Bell * refers to a case
mentioned by Prof. Haughton of young lion-cubs which all died
of hare-lip — the supposed reason of the arrest being that the keeper
fed the pregnant lioness on tit-bits, without bones. When the
supply of bones was ensured on subsequent occasions, the tendency
to hare-lip disappeared. In connection with human affairs and
qualities of mind and character, it is well to bear in mind that what
we call defectives and criminals may sometimes be just like these
hare-lip cubs, though more viable.

In a hornless breed of cattle, derived originally from a horned
breed, a calf is born with small horns. This may be plausibly
interpreted as a reversion to a horned ancestor. But when a calf

* " Discussion on Heredity in Disease," Scottish Med. and Surg. Journ.
vi. 1900, p. 307.

VESTIGIAL STRUCTURES 127

is born with a three-chambered heart, it is gratuitous to call this
a reversion to the saurians with three-chambered hearts, from which
mammals evolved. It is simply a case of arrested development.

Vestigial Structures. — It is a familiar fact that structures
of ancient origin and erstwhile importance may still linger in
dwindled expression in organisms where they do not seem to
have much or any significance. They are relics of the past,
vestiges of ancestral history, comparable, as Darwin said, to
the unsounded letters in many words, the 0 in leopard, or the
b in doubt — non-functional vestigial letters of which the spelling-
reformers would rob us so ruthlessly.

Each one of us is a walking museum of such relics, some of
which we should probably do better without. Thus the unused

I; muscles of the ear and the rudimentary third eyelid are ancestral
characters which persist in us, though without much significance
now. They are like the unused, often unusable, buttons, etc.
which survive on some parts of our every-day attire — useless,
but interesting, vestiges of bygone days. The gill-clefts of reptiles,
birds, and mammals ; the embryonic teeth of whalebone whales ;
the buried remains of pelvis and hind-limbs in whales ; the hint
' of a gill in the skate's spiracle, and so on, are familiar examples
of these " vestigial structures," traces of ancestral history, and
intelligible on no other theory.

But it goes without saying that as the occurrence of these
vestigial structures is still normal, there is no utility in calling
them " reversions " — even if now and again they are expressed
in greater strength than usual or persist beyond the time at
which many of them (e.g. all the gill-clefts save one) disappear —
namely, during development. They are very interesting, how-
ever, (1) in showing that ancestral features have great power of
hereditary persistence, and (2) inasmuch as they often show
great variability.

Acquired Modifications resembling Ancestral Characters —
When an individual exhibits a structural peculiarity not ex-

iz8 REVERSION AND ALLIED PHENOMENA

pressed in parents or grandparents, but known to occur in more
or less remote ancestors, we must try to discover how far this
peculiarity is really fart of the inheritance. That is to say,
we must inquire whether it may not be a modification induced
from without, which happens to resemble an innate character
of the ancestors. Many domesticated animals which have
become wild (feral) may show features resembling the original
wild ancestor, but these may be due to the direct influence of
the old environment and the old functions. It is safe to say that
many of the so-called reversions of feral animals are not inborn
but acquired and modificational.

Filial Regression. — We shall afterwards consider (Chapter IX.)
Mr. Galton's Law of Filial Regression, but it must be noticed
here, if only to point out that it has nothing to do with reversion.
The law, concretely stated, is that offspring are not likely to
differ from mediocrity in a given direction so widely as their
parents do in the same direction. There is a continual tendency
to sustain a specific average, or a stock-average.

Let us take a simple instance from Prof. Karl Pearson's
Grammar of Science. Suppose a group of fathers with a stature
of 72 in. : the mean height of their sons is 70*8 in. — a regression
towards the mean height of the general population. On the
other hand, fathers with a mean height of 66 in. give a group
of sons of mean height 683 in. — again nearer the mean height
of the general population. The " regression " works both ways ;
there is a levelling-up as well as a levelling-down. " The father
with a great excess of the character contributes sons with an
excess, but a less excess of it ; the father with a great defect of
the character contributes sons with a defect, but less of it."

Now this very important and normal fact of filial regression
has nothing to do with reversion, which implies the reappear-
ance of a definite ancestral character or set of characters which
have " lain latent " for several generations.

Independent Variations resembling Reversions. — If we

VARIATIONS RESEMBLING REVERSIONS 129

mean by reversion the re-expression of an ancestral character after
a period of latency, it is obviously a particular mode of inheritance.
From another point of view it is a variation, and due to some
unknown germinal conditions which permit a long-latent, but
never lost, character to re-assert itself. When we consider the
intricate reductions which occur in the maturation of the germ-
I cells, and the not less intricate reinforcements involved in
j amphimixis, it is not impossible to imagine how an ancient
latent character may come to the front again after many genera-
tions.

But we have also to remember that, apart from reasser-
tions of what is relatively old, there is a continual emergence
of what is relatively new. What occurred once as a new variation
may occur again, and it is a certain fact that the same type of
variation occurs over and over again in varieties of different
species. How many red and blue flowers have white varieties !
how many trees have weeping varieties ! how many Arthropods
show similar increase or decrease in the number of their joints!
how many birds show albinism ! There are limits to the varia-
tions of the kaleidoscope, and to the kaleidoscope of variations.
Therefore it is always possible that a variation really occurring
de novo, and apart from latent characters, may happen to coincide
with an ancestral trait. It may be described as a reversion, but
it is really an independent variation.

Supernumerary mammae occasionally occur in human beings
in both sexes. Ammon found them in 3 per cent, of German
recruits. They obviously suggest the several pairs of mammae which
occur in many mammals — e.g. in the half-monkeys or Lemurs.
Weismann (1893, p. t,^^) says, " They are undoubtedly to be looked
upon as reversions to extremely remote characters possessed by our
lower mammalian forefathers." But it seems simpler to regard
them as independent variations, comparable to many other ab-
normal multiplications of parts. They happen to suggest bygone
conditions, but that is probably all that we are warranted in saying.

Polydactylism in man has been interpreted as a reversion to an

9

130 REVERSION AND ALLIED PHENOMENA

ancestor with more than five digits ; but this is illegitimate, for the
so-called " heptadactylous ancestor " is a pure myth. Polydacty-
lism in man can only be called a reversion when there is in the
family history a previous occurrence of the same abnormality some
generations back.

It occasionally happens that a particular part of the skin in man
exhibits a mouse-like covering of close-set hair. To interpret this —
a mere random variation — as a reversion is credulous in the ex-
treme. It may also be noted, incidentally, that to call the wool-like
covering of small hairs (the " lanugo ") on the human foetus a re-
version to a hairy ancestor is quite absurd ; it is a normal stage in
development quite outside the rubric of reversion. It may be an
inheritance from a distant past, but it is no more a reversion than
the occurrence of a notochord as a constant antecedent to the
development of its substitute, the backbone. Similarly the dog's
habit of turning round and round before it settles down to sleep may
be interpretable in the light of past history, but it has nothing
to do with reversion.

" When horses are occasionally born at the present day in which
one or two accessory toes are present on two or even all four feet,
we are perfectly right in considering the development of these toes
to be due to reversion to an ancestor of the Miocene period." That
the modern horse which steps daintily on the tip of a single (third)
toe for each limb, and has merely hidden rudiments of the second
and fourth, has been evolved from a many-toed ancestor, is one of
the most certain of evolutionist inferences, but are we " perfectly
right " in interpreting the occasional development of supernumerary
toes, as on Julius Caesar's horse, to the reassertion of latent ancestral
items in the inheritance ? Is it not simpler to regard this as an
independent variation, comparable to multiplications of other parts
to which reversionary interpretations are inapplicable ? We must
remember, also, that vestigial organs are in many cases peculiarly
liable to vary.

It ought not to be necessary to remark that the ancestor to
whom the organism is supposed to revert must be real, not hypo-
thetical.

Some enthusiastic exponents of the reversion theory have not
scrupled to name or even invent the ancestor to whom the

IMPROBABLE CASES 131

peculiarity in question is supposed to be a reversion, although
evidence of the pedigree is wanting. And the terribly vicious
circle is not unknown of arguing to a supposed ancestor from the
supposed reversion, and then justifying the term " reversion " by
its resemblance to the supposed ancestor. Playing with biology
can hardly go further than this! Moreover, the postulate of
characters remaining latent (save for occasional more or less
hypothetical reawakenings) for millions of years, is made as
glibly as if it were just as conceivable as a throw-back to a
great-grandfather.

There are many reasons why it is absurd to describe a Cyclopean
one-eyed human monster as a reversion to the one-eyed larval
ascidian. One is that there is no warrant for believing that
the ascidian type was in the direct line of our long pedigree.

One of the diagnostic features of gout is the presence of uric acid
in the blood, and its deposition in various tissues of the body (doubt-
less helped by the frequently associated degeneration of the kidney,
which is normally competent to filter out the normal nitrogenous
waste-product, which is mostly in the form of urea). It is known,
however, that reptiles, for instance, like many backboneless animals,
normally excrete most or a large part of their nitrogenous waste in
the form of uric acid. This has led even such an eminent pathologist
as Prof. Hamilton (1900, p. 297) to say, "May we not entertain,
as a possibility, that the gouty constitution, so-called, is in part a
reversion to some far-back ancestor, in which uric acid was excreted
normally to a much larger extent than it is at present in an average
member of the human race ? " That is to say, the gouty person
reverts to the physiological habit of a far-back ancestral organism
(not even any known mammalian type), which had uric acid as a
characteristic waste-product, but he does not, unfortunately, revert
to the associated condition of having kidneys able to excrete the
uric acid adequately. But our simple point is that the supposition
of gouty tendencies lying latent in some form or other through
literally millions of years taxes our imagination too severely. Such
instances are almost sufficient to damn the reversion hypothesis
altogether.

132 REVERSION AND ALLIED PHENOMENA

§ 5. " Skipping a Generation "

It is often remarked in human inheritance that a child re-exhibits
the peculiarity of a grandfather or grandmother, which the parents
did not show. A Mendclian interpretation of this is in some cases
possible. ' If the two grandfathers have blue eyes and both grand-
mothers brown eyes, then the parents may both have simplex brown
eyes * ; they will both form germ-cells of which 50 per cent, have
and 50 per cent, lack the determiner to form brown iris pigment.
From such brown-eyed parents one child in four will have blue
eyes like the grandfathers. This is atavism. Cases of atavism
can, in general, be explained on the same ground as atavism to
blue-eyed grandparents " (Davenport, 1910, p. 292).

In case of sex-limited characters, such as bleeding or haemophilia,
the phenomenon of " skipping a generation " may be illustrated.
For the haemophilia is usually transmitted through unaffected
daughters to grandsons. This may be comparable to other cases
of sex-limited inheritance, e.g. in certain strains of sheep where
the horns are confined to the males.

It is likely that skipping a generation is less frequent than it is
supposed to be ; thus features which the parent thinks he never
had may have been plain enough when he was of the same age as
his son now is. Moreover, in the case of characters that blend it is
an obvious possibility that a grandson should sometimes show his
grandfather's pattern. Finally, some cases of the disappearance of
exceptional ability and the return to mediocrity come within the
rubric of " filial regression."

But our present point is that there seems little utility in calling
" skipping a generation " a " reversion," or even an atavism.

A drone-bee arises from an unfertilised egg ; it has a mother
and two grandparents, but no father. But it seems rather absurd
to call its resemblance to its grandfather either atavistic or rever-
sionary. This is a reductio ad absurdum, for the drone-bee would
resemble its father if it had one 1

* " Ordinarily when parents are similar, each unit character of the
offspring develops from two similar determiners — one paternal and one
maternal. Thus in its origin any unit character is duple*. When,
however, the determiner is found in only one of the parents the character
is simplex." This will be clearer after the chapter on Mendelism has been
read.

NEW VIEW OF REVERSION 133

§ 6. Mendelian Interpretation of Reversion

As we have already indicated, the number of alleged rever-
sions has been greatly reduced by the results of the study of
Mendelian inheritance. An interesting re-interpretation of " re-
versions " has been supplied.

Some red guinea-pigs, as Castle has shown, produce in crosses with
a black race the " agouti " type of coat found in all wild guinea-pigs,
and various experiments prove that this is due to the coming to-
gether of three colour-factors — simple red, simple black, and a third
which is carried by the red but can become visible only in the presence
of both black and red.

In certain instances, which are quite well defined by the Mendelian
experimenters, a cross between a black and an albino mouse, or be-
tween a black and an albino rabbit, results in a complete reversion
to the wild grey form.

Crosses between the tall, upright, bush-like " Bush " sweet-pea
and the dwarf prostrate " Cupid " variety resulted in a procumbent
plant with long internodes, like the wild type that is found growing
in Sicily.

In these and in similar cases it has been possible by various
experimental tests to give convincing proof that the reversion is
a re-synthesis of characters that had been analysed apart. As
Prof. R. C. Punnett concludes : " Reversion, therefore, in such
cases we may regard as the bringing together of complementary
factors which had somehow in the course of evolution become
separated from one another " (1911, p. 54).

§ 7. Reversion in Crosses

False Reversion or Yicinism. — In his criticism of cases which
have been labelled " reversions," De Vries draws a sharp dis-
tinction between "true reversion," due to unknown internal causes
which induce long-lost latent ancestral characters to assert
themselves, and " false atavism or vicinism," which is due to
crossing. His investigation of a large number of cases led him

134 REVERSION AND ALLIED PHENOMENA

to conclude that " true atavism, or reversion caused by an innate
latent tendency, seems to be very rare," and that most of the
botanical instances are due to crossing. He calls this false
reversion " vicinism," as indicating the sporting of a variety
under the influence of others in its vicinity. " Crossing and pure
variability are wholly distinct groups of phenomena, which
should never be treated under the same head, or under the same
name." He does not deny in any way the numerous " rever-
sions " which gardeners describe ; he simply points out (with
much circumstantial evidence to warrant his contention) that
nearly all these ordinary " reversions " are due to crosses. He
shows, for instance, how a famous case, the reversion of the
" Tuscarora " variety of American corn cultivated by Metzger
in Baden, may be readily interpreted as a typical instance of
vicinism. Why the offspring of hybrids should revert to the
parental type is another question, to which we shall return in
the chapter on Mendelism (Chapter X.).

Two white-flowered sweet-peas are crossed, and the result
is a progeny with the wild, purple flowers. Two smooth
stocks are crossed, and the result is a progeny with the original
hoary, ancestral type. These cases are what Darwin called " re-
version on crossing." But, as Mr. Bateson says, " such reversion
is nothing but the meeting of two parted complementary
elements, which have somehow been separated by variation."

Thus it is possible that many so-called reversions may be
simply Mendelian phenomena in disguise.

§ 8. Reversion of Retrogressive Varieties

Within a species it is often possible to distinguish several
subspecies or " elementary species " (De Vries), which differ
from one another in many characters affecting many organs.
Thus in the species called Draba verna, or whitlow grass, there
are two hundred or so minor groups, like constellations within

RETROGRESSIVE VARIETIES 135

constellations. But the species may also include " varieties,"
i more or less sharply distinguished from the rest of the species by
the apparent absence of some notable specific feature, or, more
rarely, by the acquisition of some peculiarity already seen in
closely allied species. They stand aside, as it were, like far out-
lying parts of the constellation. " Varieties," thus defined,
usually differ from their parent species in a single sharp character
only, or in several correlated characters ; they usually arise in
a negative way by the apparent loss of some quality ; and they
have great stability. They are comparable to the familiar colour-
varieties in rabbits, guinea-pigs, mice, etc., which seem to arise
by the dropping out of part of the ancestral equipment of char-
acters. They are in no sense reversions.

Illustrations.

White "varieties " of red and blue flowers — e.g. of red-flowering
currant.

Smooth " varieties " of hairy plants — e.g. nectarine (from peach).

Smooth "varieties " of prickly plants — e.g. holly and gooseberry.

Rayless "varieties" of many composites normally with ray
florets — e.g. white mangold, camomile, daisy.

Radiate "varieties" of many composites, normally with no
ray-florets — e.g. tansy and groundsel.

Red " varieties " of white flowers — e.g. hawthorn.

Red " varieties " of green trees and shrubs — e.g. beech and birch.

Weeping " varieties " of ash, willow, etc.

Starchless seeds — e.g. sugar-corn.

Seedless fruits — e.g. banana and mandarin orange.

Mr. Burbank's stoneless plum.

As these varieties are most frequently in a negative direction
having apparently lost some character which their parent-species
possesses, De Vries includes most of them in the term " retrograde
varieties." Perhaps "retrogressive varieties " would be a clearer
term.

They usually breed true, but some of them are perpetuated
asexually — e.g. of course, the seedless fruits. Sometimes,
however, the apparently lost ancestral character re-appears, as
when the smooth nectarine, a " variety " of peach, becomes

136 REVERSION AND ALLIED PHENOMENA

downy, or when the white-flowering currant puts forth red
flowers. Such cases may be described as reversions to the
specific type, and they can be interpreted only in two ways.
Either we have to do with new variations which happen to hit
the old mark, or, as seems more probable, latent ancestral
characters have re-asserted themselves.

It is a current belief that these " varieties " have a strong
tendency to "revert" to the parent species, but, according to
De Vries, this is, as regards pure varieties, not of hybrid origin,
and ordinarily propagated by seeds, a popular delusion. " In
the present state of our knowledge it is very difficult to decide
whether or not true reversion occurs in constant varieties. If
it does occur it surely does so very rarely, and only under unusual
circumstances, or in particular individuals " (1905, p. 155). It
must be noticed, however, that De Vries distinguishes true
reversion (due to a spontaneous germinal change) from false
reversion which is induced by hybridising.

In illustration of the constancy of varieties he cites the wide-
spread rayless form of the wild camomile {Matricaria chamomilla
discoidea), which is so constant that many botanists have made
a species of it. De Vries raised in successive years between
1,000 and 2,000 seedlings, but observed no trace of reversion.
Similarly, the rayless " variety " of the common tansy ragwort
(Senecio jacobcsa) is quite as stable as the radiate species. De
Vries also refers to the stability of white strawberries, green grapes,
white currants, crisped lettuce, crisped parsley, smooth spinach,
white flax, sugar-corn, and strawberries without runners.

Seed- reversion Yery Rare. — Excluding cases where it is
doubtful whether the variety has not a hybrid origin, and is
therefore liable to the peculiar phenomenon known as the
splitting up of hybrids, excluding also all cases of "sporting
varieties/' where an apparent reversion might be a mere
coincidence in the crowd of variations, De Vries concludes
that "seed-reversions must be said to be extremely rare. . , .

INSECURE INTERPRETATIONS 137

It would be bold indeed to give instances of seed-atavism,
and I believe that it will be better to refrain wholly from doing
so. . . . It is by far safer in the present state of our know-
ledge to accept bud-variations only as direct proofs of true
atavism. And even these may not always be relied on, as
some hybrids are liable to split up in a vegetative way, and in
doing so to give rise to bud-variations that are in many respects
apparently similar to cases of atavism " (1905, p. 176).

§ 9. Interpretations in Terms of Reversion

As in many other cases, one of the difficulties in regard to the
reversion theory is that in terms of it much can be interpreted
and relatively little demonstrated. In regard to the origins
of domesticated animals and cultivated plants, we remain in
great obscurity. In regard to the actual pedigree of wild species
our ignorance is even greater. Thus, while it is often easy to
interpret an unexpected variation as a reversion to a plausible
ancestral type, we have little security in so doing.

Thus De Vries distinguishes between experimentally demon-
strable reversion and what he calls " systematic atavism," where
the ancestral type is merely presumed to be so-and-so on the
basis of taxonomic considerations.

It is probable that the common ancestors of the " elementary
species " {Primula officinalis, P. elatior, and P. acaidis), which
make up the systematic species of primrose, Primula vera, were
" perennial plants with a rootstock bearing their flowers in
umbels or whorls on scapes. Lacking in Primula vera, these scapes
must obviously have been lost at the time of the evolution of
this form." But in the common acaulescent " elementary
species," P. acattlis, a scape sometimes develops. It may be
reasonably interpreted as due to the re-vitalising of a dormant
scape-character inherited from the presumed ancestor. "Simi-
larly with the appearance of bracts in the usually bractless

138 REVERSION AND ALLIED PHENOMENA

Crucifers, and with the unexpected appearance of upright
tomatoes. Similarly, the twisted teasels lose their decussation,
but in doing so the leaves are not left in a disorderly dispersion,
but a distinct new arrangement takes its place, which is to be
assumed as the normal one for the ancestors of the teasel family."

§ 10. Further Examples of Reversion

In one of Prof. Cossar Ewart's experiments a pure white
fantail cock pigeon, of old-established breed, which in colour
had proved itself prepotent over a blue pouter, was mated with
a cross previously made between an owl and an archangel,
which was far more of an owl than an archangel. The result was
a couple of what were, theoretically, fantail-owl-archangel
crosses, but the one resembled the Shetland rock-pigeon, and the
other the blue rock of India. Not only in colour (slaty-blue),
but in shape, attitude, and movements there was an almost
complete reversion to the form which is believed to be ancestral
to all the domestic pigeons. The only marked difference was
a slight arching of the tail, but there were only twelve tail-
feathers, as in the rock-dove, whereas the father fantail had
thirty.

A dark bantam hen, crossed with an Indian game Dorking
cock, produced amongst others a cockerel almost identical with
a jungle fowl (Gallus bankiva) — i.e. with the original wild stock
(Ewart).

Similarly, in his horse-zebra hybridisations, Ewart obtained
forms whose stripings were at least plausibly interpreted as
reversions to an extremely old type of horse, such as is hinted
at in the striped ponies of Tibet.

A smooth-coated white rabbit, derived from an Angora
and a smooth-coated white buck, was mated with a smooth-
joated, almost white doe (grand-daughter of a Himalaya doe),
frith very interesting results, significant of the complexity of

FURTHER EXAMPLES

139

the conditions. In the litter of three, one was the image
of the mother, one was an Angora like the paternal grand-

Fig. 26. — Varieties of domestic pigeon arranged around the ancestral
rock-dove (Columba livia). (Based on Darwin's figures.)

mother, and the third was a Himalaya like the maternal great-
grandmother.

140 REVERSION AND ALLIED PHENOMENA

For all these cases, except that of the horse-stripings, as
also for similar cases given by Darwin, Mendelian interpreta-
tions are now forthcoming, and the hypothesis of there-assertion
of long latent ancestral characters is unnecessary.

When the swimming-bell or medusoid Epenthesis folleata
appears with pentamerous symmetry instead of the usual
arrangement of its organs in fours or multiples of four, no one
would dream of calling this discontinuous variation an instance
of reversion, for we only know of one medusoid (Pseudoclytia
pentata) where five is normally the ruling number (Mayer, 1901).
But when the last-named medusoid occurs with four oral lips,
as it occasionally does, it may be said that this variation is
reversionary, since there is good reason to believe that Pseudo-
clytia pentata is a pentamerous derivative of the Epenthesis
stock. Even in this case the interpretation of the four lips
as reversionary may not be correct, since, as a matter of
fact, the number of lips in Pseudoclytia varies from one to
seven.

Reversion in Parthenogenesis. — Weismann (1893, p. 344)
reports a very interesting case which he observed in varieties
of a small Ostracod crustacean (Cypris reptans) which multiplies
parthenogenetically. In the course of observations extending
over eight years he found that, amidst the expected uniformity
of resemblance between parent and offspring, exceptions occa-
sionally occurred. These were of such a nature that he could
only interpret them " as exhibiting reversions to an ancestral
form many generations removed."

Other Instances of Reversion

White-flowering Currant. — The white-flowering variety of the
red-flowering currant (Ribcs sanguineum) is said to have originated
many years ago from seed in Scotland. " Occasionally this white-
flowered currant reverts back to the original red type, and the
reversion takes place in the bud. . . . Once reverted, the branches
remain for ever atavistic. It is a very curious sight, these smal}

OTHER INSTANCES OF REVERSION I41

groups of red branches among the many white ones " (De Vries,
1905, p. 167). This case is peculiar, however, because the white
variety is propagated only by cuttings or grafting. " If this is
true, all specimens must be considered as constituting together
only one individual, notwithstanding their wide distribution in the
gardens and parks of so many countries. This induces me to sup-
pose that the tendency to reversion is not a character of the variety
1 as such, but rather a peculiarity of this one individual " (p. 168).
Wheat-ear Carnations. — Large beds of carnations sometimes
show peculiar anomalous forms known as " Wheat-ears," with
small green ears instead of flowers. There has been a loss of flowers
and a multiplication of bracts. On a specimen of this De Vries
observed that some branches reverted wholly or partially to the
production of normal flowers. " The proof that this retrograde
modification was due to the existence of a character in the latent
state, was given by the colour of the flowers. If the reverted buds
had only lost the power of producing spikes, they would evidently
have returned to the characteristics of the ordinary species, and
their colour would have been a pale pink. Instead of this, all
flowers displayed corollas of a deep brown. They obviously reverted
to their special progenitor, the chance variety from which they had
sprung, and not to the common prototype of the species " (1905,
p. 229).

A Picturesque Case. — The long-headed green dahlia originated
twice from two different double-flowrered varieties — a deep carmine
with white tops on the rays, and a pale orange known as " Sunrise."
They were quite sterile and were progagated asexually, one in
Prof. De Vries's garden, the other in the nursery at Haarlem, where
both arose. " In the earlier cultures both remained true to their
types, never producing true florets. No mark of the original differ-
ence was to be seen between them." But in 1903 both reverted to
their prototypes, and bore ordinary double flower-heads. " Thus
far we have an ordinary case of reversion. But the important side
of the phenomenon was, that each plaat exactly ' recollected '
from which parent it had sprung. All of those in my garden re-
verted to the carmine florets with white tips, and all of those in the
nursery to the pale orange colour and the other characteristics of
the ' Sunrise ' variety " (1905, p. 231). It seems impossible not to
admit that characters of the parent-varieties had lain for a time
latent and had eventually reasserted themselves.

142 REVERSION AND ALLIED PHENOMENA

Conclusion. — In his Locksley Hall Sixty Years After Tennyson
spoke of —

Evolution ever climbing after some ideal good,

And Reversion ever dragging Evolution in the mud;

but this is making a bogey of reversion. Many of the phenomena
commonly labelled as " reversions " are wrongly labelled, and
true Reversion does not seem to be of frequent occurrence.
Moreover, when it does occur, it may mean, not a deterioration,
but a return to a position of greater organic stability. What
acts as a drag or brake — often advantageously — on progressive
variation is not so much reversion as filial regression.

But the great step of progress that has been made of recent
years is due to the Mendelian experimenters who have shown
that many of the reversions which follow crossing are due to the
re-combination of complementary factors which had become
separated in the course of domestication and cultivation.

Wherever this can be shown there is, of course, no warrant for
the hypothesis that reversion is due to the sudden activation of
a long latent ancestral character. But this hypothesis may be
in the meantime retained for any cases that appear to demand it,

CHAPTER VI

TELEGONY AND OTHER DISPUTED QUESTIONS

"The mysterious wireless telegraphy of ante-natal life." — J. W.
Ballantyne.

§ i. What is meant by Telegony.

§ 2. The Classic Case of Lord Morton's Mare.

§ 3. Representative Alleged Cases of Telegony.

§ 4. Ewart's Penycuik Experiments.

§ 5. Suggestions which explain away Telegony.

§ 6. Suggestions as to how Telegonic Influence might be
effected.

§ 7. A Statistical Suggestion.

§ 8. The Widespread Belief in the Occurrence of Telegony.

§ 9. An Instructive Family History
§ 10. A Note on Xenia.
§ 11. Maternal Impressions.

§ 1. What is meant by Telegony

The term "telegony" is applied to doubtful, certainly rare, but,
if true, very remarkable cases where an offspring resembles a
sire which, though not its father, had previously paired with its
mother. More theoretically expressed, telegony is the supposed
influence of a previous sire on offspring subsequently borne by
the same female to a different sire. The ovum or the embryo
is supposed to be influenced by the mother's previous impregna-
tion or by the consequences thereof.

H3

144 TELEGONY

To take a simple instance, the racehorse Blair-Athol had a
very characteristic blaze or white bald face, and it is said that
mares which had once borne foals to Blair-Athol subsequently
produced to quite different stallions foals which exhibited the
Blair-Athol blaze. It is very generally asserted by dog-breeders
that if a thorough-bred bitch has had pups to a mongrel, her
value is greatly decreased, for she will not afterwards breed
true.

The alleged phenomena are of much interest, but the evidence
of their actual occurrence is far from satisfactory, and their
theoretical interpretation in terms of telegony is beset with
physiological difficulties. But as a belief in telegony is still
widespread, it will not be unprofitable to consider (a) the alleged
facts, and (b) the interpretations suggested.

§ 2. The Classic Case of Lord Morion's Mare

The classic case, given by Lord Morton (1821), is thus sum-
marised by Darwin : "A nearly purely bred, Arabian, chestnut
marc bore a hybrid to a quagga ; she was subsequently sent to
Sir Gore Ouseley, and produced two colts by a black Arabian
horse. These colts were partially dun-coloured, and were striped
on the legs more plainly than the real hybrid, or even than the
quagga. One of the two colts had its neck and some other
parts of its body plainly marked with stripes. Stripes on the
body, not to mention those on the legs, and the dun-colour, are
extremely rare — I speak after having long attended to the
subject — with horses of all kinds in Europe, and are unknown
in the case of Arabians. But what makes the case still more
striking is that the hair of the mane in these colts rcsemWed
that of the quagga, being short, stiff, and upright. Hence there
can be no doubt that the quagga affected the character of the
offspring subsequently begot by the b^ack Arabian horse "
(Darwin, 1868, vol. i. pp. 403-4).

THE CASE OF LORD MORTON'S MARE 145

In 1823 the mare had again a foal by an Arab stallion, and
this also showed some quagga characters.

It may well be asked : If this was not tclegony, what was it ?
But the case is not quite so satisfactory as it seems. Settegast *
remarks that the drawing made of the foal with the alleged
quagga characters merely shows indistinct dark stripes on the
neck, withers, and legs, and that similar stripes not uncommonly
occur on pure-bred foals. A stiff mane may also occur as a
variation in horses. It is possible that the alleged quagga-like
characters had nothing to do with the original quagga sire, but
were reappearances of latent ancestral characters.

Sanson (1893) sets another case against Lord Morton's. A bay
mare had by two different stallions seven foals of a uniform
colour, and then by a third stallion a foal more zebra-like than
Lord Morton's. To which Delage adds that this eighth foal was
pommelled grey — a colour with which zebra-like stripes are
not infrequently associated.

Cornevin cites a breeder from the Pyrenees to the effect that
a mare served by an ass and producing a mule was thereafter
served by a horse and cast a foal which had hoofs more mule-
like than horse-like. But this is too vague to be of much use,
and besides, " asinine " variations sometimes occur in horses
where there has been no hybridising (Sanson, 1893).

Moreover, the opposite result has been often obtained. Sette-
gast (1888) gives the case of four stud mares which were served
by asses and bore mules. They were subsequently served by
horses, and the foals showed no asinine traits.

§ 3. Representative Alleged Cases of Telegony

Man. — Herbert Spencer cites from Flint's Human Physiology
(1888) the case of a white woman who had intercourse with a negro
and afterwards with a white man. There were some negro-pecu-

* Thierzuc/it, Barslau, Bd. i. 1878, pp. 223-34.

10

146 TELEGONY

liarities in the children by the second male. But it is perhaps
enough to say that it is difficult to get at the truth in such cases.

Cornevin (1891, p. 356) gives the following case. The widow oi
a hypospadic man had by a second and normal husband four hypo-
spadic sons, two of whom transmitted the abnormality {Lancet, 1884).
But in a case like this we require further particulars — e.g. as to the
normality of the mother, and as to any tendency to hypospadism
both in her ancestry and in that of her second husband.

Cornevin also cites the case of a woman married to a deaf-mute,
by whom she had one deaf-mute child. By a second normal husband
she had a deaf-mute child, and then others who were normal
(Ladreit de Lacharriere, in preface to Goguillot's Comment on fait
parley les sourds-muets, Paris, 1889). But here again it is necessary
to know whether there was any tendency to deaf-mutism on the
mother's side or in the ancestry of her second husband.

Dogs. — It is the deeply rooted opinion of dog-breeders — doubtless
resting on a basis of experience, though it may be misinterpreted
experience — that a bitch of good stock once lined by a mongrel is spoilt
for further prize -breeding. It is said that many valuable bitches
have been sacrificed because of this deeply rooted opinion.

The following case is cited by Cornevin (1891, pp. 356-7), from
Kiener (1890). An Artesian bitch was first lined by a wall-ej'ed
mastiff, and afterwards by an Artesian dog. Among the pups born
to the latter one was wall-eyed. One requires to know how fre-
quently a wall-eyed variation crops up, and whether there was any
occurrence of it in the ancestry of the mother or of the second
male.

Darwin (1868) gives the case of a hairless Turkish bitch which was
lined by a spaniel, and had some hairless pups and some with short
hair. She was subsequently paired with a hairless Turkish dog, but
the offspring were as before. It must again be asked whether there
may not have been some spaniel strain in the previous ancestry.

Spencer (1893) tells of a Dachshund bitch which was paired with
a collie and had a hybrid litter. The following year she bore to a
Dachshund a similar hybrid litter. But we require to know how
thoroughly pure-bred the Dachshund mother and father were.

Perhaps the most useful comment on the cases of reported telegony
in dogs is that made by Prof. Cossar Ewart (1901): "When it
is remembered that we are surprisingly ignorant of the origin of the
various breeds of dogs, and that, however pure the breed, reversion

ALLEGED CASES 147

to a former ancestor may at any moment occur, it will, I think, be
admitted that, for the purpose of testing the ' infection ' doctrine,
the dog, of all our domestic animals, is the least satisfactory." Mr.
C. H. Lane, discussing toy spaniels in his book, All about Dogs,
says, " I have been told by breeders that they have had in one litter
a specimen of all four breeds [i.e. of King Charles, Prince Charles,
Blenheim, and Ruby spaniels]. In the same way rough and smooth
terriers often occur in the same litter, not because of infection, but
because of reversion."

Cats. — Dr. H. de Varigny tells of a normal cat which, after pro-
ducing kittens to a Manx cat, had several tail-less kittens to an or-
dinary cat (Journal des Debats, September 9th, 1897; cited by Ewart,
1 90 1 ). But the mother, or the second father, or both, may have had
a tail-less ancestor, to which some of the kittens happened to revert.
Or even if there were no such ancestor, the tail-lessness may have
been merely a variation that happened to coincide with the pecu-
liarity of the first sire, but was not in any way due to him. For
tail-lessness is not a very rare " sport."

As a counter-case, Prof. Ewart refers to " a pair of young cats,
of a somewhat peculiar variety, obtained from Japan. These cats
belonged to a small breed, bluish in colour, with the exception of the
ears and extremities, which were black. When the female grew up
she first had kittens to a common tabby cat. These kittens showed
the characteristic tabby markings. Her next kittens were by her
Japanese mate, but in no respect did they suggest the previous
tabby-coloured mate. No better experiment than this could be
made with cats. The imported breed was quite distinct, and yet
not sufficiently prepotent to swamp the common domestic English
i cat. Yet, though the first litter was sired by a common tabby, there
was no indication whatever of the previous tabby mate in her second
and pure-bred litter." (Case cited by Sydney Villar, F.R.C.V.S.,
Proc. Nat. Vet. Assoc. 1900, p. 130.)

Sheep. — Dr. Alexander Harvey, in a paper " On a Curious Effect
of Cross-breeding " (1851), gives on the authority of W. McCombie
of Tilliefour, Aberdeenshire, the following case :

Six pure-bred black-faced horned ewes were put, in the autumn
: of 1844, some to a Leicester ram (white-faced and polled), and others
to a Southdown ram (dun-faced and polled), and produced cross-
bred lambs.

In the autumn of 1845 the same ewe9 were put to a pure black-

148 TELEGONY

faced horned ram of their own breed. The lambs were all polled
and brownish in the face.

In the autumn of 1846 the ewes were again put to another fine
ram of their own breed. Again the lambs were mongrels, but not
so markedly as before. Two were polled and dun-faced, with very-
small horns ; while the other three were white-faced, with small
round horns. At length the owner parted with his ewes without
getting from them a single pure-bred lamb.

Perhaps, however, the ewes were not so pure-bred as was supposed.

Cornevin cites from Magne the statement that white ewes, first
crossed by black rams and then by white rams, bear to the latter,
lambs which are piebald or which have blackish eyelids, lips, and
limbs (Magne, J. H., Hygidne veterinaire appliquee, p. 206). But
black variations are common even when no black rams have been
used for several generations.

Cattle. — Weismann (1893, P- 3S5) refers to a case reported by
Carneri. A cow of a dark grey Miirzthal herd was put to a " light-
coloured Pinzgau bull " ; it bore a calf with the characteristic
brown and white patches of the Pinzgau breed, as well as with dis-
tinct traces of the dark grey Miirzthal cross. It was subsequently
served by a Miirzthal bull, and the second calf, while for the most
part grey, showed " large brown spots like those of the Pinzgau
breed." But this case is also inconclusive, since it is possible, as
Carneri admitted, that " a drop of Pinzgau blood " may have pre-
viously got into the Miirzthal herd without his being aware of it.

Pigs. — Another circumstantial case cited by Darwin is that of a
sow of Lord Western's black-and-white Essex breed, which Mr. Giles
put first to a deep chestnut wild boar and after a time to a boar of
the black-and-white breed. The offspring of the first union showed
the characters of both parents, but in some the chestnut colour of
the boar prevailed. From the second union the sow produced some
young plainly marked with the chestnut tint, which is never shown
by the Essex breed (Darwin, 1868, vol. i. p. 404).

Rodents. — Breeders of rabbits, rats, and mice have sometimes
reported phenomena which suggest telegony ; but the great varia-
bility of these rodents makes them very unsuitable subjects of ex-
periment.

Prof. Cossar Ewart refers to two cases. Mr. C. J. Pound, bac-
teriologist to the Queensland Government, " crossed a grey rabbit
with a grey-and-white buck, and then mated her with a black buck

E WART'S PENYCUIR EXPERIMENTS 149

with the result that in the second litter there were grey-and-white
as well as grey-and-black young. Again, a female black rat after
breeding with a pure white rat produced, to a brown rat, white,
brown, and piebald offspring. . . . Had Mr. Pound made a number
of control experiments he would doubtless have discovered that
black female rats sometimes yield to a brown rat white, brown,
and piebald offspring, without having been first mated with a white
rat, and that grey doe rabbits often produce to a black buck grey-
and-white as well as grey-and-black young."

Experiments on rats and rabbits made by Dr. Bond (Trans.
Leicester Literary and Philosophical Society, vol. v. October, 1899)
yielded no results which could not be readily interpreted as due to
reversion and other forms of variation.

Birds. — A case of supposed telegony in birds is referred to by
Darwin (1868, vol. i. p. 405) : "A careful observer, Dr. Chapuis,
states (Le Pigeon Voyageur Beige, 1865, p. 59) that with pigeons
the influence of a first male sometimes makes itself perceived in
the succeeding broods ; but this statement, before it can be fully
trusted, requires confirmation." Mr. Frank Finn, in a paper
entitled " Some Facts of Telegony " (Natural Science, iii., 1893,
PP- 436-4o). cites a number of cases which seem to him to
afford evidence of telegonic phenomena in birds, but they are not
convincing.

From the above citations it appears that the evidence of the
occurrence of telegony is in great part, at least, of the same un-
satisfactory character as that adduced in favour of use-inheritance
— largely anecdotal, impressionist, and uncriticised. The need for
careful experiments like those begun by Prof. Ewart ( 1 896) is obvious.

§ 4. Ewart's Penycuik Experiments.

The position of affairs being that a number of great authorities
— e.g. Darwin and Spencer — had expressed their belief in the
occurrence of telegony, and that a number of equally competent
authorities had expressed themselves extremely sceptical on
the subject, Prof. Ewart resolved on definite experiment— the
only secure path.

In general terms, he made a number of experiments likely

150 TELEGONY

to give telegony the best possible chance of declaring itself, and
although he has displayed his scientific mood in abstaining from
dogmatic conclusion, and in suggesting many other experiments
which should be made, there is no ambiguity in his verdict that
the evidence of any undoubted telegony is very unsatisfactory.
The Penycuik experiments proved this, at least — that telegony
does not generally occur, even when what were considered to
be favourable conditions were secured ; indeed, anything sug-
gestive of telegony occurred only in a very small percentage
of cases. Moreover, where peculiar phenomena of inheritance
were observed, they seemed to be readily explicable on the
reversion hypothesis.

The general nature of the experiments may be understood by
taking one of the best cases, which loses much, however, when
summarised apart from the beautiful pictures illustrating the
book (Ewart, 1899). A Rum pony mare, Mulatto, of remarkably
pure breed, was served by a Burchell zebra stallion, Matopo,
and the result in August, 1896, was Romulus, whose markings
were quite different from those of his sire, being suggestive rather
of the Somaliland zebra. In 1897 Mulatto had a bay colt foal
to a grey Arab stallion, and this foal — unfortunately short-
lived— gave no proof of telegony. The stripes which most
frequently occur in horses were absent ; there were others which
are not uncommon in horses ; but the most distinct markings
(not that any were strongly developed) — namely, those across
the croup — were of a sort extremely rare in both foals and horses.
In short, the markings of Mulatto's second foal were puzzling,
but in no definite way suggestive of the influence of the previous
zebra sire. In this, as in the other cases, the verdict as to the
occurrence of telegony was " non-proven."

In regard to experiments it should be remembered, however,
that if telegony (supposing it to be a fact) be due to some strange
persistence or unusual influence of the spermatozoa of a previous
sire, then many isolated cases with negative results do not prove

SUGGESTED EXPLANATIONS 151

much. As Pearson observes (1900, p. 462), " should it occur
once in a hundred trials we are hardly likely just to hit upon
the successful instance."

§ 5. Suggestions which explain away Telegony

(a) It has been repeatedly suggested, by those who do not
believe in the reality of telegonic influence, that the phenomena
are simply illustrations of reversion. A normal cat has kittens
to a Manx cat, and afterwards to a normal cat. In the second
litter some are tail-less. " It does not follow, however, that some
of the subsequent kittens were tail-less because their dam had
been previously mated with a cat of the Manx breed. . . .
The most likely explanation is that tail-less individuals occurred
in the ancestry of one or both of the parents ; in other words,
the absence of the tail is due to reversion to an ancestor "
(J. Cossar Ewart, Trans. Highland and Agricultural Society of
Scotland, 190 1).

This view amounts to denying telegony in the strict sense.
We are asked to believe that there is no causal nexus between
the previous sire and the subsequent offspring who resemble
him. They happen to resemble him because he resembled one
of their ancestors. This seems to us easier than believing in
telegony.

The plausibility of this explanation will vary in different cases.
Thus Finn points out that the occurrence of feather-legged fowls
in a pure Dorking breed, or of polled lambs from black-faced
horned ewes, cannot be set down to reversion, " feather-legged
fowls and polled sheep not being ancestral types."

(b) It has also been suggested that the subsequent offspring
have accidentally varied in the direction of resemblance to the
previous sire. The resemblance is a mere coincidence. As
the reliable facts are few and far between, there is much to be
said for this view.

152 TELEGONY

(c) Another suggestion explains away the alleged facts of tele-
gony by referring them to maternal impression, the supposition
being that the mental image, etc., produced in the mother by
the first sire exerts an influence on subsequent germs or on their
development after fertilisation by another sire. There is little
to be said in favour of this interpretation !

§ 6. Suggestions as to how a Telegonic Influence might be effected

(a) It is well known that in most European bats sexual union
usually occurs in autumn, but the spermatozoa are simply
stored in the uterus, ovulation and fertilisation taking place
in spring after the winter sleep. A somewhat similar retention
of stored spermatozoa, which become operative long after
impregnation, is familiar in insects : thus, in some queen bees
the store has been known to last for two or three years, and
Sir John Lubbock gives the remarkable instance of an aged
queen ant which laid fertile eggs thirteen years after the last
union with a male. From a consideration of such facts the
suggestion has emerged that the second offspring are really
fertilised by persistent spermatozoa derived from the first sire.

Weismann (1893, p. 385) suggests the possibility that " sper-
matozoa had reached the ovary after the first sexual union had
occurred, and had penetrated into certain ova which were still
immature." When these ova mature amphimixis might occur,
and coincide in time with a second coitus to which the subse-
quent offspring would be ascribed.

But were this the explanation, we should sometimes find,
as Weismann remarks, that offspring were produced without
any second sire at all. No such phenomenon is known among
higher animals.

Moreover, there is no warrant for supposing that spermatozoa
can persist as such through a period of gestation. " There is
abundant evidence," Prof. Cossar Ewart says, " that in the

SUGGESTED EXPLANATIONS 153

rabbit, as in other mammals, unused sperms lose their fertilising
power and disintegrate long before the period of gestation comes
to an end."

For these two reasons the above interpretation may be
rejected.

(b) Somewhat subtler is the suggestion — often also called the
" infection hypothesis " — that although the sperms of the first
sire cannot be supposed to persist and fertilise ova discharged
long afterwards, yet it is conceivable that the disintegrated
substance of the sperms may persist and influence the ovaries
and the ova, or that the sperms may exert an influence which
does not amount to fertilisation.

So great a physiologist as Claude Bernard seems to have
believed in the possibility of such an influence, though it is
somewhat suggestive of the " aura seminalis " of the ancients.
In this connection, however, Cornevin recalls the facts that a
turkey-cock's impregnation of the female suffices for the score
or so of fertile eggs which are laid during the season, and that
the common cock's act suffices for seven or eight eggs. In both
cases the fertile eggs are succeeded by other ".clear " eggs, which
are incapable of developing, and Cornevin asks whether we can
believe that there is a brusque separation between the two sets,
or whether the first at least of the " clear " set may not illustrate
this supposed partial fertilisation. Romanes also suggested
that the supposed effect was due to an absorption by the eggs
of surplus sperm-material.

(c) Another slightly different suggestion is that the surplus
sperms derived from the first sire exert a physiological influence
on the constitution of the mother, such that subsequent gestations
are affected. Perhaps no one will deny that the male may in
this way affect the constitution of the female, and Brown-
Scquard's experiments on injections of spermine or testicular
extract may be recalled in this connection ; but it is difficult to
conceive that the influence should be of so precise a nature as to

i54 TELE GO NY

evoke, for instance, the alleged quagga mane and quagga stripes
in the second foal of Lord Morton's mare.

Baron compares this supposed influence to the influence of
pollen upon fruit (see § 10), and Darwin says that this analogy
" strongly supports the belief that the male element acts directly
on the reproductive organs of the female " (Darwin, 1868,
p. 405). But no specific effect on the female animal has ever
been demonstrated.

(d) Perhaps the most plausible theory is that the mother is
influenced through the fcetus during pregnancy, and that the
influence re-acts on subsequent offspring. On this so-called
" saturation hypothesis " the suggestion is that the characters
of the sire, while expressing themselves in the unborn embryo,
also saturate into the dam and affect her constitution in such
a precise way that her offspring by subsequent sires may through
maternal influence acquire (or inherit ?) some of the character-
istics of the first. Thus Sir William Turner (1889), in dis-
cussing Lord Morton's case, says, " I believe that the mother
had acquired, during her prolonged gestation with the hybrid,
the power of transmitting quagga-like characters from it, owing
to the interchange of material which had taken place between
them in connection with the nutrition of the young one. . . .
In this way the germ-plasm of the mother, belonging to ova
which had not yet matured, had become modified whilst still
lodged in the ovary. This acquired modification had influenced
her future offspring, derived from that germ-plasm, so that they
in turn, though in a more diluted form, exhibited zebra-like
markings."

Similarly, Cornevin (1891) asks, may not the fcetus have in
its blood special properties derived from the father, and may
not these act like a vaccine on the blood of the mother ? The
blood of the mother, thus affected, will act on the ova subsequently
fertilised by another sire (Cornevin, 1891, p. 359). So also
Harvey, 185 1. A similar hypothesis has been suggested to explain

A STATISTICAL SUGGESTION

l55

certain facts connected with the so-called transmission of
syphilis.

This view did not, however, commend itself to Darwin, for he
says (1868, vol. i. p. 405) : " It is a most improbable hypothesis
that the mere blood of one individual should affect the repro-
ductive organs of another individual in such a manner as to
modify the subsequent offspring." He also points out that this
hypothesis would not apply to telegony in birds, which has been
alleged, though denied by Harvey and still requiring confirma-
tion (Darwin, 1868, vol. i. p. 405).

It is conceivable that something like the " saturation" above
indicated may occur in a case of a poison or protective anti-toxin,
which might diffuse in and out. We can imagine that a sire in-
fected with some virulent disease, and showing certain structural
disturbances associated therewith, may have offspring which are
similarly affected, and that the influence from them may pass
before their birth into the constitution of the mother, and so
affect her that subsequent offspring by a healthy sire are diseased
after the manner of the first. But while we have some facts
to go upon in regard to the diffusion of toxins and anti-toxins,
we have none as yet which justify us in supposing the diffusion
of structural characteristics or of representatives of these.

§ 7. A Statistical Suggestion

Prof. Karl Pearson (1900, p. 461) has approached the problem
from the statistical side. If the female can be influenced at
later reproductions by a male who has been associated with her
in earlier ones, and if the alleged telegony is not due to some
abnormal persistence of the spermatozoa of earlier unions, then
in the permanent union of a pair we ought to find an increasing
influence of the paternal type. But there seems to be, as regards
stature, no evidence of any increase in the " hereditary influence "

156 TELEGONY

of the father, therefore " no evidence of any steady telegonic
influence."

But an increasing hereditary influence of the same father
seems to us rather different from the precise point at issue in the
controversy over the occurrence or non-occurrence of telegony.
It must be remembered that the bias of the child this way or that
depends on the relative potency of the various items in the
paternal and maternal contributions to the fertilised egg-cell,
and that this relative potency may be affected by a variety of
circumstances — e.g. the relative age or vigour of the gametes at
the time of amphimixis.

Careful comparisons of the families of the same mother by two
successive husbands would be interesting— especially if there
be anything in the suggestion that the telegonic influence is an
influence exerted on the mother during gestation by the previous
offspring, rather than directly through the previous father.

§ 8. The Widespread Belief in the Occurrence of Telegony

The belief that offspring sometimes resemble not so much
" the father, but an earlier mate of the mother," is widespread
among experienced breeders, and, like the belief in the influence
of maternal impressions upon the offspring, is probably very
ancient. Apart from stock, the belief is often expressed in
regard to man himself. " We certainly know that what used
to be spoken of as the ' infection of the germ,' but which, following
Weismann, we nowadays call ' telegony,' was considered possible
by physiologists at the end of the seventeenth century ; we know
the infection tradition has long influenced Arab breeders, and
that believers in this hypothesis may now be found in every
part of the world, more especially wherever an overlapping of
distinct races occurs — as e.g. in the southern states of America
and in certain Turkish provinces. Further, until quite recently
many biologists considered that what is commonly and conveni-

WIDESPREAD BELIEF IN ITS OCCURRENCE 157

ently known as Lord Morton's experiment has proved ' infection
of the germ' to at least occasionally take place " (Ewart, 1899,

P- 57)-

It is psychologically interesting, therefore, to ask for some

explanation of the widespread belief in the occurrence of a

phenomenon the scientific evidence of which seems so slender.

There is no doubt, we are told, that the value of a pure-bred bitch

at once goes down if she has been accidentally lined by a mongrel,

and it is possible that there may be good reason for this apart

from the fact that the episode is not one which figures well in the

record. It is possible that the constitution of the bitch may be

subtly affected by a crossing— especially a fertile crossing — with

a dog of inferior strain ; and that the deteriorated constitution

may react upon future offspring although real telegony does not

ensue.

One must remember, however, that the statements one hears
are often fairly precise. " If a pointer bitch gets accidentally
served by a collie dog and produces a litter, the pups will be of
various types, some like the pointer, some like the collie, and
some a blend. And let that pointer bitch be afterwards served
by a pure pointer dog, the result will be a litter among which the
collie type can be unmistakably observed." It is desirable that
some effort should be made to secure absolutely definite state-
ments, supplemented by photographs.

It is hardly sufficient to remind ourselves that people are
indescribably careless about their beliefs, and that breeders are
notoriously superstitious ; for considerations of money value
have a potent effect in evolving carefulness, and breeding is
gradually becoming an art based on scientific conclusions. There
must be some basis for the widespread belief, and the answer
given by the practical men themselves is that they have had
abundant experience of the occurrence of telegony. This asser-
tion leads us to look for phenomena which might be readily
mistaken as telegonic, and there can be little doubt that Prof.

158 TELEGONY

Ewart is right in maintaining that the mistake is in the mis-
interpretation of reversions.

A glance at the chapter on reversion (Chapter V.) will remind
the reader that the crossing *of different strains often results
in apparent " throw-backs." A dark bantam hen paired with
an Indian game Dorking produced, amongst others, a cockerel
almost identical with a jungle fowl (Gallus bankiva) — that is, with
the original wild stock. What occurs when different breeds
are crossed may occur on a smaller scale when individuals of
the same breed, but of different strains, are crossed. When
reversionary phenomena occur they usually spell disappointment
to the practical breeder. In search of an explanation, he some-
times thinks that he finds one in telegony ; that is to say, gives
the blame of the reversion not to the immediately preceding
crossing, which was theoretically correct, and should have turned
out well, but to some remoter, less careful, or perhaps accidental
crossing. In this way the remoter sire is made the scapegoat
for the reversion, and the belief in telegony has grown.

§ 9. An Instructive Family History

A good instance of the way in which cases of alleged telegony
evaporate when analysed has been given by Dr. O. vom Rath.
It concerns the somewhat intricate family history of certain cats.

A family who had lived for many years in Tunis migrated in
1888 to Baden, taking with them a beautiful pair of kittens.
These were none the worse for the change, except that they grew
up very unwilling to leave the house, and more or less vicious.
The female cat (F) was grey-brown with black stripes ; the torn
(M) was pitch-black, except a large white spot on the right breast,
and had a naturally half-sized left ear. In each litter which they
cast there were some abnormal kittens, with rudimentary ear
and tail. All these and all the males were destroyed ; the normal
females were given away. But as the torn (M) became more and
more vicious he was castrated, and became peaceful and lazy.

A NOTE ON XENIA 159

The she-cat (F) was then crossed with an unblemished German
torn, but she still produced abnormal kittens in each litter. Thus
a strong suggestion of telegony arose.

Further inquiries showed, however, that a normal daughter
of F, crossed with a normal German torn, had borne a red male
with rudimentary left ear and rudimentary tail. Inquiries as
to the pedigree of F and M showed that /, the mother of F, had
a rudimentary tail, but no rudimentary ear, and was like F in
colour. This / had been crossed with a red torn (R), who had
a rudimentary ear and tail ; there was but one litter, which was
destroyed, and R soon afterwards died. Then / was paired
with a normal black younger brother (S) of the deceased (R).
From this normal S and from this / with a rudimentary tail,
F sprang. But the two parents of / and the two parents of
R and S were relatives, belonging to a family in which a rudi-
mentary ear and tail were common — all springing from a pair
which the owner of F and M had found in a hollow tree near
Tunis.

Dr. vom Rath has more to tell, but enough has been quoted
to show the correctness of his conclusion that there was no tele-
gony at all. There was a strong family tendency to having a
rudimentary ear and tail. But it is evident that if Vom Rath
had not had patience to search out the family history, the case
for the occurrence of telegony would have been fairly good — at
least as good as many others.

§ 10. A Note on Xenia

The mysterious name "xenia," which seems to mean " guest-
gifts," was applied by the botanist Focke to cases where the
pollen from the " male " parent seemed to affect the tissue of the
maternal ovary — the substance of the seed, or even the fruit,
as distinguished from the embryo itself.

Correns has made careful experiments with maize and estab-

160 TE LEGO NY

lished that there at least xenia occurs. When the white -grained
variety {Zea alba) is pollinated from the blue-grained variety
(Zea cyanea), the majority of the seeds have white endosperm
around the embryo, but a few have blue endosperm. The con-
verse is likewise the case. It must be noted that the effect is
only on the so-called " endosperm," or nutritive layer around
the embryo ; the envelope of the seed, for instance, is never
affected.

What happens seems to be this. The pollen-tube arising from
the pollen-grain contains two generative nuclei, which arise by
the division of one. Of these two nuclei, one fertilises the egg-
cell, the other fuses with what are called the polar-nuclei (a fact
discovered by Nawaschin and Guignard). Thus there is a sort of
double fertilisation within the embryo-sac ; the one results in the
embryo, the other gives origin to the endosperm.

Thus we see that xenia (in the well-authenticated case of
maize) is no mysterious influencing of maternal tissue by the
pollen-tube, and that it does not require Darwin's hypothesis of
a migration of " gemmules " from the fertilised ovum into the
surrounding tissue. It is a phenomenon sui generis, due to the
very peculiar " double fertilisation." As Weismann points out,
it corroborates the view that the nuclei are the vehicles of the
hereditary qualities.

Many of the alleged cases of xenia are cited in Prof. Delage's
great work (1903, p. 252), the most picturesque being that of an
apple-tree of Saint Valery. " This tree was sterile through the
abortion of its stamens. Every year the young girls gathered
branches from other apple-trees in flower, and shook them over
the flowers of the non-staminate tree to fertilise them. Tillet
De Clermont-Tonnerre (1825) relates that the resulting fruits
recalled in their size, colour, and taste, those of the trees which
had furnished the pollen."

It is to be feared, however, that many of the alleged cases of
xenia will not stand examination. Thus the records in regard

MATERNAL IMPRESSIONS 161

to peas do not seem to be relevant, since the two halves of the
pea-seed are of course the cotyledons and part of the embryo.
Some of the phenomena seem simply ordinary cases of Mendelian
inheritance (see Chapter X.).

Some of the cases where it is said that the whole fruit is affected
— e.g. in grapes and oranges — well deserve further investigation.

§ n. Maternal Impressions

It is a time-honoured belief that the mental states — especially
vivid sense-impressions and strong emotions — of a pregnant
mother may so affect the unborn offspring that structural changes
result which have some correspondence with the maternal ex-
perience. The belief was hardly doubted till Blondel began to
criticise it early in the eighteenth century.

Every one allows that the mother's health in the widest sense
may react on the offspring, within what limits we hardly know;
but it is a very different matter to believe in definite and specific
structural effects. There can be no doubt that the firmly rooted
theory is in the main quite unscientific, except in the sense that
it expresses the instinct to discover some cause for peculiar
phenomena. A child has hypertrichosis : did not the mother
look too long at a picture of John the Baptist in a hairy robe ?
A white mother has a dark child : what can she say but that
she was frightened by a Moor ?

The abundant literature on the subject has been carefully
studied by Dr. J. W. Ballantyne, and it need hardly be said that
his general verdict is wholly against the tenability of the theory,
except in a very refined form.

The mental experiences of the mother have been held to ex-
plain peculiarities of colour, abnormal hairiness, birth-marks,
malformations, and even conception itself. The post hoc ergo
propter hoc argument has never been more wildly used, and the
result has been a retardation of the study of ante-natal pathology.

Jacob's trick of using peeled wands to influence the colour of

ii

i62 TELEGONY

his stock is still practised in modified form. A famous breeder
of cattle has assured me that to obtain a particular colour of calf
from a cow which persistently refused to produce what he wanted,
he followed the patriarch's prescription with success. He had
her covered blindfold ; after the sire had gone he brought to her
a heifer of the desired colour, and that was the first object she saw
when the bandages were removed ; she was left with the heifer
as a companion to occupy her mind, and the result in due time
was a calf of the desired colour. Nor was this an isolated case.

What can one say — the credibility of the witness being secure —
except the unsatisfactory word " coincidence " ? One requires to
know in what direction, as regards colour, the sire was prepotent.
One requires to know how many failures are forgotten in pro-
portion to the successes remembered ?

It is admitted that shock and distress and the like may have
prejudicial effects on the unborn offspring. It is stated that after
the Irish famine and after the siege of Paris there were many
children born with stigmata of various sorts, and these were
sometimes referred back to particular experiences instead of to
the general state of malnutrition and nervous exhaustion. But
to associate a particular structural defect with a particular
mental impression seems an untenable position. The modus
operandi is difficult to conceive of. Sometimes, indeed, the
maternal-impression theory is demonstrably untenable, when
the impression occurs late in pregnancy, for most of the great
events in development occur very early. We have also to re-
member the multitude of cases in which, in spite of very startling
maternal experiences, the offspring is quite normal. In com-
parison with this multitude of cases where nothing happens, the
number of really puzzling cases is very small, and may be
dismissed as coincidences.

At the same time it is always unwise to speak of impossi*
bilities in regard to matters which are inadequately known and
imperfectly understood. That we cannot imagine the nature of a

A PUZZLING CASE

■63

physiological nexus does not prove its non-existence. Thus, as in
regard to the transmission of acquired characters and telegony,
we may be scientifically sceptical and give a verdict "non-
proven," without dogmatically saying " impossible."

We can understand how contact with a puzzling case gives the
observer pause. A medical practitioner of keen scientific intelli-
gence told me of a patient who, during pregnancy, had seen her
husband suffer a serious accident. His arm was cut open by a
falling block. As the impression seemed to weigh on the woman's
mind in its relation to the unborn child, the doctor was asked to
reassure her— which he did, with confidence and no doubt with
skill. He was rather startled, however, when the time came, to
find that the child he ushered into the world had a mark on the
arm suggestive of the father's wound, and on the same arm.

We must remember that for a prolonged period the unborn
child is part and parcel of the mother — almost an integral part
of herself — and we are beginning to know enough of the influence
of mind upon body to make us cautious in dogmatising as to
the possibilities of what Ballantyne * finely calls " the mysterious
wireless telegraphy of ante-natal life."

* While expressing his disbelief in the potency of maternal impressions
to cause conditions in the foetus resembling the impression, Dr. J. W.
Ballantyne cautiously adds (" Discussion on Heredity in Disease," Scottish
Med. and Surg. Journ. vi. 1900, p. 3 10) that " to whatever extent we believe
the mind capable of influencing the state of a part of the body, to that same
extent, or to a degree rather less, the mother's mind might influence her
parasitic growth— i.e. the foetus in utero. But this amount of belief
would of course vary very much in accordance with the elasticity of our
belief regarding the influence of the mind over the body."

CHAPTER VII

THE TRANSMISSION OF ACQUIRED CHARACTERS

" A right answer to the question whether acquired characters are or are
not inherited underlies right beliefs, not only in Biology and Psychology,
but also in Education, Ethics, and Politics." — Herbert Spencer.

" II n'est pas demontre que les modifications acquises sous l'influence
des conditions de vie soient generalement hereditaires, mais il parait bien
certain qu'elles le sont quelquefois. Cela depend sans doute de leur
nature." — Yves Delage. [This is the opinion of one of the acutest of
living biologists, but we find ourselves forced to a negative position.]

§ I. Importance of the Question.

§ 2. Historical Note.

§ 3. Definition of the Problem.

§ 4. Many Misunderstandings as to the Question at Issue.

§ 5. Various Degrees in which Parental Modifications

might affect the Offspring.
§ 6. Widespread Opinion in favour of Affirmative Answer.
§ 7. General Argument against the Transmissibility of

Modifications.
§ 8. General Argument for the Transmissibility of Modi-
fications.
§ 9. Particular Evidences in support of the Affirmative
Answer.
§ 10. As regards Mutilations and the Like.
§ 11. Brown-Sequard's Experiments on Guinea-pigs.
§ 12. Negative Evidence in favour of the Affirmative Answer.
§ 13. The Logical Position of the Argument.
§ 14. Indirect Importance of Modifications.
§ 15. Practical Considerations.

164

THE PROBLEM NOT MERELY ACADEMLC 165

§ 1. Importance of the Question

No one is at present entitled to rank the transmission of
" acquired characters " — i.e. somatic modifications — among the
facts of inheritance, and the logical place for a discussion of this
subject should be beside other disputed questions, like the
occurrence or non-occurrence of telegony. But we have given
special prominence to a discussion of this problem because
of its great importance both practically and theoretically, and
because of the abundant debate which has been aroused over it.

Not a Merely Academic Problem. — The question as to the
transmissibility of characters acquired during life by the body
of the parent as the result of changes in environmental or func-
tional influences is much more than a technical problem for
biologists. Our decision in regard to it affects not only our
whole theory of organic evolution, but even our every-day
conduct. The question should be of interest to the parent,
the physician, the teacher, the moralist, and the social reformer
— in short, to us all.

If the particular results of changes or peculiarities in individual
nurture, education, and experience do not directly and specifi-
cally affect the inherited nature of the offspring, there must
be a revision of some current psychological and pedagogical
opinions ; but it must be borne in mind that man's rich external
heritage of tradition and convention, custom and institution,
law and literature, art and science, makes his case quite peculiar,
for the results of man's external heritage are often such as
might have come about if acquired characters were heritable.

If the particular results of changes or peculiarities in individual
" nurture " do not directly and specifically affect the inherited
nature of the offspring, there must be a revision of that theory
of organic evolution which is usually called Lamarckian, in
which it is a central postulate that whatever is acquired may
also be transmitted.

166 TRANSMISSION OF ACQUIRED CHARACTERS

Spencer's Estimate of the Importance of this Question.—

After contrasting the two hypotheses of the transmissibility
and the non-transmissibility of acquired characters, Herbert
Spencer said : " Considering the width and depth of the effects
which the acceptance or non-acceptance of one or the other
of these hypotheses must have on our views of life, the question,
Which of them is true ? demands beyond all other questions
whatever the attention of scientific men. A grave responsi-
bility rests on biologists in respect of the general question,
since wrong answers lead, among other effects, to wrong belief
about social affairs and to disastrous social actions." This
authoritative statement removes all need of apology for the
prominence which we have given to the question.

An Interminable Question. — The attention of scientific men
which Herbert Spencer demanded for this problem has not
been grudgingly given. The subject has been keenly debated
for many years ; there are, as our bibliography will show, scores
of papers and not a few books devoted to its discussion. Indeed,
one of the most tolerant of biologists, Prof. W. K. Brooks,
has spoken of it as " the interminable question." Those who
give the affirmative answer have not succeeded in proving their
case ; as for the other side, how can they prove a negative ?
Therefore, while we have no hesitation as to the verdict of " non-
proven " to which the evidence at 'present available points, we do
not expect a satisfactory issue until many years of experimental
work have supervened.

Why, then, if a satisfactory termination be not at present
possible, and if no unanimity even among experts can be looked
for, should we enter upon the discussion once more ? Prof.
Brooks states our warrant in a quotation from Berkeley's Siris :
" It is Plato's remark in his Theatetus, that while we sit still we
are never the wiser, but going into the river and moving up and
down is the way to discover its depths and shallows. If we exer-
cise and bestir ourselves we may even here discover something."

HISTORICAL NOTE 167

Experiment is doubtless most urgent, but misunderstandings
in regard to the problem are still so prevalent that we take
courage in attempting a re-discussion, from which we have tried
to eliminate obscurity and prejudice.

§ 2. Historical Note

Doubt as to the transmission of acquired characters is certainly
not novel, though Galton and Weismann deserve credit for
denning the scepticism.

Brock has pointed out that the editor, whoever he was,
of Aristotle's Historia Animalium seems to have differed
from his master on this subject. Aristotle had referred to the
transmission of the exact shape of a cautery mark, but the
editor insinuated a doubt as to credibility of instances of this
sort.

Kant. — In modern times Kant was one of the first to express
a firm disbelief in the transmission of individual peculiarities ;
Blumenbach inclined to the same opinion ; but neither seems
to have defined precisely what he intended to exclude from the
bundle of inheritance.

Prichard. — James Cowles Prichard (b. 1786), a well-known
anthropologist, anticipated as early as 1826 some of the character-
istically modern views on evolution. His importance has been
pointed out by Prof. Edward B. Poulton. In the second
edition of his Researches into the Physical History of Mankind
(1826), Prichard stated the case in favour of the general evolu-
tionist interpretation of animate nature, recognised the operation
of natural and artificial selection, and not only drew a clear
distinction between acquired and inborn peculiarities, but argued
that the former were not transmitted. He was not rigidly
consistent, however, and his convictions seem to have weakened
in after years ; yet his anticipation of one of Weismann's positions
by more than half a century is very interesting.

In more recent times we find sporadic expressions of scepticism

1 68 TRANSMISSION OF ACQUIRED CHARACTERS

as to the transmission of acquired characters — e.g. by the mor-
phologist His and the physiologist Pfliiger; but, as we have
said, the focussing of the question was due to Galton and
Weismann.

Galton. — Thus Galton in 1875 stated his opinion that the
current theory of the inheritance of characters acquired during
the lifetime of the parents " includes much questionable evidence,
usually difficult of verification. We might almost reserve our
belief that the structural cells can react on the sexual elements
at all, and we may be confident that at the most they do so in
a very faint degree — in other words, that acquired modifications
are barely, if at all, inherited in the correct sense of that word."

Galton's position at that time may be summed up as follows :

( 1 ) In regard to climatic variations, Galton doubted the reality of

any reaction of the " body " upon the germs, but believed
that the germs are themselves directly affected.

(2) The same is true in regard to many diseases that have been

acquired by long-continued irregular habits.

(3) The cases of the apparent inheritance of mutilations are out-

numbered by the overpowering negative evidence of their
non-inheritance.

(4) It is hard to find evidence of the power of the personal structure

to react upon sexual elements that is not open to serious
objection. That which appears the most trustworthy lies
almost wholly in the direction of nerve changes, as shown
by the inherited habits of tameness, pointing in dogs,
and the results of Dr. Brown-Sequard's experiments on
guinea-pigs.

Weismann. — But Weismann gave the scepticism an even
sharper point. He denied all transmission of acquired modifi-
cations, partly because he found the evidence so flimsy and
anecdotal, partly because he could not conceive of any mechanism
whereby the transmission of a particular acquired modification
could be effected, and partly because his whole theory of heredity
and variation raised strong probabilities against the view that

VIEWS OF G ALTON AND WEISMANN 169

acquired characters were transmitted. On Weismann's view
the sole fountain of specific change is in the germ-plasm of the
sex-cells. It is true that the environment makes dints on the
organism, but only upon its body ; the reproductive cells,
through which alone the change could be transmitted, are
either unaffected or are not affected in such a definite way as to
bring about the transmission of the parental modification. It is
true that the results of changed function (use and disuse) are often
very marked, and very important for the individual; but they
are not transmitted as such or in any representative degree, and
therefore are of no direct account in the evolution of the species.
Thus the ground is taken from under the feet of Buffonians and
Lamarckians, and the whole burden of organic progress is laid
upon germinal variation and the processes of selection.

The following sentences indicate Weismann's original posi-
tion:

(1) "Acquired characters are those which result from external
influence upon the organism, in contrast to such as spring
from the constitution of the germ."

(2) " Characters can only be inherited in so far as their rudiments
(' Anlagen ') are already given in the germ-plasm."

(3) " Modifications which are wrought upon the formed body,
in consequence of external influences, must remain limited
to the organism in which they arose."

(4) " So must it be with mutilations, and with the results of use
or disuse of parts of the body."

(5) " No such modifications of the soma (affected by environment
or by use and disuse) can be transmitted to the germ-cells,
from which the next generation springs. They are, there-
fore, of no account in the transformation of the species."

(6) "The only principle that remains for the explanation of the
transformation of the species is direct germinal variation."

On germinal variations natural selection operates in the
usual way. The helpful subsidiary theory of germinal selection
was afterwards suggested, and various saving clauses were added,
which do not, however, affect the clearness and strength of
Weismann's original position.

170 TRANSMISSION OF ACQUIRED CHARACTERS

Lamarck's Laws. — It may be fairly said that the fons el origo
of the affirmative position was Lamarck. Though he did not
originate, he formulated and illustrated the theory of the in-
heritance of acquired characters. He maintained the trans-
missibility of modifications due to increased and decreased and
changed use, and also of modifications due to environmental
change, whether directly induced, or indirectly induced by
altered function. The giraffe has attained its long neck by
stretching it for many generations ; swimming birds have got
webbed feet because they stretched their toes in the water ;
wading birds have got long legs because they stretched them ;
the mole has very small eyes because it has ceased to use them ;
the whalebone whale has no functional teeth because it has
acquired the habit of swallowing its food without mastication ;
and so on.

Lamarck's two laws of nature, which he said no observer
could fail to confirm, were : *

(i) In every animal that has not passed beyond the term of its
development, the frequent and sustained use of any organ
strengthens it, develops it, increases its size, and gives it
strength proportionate to the length of time of its employ-
ment. On the other hand, the continued lack of use of
the same organ sensibly weakens it ; it deteriorates, and
its faculties diminish progressively, until at last it disappears.
(2) Nature preserves everything that she has caused the individual
to acquire or to lose by the influence of the circumstances
to which the race has been for a long time exposed, and
consequently by the influence of the predominant use of
certain organs (or in consequence of their continued disuse).
She does this by the generation of new individuals, which
are produced with the newly acquired organs. This occurs,
provided that the acquired changes were common to the two
sexes, or to the individuals that produced the new forms.

Prof. E. Ray Lankester has pointed out (1894) that Lamarck's

* I have taken the translation from T. H. Morgan's Evolution and
Adaptation (1903), p. 226.

CRITICISAf OF LAMARCK'S LAWS 171

first and second laws are contradictory the one of the other. In
correspondence with the normal conditions of the environment,
organisms show " responsive " quantities in their parts ; but
change a young organism to an environment quantitatively
different, and it shows new responsive quantities in the parts of
its structure concerned, new or acquired characters.

" So far, so good. What Lamarck next asks us to accept, as
his ' second law,' seems not only to lack the support of experi-
mental proof, but to be inconsistent with what has just preceded
it. The new character, which is ex hypothesi, as was the old
character (length, breadth, weight of a part) which it has re-
placed— a response to environment, a particular moulding or
manipulation by incident forces of the potential congenital
quality of the race— is, according to Lamarck, all of a sudden
raised to extraordinary powers." It is declared to be trans-
missible, that is, it alters the potential character of the species,
so as to persist when other quantitative external conditions are
substituted for those which originally determined it. But this
has never been experimentally proved, and there is strong
reason for holding it to be improbable.

" Since the old character (length, breadth, weight) had not
become fixed and congenital after many thousands of successive
generations of individuals had developed it in response to
environment, but gave place to a new character when new
conditions operated on an individual (Lamarck's first law),
why should we suppose that the new character is likely to
become fixed after a much shorter time of responsive existence,
or to escape the operation of the first law ? Clearly there is no
reason (so far as Lamarck's statement goes) for any such suppo-
sition, and the two so-called laws of Lamarck are at variance
with one another.

" In its most condensed form my argument has been stated
thus by Prof. Poulton (Nature, vol. li., 1894, p. 127) ; Lamarck's
' first law assumes that a past history of indefinite duration

172 TRANSMISSION OF ACQUIRED CHARACTERS

is powerless to create a bias by which the present can be con-
trolled ; while the second assumes that the brief history of the
present can readily raise a bias to control the future.' ,: (See
E. Ray Lankester's Kingdom of Man, 1907, pp. 128-130.)

Laraarckism remains alive. — The Lamarckian position is still
stoutly maintained — usually in more or less modified form — by
many prominent naturalists, especially in France and America.
It is often held along with a more or less half-hearted Darwinism,
just as Darwin combined some Lamarckism with his own selec-
tionist doctrine — even in spite of his protest, " Heaven for fend
me from Lamarck nonsense of a tendency to progression, adapta-
tions from the slow willing of animals, etc." Though Alfred
Russel Wallace has said, " The hypothesis of Lamarck has been
repeatedly and easily refuted by all writers on the subject " ;
though Huxley said, " The Lamarckian hypothesis has long
since been justly condemned" ; though Ray Lankester has said
that perhaps the greatest step of progress in modern aetiology
will be the complete removal of all taint of Lamarckism, — there
remains a vigorous school of Lamarckians and a still more
vigorous school of Neo-Lamarckians, who, whatever be the
truth in regard to the transmission of acquired characters, have
got a firm grip of the often-overlooked commonplace that the
organism is an active, self-assertive, self-adaptive living creature —
to some extent master of its fate.

§ 3. Definition of the Problem

A Protest. — Much time and energy have been wasted on the
discussion as to the transmissibihty or non-transmissibility of
" acquired characters " or somatic modifications, through lack
of precise definition of the terms. Usually, though not always,
the fault has been with the supporters of the affirmative position,
who have failed to observe the rules of the game by ignoring the
definitions of those who find themselves forced to a negative

DEFINITION OF THE PROBLEM iy6

conclusion. By all means let there be a critical discussion as
to the best definition of " an acquired character," " a modifica-
tion," " a somatic change induced on the body by environmental
or functional influences " ; by all means let there be a criticism
of terms and categories — the minting of a perfectly unambiguous
word for somatic modifications would be most welcome : but if
the sheaves of facts and alleged facts are to be thrashed out
with the end of getting at the wheat of truth, we must adhere
to certain definitions — notably, of course, to those given by
Weismann, who brought the problem in its modern aspect into
focus. Even a sense of humour should hinder a young medical
practitioner from thinking that he makes for progress by ad-
vancing an argument which has no cogency unless the biological
dictionary be first re-edited. It should be evident that a dis-
cussion over which some of the wisest heads in Europe and
America have pondered cannot be, as some have had the
effrontery to declare it, a mere play of words. Is it too much
to ask of those who are keen to break a lance with the Biologist
of Freiburg that they should first at least read The Germ-
Plasm ?

What is an Acquired Character? — In our previous dis-
cussion of " heredity and variation" we have briefly expounded
the distinction between germinal, blastogenic, constitutional,
endogenous " variations," and bodily, somatogenic, acquired,
exogenous " modifications." An acquired character, or a
somatic modification, may be defined as a structural change
in the body of a multicellular organism, involving a deviation
from the normal, directly induced during the individual lifetime
by a change in environment or in function (use and disuse), and
such that it transcends the limits of organic elasticity, and there-
fore persists after the factors inducing it have ceased to operate.

Illustrations. — Dwarfing of Japanese trees, deformation of trees
by the wind, blanching of plants grown in darkness, changes
directly induced by transplantation, persistent sun-burning, change

i74 TRANSMISSION OF ACQUIRED CHARACTERS

of colour after particular diet, callosities induced on the skin by
pressure, e.g. those at first produced on the finger-tips of one who is
learning to play the violin, dwarfing of animals in confined space,
increased muscular development by exercise, atrophy of muscles
through disuse, chronic fatigue of nerve-cells, alterations in the walls
of the food-canal through particular diet, changes in the skeleton
as the result of specialised activities, increased growth of hair, etc.,
after importation to a warm climate, accumulation of fat as the result
of modified nutrition, and so on through a long list.

To understand the question clearly we must spend a little
time and thought over it. Let us briefly consider the various
relations between an organism and its surroundings.

i. Relation of Dependence between Organism and Environ-
ment.— It is a familiar fact that a living creature is de-
pendent upon its surroundings. A great part of life consists
in action and reaction between the organism and its environ-
ment. It is a profound commonplace that between the animate
system — so incomprehensibly unified — and its inanimate milieu,
there is a continual coming and going of matter and energy.
On this life depends. The may-fly during its short aerial life
must breathe even if it does not feed ; the philosopher requires
his dinner, just as his dog does. This may be called the relation
of constant and normal environmental dependence — necessary
to the development and to the continuance of the organism.

2. Transient Adjustments. — But surroundings are changeful,
and the living creature changes with them. A great part of
life consists of effective responses to external changes ; consciously
or sub-consciously the organism adjusts itself to changes in its
environment, or works in the direction of adjustment. There
is bright sunshine and our pulse beats more quickly ; the external
temperature rises and we perspire. Thousands of these changes
are familiar, saving life from monotony. Yet in regard to
many there remains no abiding result that can be detected.
There are structural changes attendant on normal nerve-fatigue,
but in rest and food we gain almost complete recuperation. No

LONG PERSISTING ADJUSTMENTS 175

doubt there is always some, lasting impression, for even the bar
of iron is never quite the same after it has been once struck ;
but the results of the slight organic changes we have been
alluding to are usually lost as the sand-ripples are lost when
the tide turns. They are the merely transient results of re-
sponses to frequently recurring environmental changes to which
the organism is well accustomed.

3. Adjustments which persist for a Considerable Time.—
Insensibly, however — for it is all a matter of degree — we pass
from transient results to others which last for a considerable
time. We are browned by the sun on our summer holiday, and
the result may last far into the autumn. The change, though
still very superficial, has taken a firmer hold. The world is
full of illustrations — the increase in the child's weight after
a month at the farm, the increase in the size of the muscles after
a course of Sandow exercises, the warping of the plant-stem
which has been illumined from one side only, the blanching of
the banked-up celery. But these results do not last long after
the inducing conditions have ceased to operate. Sooner or
later there is a return to the normal. Like a bow unstrung,
the organism rebounds approximately to its previous state.
The stimulus ceases or the absent stimulus is restored, and
the organism, as if at the command " As you were," returns
to the status quo.

4. Modifications. — Insensibly, however — for it is still only a
matter of degree — we pass from these temporary changes to
others which are demonstrably permanent. For there are cases
where the new stimulus provokes a structural change, which
persists after the stimulus has ceased. As we have put it,
metaphorically, the limit of organic elasticity has been trans-
cended. These are what in technical language we call " acquired
characters " or " modifications."

The Englishman who works half his lifetime under a tropical
sun may become so tanned that the result does not disappear

176 7RANSMISSI0N OF ACQUIRED CHARACTERS

during all the years in which he enjoys his pension at home.
He has changed his skin, but he cannot by any means
change it back again. Through prolonged disuse from early
years a muscle may pass into a state of atrophy, and may
so remain throughout life. Pressure on the little toe may so
deform it, that even in the " easiest " shoes it can never right
itself. A tree may be blown out of shape by the wind, and the
crooked bough may never be straightened. Over-exertion may
strain the heart permanently. A sudden shock may be followed
by a whitening of the hair from which there is no natural
recovery.

5. Modifications and Variations. — When we analyse the
observed differences between fellow members of a species, we find
that some of them can be definitely associated with peculiarities
of function and environment. They can be more or less ac-
counted for physiologically in terms of some change in surrounding
influences or of some change in function thereby induced. They
may not be* hinted at in the young forms, but they begin to
appear when the peculiar conditions begin to operate, and they
are usually exhibited in some degree by all organisms of the same
kind which are subjected to the same change of conditions.
Furthermore, they can be experimentally brought about. These
are " modifications."

By those who measure observed differences they are usually
slumped along with true variations, but this appears to us
to lead to confusion. True variations are those peculiar ities
which remain when all the modifications are subtracted from the
total of observed differences.

It goes without saying that the distinction cannot always
be drawn in practice. Often, however, it is quite apparent,
and in any case the theoretical distinction is clear. Variations,
in the strict sense, cannot be causally related to peculiarities in
habit or surroundings ; they are often hinted at in the earliest
stages- — even before birth ; and they are very unequal even

MODIFICATIONS AND VARIATIONS 177

among organisms whose conditions of life seem absolutely identi-
cal. We refer them to changes in the germinal material before
or during fertilisation. We call them endogenous, constitutional,
blastogenic ; and there is no doubt that they are transmissible,
though they are not always transmitted.

Is there really an Antithesis? — Some subtle minds have
found satisfaction in maintaining that the distinction between an
acquired modification and an inborn variation is a distinction
without a difference. In his interesting Problems of Biology
Mr. George Sandeman points out that every acquired quality
is congenital (i.e. there are in the organisation the rudimental
possibilities of it), and that every congenital quality is also
acquired (i.e. it requires to be nurtured by appropriate con-
ditions if it is to develop). In this epigram there is undoubtedly
truth, but is it relevant ?

No doubt the possibility of the modification must be in the
organism, just as the possibility of an explosion is in the barrel
of gunpowder. The environment is not creative ; yet, as a matter
of fact, it seems possible to distinguish between the actual
modification which we see and measure and the possibility of it
which we presuppose.

Similarly, it is very true that the potentialities so marvellously
embodied in the fertilised egg-cell require appropriate environing
conditions if they are to be realised, for, as His observed long
ago, "it is a piece of unscientific mysticism to suppose that
heredity will build up an organism without mechanical means."

The common jelly-fish (Aurelia aurita) often has a pentamerous
instead of a tetramerous symmetry. This is a variation of
germinal, endogenous origin. Of course it requires an environ-
ment to develop in, but we cannot causally relate the structural
peculiarity to any peculiarity in the environment. It seems
to be logically quite distinguishable from a modification.

Discussing words is often indescribably tiresome, but it is better
than misunderstanding them. " Inheritance of acquired characters "

12

178 TRANSMISSION OF ACQUIRED CHARACTERS

may be a most unfortunate phrase, but it has come to have a perfectly
definite technical meaning and usage, which any normal person
can understand in a few minutes. Prof. W. K. Brooks, in his
Foundations of Zoology, says that he never uses the phrase " inherit-
ance of acquired characters " except under protest, and this may
be commendable restraint ; but it seems to us inconsistent with his
usual wisdom to go on to say, " If any assert that the dog inherits
anything which his ancestors did not acquire, their words seem
meaningless ; for, as we use words, everything which has not existed
from the beginning must have been acquired — although one may
admit this without admitting that the nature of a dog is, wholly
or to any practical degree, the inherited effect of the environment
of his ancestors." But as the word " acquired " is now a technical
term, meaning wrought out on the body as the result of changes in
environmental or functional stimuli, we fail to see that, as we use
the words, there is anything meaningless in the first assertion, or
any warrant for the second.

Summary. — What forms the material basis of all inheritance,
in all ordinary cases of sexual reproduction among multicellular
organisms, is the fertilised ovum. The question under dis-
cussion is, physiologically stated, whether we can conceive that
structural changes in the body of a parent, induced by changes
in functional or environmental influence, can so specifically
affect the reproductive cells that these will, if they develop,
reproduce in any degree the modification acquired by the parent
or parents. The question under discussion, logically stated,
is whether there are any secure phenomena of inheritance
which forcibly suggest the reality of the transmission of acquired
characters ; or whether, if such phenomena there be, a simpler
interpretation may not be found. If, summing up in Galton's
phrase, we call environmental and functional influences " nurture,"
our question is seen to be the exceedingly important one, May the
results of peculiarities in parental " nurture" be as such trans-
mitted, or is it the germinal " nature" alone that constitutes the
inheritance ?

SOME MISUNDERSTANDINGS CONSIDERED 179

§ 4. Many Misunderstandings as to the Question at Issue

The precise question is this : Can a structural change in the
body, induced by some change in use or disuse, or by a change in
surrounding influence, affect the germ-cells in such a specific or
representative way that the offspring will through its inheritance
exhibit, even in a slight degree, the modification which the parent
acquired ?

Before we pass to discuss the evidence pro and con it will
be useful to notice some frequently recurring misunderstandings,
the persistence of which would make further argument futile.

Misunderstanding I — How can there be progressive evolution
if acquired characters are not transmitted ? — Those who have
not thought clearly on the subject often shake their heads sagely
and remark that they " do not see how evolution could have
been possible at all unless what is acquired by one generation
is handed on to the next." To this we have simply to
answer (1) that our first business is to find out the facts of the
case, careless whether it makes our interpretation of the history
of life more or less difficult, and (2) that in the supply of
germinal variations, whose transmissibility is unquestioned,
there is ample raw material for evolution. We know a little
about the abundant crop of variations at present supplied ;
there is no reason to believe that it was less abundant in the
past.

Misunderstanding II — Interpretations are not facts. — There
are many adaptive characters in plants and animals which
may be superficially interpreted as due to the direct result
of use and disuse or of environmental influence. The
Lamarckians have so interpreted them, and the Lamarckian
way of looking at adaptations has become habitual to many
uncritical minds. They see on modern flowers the footprints
of insects which have visited them for untold ages ; they speak
of the dwindling of the whale's hind-limbs through disuse, of

180 TRANSMISSION OF ACQUIRED CHARACTERS

the hardening of the ancestral horses' hoofs as they left the
marshes and ran on harder ground ; they picture the giraffe by
persistent effort lengthening out its neck a few millimetres every
century, as the acacia raised its leaves higher and higher off
the ground ; and they say that animate nature is so full of
evidences of the inheritance of acquired characters that no
further argument is needed.

But all this is a begging of the question. It is easy to find
structural features which may be interpreted as entailed acquired
characters, if acquired characters can be entailed. Obviously,
however, we must deal with what we can prove to be modifi-
cations, or with what we can plausibly regard as modifications
because we find their analogues in actual process of being
effected to-day.

It is easy to say that the blackness of the negro's skin was
produced by the tropical sun, and that it is now part of his
natural inheritance. It is easy to say this, but absolutely
futile. Let us first catch our modifications.

The Golden Rod (Solid ago virgaurea) growing on the Alps is
precocious in its flowering when compared with representatives
of the same species growing in the lowlands. Hoffmann found
that Alpine forms transplanted to Giessen remained precocious,
therefore the acquired precocity had become heritable. But
there is no evidence that the precocity was acquired ; it may
have been the outcome of the selection of germinal variations.

The African Wart-hog (P ha co cheer us) has the peculiar habit
of kneeling down on its fore-limbs as it routs with its huge tusks
in the ground and pushes itself forward with its hind-limbs.
It has strong horny callosities protecting the surfaces on which
it kneels, and these are seen even in the embryos. This seems
to some naturalists to be a satisfactory proof of the inheritance
of an acquired character. It is to others simply an instance
of an adaptive peculiarity of germinal origin wrought out by
natural selection.

SOME MISUNDERSTANDINGS CONSIDERED 181

Misunderstanding III — Begging the question by starting
with what is not proved to be a modification. — There is no rele-
vancy in citing cases where an abnormal bodily peculiarity
re-appears generation after generation, unless it be shown that
the peculiarity is a modification, and not an inborn variation
whose transmissibility is admitted by all. Short-sightedness
may recur in a family-series generation after generation, but there
is no evidence to prove that the original short-sightedness was a
modification. In all probability, short-sightedness is in its origin
a germinal variation, like so many other bodily idiosyncrasies.

In regard to some diseases, such as rheumatism, it is often said

dogmatically by those who know little about the matter that

the original affection in the ancestor was brought about by some

definite external influence — such as a cold drive or a damp bed ;

but it seems practically certain that in all such cases we have

to do with an inborn predisposition, to the expression of which

the cold drive or the damp bed was merely the liberating stimulus,

comparable to the pulling of the trigger in a loaded gun. The

liberating stimulus is, of course, of great importance, both in

the case of the gun's discharge and the organism's disease, but

it only goes a little way towards a satisfactory interpretation in

5, either case. Not that we can explain the origin of rheumatism

or shortsightedness or any such thing — there is no explanation

iv in calling them germinal variations that cropped up ; but we

, are almost certain that they never are modifications or acquired

ii characters.

ib Herbert Spencer twits those who are sceptical as to the trans-

v mission of acquired modifications with assigning the most flimsy
ic'al reasons for rejecting a conclusion they are averse to ; but when
Spencer cites the prevalence of short-sightedness among the
ii:: " notoriously studious " Germans, the inheritance of musical
talent, and the inheritance of a liability to consumption, as
evidence of the inheritance of modifications, we are reminded of
the pot calling the kettle black.

1 82 TRANSMISSION OF ACQUIRED CHARACTERS

Over and over again in the prolific literature of this discussion
the syllogism is advanced, either in regard to gout or something
analogous —

Gout is a modification of the body, an acquired character ;
Gout is transmissible ;
Modifications are sometimes transmissible.

It may be formally a good argument, but there is every reason
to deny the major premiss. There is no proof that the gouty
habit had an exogenous origin — that it was, to begin with, for
instance, the direct result of high living ; though it is generally
admitted that excesses in eating or drinking may give a stimulus
to its expression. " The conclusion I have arrived at," says
Prof. D. J. Hamilton (1900, p. 297), " is that the gouty habit of
body has arisen as a variation, and as such is hereditarily trans-
missible, and that excess of diet and alcohol merely renders
the habit of body apparent." It may also be pointed out that
gout and rheumatism and the like are rather processes of meta-
bolism than structural modifications, though the latter may
ensue.

After pointing out the irrelevancy of citing cases of the here-
ditary recurrence of polydactylism, haemophilia, colour-blindness
in man, or the absence of horns in cattle or of tails in cats, as
instances of the transmission of acquired characters, Prof. Ernst
Ziegler says (1886, p. 13) : " Only that can be regarded as ' ac-
quired ' which is produced in the course of the individual life,
during or after the period of development, exclusively under
the influence of external conditions ; the term is in no wise
applicable to peculiarities which, as one says, arise of themselves
from a predisposition already present in the germ."

Let us state the case once more. There is no doubt that the
expression of a germinal variation during the lifetime of an individual
may be sometimes definitely associated with a particular external
stimulus. It may thus be mistaken for a modification, and mis-
takenly spoken of as " acquired." But the relation between the

MISUNDERSTANDINGS : IRREIEVANT CASES 183

provoking stimulus and the expression of the innate tendency or
predisposition is more or less arbitrary— various kinds of stimuli
will have the same result ; whereas the relation between an environ-
mental influence and the induced modification is more or less-
constant — similar influences having similar results — and is more
strictly causal. An external stimulus may provoke the expression
of a germinal variation, as when a mouse provokes hysteria ; but
this is different physiologically from what occurs when the sun
produces sun-burning.

A certain abnormal psychosis, which may not have been hinted
at during early years, suddenly emerges under provocation. It is
carelessly spoken of (even in the law courts) as due to that
provocation — a fright, a wound, a debauch, a railway accident,
a night's exposure, and so on, and it is carelessly thought of as
" acquired " ; it is recovered from, but it re-appears in the off-
spring : therefore an acquired character may be transmitted. But
there is the strongest probability that what was called an acquired
psychosis was primarily germinal, and might have emerged under
quite different stimulation — for instance, under the normal events
of puberty and parturition.

Another version of this misunderstanding is seen in references
to the improvement of a breed in the course of generations, as the
result, it is supposed, of functional modifications. Practice makes
perfect in the individual, therefore also in the race. But we have
seen no cases cited where the results were not hopelessly complicated
by the occurrence of selection and elimination, which, by acting on
constitutional variations, may quite well account for what is hastily
referred to modification-inheritance.

Herbert Spencer was keenly aware of the misunderstanding which
we have been discussing. " Such specialities of structure as are
due to specialities of function are usually entangled with specialities
which are, or may be, due to selection, natural or artificial. In
most cases it is impossible to say that a structural peculiarity which
seems to have arisen in offspring from a functional peculiarity in
a parent is wholly independent of some congenital peculiarity of
structure in the parent, whence this functional peculiarity arose.
We are restricted to cases with which natural or artificial selection
can have had nothing to do, and such cases are difficult to find."

Yet it is strange that he should point to such facts as the following :
the bones of the wing in the domestic duck weigh less and the bones

1 84 TRANSMISSION OF ACQUIRED CHARACTERS

of the leg more in proportion to the whole skeleton than do the
same bones in the wild duck ; in cows and goats which are habitually
milked the udders are large ; moles and many cave-animals have
rudimentary eyes. Cases like these may be in part regarded as
instances of individually re-acquired modifications, but they are
for the most part readily interpreted as due to the selection of
germinal variations.

Misunderstanding IY — Mistaking the reappearance of a
modification for transmission of a modification. — It is of little
service to cite cases where a particular modification reappears
generation after generation unless it be shown that the change
recurs as part of the inheritance, and not simply because the
external conditions which evoked it in the first generation still
persisted to evoke it in those that followed. Reappearance
is not synonymous with inheritance.

Illustration. — When Prof. Nageli brought Alpine plants (Hier actum,
etc.) to the Botanical Garden at Munich, many became in the
first year so much changed that they were hardly recognisable
as the same species, and their descendants in the garden were
likewise quite different from their Alpine ancestors. The small
Alpine hawkweeds became large and thickly branching, and
blossomed freely. In some cases many generations were observed —
even for thirteen years ; there was no doubt as to the reappearance
of the acquired characters ; but it was not thereby proved that the
reappearance was due to the inheritance. On the contrary, that
the reappearance was due to the persistence of the novel conditions,
to the changes which these directly impressed on each successive
crop, was shown by the fact that when the plants were removed to
poor, gravelly soil, the acquired characters disappeared, and the
plants were re-transformed into their original Alpine character.
" The re-transformation was always complete, even when the species
had been cultivated in rich garden soil for several generations."

Misunderstanding Y — Mistaking re-infection for transmission.
> — A particular form of the fourth misunderstanding has to do
with facts so special that it may be conveniently treated of
separately. It has to do with microbic diseases. It is ad-

MISUNDERSTANDINGS: MORE IRREIE FANCIES 185

mitted that a parent infected with tubercle-bacillus or with the
microbe of syphilis may have offspring also infected. But such
cases are irrelevant in the discussion. Infection, whether before
or after birth, has nothing to do with inheritance. As Dr. Ogilvie
says (1901, p. 1072), " Wherever the transmission of infectious
disease from parent to offspring has been adduced to support
the doctrine of the inheritance of acquired characters, it has
been done in utter misconception of its meaning and scope."

Medical men have sometimes condescended to make a subtle
distinction between " hereditary " and " congenital " syphilis —
the latter manifested at birth, the former some time afterwards !
It seems strange that they have failed to recognise that there
is no reason to use the word " hereditary " at all in this con-
nection. What occurs is an infection, and it is theoretically
immaterial at what stage the infection occurs.* A microbe
cannot be part of an inheritance. f

Misunderstanding YI — Transmission in unicellular s is not
to the point. — It is not to the point to cite cases where uni-
cellular organisms, such as bacteria or monads, have been
profoundly and heritably modified by artificial culture, so
that, for instance, the descendants of a virulent microbe have
been made to lose their evil potency. It is irrelevant because in
regard to unicellular organisms we cannot draw the distinction

* It may be the germ-cells that are infected — especially when the direct
source of infection is the father ; or it may be the embryo that is infected
through the placenta : but the difference in the time of the infection is of
no theoretical interest, nor can it be inferred from any difference in the
outward symptoms, as these appear in the offspring.

t The egg of the green freshwater polyp {Hydra viridis) always contains
little greenish corpuscles which are not present in the youngest stages
oi oogenesis. It is almost certain that these are minute unicellular Algce
'Zoochlorcllce). But no one can regard these useful symbions as actually
part of the inheritance. The eggs of the silk-moth are often infected by
a. minute but fatal Protozoon which is present in the body of the moth.
It seems uncertain at what precise point these pebrine organisms become
associated with the egg, but however early it may be, the infection has
nothing to do with inheritance. (See Ziegler, 1905, p. 5.)

1 86 TRANSMISSION OF ACQUIRED CHARACTERS

between body and germinal matter, apart from which the
concept of modifications is of no value. In artificial culture
the whole character of the unicellular organism — its particular
metabolism — is altered ; it multiplies by dividing into two or
more parts, which naturally retain the altered constitution.
But this is worlds away from the supposed case of an alteration
in the structure of the little toe so affecting the germ-cells that
the offspring inherit a corresponding deformation.*

Prof. Adami (1901, p. 1319) says: "By subjecting a growth
of pigment-producing bacteria to the action of a temperature just
below that which will cause their death, we can bring about a
loss of pigment production, so that the rapidly-succeeding genera-
tions are perfectly colourless ; but gradually, in the course of time,
the cultures made from the original (heated) tube regain the power
of pigment production. This may be in two or three days, or, again,
only after several transplantations at the end of two or three
weeks ; and when we remember that a bacillus divides and so forms
a new generation in, on the average, something considerably less
than an hour, it is seen that the acquired character may be impressed
upon a race for some hundreds of generations. The more intense
the alteration to which the bacillus is subjected, the longer and the
more frequently the race is subjected to the altered temperature
conditions, the longer it is before there is a sign of return to the
normal."

These are interesting and reliable facts, but their citation as
evidence of the inheritance of " acquired characters " is misleading,
since no bacilli show any hint of the distinction between somatic
and germinal material on which the definition of " acquired char-
acters " depends, nor do they multiply except by division and

* It is surprising that even Prof. Oscar Hertwig (1898) supports his
argument in favour of the transmissibility of somatic modifications by
citing cases of inheritance in unicellular organisms. We are told that
the irritability of certain Algae to light may be modified by exposure to
strong light and to high temperature, and that " nobody would be sur-
prised " if the progeny also showed " some similar property." But this
is hardly evidence of the transmission of a modification ! We are also
told that under artificial conditions some bacteria may lose their toxic
properties, and may transmit this somewhat negative character of lost
virulence. This is admitted by all, but it is an ignoratio elenchi.

TRANSMISSION IN UNICEIIULARS 187

spore-formation. What occurred in the cases referred to was
probably a temporary dislocation or disturbance of the character-
istic organisation of the cells, with the result that pigment pro-
duction was suppressed. When the inhibiting conditions were
removed the original organisation recovered itself in the course of
generations. But there is a great difference between such cases
and, let us say, the transmission of sun-burning, or of specially
strong muscles, or of a callosity on the skin, or a dwarfed form,
which are instances of bodily modifications, technically called
acquired characters. In the case of the bacilli the disturbed organi-
sation was halved or multiplied in each reproductive process, and
the effect originally induced was inherited from generation to
generation, eventually disappearing as the restoration of normal
conditions allowed the original organisation to re-assert itself in its
integrity ; in the case of the supposed inheritance of a callosity we
have to assume either that the influence which induced this, or the
influence of it after it had been induced, also affected the germinal
material in the reproductive organs in such a way that the contained
germ-cells, when liberated, developed into an organism with more
or less of the callosity. It must be evident, without further dis-
cussion, that the cases are not at all on a par, and that inheritance
in unicellulars has not been considered with sufficient carefulness
even by experts.

Prof. L. Errera (1899) reported an experiment with a simple
but multicellular mould [Aspergillus niger), which adapted
itself to a medium more concentrated than the normal. The
second generation of the mould was more adapted than the first,
and the adaptation to the concentrated medium was not wholly
lost after rearing in the normal medium again. This looks like
evidence of the inheritance of the acquired adaptive quality
which was brought about as a direct modification. But the
case does not really help us, since the distinction between soma
and germ-plasm is not more than incipient in the mould in ques-
tion. And even if the distinction were more marked, it would
only show that the germ-plasm is capable of being affected along
with the body, by a deeply saturating influence, which nobody
as ever denied.

1 88 TRANSMISSION OF ACQUIRED CHARACTERS

Misunderstanding YII— Changes in the germ-cells along with
changes in the body are not relevant. — Another misunderstanding
is due to a failure to appreciate the distinction between a change
of the reproductive cells along with the body, and a change
in the reproductive cells conditioned by and representative
of a particular change in bodily structure. The supporters of
the hypothesis that modifications may be transmitted point
to the tragic cases where some poisoning of the parent's system,
by alcohol, opium, or some toxin, is followed by some deteriora-
tion in the offspring. There is no doubt as to the fact; the
question is as to the correct interpretation.

(i) In some cases it may be that the whole system of the
parent is poisoned — reproductive cells as well as body ; the
effect may be as direct on the germ-cells as on the nerve-cells.
These, therefore, are not cases on which to test the transmissi-
bility of an acquired character — i.e. of a particular somatic
modification. If a local poisoning had a structural effect on
some particular organ, and if that structural effect was repro-
duced in any degree in the offspring, the case would be relevant ;
but when the whole organism is soaked in a poison the case is
irrelevant. If it could be said that the sunshine, which brings
about sun-burning in the skin, soaks through the organism even
to its reproductive cells and specifically affects them, in a
manner analogous to the saturating poison, we should have a
physiological basis for expecting the inheritance of sun-burning.
But we cannot make this assumption. We have no warrant
for believing that the modification of a part re-echoes in a definite
specific way through the organism until even the penetralia of
the germ-cells reverberate.

(2) A parent organism is poisoned, and there are structural
results of that poisoning. The offspring are born poisoned,
and show similar structural peculiarities. This may be due
to the fact that the germ-cells were poisoned along with the
parental body ; but it may also be due, in the case of a mother,

CHANGES IN GERM-CELLS 189

to a poisoning of the embryo before birth, in a manner comparable
to pre-natal infection.

(3) In some cases — e.g. of alcoholism in successive generations-
there may be poisoning of the germ-cells along with the
body, there may be poisoning of the embryo before birth,
and of the infant after ; but it may also be that what is really
inherited is a specific degeneracy of nature, an innate deficiency
of control, perhaps, which led the parent to alcoholism, and
which may find the same or some other expression in the child.

Cases are known in which the children of a dipsomaniac father
and a quite normal mother have exhibited a tendency to
alcoholism, insanity, and the like. In this case the possibility of
poisoning the unborn child is eliminated, but there remain three
possibilities of interpretation, — that there was specific poisoning
of the paternal germ-cells ; that what was inherited was the
constitutional weakness which expressed itself as alcoholism
in the father ; and that there were detrimental influences in the
early nutrition, environment, education — " nurture," in short
— of the offspring.

But while we have admitted a good deal, we have not admitted
the transmissibility of a particular structural modification brought
about in the parental body as a result of the toxin.

An illustration of what we mean by the distinction " along with,
but not through the body," is afforded by an experiment of Paul
Bert's. He tried to acclimatise some Daphnia; (small fresh-water
crustaceans) to salt water by gradually adding salt to the aquarium.
At the end of forty-five days, when the water contained 1*5% of
salt, all the adults had died ; but the eggs in their brood-chambers
Burvived, and the new generation arising from these flourished well
in the salt medium {pit. Packard, 1894, p. 345). Packard sees
in this case an argument for the heritability of a modification, but
it seems to us merely an instance of the direct modification of
the germ-cells or of the embryos. Cuenot, whom Packard cites,
gives the correct interpretation : " This experiment shows with
admirable clearness that the germ-plasm has, owing to the modifi-

igo TRANSMISSION OF ACQUIRED CHARACTERS

cation, become accustomed to the salt, causing it to produce a
generation so different from the preceding."

Misunderstanding YIII — Failure to distinguish between the
possible inheritance of a particular modification and the possible
inheritance of indirect results of that modification, or of changes
correlated with it. — At first sight this seems hair-splitting, but it
is a crucial point. Through his vigorous exercise the blacksmith
develops a muscular arm worthy of admiration ; the shoemaker
acquires skeletal and muscular peculiarities less admirable. There
are many permanent and profound modifications associated with
particular occupations. Are we to believe, it is asked, that
the occupation of the parents has no influence on the offspring ?
Are we to believe, it is asked, that the children of soldier, sailor,
tinker, tailor, are in no way affected by the parental functions ?

It would be interesting to have precise data in regard to
this, but it is generally admitted that when parents have healthful
occupations their offspring are likely to be more vigorous.
The matter is complicated by the difficulty of estimating how
much is due to good nurture before and after birth. It is not
unlikely, too, that some profound parental modifications may
influence the general constitution, may even affect the germ-
cells, and may thus have results in the offspring. But unless
the offspring show peculiarities in the same direction as the
original modifications, we have no data bearing precisely on
the question at issue.

A belief in the inheritance of modifications was perhaps
expressed in the old proverb, " The fathers have eaten sour
grapes, and the children's teeth are set on edge " — a proverb
which Ezekiel with such solemnity said was not any more
to be used in Israel. Now if " setting on edge " was a structural
modification, and if the children's teeth were " set on edge "
as their fathers' had been before them, there would be a pre-
sumption in favour of the transmission of this acquired character,
though it would be still necessary to inquire carefully whether

INDIRECT RESULTS OF MODIFICATIONS 191

the children had not been in the vineyard too. But if, as Ro-
manes said, the children were born with wry necks, we should
have to deal with the inheritance of an indirect result of the
parents' vagaries of appetite, and not with any direct repre-
sentation in inheritance of the particular modification produced
in the paternal dentition.

Misunderstanding IX — Appealing to data from not more than
two generations. — It has often been pointed out that animals
transported to a new country or environment may exhibit
some modification apparently the result of the novel influence,
and that their offspring in the same environment may exhibit
the same modification in a greater degree. Thus sheep may
show a change in the character and length of their fleece,
and their progeny may show the same change more markedly.

But it is perfectly clear that if the evidence does not go beyond
this, nothing is proved that affects the question at issue. It
was to be expected that the offspring should show the modifica-
tion in a more marked degree than their parents did, since the
offspring were subjected to the modifying influences from birth,
whereas their parents were influenced only from the date of
their importation.

What would be welcome is evidence that the third generation
is more markedly modified than the second ; then there would
be data worth considering. Only then would it be necessary
to consider Weismann's somewhat subtle discussion as to the
influence of climate.

§ 5- Various Degrees in which Parental Modification might affect

the Offspring

It may seem, at first sight, unscientific to discuss various
hypothetical degrees in which parental modifications might
affect the offspring, when we do not know that modifications can
be in any degree transmitted. But unless we are greatly mis-

192 TRANSMISSION OF ACQUIRED CHARACTERS

taken, our theorem, if carefully attended to, will serve to make
the issue clearer.

In regard to germinal variations, whose transmissibility is
undoubted, it is well known that there may be different degrees
of transmission, or, more cautiously stated, that the offspring's
hereditarily determined reproduction of the parental variation
may have diverse expressions. It seems just, therefore, to
imagine that there might be different degrees in the transmission
of modifications.

(i) The first degree of transmissibility would be illustrated
if the offspring showed in any measure the same modification
as the parent had acquired. If the sun-burnt parent had a
congenitally swarthy child, that might be an indication of
modification-transmission of the first degree of directness. It
might be an illustration of what has been so carefully searched
after — the transmission of a particular acquired character. We
cannot too strongly emphasise that this and nothing else is
what Weismann has denied ; this and nothing else is the crux
of " the interminable argument." And for the sake of argument,
the possibility (i) must be kept quite distinct from the possibilities
(2) and (3).

(2) If the offspring exhibited a new character, not the same
as the parent's acquired modification, but affecting similar tissue,
though in a different fashion, we might be justified in speaking
of this as modification-transmission of the second degree of
directness. It might be an illustration, not of the inheritance
of a particular acquired character, but of something correlated
therewith, if the much sunburnt parent of a thoroughly blond
stock had a child with very dark hair on a very white skin. But
the inference would not be certain.

(3) If the offspring exhibited a novel character, analogous to
a modification, yet neither similar to the modification acquired
by the parent nor affecting the same region of the body, it
might be said that we had to deal with modification-inheritance of

AFFIRMATIVE ANSWER 193

the third degree of directness. It might be an illustration of the
inheritance of an indirect effect of a parental modification if
the sons of fathers who had eaten sour grapes had wry necks.
But we should require many instances before admitting the
hereditary nexus.

§ 6. The Widespread Opinion in favour of Affirmative Answer

It seems to be a widespread opinion that acquired characters
may be transmitted, but often the opinion wavers when it is
explained what this precisely means — namely, that a modification
in the body, brought about by a change in function or environ-
ment, may so specifically affect the reproductive elements that
when these develop there is in the offspring something corre-
sponding to the parental modification.

Opinion of " Practical Men." — In fairness we must admit that
the verdict of the practical man, whether physician or breeder,
gardener or farmer, is still in many cases an unhesitatingly
affirmative answer. One of the keenest of physicians has said
that a few months in practice would dispel all doubt as to
the inheritance of acquired characters ; but there are equally
keen physicians who have taken a different view. It may
also be that the first had not freed himself from Misunder-
standings V and VII.

Prof. Brewer, an American authority on breeding, who
gives an emphatic affirmative answer, notes :

" The art of breeding has become in a measure an applied
science ; the enormous economic interests involved stimulate
observation and study, and what is the practical result ? This
ten years of active promulgation of the new theory has not resulted
in the conversion of a single known breeder to the extent of inducing
him to conform his methods and practice to the theory. My
conclusion is that they are essentially right in their deductions
founded on their experience and observations — namely, that ao

13

i94 TRANSMISSION OF ACQUIRED CHARACTERS

quired characters may be, and sometimes are, transmitted, and
that the speculations of the Weismann school of naturalists are
unfounded."

But perhaps this widespread opinion does not mean so much
as it seems ; for it is very difficult to get busy practical men to
take the trouble to appreciate an exact distinction such
as is involved in the phrase, " the inheritance of an acquired
character."

Against the opinion quoted we may balance that of an ex-
perienced botanical physiologist, Prof. MacDougal. "Despite
general assertions to the contrary, no evidence has yet been
obtained to prove that the influence of tillage, ' cultivation,'
or the mere pressure of environment factors has produced any
permanent changes in hereditary characters of unified strains
of plants."

Great Variety of Opinion. — There is little to be gained by a
citation of opinions, for there are equally authoritative names
on both sides. But there are some points of interest. Thus we
have already noticed that the scepticism as to the inheritance
of acquired characters is not a modern fad. It is also note-
worthy that, while the majority of zoologists disbelieve in modi-
fication-inheritance, the reverse seems to be the case with
botanists. Is this because modifications are even more marked
and more recurrent in plants than in animals, or because the
distinction between soma and germ-plasm is much less definite
in plants than in animals ?

But there is this use at least in noting the discrepancy of
opinions, that we are warned from dogmatism. It cannot be
an easy question when we find Spencer on one side and Weismann
on the other, Haeckel on one side and Ray Lankester on the
other, Turner on one side and His on the other, and so on.

Herbert Spencer was so convinced that he went the length
of writing : " Close contemplation of the facts impresses me
more strongly than ever with the two alternatives — either there

REASONS FOR AFFIRMATIVE ANSWER 195

has been inheritance of acquired characters, or there has been no
evolution.'" *

Haeckel is so convinced for the affirmative that he stakes
his particular form of religion upon it, asserting that " belief
in the inheritance of acquired characters is a necessary axiom
of the monistic creed " ; and what may sound to some even more
serious is his declaration that, rather than agree with Weismann
in denying the inheritance of acquired characters, " it would be
better to accept a mysterious creation of all the species as
described in the Mosaic account."

Sir William Turner has said that " to reject the influence which
the use and disuse of parts may exercise, both on the individual
and on his offspring, is like looking at an object with only a single
eye " — which is not perhaps a very emphatic condemnation,
since most microscopic research is monocular. Moreover, the
doyen of British anatomists does not state the case with his
usual precision.

Why is the Affirmative Position so widely held? — Even
in regard to our own muscular and nervous systems, we are
familiar with illustrations of the fact that practice increases
capacity, and that desuetude is apt to be followed by loss of
power. A force de forger on devient forgeron. Organs improve
with the using and deteriorate in disuse. We are also well aware
that changes in the environment or conditions of life, and notably
in our food, cause changes in our body. It seems a " natural "
assumption to suppose that these gains and losses and changes
may be in some degree transmissible.

Apart from the " naturalness " of this assumption, there are
probably four reasons why the affirmative position is so widely
held:

(1) There are many facts which suggest modification-inheritance

* The italics are ours. See Herbert Spencer, " The Inadequacy of
Natural Selection," Contemporary Review, February and March, 1893.
Appendix B, Principles of Biology, 2nd ed. vol. i. 1898, p. 621-

196 transmission of acquired characters

until they are examined critically. The late Duke of Argyll,
in one of his scientific excursuses, said the world was strewn with
illustrations of the inheritance of acquired characters, and Dr.
W. Haacke, a very wide-awake evolutionist, has compared the
evidences for the affirmative to the sand on the sea-shore for
multitude, yet neither furnishes us, so far as we are aware, with
a single case that will bear analysis. The affirmative may be an
obvious interpretation of the results of evolution, but the ob-
vious interpretation is seldom the right one. The sun does not
go round the earth.

(2) The affirmative is an interpretation which seems to make
the theory of organic evolution simpler ; it suggests a more direct
and rapid method than the natural selection of germinal varia-
tions. If to a growing and varying nature or germinal inheritance
there were continually being added the results of peculiarities in
nurture, the rate of evolution would be quickened, both upwards
and downwards. But our first business is to find out whether
the hypothesis actually consists with experience.

Dr. Walter Kidd has argued carefully and ingeniously that all
departures of hair-direction from a simple and primitive tj^pe
may be interpreted as due to mechanical causes, namely, stimuli
repeated immensely often. The difficulty here and always is
with the presuppositions of the interpretation.

(3) We are so accustomed in human affairs to the entailment
of acquired gains from generation to generation, to standing on
the shoulders of our ancestors' achievements, that many find
it difficult to refrain from projecting this on organic nature.
They forget that the greater part of our entailing process comes
about through our social heritage, which is altogether apart
from our natural inheritance.

(4) A fourth reason is that many fictitious or anecdotal cases
of the inheritance of acquired characters continue circulating.
The inheritance of a letter branded upon the arm, which Aristotle
notes, is still in the popular currency, though it is perhaps an

GENERAL ARGUMENT AGAINST 197

extreme type of what His calls a handful of anecdotes. It is
reported that Sioux Indians tattoo discs on the cheekbone
prominences of their squaws, and it is said that similar marks
may be seen on some new-born children (Nature, iii., 1870,
p. 168). And besides fictitious cases there are some puzzling
phenomena, which the supporters of the negative position are
wont to dismiss as " coincidences " — which, it must be confessed,
is never a very satisfactory way of dealing with difficult cases.

§ 7. General Argument against the Transmissibility of

Modifications

Most of the evidence brought forward in support of the belief
in the inheritance of acquired characters is terribly anecdotal ;
but apart from this Weismann was led to a position of entire
scepticism by his realisation of the continuity of the germ-plasm.

The Apartness of the Germ-cells. — If the germ-plasm or the
material basis of inheritance be something relatively apart from
the body, and from its everyday metabolism, something often
segregated at a very early state in development, there is a pre-
sumption against its being readily affected in a specific manner by
detailed exogenous changes wrought on the structure of the body.

It seems accurate to say that the reproductive cells which
have the potentiality of becoming offspring never arise from
differentiated body-cells. Whether they are recognisable as
such, late or early, the germ-cells are simply those cells which
retain in all its integrity the complex, definite, and stable organi-
sation of the fertilised ovum from which the whole organism
develops. They have their power of reproducing creatures
more or less like the parents just because they are continuous,
through an unspecialised cell-lineage, with the fertilised ovum
from which the parental body arose. All the somatic cells are,
of course, likewise the progeny of the fertilised ovum, but in their
lineage there is differentiation and specialisation. We imagine
that in them the numerous items or potentialities in the fertilised.

198 TRANSMISSION OF ACQUIRED CHARACTERS

ovum are distributed and allowed to express themselves. In
the germ-cell lineage they are kept concentrated and latent.*

In any case the germ-cells in the reproductive organs are not
actively functioning elements of the body ; they are in a quite
peculiar way apart from the general soma ; and Weismann has
reasonably emphasised the difficulty of picturing any means
whereby the modification of a particular corner of the body can
react upon the germ-cells in a manner so specific that these can,
when they develop, reproduce the particular parental modification
or any approach to it. This argument, and the answers to it,
must be carefully considered.

i. The Germ- cells may be affected by the Body.— In the
first place, it has been answered that the body does undoubtedly,
in some cases, exert some influence on the gonads, so that the
difficulty is reduced to this : Can a modification of part of the
body exert a specific or representative influence on the germinal
material ?

But what is the precise nature of the alleged influence of
the body on the gonads ? It is pointed out that nervous
changes can excite the reproductive organs, that food-stuffs
may increase their activity, that alcohol and other stimulants
may influence them, and so on. But there is a great difference
between any such excitation of the gonads and the propagation
of a particular modification, let us say, from the skin to the
germ-cells. And there is a great difference between a poisoning
of the germ-cells along with the body, and the influencing of them
in a manner so specific that they can, when they develop, reproduce
the particular parental modification. (See Misunderstanding VII.)

* In certain conditions, as yet unknown, certain body-cells may revert
to a primitive mode of behaviour — like some kinds of criminals in society.
Thus the cells which develop into cancerous growths behave in some
ways like germ-cells, especially in their mode of division. (See the
researches of Farmer, Walker, and Moore.) But such cases need not
lead us to Hertwig's extreme conclusion that every cell is potentially a
germ -cell.

DARWIN'S AND SPENCERS THEORIES 199

2. Hypotheses as to Possible Mechanism of Transmission. —

In the second place, attempts have been made to construct
hypotheses by aid of which we might conceive how a modifi-
cation of, say, the skin, can exert a specific or representative
influence on the germinal material.

Thus, Darwin suggested his provisional hypothesis of pan-
genesis, according to which the parts of the body give off gemmules
which pass as samples to the germ-cells. But his suggestion
remains a pure hypothesis — and an unnecessary one unless new
facts come to light — and is nowadays maintained by no one
except in extremely modified form — e.g. in the Pangen-theory
of De Vries.

Spencer deserves credit for at least facing the difficulty of
conceiving a modus operandi whereby a particular modification
in, say, the brain or the thumb, can specifically affect the ger-
minal material in such a way that the modification or a tendency
towards it becomes involved in the inheritance. Briefly stated,
his theory is as follows :

Spencer's Theory of the Mechanism of Transmission. — Spencer
made the legitimate postulate that, intermediate between the
biological unit or cell and the chemical molecule, there were " con-
stitutional units," the vehicles of specific characters, ancestral and
parental traits, and the individual peculiarities of the organism
itself.

He supposed that they were very stable in their " fundamental
traits," but plastic as regards their " superficial traits."

He supposed that they had " such natures that while a minute
modification, representing some small change of local structure, is
inoperative on the proclivities of the units throughout the rest of
the system, it becomes operative in the units which fall into the
locality where that change occurs."

He supposed " an unceasing circulation of protoplasm throughout
an organism," such that, " in the course of days, weeks, months,
years, each portion of protoplasm visits every part of the body " — a
wild assumption.

Finally, " we must conceive that the complex forces of which

2oo TRANSMISSION OF ACQUIRED CHARACTERS

each constitutional unit is the centre, and by which it acts on other
units while it is acted on by them, tend continually to re-mould
each unit into congruity with the structures around, superposing
on it modifications answering to the modifications which have arisen
in these structures. Whence is to be drawn the corollary that
in the course of time all the circulating units — physiological, or
constitutional, if we prefer so to call them — visit all parts of the
organism ; are severally bearers of traits expressing local modifica-
tions ; and that those units which are eventually gathered into
sperm-cells and germ-cells (i.e. egg-cells), also bear these superposed
traits."

Thus the constitutional units are supposed to circulate and to
visit one another throughout the body. When they come to a
modified structure and visit its modified constitutional units, they are
supposed to be themselves impressed ; thus impressed, they are
supposed to be gathered into the germ-cells, which thus come to bear
the " superposed traits " resulting from modifications.

If we were sure that modifications were ever transmissible,
we might be glad of this hypothetic interpretation of the business.
But it is a difficult hypothesis to think out, and it would hardly
be tolerable even if there were facts which it was needed to
interpret. In particular, the conception of " an unceasing
circulation of protoplasm," so that " each portion of protoplasm
visits every part of the body," seems not only unwarranted,
but contradicted by well-established facts.

3. A Mechanism may exist though it remains Unknown.—
In the third place, we must recall Prof. Lloyd Morgan's warning
that although we cannot imagine how a modification might,
as such, saturate from body to germ-cells, this does not exclude
the possibility that it may actually do so. Oscar Hertwig
also maintains that our ignorance of any mechanism which
could secure the transmission of an acquired character is not
a good argument against the possibility of its occurrence. There
are, he says, many facts in biology which are quite secure,
though no causal nexus can be worked out at present (All-
gemeine Biologie, 1906, p. 621). It must be noted, however.

A CONCRETE CASE: SPENCER'S HANDS 201

that, so far as we can understand, a very complex and special
mechanism would be necessary if a modification in, say, the eye
is specifically to affect the germinal material.

Dr. George Ogilvie (1901) writes : "In a subject so involved
in obscurity the present incomprehensibility of certain relations
can hardly serve as an argument against their existence. The
development of the apparently uniform germ-plasm into the
infinite differentiation of a complex cell-state is, although no
longer a matter of doubt, perhaps not less inconceivable." But
this illustration is not altogether appropriate, since our inability
to conceive the precise " how " of development rests on our
inability to restate in simpler terms any of the fundamental
facts of life, such as growth, assimilation, or reproduction, whereas
the supposed relation between soma and germ-cells is inconceiv-
able in rather a different sense.

A better illustration, it seems to us, would be found in the
difficulty of exactly stating how particular changes in the gonads
are correlated with particular changes in the body — e.g. in the
changes associated with puberty, conception, ovarian and testi-
cular disease. Yet here we can at least imagine what the
general nature of the physiological nexus may be — in terms, for
instance, of internal secretions or " hormones."

A Concrete Case : Spencer's Hands. — It may illumine the
abstract argument to take a concrete case. Why had Herbert
Spencer small hands ? He says that it was because his grand-
father and father were schoolmasters, who did little manual
work from day to day, save in wielding the pen and sharpening
the pencil. Through disuse of the sword and the spade their
hands were " directly equilibrated " towards smallness. But
since Mr. Spencer senior was " a combination of rhythmically
acting parts in moving equilibrium," the dwindling of the hands
and the moulding of the physiological units thereof reverberated
through the whole aggregate ; a change towards a new state of
equilibrium " was propagated throughout the parental system—

202 TRANSMISSION 01 ACQUIRED CHARACTERS

a change tending to bring the actions of all organs, reproductive
included, into harmony with these new actions," or inactions.
The modified aggregate impressed some corresponding modifica-
tion on the structures and polarities of the germ-units. And
this was how Herbert Spencer had small hands. At least, so
he tells us.

Disuse of Parts. — It seems " natural " to suppose that
organs have dwindled pari passu with their disuse, and because of
their disuse. But the two statements are not synonymous. The
dwindling may be due to germinal variations in the fine of reduc-
tion, which are appropriate because of some change in the ani-
mal's habits and environment. It may even be that the organism
meets an endogenous reduction of certain parts by itself changing
its habits and habitat. Moreover, it is important to notice, as
Emery, Kennel, and Ziegler have pointed out, that there has pro-
bably been a " Kampf der Theile im Organismus " (a struggle
of parts within the organism) not merely in individual ontogeny,
but also in the racial phylogeny. Dwindling of one part occurs
when some adjacent part attains increased differentiation.
" Thus snakes have not lost their limbs because they did not use
them, but because of their evolution in the direction of excep-
tionally large trunk and tail musculature. In man the strong
dentition of his Simian forebears has become weaker, not through
disuse, but because the extraordinary increase of the brain has
been correlated with a weaker development of other parts of the
head " (H. E. Ziegler, 1905, p. 3)..

§ 8. General Argument for the Transmissibility of Modifications

The Germ-cells are not Insulated. — While it must be ad-
mitted that the germ-cells have a certain apartness from the daily
life of the body, and that they are unspecialised cells that have
not shared in the differentiation characteristic of the body-cells,
is there not some risk of exaggerating the distinction between
somato-plasm and germ-plasm ?

GENERAL ARGUMENT FOR 203

In many simple animals, such as sponges and hydroids, the
germ-cells simply make their appearance at certain times of year
among the commonplace somatic cells. In many plants the
distinction between body and germ-cells can hardly be drawn
until the period of reproduction sets in. Thus Spencer refused
to accept the contrast between body-cells and germ-cells as
expressing a fact, and referred to the numerous cases in which
small pieces of a plant or a polyp may grow into entire organisms.

To this objection Weismann answers, — (1) that the distinction
between somatic cells and germ-cells has been gradually em-
phasised in the course of evolution, and that in the simpler
multicellular organisms it is still incipient ; (2) that it is quite
conceivable that, even in some complex organisms, the body-cells,
though differentiated, may retain some residual unused germ-
plasm ; and (3) that there may be a quite definite and distinct
germ-plasm, though there is no demonstrably distinct lineage
of germ-cells.

Again, however, we must remember that the blood, or lymph,
or other body-fluids form a common medium for all the parts
of the animal, gonads included ; the results of changes in nutrition
may saturate throughout the body and affect the germ-cells inter
alia. The nervous system makes the whole organism one in a
very real sense ; in plants there are often intercellular bridges
of protoplasm binding cell to cell, and this is true in not a
few cases among animals. Moreover, there are subtle, dimly
understood correlations between the reproductive organs and
the rest of the system. If changes in the reproductive organs
can effect changes in remote parts, such as the larynx and the
mammary glands, why may not there be reciprocal influences ?
In short, the organism is a unity, and to divide it up, in any hard-
and-fast way, into soma and germ-cells may land us in the same
fallacy as parcelling the mind into separate faculties.

It must be admitted, therefore, that it is quite erroneous to
think of the germ-cells as if they led a charmed life, uninfluenced

2o4 TRANSMISSION OF ACQUIRED CHARACTERS

by any of the accidents and incidents in the daily life of the body
which is their bearer. But no one believes this, Weismann least
of all, for he finds the chief source of germinal variations in the
stimuli exerted on the germ-plasm by the oscillating nutritive
changes in the body.

Weismann's Concessions. — There are some who find in this
" a concealed abandonment of the central position of Weismann,"
and who say : "If the germ-plasm is affected by changes in
nutrition in the body, and if acquired characters effect changes
in nutrition, then acquired characters or their consequences
will be inherited." But it is quite illegitimate (§ 5) to slump
acquired characters and their consequences as if the distinction
were immaterial. The illustrious author of The Germ-Plasm has
made it quite clear that there is a very great difference between
admitting that the germ-plasm has no charmed life, insulated
from bodily influences, and admitting the transmissibility of
a particular acquired character, even in the faintest degree. The
point, let us repeat, is this : Does a structural change in a part
of the body, induced by use or disuse, or by change in surround-
ings, influence the germ-plasm in such a specific or representative
way that the offspring will thereby exhibit the same modification
that the parent acquired, or even a tendency towards it ?

The Real Difficulty. — Even when we recognise, as fully as we
can, the unity of the organism, that each part shares in the life
of the whole, it is very difficult to think of any modus operandi
whereby a local modification can specifically affect the germ-
plasm. The argument that we can as little understand the
modus operandi whereby an influence passes from the gonads
to distant parts of the body is not really sound. For we know
that in some cases the reproductive organs, besides being areas
for the multiplication of germ-cells, are organs of internal secre-
tion, producing specific substances which are carried away by
the blood-stream, and serve as the stimuli awakening the dormant
potentialities of distant parts.

THE REAL DIFFICULTY 205

Nor does the fact that morbid processes in a particular part
may result in a diffusion of toxins, which saturate even the germ-
cells, help us much in our attempt to picture how a modification
could become transmissible. For there is not the slightest
reason for supposing that the ordinary modifications in which
naturalists are interested, which experimental evolutionists can
bring about, are associated with the formation of specific toxins
which might diffuse through the whole system.

Spencer's Statement of the a priori Argument. — As Herbert Spencer
was perhaps the keenest and most convinced upholder of the affirma-
tive position, it seems just to give his statement of the a priori
argument. We have made a comment on each of the steps.

(1) "That changes of structure caused by changes of action

must be transmitted, however obscurely, appears to be
a deduction from first principles — or if not a specific
deduction, still, a general implication."

"For if an organism, A, has, by any peculiar habit or
condition of life, been modified into the Form A1, it follows
that all the functions of A1, reproductive function included,
must be in some degree different from the functions of A."

"An organism being a combination of rhythmically
acting parts in moving equilibrium, the action and structure
of any one part cannot be altered without causing altera-
tions of action and structure in all the rest."
Comment. — (a) It is not denied that some deeply saturating
modifications of the body, affecting the nutritive stream,
may affect the reproductive organs. This is not the point
at issue, (b) How far a modification is likely to affect the
reproductive organs must be determined by observation
and experiment. The appreciability of the change will
depend on the amount and nature of the modification,
and on the intimacy of the correlation subsisting in the
organism. Dislodging a rock may alter the centre of
gravity of the earth, but it does not do so appreciably.

(2) " And if the organism A, when changed to A1, must be changed

in all its functions, then the offspring of A1 cannot be the

same as they would have been had it retained the form A."

Comment. — This is logical, but is it true ? The change from

A to A1 may be important, it may appreciably alter the

206 TRANSMISSION OF ACQUIRED CHARACTERS

metabolism, but it does not follow that it can appreciably
alter the architecture of the germ-plasm. Spencer's as-
sumption that the change in the constitutional units of
the body must affect the constitutional units in the germ-
cells remains an assumption.
(3) "That the change in the offspring must, other things equal,
be in the same direction as the change in the parent, appears
implied by the fact that the change propagated throughout
the parental system is a change towards a new state of
equilibrium — a change tending to bring the actions of all
organs, reproductive included, into harmony with these
new actions."
Comment. — It seems to us to pass the wit of man to conceive
how or why an improved equilibrium in, let us say, the use
of the hand should involve any corresponding or represen-
tative change of equilibrium in the germinal material.
The drawback to abstract biology based on first principles
is that it enables its devotees to develop arguments which
seem plausible until they are reduced to the concrete.

§ 9. Particular Evidences in support of the Affirmative Answer

The question is whether modification-inheritance does or does
not occur, and we must no longer postpone our consideration
of the concrete evidence used to support the affirmative position.
Our reason for not placing this section in the foreground of the
chapter is mainly that a multitude of misunderstandings have
had to be cleared away before the so-called direct evidence could
be profitably considered. When one naturalist, Dr. W. Haacke,
declares that instances of modification-inheritance are as plentiful
as sand on the shore, and another, Prof. E. Ray Lankester,
declares that the Lamarckian position has its only remaining
defence, and that no secure one, in Brown-Sequard's experiments,
we have obvious justification for our preliminary discussion.

The instances adduced as evidence of modification-inheritance
might be classified according to the errors involved, but we have
arranged them rather in reference to the general nature of the
modifications discussed, whether environmental or functional,

IMPROVEMENT IN TROTTING HORSES 207

whether tending to increase or decrease, and so on. The alleged
inheritance of the direct effects of mutilations, injuries, and the
like is discussed separately in §§ 10 and 11.

Improvement in Trotting Horses. — Over a hundred years
ago (1796) the utmost speed of the English trotter was stated
at a mile in 2 min. 37 sec. Since 1818, accurate records have
been kept, which show a gradual increase decade after decade
in the speed and in the percentage of swift trotters. The
standard has risen and the breed has improved. The mile can
now be run in 2 min. 10 sec, or less. It is claimed by Cope
and others that we have here direct evidence of the trans-
mission of the structural results of exercise.

Brewer (cit. Cope, 1896, pp. 426-30) relates that about 1818
the record speed of the trotting horse was 3 min. to the mile ;
in 1824 it was reduced to 2 min. 34 sec. ; in 1848, to 2 min.
30 sec. ; in 1868, to 2 min. 20 sec. ; in 1878, to 2 min. 16 sec. ;
in 1888, to 2 min. n| sec. ; and finally to 2 min. 10 sec.
" The gain in speed has been cumulative. ... It has gone
on along with systematic exercise of special function in suc-
cessive generations ; . . . there is nothing that would lead us
to even suspect that the changes due to exercise of function
had not been a factor in the evolution ; . . . there is every
appearance and indication that the changes acquired by in-
dividuals through the exercise of function have been to some
degree transmitted, and have been cumulative, and that this
has been one factor in the evolution of speed."

It is impossible to prove the negative above suggested —
namely, that function has not been a factor ; but the affirmative
position is robbed of all cogency by the admitted occurrence of
rigorous artificial selection. The improvement supposed to be
entailed ma}7 not have been a modification at all ; but, supposing
it was, the interpretation of the result simply by the hypothesis of
use-inheritance gives a false simplicity to the case. It overlooks
the selective breeding which increases the constitutional swiftness,

208 TRANSMISSION OF ACQUIRED CHARACTERS

and the process of elimination which persistently weeds out the
less swift from the stud. And even apart from artificial selection
and elimination there may be a progressively cumulative suc-
cession of variations making for greater and greater swiftness.
We may even picture how this might come about, if we adopt
Weismann's conception of germinal selection.

Case of Squatting Punjabis. — It has been stated that the
Punjabis of India show certain peculiarities of musculature and
skeleton which are associated with the frequency with which
these people assume on all possible occasions the squatting
posture. It is asserted that the peculiarities of structure are
due to the peculiarities of function, but this requires definite
proof (Misunderstanding III). They may be adaptations origina-
ting in germinal variations. It is necessary to know whether the
peculiarities are in any degree represented on new-born Punjabi
babies, but even then it would be simpler to regard them as
variations than as transmitted modifications. There can be
no conclusiveness in regard to peculiarities whose first appearance
is hidden in obscurity. If squatting increased from generation to
generation, and if the structural peculiarities increased -pari passu,
the case would be interesting ; but even then we should have to
inquire whether we were not dealing with a progressive variation.

Peculiarities of Occupations. — In his interesting paper on
the anatomy of the shoemaker, Dr. Arbuthnot Lane describes
the peculiarities induced by this occupation, which tends to
form a distinct anatomical type. The same is true of the tailor.
" The bent form, the crossed legs, thumb-and-forefinger action,
and peculiar jerk of the head while drawing the thread, are the
main features of the sartorial habit," and they are associated
with permanent changes in muscles, insertion surfaces, and
articulations. These are indubitable modifications : what of
their transmission ? No one, Dr. Lane says, would expect any
perceptible changes in the first generation, but he thinks that he
has observed inherited effects in the third.

LARGE AND SMALL HANDS 209

We can only say that this line of inquiry deserves to be
followed up, especially since our minute acquaintance with the
human body and the accumulation of facts in regard to its varia-
tions make a discrimination between modification and variation
more secure than is possible in many other cases. It should be
remembered, however, that if the shoemaker's sons and grandsons
and subsequent descendants all " stuck to the last," there might
tend to be an accumulation of general constitutional peculiarities
—e.g. of meditativeness and of the physical effects of persistent
sedentary work, which might dispose the organism to re-acquire
particular modifications in a more marked degree.

Large and Small Hands. — Darwin (Descent of Man, p. 18) refers
to the alleged fact that the infants of labourers have larger hands
than those of the children of the gentry ; but this, and many
similar cases of which it is a type, may be sufficiently accounted
for by interpreting the observed differences as constitutional char-
acteristics of different stocks probably accentuated by various
forms of selection. Spencer notes, " That large hands are in-
herited by those whose ancestors led laborious lives, and that
those descended from ancestors unused to manual labour com-
monly have small hands, are established opinions." But if we
accept the " opinions " as correct, it is easy to interpret the size of
the hands as a stock character correlated with different degrees of
muscularity and vigour, and established by selection. The hands
of Japanese are in many details anatomically different from the
hands of Europeans, but there is no warrant for regarding these
detailed differences as other than constitutional racial differences
of germinal origin accentuated modificationally in the individual
lifetime.

Dwindling of Little Toe.— The alleged dwindling of the little
toe has been repeatedly cited as a case in point — proving the
inheritance of a modification produced by tight boots. But
precise data are wanting ; a dwindling has also been observed
in savages who do not wear boots ; it is possible that there may

210 TRANSMISSION OF ACQUIRED CHARACTERS

be in man, as there was in the ancestors of the modern horse, a
constitutional variation in the direction of reducing digits ;
and there are other possible explanations of the rather vague
assertions. It need hardly be pointed out that unless there is
a measurably progressive dwindling with similar boots in the
course of generations the case has no point. A control experi-
ment comparing the toes in sets of brothers respectively booted
and bootless would be interesting.

Results of Pressure. — Darwin (Descent of Man, p. 18) regards
the thickened sole of even unborn infants as due to " the in-
herited effects of pressure during a long series of generations."
But here again it is impossible to exclude the interpretation that
a variation in the direction of thickened solar epidermis might
have selection-value from very ancient days, to the arboreal
ape as well as to the bootless man. H. H. Wilder, in a paper
in which he gives a detailed comparison of the palms and soles
of Primates and Man (Anat. Anzeig. xiii. (1897), pp. 250-6),
distinctly refuses to commit himself to a Lamarckian theory,
believing that the facts may be equally well interpreted in
terms of variation and selection.

Bollinger (1882) suggests that the weak development of the
breasts in women of the Dachauer district is due to the old-
established fashion of wearing tight corsets which are pressed flat
on the breasts. It is necessary to inquire (a) whether the pecu-
liarity is not a modification inflicted on each successive generation,
or whether it is ever exhibited by a Dachauer woman who does
not wear a corset ; and (b) whether the same peculiarity does
not occur where the fashion is entirely different.

Climatic Changes. — Virchow and others have laid stress on
the fact that many peculiarities in races of men and of other
living creatures are climatic in origin, and yet are now part of
the natural inheritance. But acclimatisation is usually a slow
and gradual process, involving selection of germinal variations,
and it is difficult to get clear-cut cases of climatic modifications.

PARTICULAR CASES 211

It must also be remembered that Weismann expressly admits
that climatic influences, especially if long-continued, may influ-
ence the germ-plasm along with the whole system, and may induce
germinal variations that come to stay ; but this " has certainly
nothing to do with the view that functional modifications of any
particular organ can cause a corresponding change in the
germ-plasm." (See The Germ-Plasm, 1893, p. 408.)

In adjacent areas with different climatic and other environ-
mental conditions we not infrequently find closely related
species or local races. It seems impossible to doubt that these
are blood-relations, derived from a common ancestor. Are
thej' not due to the environmental differences ? In some way,
surely, the organismal differences are causally correlated with the
environmental differences, and it is granted by all that pecu-
liarities of climate induce changes in the nutrition, respiration,
circulation, and so on. If so, the germ-plasm may be affected
and variations may be provoked, some of which are adaptive.
But the result of these variations may be something different
from and much more profitable than the modifications directly
induced. They may be expressed in relation to quite different
organs. Thus it seems quite unnecessary to believe in the trans-
mission of climatic modifications as such, or in any representa-
tive degree. Moreover, we must never forget that the active
organism must be credited with the power of seeking out en-
vironments which suit its inborn nature — variations included.

Plants in New Environment.— Much has been made of the
changes which follow a radical change of environment. When
a plant is transferred to a new soil and climate it may undergo
a very marked change of habit ; its leaves may become hairy,
its stem woody, its branches drooping. " These," Herbert
Spencer said, " are modifications of structure consequent on
modifications of function that have been produced by modifica-
tions in the actions of external forces. And as these modifications
reappear in succeeding generations, we have, in them, examples

212 TRANSMISSION OF ACQUIRED CHARACTERS

of functionally established variations that are hereditarily trans-
mitted. But this is a non-sequitur, since the modifications may
reappear merely because they are re-impressed directly on each
successive generation. It is Misunderstanding IV.

At the same time it should be noted that radical change of en-
vironment may induce germinal variations or mutations which
breed true. These must be distinguished from modifications, as
already explained, since we cannot interpret them physiologically
as the direct somatic results of the environmental change.

Another case requiring consideration is that of a Turkestan
relative of our common Shepherd's Purse (Capsella bursa pastoris).
It has apparently spread from the low country to the uplands, and
the specimens growing at the higher altitudes are smaller than those
below, and pink instead of white. Seeds of lowland forms sown in
the uplands develop into small plants with pink flowers, but the
upland forms keep their characters (except the xerophytic leaves)
when grown in the low country. It is possible that we have here
to do with a variation coincident with a modification ; it seems,
however, that the experiments require to be repeated and extended.

Experiments on Brine-shrimps. — Reference is often made
to the observations and experiments of Schmankewitsch (1875)
on certain brine-shrimps belonging to the genus Artemia. By
lessening the salinity of the water he was able to transform one
type, Artemia salina, in the course of generations into another
type, Artemia milhausenii. By increasing the salinity, he was
able to reverse the process. Although he did not himself make
any such claim, his work has often been referred to as an illus-
tration of changing one species into another, and of the inheritance
of acquired characters.

It seems very doubtful, however, whether we have here to
do with modifications at all. Schmankewitsch did not modify
any one Artemia salina into Artemia milhausenii ; with a pro-
gressively changing environment and in the course of generations
he observed a transition of the population from the one type to
the other ; it is probable that the change of salinity operated

EFFECTS OF CHANGED ENVIRONMENT 213

directly on the eggs. This seems the more likely since the differ-
ences between the two types (in shape of tail, details of bristles,
etc.) are not such as we can interpret as the natural direct results
of altered salinity. It is well known that slight alterations in the
physico-chemical composition of the water have sometimes a
great and mysterious influence on eggs and developing embryos.

Fig. 27. — Side view of male Artemia salina (enlarged).

Encyclopedia.)

(From Chambers's

Bateson and others have shown that there is great variability in
the character of the tail and bristles of Artemia salina, of which A.
milhausenii seems to be only an extreme form without tail-lobes.

Fig. 27a. — Tail-lobes of Artemia salina (to the left) and of Artemia mil-
hausenii (to the right) ; between these four stages in the transforma-
tion of the one into the other. (From Chambers's Encyclopedia ;
after Schmankewitsch.)

But if the changes were somatic modifications, it is still open
to the critic to point out that Schmankewitsch experimented
with a progressively changing environment on a series of genera-
tions, and that the results were due to modifications hammered
afresh on each successive generation, without there being any
inheritance of these modifications.

A Typical Case. — An often-quoted and typical instance was
communicated to Darwin by Moritz Wagner. Some pupae of

214 TRANSMISSION OF ACQUIRED CHARACTERS

a Texan species of Saturnia were brought in 1870 to Switzerland.
In May, 1871, the moths emerged and were entirely true to type ;
they had young, and these were fed on the leaves of Juglans rcgia
(the Texan form feeding on Juglans nigra) ; these young developed
into moths so different in colour and form from their parents
that some entomologists referred them to distinct species. This
was a well-marked individual modification, but the story stops
just where it was beginning to be interesting. We are not told
about the subsequent generations. If they, too, were fed
on Juglans rcgia, and reared in Switzerland, they probably
reproduced the new type, but this would simply mean that
the modification was re-impressed on successive generations.

Experiments on Lepidoptera. — Standfuss reared pupae of
Vanessa urticce at a lower than the normal temperature, and
obtained a northern type (var. polaris) ; he reared them at a
temperature higher than the normal, and obtained a southern
variety (var. ichnusa). In the progeny he found a very small
percentage (all males) which showed a change in the same
direction as the parents.

Fischer worked with Arctia caja, reared the pupae at 8° C,
and obtained some unusually dark forms. Two of these were
paired and their progeny was reared at the normal temperature.
A small percentage of these — the last of the brood to emerge
from the pupa-state — showed the same kind of melanistic pecu-
liarity as the parents had shown.

Fischer pointed out, however, that the colour-aberration in the
offspring was not a repetition of the parental peculiarity, though it
was in the same direction and sometimes went farther. He did
not regard the case as illustrating the transmission of a specific
modification, but agreed with Wcismann's interpretation that the
germ-cells had been prompted to vary by the lowered temperature.
It should also be noted that in many butterflies there is a strong
constitutional — i.e. germinal — tendency to melanistic variation,
that the aberration docs not occur in all the individuals subjected
to the low temperature, that it occurs in very diverse degrees, and
that the experimenter selected two forms to pair together.

EFFECTS OF CHANGED ENVIRONMENT 215

Fresh Experiments. — Among the twentieth century experi-
ments on the transmission of modifications, there are a few
which suggest that a dogmatic denial of the possibility is very
unwise. As a striking instance let us take Kammerer's experi-
ments on salamanders.

(a) The common yellow and black salamander (Salamandra macu-
losa) is either viviparous, producing a large number of larvae 25-30
mm. in length with four limbs and short gills, or ovo- viviparous, lay-
ing large eggs which hatch out immediately into similar larvae 23-25
mm. in length. After a few months of larval life in the water they
undergo metamorphosis into land-salamanders 45-56 mm. in length.

(b) The black Alpine salamander (Salamandra atra) produces at a
birth two fully formed terrestrial young ones 38-40 mm. in length,
the larval stage being skipped — in obvious relation to the Alpine
conditions of life.,

(c) Kammerer kept the spotted salamander in the cold and got it,
after a few pregnancies, to produce only two young ones, as in the
black salamander.

(d) He kept the black salamander in a warm place with plenty of
water, and got it to produce 3-9 gilled larvae, thus approaching the
condition in the spotted salamander.

(e) Now the offspring of the salamanders thus treated (c and d) were
kept for two and a half years in a vivarium, but did not become
sexually mature until they were placed in the open in conditions
normal to 5. maculosa. They became mature when three and a
half years old.

The offspring of (c) gave birth to (1 ) very advanced larvae, 45 mm.
long with much reduced gills, metamorphosing several days after, or
moderately advanced larvae, 20 mm. long, with large gills; or (2)
to small larvae 26 mm. long with rudimentary gills, laid on land,
and metamorphosing after four weeks into salamanders 29 mm. in
length. Thus there was a partial persistence of a modified mode of
reproduction in the absence of the modifying conditions .

(/) The offspring of (d) bore in the water 3-5 larvae, 33-40 mm.
or 21-23 mm. in length, light in colour, and possessing gills. In
this case there was practically a continuance of the modifying con-
ditions and there was an augmentation of the parental modification.

The difficulties in regard to these very interesting experiments
are: (1) they do not deal with a structural modification in the
ordinary sense ; (2) it may be that the experimental conditions of

216 TRANSMISSION OF ACQUIRED CHARACTERS

(c) and (d) acted directly on the germ cells of the original subjects
of experiment ; and (3) there was some measure of artificiality in
the conditions under which the second generation developed which
may have disturbed the normal routine of reproduction.

Breeders' Evidence. — The evidence given by breeders in sup-
port of the theory of modification-inheritance, which is a tacit
or an avowed belief of many, if not of most, appears to us in most
cases too full of vagueness and misunderstanding to be of signifi-
cance ; but it has been often adduced by expert biologists, notably
by Cope (1896), who cites his cases from Brewer (1892-3), an
acknowledged agricultural authority. The first argument relates
to the inheritance of characters due to nutrition, and is as follows :
The size of domestic animals is often of much practical import-
ance, and has been attended to for many years with all the
carefulness which a pecuniary stake ensures. It used to be said
that " feed is more than breed," but it is now recognised that
"heredity or 'breed' is the more important." There is also, of
course, careful selection, " but no breeder claims that a breed is
or can be kept up to extra size by selection alone." " Breeders
do not believe that the characters acquired through the feeding
of a single ancestor, or generation of ancestors, can oppose more
than a slight resistance to that force of heredity which has been
accumulated through many preceding generations, and is con-
centrated from many lines of ancestry. Yet the belief is universal
that the acquired characters due to food during the growing
period have some force, and that this force is cumulative in suc-
cessive generations. All the observed facts in the experience
with herds and flocks point in this direction." The breeding of
small and delicate Alderney cows was furthered by systematic
underfeeding of the calves. Large-sized breeds have originated
m regions of abundant food, and smaller breeds in districts of
scantier forage. " This can hardly be due to accident." In
short, " if these acquired characters are in no degree whatever
transmitted, then certain practices of breeders, which are

BREEDERS' EVIDENCE 217

founded upon the contrary belief, are delusive and expensive
mistakes."

We have given this argument at some length, since it deals
with a subject of great practical importance, and since it is pre-
sented to us with the double authority of Cope and Brewer. It
is, however, on every count most disappointingly inconclusive.
If the size is a function of four variables, — (a) the inheritable
constitution of the stock (statistically determinable in certain
of its expressions at the beginning of a period of observation) ;
(b) the individual modifications produced by altered nutrition
(approximately determinable by control experiments and ob-
servations) ; (c) the possible occurrence of modification-inherit-
ance ; and (d) the amount of discriminate selection within a
given period (also admitting of mere or less precise statement), —
then the only feasible way of reaching a conclusion as to the
importance of any one factor — say the third in this case — is to
eliminate the others one by one.

As to the Alderney cows, it is admitted by all that the skilful
breeder can breed small or breed large, either by relying wholly
on the selection of a sufficiently variable stock, or by assisting
selection by modification kept up for each generation ; but this
does not touch the question at issue.

And if it be a fact that large-sized races always come from
regions of abundant nutrition, and vice versa, it is plainly
consistent both with natural and artificial selection.

As to the argument that unless modification-inheritance be
a fact the practice of breeders is an expensive mistake, one is
tempted to retort that the latter is at least as likely as the former ;
but the sufficient answer is that breeders, even though they may
think they do, never put their stake on the doubtful card.

Finally, it may be noted, though this is a point rather for the
biologist than for the breeder, that experiments on increased
size of parts are more decisive than those which refer only to
the size of the whole.

218 TRANSMISSION OF ACQUIRED CHARACTERS

Manly Miles gives two cases to illustrate what seems to him a
general fact, the occurrence of modification-inheritance in breeding :
" The fashion of raising lambs by nurses of other breeds, and drying
up the dam at once to keep her in show condition, resulted in seriously
diminishing the inherited capacity for milk production in the females
of the family as treated." " Cows on short pastures and under
careless management will form the habit of ' going dry ' early in
the season, and this habit of giving milk for a short period is not only
transmitted, but becomes a marked peculiarity of the females of the
family that is persisted in under better conditions of food supply."

But these and numerous similar cases only show, what is univer-
sally admitted, that a nutritive disturbance in the mother is apt to
affect the nutritive vigour of the offspring.

Brewer (cited by Cope, 1896, p. 436) reports what may be called
a good case. Sheep taken from a favourable region to one with
alkaline or salt soil, dry climate, and corresponding forage plants,
acquire a certain harshness in the wool. The change begins immedi-
ately, " but is more marked in the succeeding fleeces than in the
first. It is also alleged that the harshness increases with succeeding
generations, and that the flocks which have inhabited such regions for
several generations produce naturally a harsher wool than did their
ancestors, or do the new-comers." Of course, the second generation
would naturally have harsher wool than the new-comers, but if
harshness really increases with succeeding generations, the case is
one of the best as yet brought forward.

Immunity. — Another typical line of evidence is based on the
study of immunity. To this very important, but very difficult,
subject we have referred in another chapter, but the particular
point here may be briefly stated. It is well known that some
natives are relatively immune to yellow fever ; this is now a
heritable quality ; the question is whether it can be regarded as
originally an acquired character. Was it in origin a modification
of the bodily metabolism subsequent upon the disease ? It
seems very difficult to adopt this interpretation, and most
authorities incline to the other alternative of regarding immunity
as a constitutional variation which has become dominant in the
race by the elimination of those members who were not immune.

MEDICAL ARGUMENTS 219

It may be objected, however, that there are cases where a
mother rabbit or guinea-pig has been artificially rendered
I immune to certain diseases, and has afterwards had young born
immune. This may be due to a kind of infection before birth,
some anti-toxin or other having probably passed from the
mother to the unborn young. (Misunderstanding No. V.)

Medical Arguments. — A medical argument which has convinced
many is somewhat as follows. Its cogency rests on the difficulty of
drawing hard-and-fast denning lines.

It is allr g::d that a pregnant woman with smallpox may infect her
unborn offspring — a clear case of intra-uterine contagion.

A tubercular mother may have an offspring without tuberculosis,
but with something wrong with its heart. Here a constitutional
diathesis, stimulated by a bacillus, is followed by a result in the
offspring quite different from the condition in the parent.

Toxins produced by bacterial disease in the parent may affect
the offspring without inducing any special disease, but by weakening
its constitution and power of resistance.

Toxins produced, apart from bacterial disease, by a saturation of
the parent with alcohol, opium, and the like, may affect the offspring
both functionally and structurally, with the result that there are
diseases and malformations.

It has been shown experimentally that toxins (hydrocyanic acid,
nicotin, alcohol, etc.) may, directly injected into the eggs of fowls,
affect the development so that malformation results. It is stated
that the effects of lead-poisoning on the offspring may be wholly
due to the father. Therefore it seems legitimate to infer that toxins
produced in the body may have a direct effect upon the germinal
material.

It is not shown, however, that the effect on the offspring is the
same as that induced in the parent — which is the biological point
under discussion — and it is a wild hypothesis that an ordinary
modification liberates anything comparable to a toxin.

Alcoholism. — Habitual drunkenness in a parent or in the
parents produces familiar modifications, and may be followed
by dire results in the offspring. But before drawing the hasty
conclusion that definite structural results of alcoholic poisoning

220 TRANSMISSION OF ACQUIRED CHARACTERS

on the parent's body are in the strict sense transmitted to the
offspring, we do well to consider — (i) that the intemperate habits
of the parent may be the expression of an inherited psychopathic
disposition, and it is this which is transmitted to the offspring ;
(2) that the saturation of the body with alcohol may have a
direct effect on the nutrition and developmental vigour of the
germ-cells ; (3) that the children of drunkards often become
accustomed to alcohol as part of their food, from the days of
suckling onwards.

Nervous Diseases. — Prof. Binswanger of Jena, a famous
student of psychiatry, has expressed his inability to find evidence
that a mental or nervous disease acquired during the individual
life is, as such, or in partial expression, inherited by the offspring.
There are, he of course allows, numerous cases in which an
inheritance of mental or nervous diseases can be traced from
one generation to another ; but his difficulty was to find a case
where it could be securely maintained that the first occurrence
of the disease was due to external influence.

It may, of course, be urged, though it seems an untenable
extreme, that mental and nervous diseases never have an exo-
genous origin, but are always referable to germinal defect. If
so, it simply forces us to say that this line of argument is closed
as far as the question of the transmissibility of modifications is
concerned.

Modifications of Habits and Instincts. — Many animals are
very plastic in their habits, and some show some plasticity even in
their instincts. It seems an interesting line of experiment to
try to determine whether there is any evidence of transmission
of peculiar individually modified habits. For an expert discus-
sion of the subject we must refer to Principal Lloyd Morgan's
Habit and Instinct.

There are obviously many difficulties. The experimenter
must be sure that the original change of habit is really modifica-
iional, not an inborn idiosyncrasy. He must be careful to

MODIFIED HABITS AND INSTINCTS ±2\

eliminate the possibility of the offspring learning by imitation
or suggestion. He must also exclude the possibility of selection.
He must remember that the offspring are probably as docile, as
plastic, as adaptable as their parents, or perhaps more so. Moun-
taineering mules come to have an extraordinary power of adapting
themselves to peculiar exercises, but mule does not inherit from
mule !

A hen becomes an adept in rearing ducklings : will her own
children, put to a similar task, be less fussy than she was at
first ? House-martins have learned to build beneath the eaves :
has there keen any hereditary transmission of this acquired
habit, or is it merely "the result of intelligent adaptation through
the influence of tradition " ? Have grouse inherited the habit of
flying so as to avoid telegraph wires ? Is it indubitably the
case that the kittens of a cat " taught to beg for food like a
terrier" may spontaneously exhibit the same peculiar habit?
These are some of the cases which Lloyd Morgan discusses, and
his conclusion is that the evidence for the transmission of acquired
habit is insufficient.

§ 10. As regards Mutilations and the Like

When we think of the bellicose activities of our ancestors, it
seems almost absurd to discuss the question of the transmissi-
bility of the results of mutilations, wounds, and other injuries.
Moreover, it is well known that dishorning of cattle, docking of
horses' tails, curtailing of sheep, cropping of dogs' ears, and
similar practices, have been continued for many generations
without any known hereditary effect. The circumcision of the
children of Jews and Mohammedans has gone on for many cen-
turies, but there is no demonstrable structural result. Yet the
question is one of possibilities, and there is a huge literature of
observations and experiments.

Few Useful Results. — The net result, it must be confessed, is
very disappointing, and the reasons for this are not far to seek.

2*2 TRANSMISSION OR ACQUIRED CHARACTERS

(i) Many of the experiments and observations have failed to
conform to the ordinary canons of scientific method. Many of
them overlook the probability of coincidence, identify post hoc
with propter hoc, mix up observation and inference, or base a con-
clusion on a small number of instances. It may be noted that
cases suggesting the transmission of the results of mutilation and
injury are most abundant in the older, less critical literature.
What may be called good cases have been very scarce of recent
years, though many observers have been on the watch for them.

(2) Some of the kinds of experiment — e.g. the amputation of
large parts or of portions of internal organs, such as the spleen —
are evidently of a kind which must be rare in nature. Therefore,
though such "fool's experiments," as Darwin would have called
them, may have some indirect value, they tend to be of little
significance to the evolutionist.

(3) The experimental repetition of those mutilations and in-
juries which are common in nature is of little value, since nature's
experiment shows with sufficient clearness that the results are
not transmitted. If they were there would be but little now left
of man and other combative organisms. As Hartog says, " The
tendency to transmit the mutilation itself would be so ruinous
as to rapidly extinguish any unhappy race in which it was largely
developed " (Contemp. Rev., v. 64, p. 55). As a matter of fact,
even in the individual lifetime the results of mutilation are very
often repaired by regeneration, which in its specialised expression
is probably the adaptive outcome of prolonged selection.

(4) If the results of mutilation can be in any degree trans-
mitted, they must affect the germ-cells in some specific way.
The improbability of this is very great in the case of many
mutilations, such as lopping off a tail. The amputation has often
little demonstrable effect beyond a slight irritation of the tissues
at the cut surface ; the organism's reaction bears little relation
to the actual effect produced ; a considerable part of the body
has been lost, but there is no constitutional disturbance — the

mutilations and the like a%

reaction is a mere scar. Why should one expect the offspring to
have a shorter tail because its parent has been curtailed ? Might
one not as reasonably expect a longer tail ? No one has ever
observed that the descendants of much-pruned fruit-trees or
decorative shrubs are any the smaller in consequence. The
length of the hair in offspring is not known to be affected by the
frequent cropping, clipping, or shearing of their parents. In
fact, the structural results of most mutilations are not modifi-
cations in the usual sense.

(5) But there are cases in which the removal of a part has
deeply saturating effects. Thus the removal of a thyroid
gland may have an influence on many parts of the body. In
such cases, therefore, the possibility of the germ-cells being in-
fluenced is more conceivable. But, unless the change in the
offspring — supposing that there is some change — corresponds to
the direct change wrought upon the parent, we have not to deal
with modification-inheritance of the first degree, which is the
only question under dispute.

(6) Since the structural change due to a mutilation is not on
the same plane as the ordinary modifications which occur in
nature, we do not expect useful results from further mutilation
experiments. We may refer, however, to the suggestion made
by Dr. J. W. Ballantyne,* that in this connection, as with other
modification experiments, investigators err by beginning at too
late a stage, after the organism is firmly set. It may be that ex-
periments on early stages would yield more positive results. It
may be that the germ-cells in their early generations are more
reachable by, or sensitive to, somatic influences.

Illustrations. — In our brief discussion of this well-worn subject,
we shall for convenience distinguish three categories : (a) amputa-
tions, such as docking the tail ; (b) wounds, such as the rupture
of the hymen ; (c) deformations, such as the compression of the

* "Discussion on Heredity in Disease," Scottish Med. and Surg. Journal,
vi. (1900), p. 312,

224 TRANSMISSION OF ACQUIRED CHARACTERS

Chinese lady's foot. Under each category we shall notice merely
a few typical cases, which may be added to as the reader pleases by
referring to the literature cited, or by consulting the great work of
Delage.

Amputations repeated Generation after Generation. — Circum-
cision among Jews and Mohammedans, docking horses, dogs, and
sheep, cutting off parts of the ears of dogs, dishorning cattle, are
cases in point, and there is no evidence of transmitted result. Dar-
win (1879) does indeed cite Riedel to the effect that a shortened
prepuce has been induced among the Mohammedans of Celebes, but
Delage notes the inconclusiveness of Riedel's observations. Haeckel
(1875) and Leidesdorff (Wien. med. Wochenschr. 1877) have also
stated that a rudimentary prepuce occurs more frequently in races
who practise circumcision, but other statistics do not bear this out.
As Ziegler says (18S6, p. 27), "There is in this respect no difference
between Jews and Christians ; among the latter a defective develop-
ment of the prepuce is as frequent as among the former." See also
Roth, Correspondenz-Blatt f. Schweizer Aerzte, 1884.

Weismann cut off the tails of mice for nineteen generations, Bos
for fifteen, Cope for eleven, Mantegazza and Rosenthal likewise, but
in no case was any inherited result observed. An American record
of the production of a tail-less race almost certainly illustrates an
unscientific use of the imagination.

The tails of fox-terriers are often cut, and pups with short tails
are sometimes observed. The following case is representative of a
number of records. A fox-terrier, whose tail had been cut, had four
pups, one with a full-length tail, one with a rather short tail, and
two with quite short tails. But the short tails had the usual tapering
vertebrae (D. E. Hutchins, Nature, lxx., 1904, p. 6).

Delage cites Tietz ( 1 889) to the effect that kittens with an atrophied
tail are frequent in the Eiffel, where the peasants habitually curtail
their cats — in mistaken kindness, for they believe that there is a
worm at the root of the tail which keeps them from catching mice !
If abortive tails are unusually common in that district, the fact is of
much interest, and Delage does not find sufficient explanation in the
suggestion of Dingf elder (1887), that, as the peasants leave short-
tailed kittens alone, an inborn variation towards short tails has
been allowed to diffuse itself. It is, of course, easy to appeal to an
innate tendency to shortening of the tail, but it is curious that the
examples should be found so generally among domesticated animals,

MUTILATIONS AND THE LIKE 225

like cats and dogs, sheep and horses, which are so often artificially
docked.

Amputations not repeated throughout Generations. — These form
what we may call the " curtailed cat " type, the point being that a
she-cat whose tail has been cut off accidentally or otherwise has been
known to bear kittens, some or all of which have tails shorter than
the normal. The cogency of such cases is annulled when we remem-
ber,— (1 ) the existence of a Manx and Japanese breed of tail-less cats ;
(2) the occasional occurrence of tail-less or short-tailed kittens as
" sports " in the litters of quite normal parents ; and (3) the
frequently observed variability of the tail region in many mammals.
In all such cases at least two inquiries are imperative : ( 1 ) some
estimate of the probability of coincidence, since the post hoc may be
no propter hoc, but merely a variation which happens to resemble
more or less the result of the mutilation ; and (2) an investigation
into the pedigree of both parents, since there may be in either or
in both an innate tendency towards a shortening of the tail. These
inquiries are not usually made.

A number of very interesting cases are given by Delage (1903),
and it is difficult to dispose of them except by calling them " mere
coincidences." One of my colleagues has told me of a case of a
child with a peculiar bare patch among the hair, corresponding to a
similar area on the mother's head, where the bareness was due to
ringworm. The child's patch was bare save for a narrow streak
of short hair, stretching about half way across. The patch was a
little in front of the mother's, but was similarly situated above
the left ear. What can one say but " coincidence " ? Or may one
suggest that the ringworm found out a hereditarily weak spot ? —

Wounds repeated Generation after Generation. — We do not aim at
any surgical precision in distinguishing amputations from wounds.
Our point is simply that there is a difference between the effect
of an amputation which may be almost negative, and the effect of
a. wound which disturbs the relations of parts. The classification
is borne out by the fact that whereas there is not a grain of evidence,
so far as we know, to lead one to believe in the inheritance of the
results or any results of amputations, except when very important
organs are operated upon, the same cannot, at first at least, be said
in regard to the effects of wounds.

The typical case here is the rupture of the hymen in the first
sexual intercourse — a trivial lesion, perhaps, but one which has

15

226 TRANSMISSION OF ACQUIRED CHARACTERS

occurred in every generation, and one of which no inherited results
are known. Nor are there known results of a kind of circumcision
practised by Somalis and others on girls as well as boys. In some
races ear-boring, nose-boring, and the like, have been practised by
both sexes for many generations ; and no inherited result has ever
been observed.

Casual Wounds. — Darwin cites the case of a man whose thumbs
were badly injured in boyhood, as the result of frost-bite. His
oldest daughter (S) had thumbs and thumb-nails like the father's ;
his third child was similar as to one thumb ; two other children
were normal. Of the four children of S, the first and the third,
both daughters, had deformed thumbs on both hands. The cogency
of this case depends on whether there was or was not any previous
family tendency to thumb-deformity. It may have been that the
frost-bite was really an unimportant incident. Darwin gives
another case of a man who, fifteen years before marriage, lost his
left eye by suppuration. His two sons had left-sided microphthal-
mia. Here we have probably to deal with an innate eye-defect
in the father.

Bouchut * reports the case of a man of twenty-five who injured
his hands and feet by a fall from a scaffold. Of five children only
one was normal. His son had one finger on each hand and two
toes on each foot. A daughter (M) had two toes on each foot, one
finger on the right hand, and two on the left. She married a normal
man, and of her four children the oldest was normal, the others
like herself.

Cases like the last may seem puzzling to those unaccustomed to
deal critically with the facts of inheritance. But in reality they
are in most cases merely illustrations of the familiar fallacy of con-
fusing post hoc and propter hoc, of mixing observation and inference
(Ziegler, 1886, p. 26). Bouchut does not say that the children
showed the same deformity as their father acquired ; he does not
tell us about the ancestry of the father and mother, an indispensable
fact if a case is to be considered seriously, since inborn mal-
formations are common in some families ; finally, the frequency
of inborn malformations of the fingers and toes must be borne in
mind, and the possibility of coincidence allowed.

Ziegler (1886, pp. 29, 30) discusses a number of cases where defects

* Nouveaux Elements de Pathologie generate, Paris, 1882. Cited by
Ziegler, 1886, pp. 3, 4.

MUTILATIONS AND THE LIKE 227

in the eye occurred in the offspring of animals whose eyes had
been operated on, injured, or infected. But experiments in which
the eyes are infected with tubercle or the like are not relevant
until all possibility of the offspring being infected is excluded ;
and as for cases such as those given by Brown-Sequard (1880),
where the extirpation of the eye-bulb in the parent was followed
in the offspring by the loss of one eye or of both, or by corneal
obscuration, it is necessary to compare the results with the statistics
as to the frequency of various kinds of innate eye-defects.

Deformations. — We do not know all that we should like to know
in regard to the artificially deformed feet of Chinese ladies, but there
is no evidence that £he long-continued deformation has resulted
in any hereditary change.

For untold ages the herdsmen in some parts of the Nile valley
have artificially deformed the horns of their cattle, making them
bend forwards, twist spirally, and so forth ; but no effect on
offspring has ever been observed (R. Hartmann, Die Haussaugethiere
der Wildlander. Ann. Landwirthsch. ; Berlin, 1864, p. 28).

The Rook's Bill-feathers. — Settegast and others have referred
to the bristle-like feathers about the nostrils and the base of the
bill in the young rook. They are said to disappear mechanically
when the bird begins to bore with its beak in the ground, yet they
are always present in the nestling. To cite this as an example of
the non-transmission of a deformation-effect is probably quite
erroneous, for there is no proof that the disappearance is causally
connected with burrowing. It is probably a constitutional pecu-
liarity that these feathers should be moulted and not replaced.
They disappear even if the rook is not allowed to bore (see
Oudemans and Haacke, cited by Delage, 1903, p. 223). On the
other hand, to start from the fact that the bristles disappear even
if there is no boring in the ground, and to cite this as an instance
of the transmission of a deformation-effect, is equally fallacious.
There is no evidence that it was a deformation-effect to start with.

Soyne Puzzling Cases. — While the argument based on the apparent
transmission of the results of mutilation appears to us very weak,
it must be admitted that there are some cases which, if accurately
recorded, are puzzling. It is desirable that any fresh cases, similar
in nature to those which we propose to illustrate, should be studied
carefully and without prejudice. Though they may not prove
modification-inheritance, they may lead to interesting results.

228 TRANSMISSION OF ACQUIRED CHARACTERS

Prof. Haeckel * records that a bull on a farm near Jena had
its tail squeezed off at the root by the accidental slamming of
the byre-door, and that it had thereafter a tail-less progeny. This
is very interesting, but we are bound to ask — (i) how often tail-
less cattle arise apart from curtailing by the byre-door ; (2)
whether the bull had any tail-less offspring before it was cur-
tailed ; (3) how many tail-less offspring it actually had, and so on.
It may be that the answers to these questions would be quite satis-
factory, but, to make the case cogent, the questions should have been
forestalled.

In 1874 Herr W. Besler, in Emmerich on the Rhine, wrote to Prof.
L. Biichner (1882, p. 24) to report the following case. At Dobeln,
in Saxony, at Eichler's Hotel there, he saw a young dog apparently
bereft of ears and tail. When he remarked that the beast had been
far too much cut, he was told that this was not the case, for it and
its brother had been born so, out of a litter of four. The mother
was normal, the father was an " Affen-Pinscher," whose ears and
tail had been cut. The same condition had occurred once in a pre-
vious litter. Supposing that this was more than an ostler's yarn,
we should have to inquire into the ancestry of the father and mother
to see whether inborn shortness of ears and tail had ever manifested
itself in the family.

Prof. Biichner also relates that in the autumn of 1873 a build-
ing-contractor, K , in Westphalia, bought a duck whose right

" wing-bone " had been broken and had mended in a crooked fashion.
Next spring the duck had four ducklings, two of which showed on
the right wing, and two on both wings, an extra feathered wing
(4-5 in. in length), protruding immovably at an angle of 450 above
the otherwise normal wing. But this duplicity, if such it was
bore no precise relation to the original injury, and probably was
quite unconnected with it.

Biichner gives a number of other instances. Thus Williamson
saw dogs in Carolina which had been tail-less for three or four genera-
tions, one of the ancestors having lost the tail by accident. f But
tail-lessness is also known as a germinal variation.

Bronn % describes the case of a cow which lost one horn by ulcera-
tion ; it had afterwards three calves which showed on the same side

* Schbpfungs-Geschichte, ed. 1870, p. 102.

j Waitz, Anthropologic der N aturv'dlker , i. p. 93.

% Geschichte der Natur, 1 871, p. 96.

MUTILATIONS AND THE LIKE 229

of the head no true horn, but a small nucleus of bone hanging to
the skin. It may have been that an inborn weakness, which led
to the ulceration of the mother-cow's horn, took a slightly different
expression in the calves.

Dr. J. W. Ballantyne quotes Kohlwcy's experiments on pigeons :
" He cut off the posterior (first) digit of the foot, and the mutilated
bird got into the habit of turning the fourth digit backwards and
using it in perching ; he got no descendant of these mutilated birds
without a posterior digit, but he got a descendant of one of the pairs
with its fourth digit turned backwards like the first. The mutilation
was not transmitted, but the physiological adaptation tomect itwas."
Is it sufficient to regard this simply as a coincident variation ?

Some of the best cases are those in which a morbid change was
associated with the loss or injury of a particular structure. A cow
loses its left horn by suppurative inflammation ; it has subsequently
three calves in which the left horns were abortive (Thaer, 18 12).
But it may be that the original loss was due to a weakness of
germinal origin.

Prof. W. H. Brewer (1892-3) is responsible for launching a large
number of rather unseaworthy instances of modification-inheritance.
Inter alia, he tells the story of a pure-bred game-cock who lost an
eye in a fight, and transmitted his loss. While the wound was
very malignant, he was turned into a flock of game-hens of another
strain, and " a very large proportion of his progeny had the corre-
sponding eye defective." ' The chicks were not blind when hatched,
but became so before attaining their full growth. The hens after-
wards produced normal chickens with another cock."

A trustworthy correspondent writes : " My great-grandmother had
one toe broken at a dance ; all her descendants are born with one
toe bent double — my grandmother, mother, aunt, sister, and myself."
But to this almost typical story what can be said except that
congenital variations of the toes are common, and that the accident
at the dance had nothing to do with the story ?

Of great interest is the statement made by some botanists that
some peculiar effects on trees due to mites, ants, etc., are trans-
mitted. Thus Lundstrom says that the little shelters (acaro-
domatia) produced on the leaves of lime-trees, etc., by mites, may
appear when there are no mites.

But, admitting that there are some puzzling cases, we cannot
avoid the general conclusion that as regards mutilations, amputa-
tions, wounds, and deformations, the case for the affirmative is not
Strengthened by further inquiry.

230 TRANSMISSION OF ACQUIRED CHARACTERS

§ ii. Brown-Sequard! s Experiments on Guinea-Pigs

In recent discussions of modification-inheritance much pro-
minence has been given to the experiments made by Brown-
Sequard, Westphal, and others on the apparent transmission of
artificially induced epilepsy in guinea-pigs. The reason for this
prominence is that the case is not without cogency, and that
a record of precise experiments (although of a somewhat ugly
character) comes as a relief amid anecdotal evidence. Prof.
E. Ray Lankester goes the length of saying (1890, p. 375), " The
one fact which the Lamarckians can produce in their favour is
the account of experiments by Brown-Sequard, in which he pro-
duced epilepsy in guinea-pigs by section of the large nerves or
spinal cord, and in the course of which he was led to believe that
in a few rare instances the artificially produced epilepsy was
transmitted." As the case has been often discussed — e.g. by
Romanes (1895, vol. ii. chap, iv.) — we shall treat of it briefly.

What the Experiments were. — Through a long series of
years (1869-91), Dr. Brown-Sequard, a skilful and ingenious,
if somewhat impetuous, physiologist, experimented on many
thousands of guinea-pigs. He made a partial section of the
spinal cord in the dorsal region, or cut the great sciatic nerve of
the leg ; he observed that the injury was followed after some
weeks by a peculiar morbid state of the nervous system, cor-
responding in some of its features to epilepsy in man ; he allowed
these morbid animals to breed, and found that the offspring were
frequently decrepit, and that a certain number had a tendency
to the so-called epilepsy.

Results of the Experiments. — If it be understouu that we have
omitted or altered a few difficult technicalities, we may call the
following statement Brown-Sequard's summary of his results. The
inverted commas are ours :

(1) "Epileptic" symptoms appeared in the offspring of parents
who had been rendered " epileptic " by an injury to the
spinal cord.

BROWN-SEQUARD'S EXPERIMENTS 231

(2) "Epileptic" symptoms appeared in the offspring of parents

who had been rendered " epileptic " by section of the
sciatic nerve.

(3) An abnormal change in the shape of the ear was observed in

the offspring of parents in which a similar change followed
a division of the cervical sympathetic nerve.

(4) Partial closure of the eyelids was observed in the offspring of

parents in which that state of the eyelids had resulted
either from section of the cervical sympathetic nerve,
or the removal of the superior cervical ganglion.

(5) An injury to the restiform body (associated with the medulla

oblongata) was followed by a protrusion of the eya
(exophthalmia), and this reappeared in the offspring some-
times through four generations, even affecting both sides,
though the lesion in the parent had only been on one
of the corpora restiformia.

(6) An injury to the restiform body near the nib of the calamus

was followed by hematoma and dry gangrene of the
ears, and the same conditions reappeared in the
offspring.

(7) After a section of the sciatic nerve, or of the sciatic and crural,

some of the guinea-pigs gnawed off two or three of the
toes, which had become anaesthetic ; in the offspring two
or three toes were absent. Sometimes, instead of complete
absence of the toes, only a part of one or two or three was
missing in the young, although in the parent there was a
loss not only of the toes, but of the whole foot (partly eaten
off, partly destroyed by inflammation, ulceration, or
gangrene).

(8) As effects of an injury to the sciatic nerve, there followed

various morbid states of the skin and hair of the neck and
the face, and similar alterations in the same parts were
observed in the offspring.
When the sciatic nerve had been cut in the parent, the descend-
ants sometimes showed a morbid state of the nerve. There was
also a similarity in the successive appearance of the phenomena,
described by Brown-Sequard as characteristic of the periods of
development and of abatement of the " epilepsy," especially in
the appearance of the epileptogenic area and the disappearance of
hair around that area whenever the disease showed itself.

232 TRANSMISSION OF ACQUIRED CHARACTERS

Muscular atrophy of the thigh and leg followed section of the
sciatic nerve, and this was also observed in the offspring.

After cutting the restiform body one eye suffered deterioration ;
this was seen in the offspring in one eye, or even in both.

In general, the morbid conditions may affect both sides in the
parents and only one in the offspring, or vice versa, or the side
affected may be different.

One generation may be skipped, but the duration of transmission
was in some cases traced through five or even six generations.

The females seemed better able to transmit morbid states than
the males.

As to the frequency of transmission, some inherited result was ob-
served in more than two-thirds of the cases.

Brown-Sequard's results were partly confirmed by his assistants,
Westphal (1871) and Dupuy (1890), by Obersteiner (1875), and by
Romanes (1895). Dr. Leonard Hill divided the left cervical sympa-
thetic nerve in a male and a female guinea-pig, and thereby produced
a droop of the left upper eyelid. Two offspring of this pair ex-
hibited a well-marked droop of the upper eyelid. " This result is
a corroboration of the series of Brown-Sequard's experiments on the
inheritance of acquired characters."

Facts to be noted, which dispose of a Number of Criticisms. —
It is stated that the so-called " epileptic " state may also be induced
in the dog by injury to the cerebral cortex, and may, in this case also,
reappear in the offspring. If this be so it shows that we have not
to deal with a tendency peculiar to guinea-pigs.

It is stated that the " epileptic " condition does not occur spon-
taneously— i.e. apart from injury to the nervous system — in guinea-
pigs. Therefore the interpretation of the apparent inheritance as
being due to a fresh variation which happened by coincidence to
resemble the parental state, is inadmissible.

As the tendency to " epileptic " fits (which do not last long) was
seen only in the offspring of animals which had been operated on,
and was manifested only after appropriate stimulus, especially after
irritating an " epileptogenic " zone behind the ear on the same side
as the original injury, we must pass by Galton's suggestion (1875)
of the possibility of reappearance through imitation. Even if it
be allowed that there is a certain infectiousness in " fits," this would
not apply to the loss of toes, the diseased state of the ear, the pro-
truding eyes, and so on.

BROWN-S&QUARUS EXPERIMENTS 233

It is stated that the morbid condition of the parents was also in-
duced by bruising the sciatic nerve without cutting the skin, or by
striking the animals on the head with a hammer. If this be so it
seems to show that the result may occur without any associated
microbe influence, and possible infection of the offspring thereby
(Weismann's criticism, in part). The hypothesis of microbes does
not seem to be supported by any definite facts, but we note that it
is not entirely excluded by Ziegler in his review of possible ex-
planations (1886, p. 29).

Brown-Sequard experimented with both males and females, and
although he got more striking results with the latter, he did not fail
with the former. This seems to lessen the force of the criticism that
the offspring were affected during gestation, and therefore not, in
the strict sense, hereditarily.

Criticisms. — (1) The original modification was cutting, bruising,
or destroying part of the nervous system ; the subsequent result was
the " epileptic " state, and the various other diseased conditions
mentioned. It need hardly be said that the mutilation or injury
inflicted on the parent was never reproduced in the offspring, though
the subsequent results sometimes were.

(2) The conditions exhibited by the offspring were very diverse —
general feebleness, motor paralysis of the limbs, trophic paralysis
resulting in loss of toes, cornea, etc., other nervous and sensory dis-
orders, and in some cases the particular " epileptic " state. In a
number of cases the condition of the offspring was so different from
that of the parent, that the only common feature was that in both
cases there were abnormal neuroses. Romanes, while regarding his
results as corroborations of those of Brown-Sequard, admitted that
the epileptic condition was only rarely transmitted.

(3) Even numerically there was no small diversity in the results.
Thus in one set of experiments (Obersteiner, 1875), out of thirty-two
young ones born of " epileptic " parents, only two showed symptoms
of " epilepsy " and paralysis, three were paralytic, and eleven were
only weak. Romanes did not find that any of the offspring of parents
who had eaten their toes off showed, even in six generations, any
defect in these parts. Even Brown-Sequard only observed this
peculiar " transmission " in about 1 or 2 per cent, of cases.

(4) Prof. Ziegler 's criticism is partly based on the allegation
that guinea-pigs (as we keep them in captivity) are pathological and
nervous animals, very readily thrown into an epileptic state. On

234 TRANSMISSION OF ACQUIRED CHARACTERS

making a slight cut in the skin, on the occasion of a small operation
on the neck, Ziegler sent an apparently healthy guinea-pig into a
severe epileptic fit. But there seems considerable difference of
opinion as to this nervousness of captive guinea-pigs.

(5) It seems to us that the original modification was too violent
to afford satisfactory data in connection with the present discussion.
No matter how neatly the operations were effected, the partial sec-
tion of the spinal cord, the cutting of the sciatic or of the cervical
sympathetic nerve, the removal of the superior cervical ganglion,
the injuring of the restiform body, imply very serious injuries, and
it is hard to believe that others were not implied in some of the ex-
periments— e.g. on the restiform body. But if a modification is
violent it may disturb the whole organism, nutritive * and repro-
ductive f functions alike, and it may naturally lead to abnormality
in the offspring. Especially may it lead to general decrepitude,
which, it seems to us, was the most frequent result. At the same
time this hardly touches the most distinctive feature of the ex-
periments, that sometimes there appeared in the offspring morbid
conditions precisely similar to the results of the injury inflicted on
the parents. It may be, however, that only particular parts of the
body are susceptible to the influence of the original disturbance.

Prof. T. H. Morgan (1903, p. 257) directs attention to the experi-
ments of Charrin, Delamare, and Moussu, which have an interesting
bearing on some of Brown-Sequard's results. After the operation
of laparotomy on a pregnant rabbit or guinea-pig, the kidney or the
liver became diseased, and the offspring showed similar affections.
The experimenters suggested that some substance set free from the
diseased kidney of the mother affected the kidney of the young in
the uterus. "May not, therefore, Brown-Sequard's results be also
explained as due to direct transmission from the organs of the parent
to the similar organs of the young in the uterus ? " But this would
not be inheritance in the strict sense. It should be noted, however,
that what has been just said does not of course apply to those cases
in which Brown-Sequard experimented on the male parent. Charrin
maintains on experimental grounds that " cytotoxins " ma}' pass
not only from the mother to the foetus, but from either parent to
its germ-cells — ova or spermatozoa (see Revue generate des Sciences,

* Dupuy, while confirming Brown-Sequard, laid emphasis on the altera
tions of nutrition after the experiments.

■f Sommer notes a diminution of fertility after the experiments.

BROWN-SEQUARDS EXPERIMENTS 235

Jan. 15, 1896). Moreover, Voisin and Peron have found evidence
that in epilepsy a toxin is produced which causes convulsions
when injected into animals (see Archives de Neurologie, xxiv.,
1892, and xxv., 1893, and Voisin's L'Epilcpsie, Paris, 1897, pp. 125-
133). It is thus not a mere speculation to suppose that a toxin
was produced in the guinea-pig epilepsy, and that this affected the
germ-cells of both sexes. This suggestion is made by Prof. Bergson
in his remarkable book L' Evolution Creatrice (1907), and he adds
to the suggestion the query, May not something of the same sort
be true in those cases where acquired peculiarities are transmitted ?

Prof. T. H. Morgan (1903, p. 255) also notes an interesting
fact. " While carrying out some experiments in telegony with mice,
I found in one litter of mice that when the young came out of the
nest they were tail-less. The same thing happened again when the
second litter was produced, but this time I made my observations
sooner, and examined the young mice immediately after birth. I
found that the mother had bitten off, and presumably eaten, the
tails of her offspring at the time of birth. Had I been carrying on
a series of experiments to see if, when the tails of the parents were
cut off, the young inherited the defect, I might have been led into
the error of supposing that I had found such a case in these mice.
If this idiosyncrasy of the mother had reappeared in any of her
descendants, the tails might have disappeared in succeeding genera-
tions. This perversion of the maternal instincts is not difficult to
understand, when we recall that the female mouse bites off the
navel-string of each of her young as they are born, and at the same
time eats the after-birth. Her instinct was carried further in this
case, and the projecting tail was also removed.

"Is it not possible that something of this sort took place in
Brown-Sequard's experiment ? The fact that the adults had eaten
off their own feet might be brought forward to indicate the possi-
bility of a perverted instinct in this case also." On the other hand,
this interpretation cannot apply to some other results which Brown-
Sequard observed.

Sommer's Experiments far from corroborating Broivn-Seqnard's. —
In experiments the results of which were published in 1900, Max
Sommer repeated some of those which Brown-Sequard and others
had made, but without corroborating them.

The so-called " epilepsy " was induced by cutting the sciatic nerve
on one side or on both sides ; the tendency to " fits " occurred some

236 TRANSMISSION OF ACQUIRED CHARACTERS

days or some weeks after the operation ; they were brought on by rub-
bing particular areas of the body (the epileptogenic zones) ; whether
they ever occurred spontaneously remained doubtful, since any
friction on the appropriate spots — e.g. when the animal scratched
itself — served to bring them on. After some months the tendency
to the attacks disappeared, and irritation of the appropriate areas
was followed by only a slight fit or by none. (This is a noteworthy
fact.)

The fertility of the " epileptic " guinea-pigs was lessened.

Twenty-three young ones were reared (a small number compared
with those in Brown-Sequard's experiments) — six from two pairs
in which the father was " epileptic," six from four pairs in which the
mother was " epileptic," and seven from five pairs in which both
parents were " epileptic." In no case did " epilepsy " appear in the
offspring. Even paralysis of one or more of the extremities was not
demonstrated, though most carefully looked for.

In the parents there were several defects in the toes or ulcerations
of the hind extremities, but in no case was there reappearance of the
dejects or ulcerations in the offspring.

Two of the young were decrepit, and in one there was a clouding
of the cornea ; but there is no warrant for associating this directly
with the " epilepsy " of the parents.

Sommer's conclusion is as follows : "As regards the hereditary
transmission of epilepsy in guinea-pigs, or of other accidentally
acquired pathological symptoms — e.g. defects in the toes — we have
obtained an absolutely negative result ; we have not been able to
confirm the experiments of Brown-Sequard and Obersteiner ; and
we do not think that these can any longer serve as a support to the
doctrine of the inheritance of acquired characters." *

Before leaving the subject of these disagreeable experiments
we may be permitted to express our opinion that, altogether
apart from convictions as to the ethical limits of scientific inquiry,
a sound biology is not likely to gain much from experiments the
conditions of which are so utterly different from those occurring
m the state of nature. It seems to us that they are entirely

* Sommer also points out that the guinea-pig's " epilepsy " does not
correspond to true epilepsy in man, but rather to the so-called reflex
epilepsy which follows from peripheral nerve-injuries.

BROWN-S&QUARDS EXPERIMENTS 237

different from experiments on decapitated earthworms, curtailed
lizards, crabs with lost limbs, and the like, for there the investi-
gator is in touch with injuries which frequently occur in natural
conditions.

The case is certainly a difficult one, but from what we have
said it must be evident that it cannot be cited without qualifica-
tion in support of the thesis that somatic modifications are
transmissible. It is illegitimate to conclude, as Debierre does
(1897, p. 4) : 'II est done incontestable que des caracteres
acquis artificiellement pendant l'age adulte de l'animal ou
acquis naturellement pendant la vie embryonnaire peuvent etre
transmis par l'heredite."

Our general conclusion is that the results of Brown-Sequard's
experiments do not strengthen the affirmative position ; and
that their probable interpretation is that the artificially induced
epilepsy liberated a toxin which affected the germ-cells in some
cases, the germ-cells and the foetus in other cases.

§ 12. Negative Evidence in favour of the Affirmative Answer

In support of the affirmative answer Herbert Spencer ad-
duced what he called negative evidence — namely, those " cases in
which traits otherwise inexplicable are explained if the structural
effects of use and disuse are transmitted."

(1) First he referred to the co-adaptation of co-operative parts,
With the enormous antlers of a stag there is associated a large num-
ber of co-adaptations of different parts of the body, and similarly
with the giraffe's long neck and the kangaroo's power of leaping.
Spencer argued that the co-adaptation of numerous parts cannot
have been effected by natural selection ; but that it might be effected
by the hereditary accumulation of the results of use.

It must be admitted that co-adaptations are difficult to account
for in terms of the ordinary selection formula, but it is also difficult
to accept the use-inheritance interpretation. We do not really
know to what extent deep-seated co-adjustment can be effected by

238 TRANSMISSION OF ACQUIRED CHARACTERS

exercise even in the course of a long time, and the theory requires
such data before it can be more than a plausible interpretation, with
certain a priori difficulties against it.

Another interpretation may be suggested. If an animal suddenly
takes to leaping, many individual adjustments to the new exercise
may arise ; if the animals of successive generations leap yet more
freely, they may individually acquire more thorough adjustments
Meanwhile there may arise constitutional variations making towards
adaptation to the new habit, and under the screen of the individual
modifications these may increase from minute beginnings till they
acquire selection-value (Mark Baldwin, Lloyd Morgan, and Osborn).
Nor should it be forgotten that variations in different parts of the
body are often correlated. The subsidiary theory of germinal selec-
tion is also helpful. Finally, it is possible that in some of these cases
the result was not due to the gradual accumulation of minute varia-
tions, but was originated by one of those sudden discontinuous
changes which are now called mutations.

(2) Secondly, Spencer dwelt upon the notably diverse powers of
tactile discrimination possessed by the human skin, and sought to
show that while these could not be interpreted on the hypothesis
of natural selection or on the correlated hypothesis of panmixia, they
could be interpreted readily if the effects of use were inherited. But
the difficulty again is to get secure data. It is uncertain how much
of the inequality in tactile sensitiveness is due to individual exercise
and experience, though it is certain that tactility in little-used parts
can be greatly increased by use. Nor is it certain how much of the
apparent unlikeness in tactility is due to unequal distribution of
peripheral nerve-endings and how much to specialised application
of the power of central perception. As Prof. Lloyd Morgan says :
" We do not yet know the limits within which education and prac-
tice may refine the application of central powers of discrimination
within little-used areas. The facts which Mr. Spencer adduces may
be in a large degree due to individual experience, discrimination
being continually exercised in the tongue and finger-tips, but seldom
on the back or breast. We need a broader basis of assured fact."
Nor, it may be added, is the action of selection to be excluded.

(3) Spencer's third set of negative evidences was based on rudi-
mentary organs which, like the hind limbs of the whale, have nearly
disappeared. Dwindling by natural selection is here out of the ques-
tion ; and dwindling by panmixia— i.e. the diminution of a structure

LOGICAL POSITION OF THE ARGUMENT 239

when natural selection ceases to affect its degree of development — ■
" would be incredible, even were the assumptions of the theory
valid." But as a sequence of disuse the change is clearly explained.
Prof. Lloyd Morgan replies : "Is there any evidence that a structure
really dwindles through disuse in the course of individual life ? Let
us be sure of this before we accept the argument that vestigial organs
afford evidence that this supposed dwindling is inherited. The
assertion may be hazarded that, in the individual life, what the evi-
dence shows is that, without due use, an organ does not reach its full
functional or structural development. If this be so the question
follows : How is the mere absence of full development in the indi-
vidual converted through heredity into a positive dwindling of the
organ in question ? " Moreover, the convinced Neo-Darwinian is
not in the least prepared to abandon the theory of dwindling in the
course of panmixia, especially in the light which Weismann's con-
ception of germinal selection has thrown on this process.

§ 13. The Logical Position of the Argument

Before we state what appears to us at present the inevitable
conclusion, it may be useful to indicate briefly the logical position
of the argument.

Weismann has pointed out that there are two possible methods
by which the affirmative position — that modifications are trans-
missible— might be established. In the first place, there might
be actual experimental proof of such transmission ; in the second
place, there might be a collection of facts which cannot be in-
terpreted without the hypothesis of modification-inheritance.

Experiment. — The experimental method has not been followed
as often as might have been expected, and where it has been
followed the results are far from conclusive. But it is important
to remember that although a few good cases of the inheritance
of an acquired character would prove the possibility of such in-
heritance, hundreds of failures to demonstrate the transmission
experimentally do not prove that it is impossible.

The Neo-Lamarckian believes that when new conditions of life

24o TRANSMISSION OF ACQUIRED CHARACTERS

operate in an approximately similar way for many generations,
they will produce definite and slowly cumulative effects upon
the organisms subjected to them. He is by no means com-
mitted to the belief that every change of conditions will produce
appreciable hereditary effects in a few generations. The point
is not whether modifications are fully and completely transmitted,
but whether any trace, of them may be transmitted. Still less
is the Neo-Lamarckian bound to admit that any given change of
conditions, more or less arbitrarily selected by any one as being
convenient for experimental purposes, will produce recognisable
results in the following generation. Thus the fact that most of
the experimental results are inconclusive or negative does not
disprove the Lamarckian belief.

Interpretation. — As to the second method, that of the in-
terpretation of facts, it cannot be very conclusive either, since
both sides have to prove a negative in order to establish their
case. The Neo-Lamarckians have to show that the phenomena
they adduce as illustrations of modification-inheritance cannot
be interpreted as the results of selection operating on germinal
variations. In order to do this to the satisfaction of the other
side, the Neo-Lamarckians must prove that the characters in
question are outside the scope of natural selection, that they are
non-utilitarian and not correlated vath any useful characters —
a manifestly difficult task. The Neo-Darwmians, on the other
hand, have to prove that the phenomena in question cannot be
the results of modification-inheritance. And this is in most
cases impossible. Thus we seem to reach a logical dead-lock.

Cases where the Theory of Modification-Inheritance is inap-
plicable.— It is true, however, that there are certain characters
of certain organisms, in regard to which it may be said with some
security that they could not have arisen by the inheritance
of acquired characters. Thus many insects and the like have
adaptive characters in their cuticular structures — knobs suited
for crushing, saws suited for cutting, gimlets suited for boring,

LOGICAL POSITION OF THE ARGUMENT 241

and so on. But these cuticular structures are non-cellular, non-
living parts of the external investment of the body ; they are
made and re-made (after moulting), by the underlying living
skin. How then can they be interpreted in terms of modifica-
tion-inheritance ? The matter becomes even more difficult
when we consider cases in which the adaptiveness is in the colour
or markings of these inert cuticular parts. Weismann has
argued that, since there are some adaptive characters which can-
not be interpreted in terms of modification-inheritance, this
hypothetical factor need not be assumed in attempting to in-
terpret the origin of other adaptations, similar to the former,
except that the factor in question is not by the nature of the
case apparently excluded from having any connection with
them.

But it cannot be said that this application of the " law of par-
simony " is altogether successful. It may recoil on those who
use it. It might be argued that there are some adaptive charac-
ters which cannot be readily interpreted in terms of natural
selection (as is implied in the appeal of some Neo-Darwinians
to " intra-selection," " germinal selection," and so on), and that
therefore natural selection cannot be regarded as a generally
acting factor. Moreover, the Neo-Lamarckian is at liberty to
reply, that he does not regard the modification-inheritance
theory as applicable to all possible cases.

Antecedent Probabilities. — If we turn to the antecedent
probabilities of the two beliefs, we find that the assumptions of
either side are equally improbable to the other, according to
their respective points of view. Thus, the supporters of the
negative answer may say that they cannot conceive how a par-
ticular local modification of the body can so affect the germ-cells
that, when these develop into offspring, the acquired character
shall re-appear. The supporters of the affirmative answer may
say that they find it impossible to believe in the selectionist in-
terpretation of many of the adaptive characters which make up

16

24 2 TRANSMISSION OF ACQUIRED CHARACTERS

an organism, impossible to believe that the little items of im-
provement which are added generation after generation — say
in a cricket's musical instrument — can have had selection-value.
There are other difficulties on both sides, and it is likely to remain
for a long time a matter of opinion which side has the greater
difficulties to face.

A Matter of Fact. — It is plain, however, that what we have to
ask is whether interpretations in terms of modification-inheritance
have any basis in present-day experience, such as selectionist
interpretations have, for instance, in domestication on the one
hand and variation-statistics on the other. And our survey
seems to indicate that it is very difficult to find any empirical
basis whatsoever for the affirmative position.

If modification-inheritance were known to be a fact it would
in nowise exclude interpretations in terms of natural selection
and other factors, for even the most thorough-going Neo-
Lamarckian will hardly maintain that his hypothesis, if verified,
would be an all-sufficient etiological factor, and even the most
convinced Neo-Danvinians could not refuse to recognise an ad-
ditional factor if that were verifiable. There is no need to pit
one theory against the other in this fashion ; the more factors in
evolution that are discovered the better !

The question resolves itself into a matter of fact : Have we
any concrete evidence to warrant us believing that definite
modifications are ever, as such or in any representative degree,
transmitted ? It appears to us that we have not. But to say
dogmatically that such transmission is impossible is unscientific.
In regard to that, the truly scientific position is one of active
scepticism (thdtige Skepsis).

§ 14. Indirect Importance of Modifications

Importance of Nurture. — Scepticism as to the transmission
of acquired characters does not imply that we under-rate the
importance of " nurture." We have seen (1) that an appro-

INDIRECT IMPORTANCE OF MODIFICATIONS 243

priate environment is the necessary correlate of a normal in-
heritance, otherwise the organism cannot realise itself in de-
velopment ; (2) that changes in environment and function may
provoke variations in the germ-plasm ; (3) that the individual
is often very plastic and readily acquires adaptive modifications
which may be of great individual importance, and may even
preserve the life ; (4) that the secondary effects of modifica-
tions may, in certain cases, reach and influence the germ-cells ;
(5) that the state of the maternal constitution is very important
in cases where there is an intimate connection between the mother
and the unborn young.

Selection and Stimulus. — In two other ways changes in the
conditions of life are of great importance : they form part of
the mechanism of selection, whereby the relatively less fit
variants are quickly or slowly, roughly or gently, eliminated ;
and they act as a stimulus to the intrinsic self-assertiveness and
" endeavour after well-being " which characterise living crea-
tures. We must advance beyond the conventional view that
the environment is like a net closing in upon passive victims,
which can only escape if they have been fitted by germinal varia-
tion (or acquired modification) to pass through some of the
meshes ; we must recognise as a fact of life, what Lamarck and
many others have seen with clearness, that organisms actively
assert themselves against this closing net, and by active en-
deavour (also, of course, a variational character when traced
back) may win their way through.

Indirect Importance of Modifications. — But there is another
important consideration, which has been stated independently
by Profs. Mark Baldwin, Lloyd Morgan, and H. F. Osborn —
namely, that adaptive modifications may act as the fostering
nurses of germinal variations in the same direction. We have
referred to this elsewhere, but it may give greater completeness
to our survey if we quote a brief statement of the idea as ex-
pounded by Lloyd Morgan {Habit and Instinct, 1896, p. 319) :

244 TRANSMISSION OF ACQUIRED CHARACTERS

" Persistent modification through many generations, though not
transmitted to the germ, nevertheless affords the opportunity for
germinal variation of like nature.

" Suppose that a group of plastic organisms is placed under new
conditions. Those whose innate plasticity is equal to the occasion
are modified and survive. Those whose plasticity is not equal to
the occasion are eliminated. . . . Such modification takes place
generation after generation, hut, as such, is not inherited. . . . But
any congenital variations similar in direction to these modifications
will tend to support them and to favour the organism in which they
occur. Thus will arise a congenital predisposition to the modifica-
tions in question.

" The plasticity still continuing, the modifications become yet
further adaptive. Thus plastic modification leads, and germinal
variation follows ; the one paves the way for the other.

" The modification as such is not inherited, but is the condition
under which congenital variations are favoured and given time to
get a hold on the organism, and are thus enabled by degrees to reach
the fully adaptive level."

§ 15. Practical Considerations

We have seen that the scientific position in regard to the
transmissibility of modifications should be one of active scep-
ticism, that there seems to be no convincing evidence in support
of the affirmative position, and that there is strong presumption
in favour of the negative.

A modification is a definite change in the individual body,
due to some change in " nurture." There is no secure evidence
that any such individual gain or loss can be transmitted as such,
or in any representative degree. How does this affect our esti-
mate of the value of "nurture " ? How should the sceptical or
negative answer, which we believe to be the scientific one, affect
our practice in regard to education, physical culture, ameliora-
tion of function, improvement of environment, and so on ? Let
us give a practical point to what we have already said.

(a) Every inheritance requires an appropriate nurture if it is

PRACTICAL CONSIDERATIONS 245

to realise itself in development. Nurture supplies the liberating
stimuli necessary for the full expression of the inheritance. A
man's character as well as his physique is a function of " nature "
and of " nurture." In the language of the old parable of the
talents, what is given must be traded with. A boy may be truly
enough a chip of the old block, but how far he shows himself such
depends on " nurture." The conditions of nurture determine
whether the expression of the inheritance is to be full or partial
It need hardly be said that the strength of an (inherited) indi-
viduality may be such that it expresses itself almost in the face
of inappropriate nurture. History abounds in instances. As
Goethe said, Man is always achieving the impossible. Corot was
the son of a successful milliner and a prosperous tradesman, and
he was thirty before he left the draper's shop to study nature.

(b) Although modifications do not seem to be transmitted as
such, or in any representative degree, there is no doubt that they
or their secondary results may in some cases affect the offspring.
This is especially the case in typical mammals, where there is
before birth a prolonged (placental) connection between the
mother and the unborn young. In such cases the offspring is for
a time almost part of the maternal body, and liable to be affected
by modifications thereof— e.g. by good or bad nutritive conditions.

There is considerable evidence that the mammalian mother passes
on the surplus nourishment to the foetus, and that the size of the
offspring in mankind depends very directly on the diet and nutrition
of the mother during pregnancy. (See Noel Paton, 1903.)

In other cases, also, it may be that deeply saturating parental
modifications, such as the results of alcoholic and other poisoning,
affect the germ-cells, and thus the offspring. A disease may saturate
the body with toxins and waste-products, and these may provoke
prejudicial germinal variations.

(c) Though modifications due to changed " nurture " do not
seem to be transmissible, they may be re-impressed on each
generation. Thus " nurture " becomes not less, but more, im-
portant in our eyes.

246 TRANSMISSION OF ACQUIRED CHARACTERS

" Is my grandfather's environment not my heredity ? " asks
an American author quaintly and pathetically. Well, if not, let
us secure for ourselves and for our children those factors in the
" grandfather's environment " that made for progressive evolu-
tion, and eschew those that tended elsewhere.

" Was du ererbt von deinen Vatern hast
Erwerb cs, um es zu besitzen."

Are modifications due to changed nurture not, as such, entailed
on offspring ? Perhaps it is just as well, for we are novices at
nurturing even yet ! Moreover, the non-transmissibility cuts
both ways : if individual modificational gains are not handed on,
neither are the losses.

Is the " nature " — the germinal constitution, to wit — all that
passes from generation to generation, the capital sum without
the results of individual usury ; then we are freed, at least, from
undue pessimism at the thought of the many harmful functions
and environments that disfigure our civilisation. Many detri-
mental acquired characters are to be seen all around us, but if
they are not transmissible, they need not last.

(d) The plasticity of the organism admits of definite modi-
fications being re-impressed on successive generations of indi-
viduals, and this is the more important when we consider what
has been said in the section on "The Indirect Importance of
Modifications." They may serve as modificational screens until
coincident variations in the same direction can emerge and
establish themselves. This also cuts both ways in human
societies, where natural selection is interfered with, and where
naturally prejudicial deviations from the norm are not neces-
sarily punished by elimination.

(e) Of particular importance is the fact that man, in contrast
to other creatures, has developed around him an external heritage,
a social framework of customs and traditions, of laws and in-
stitutions, of literature and art — by which results almost equiva-

INHERITANCE OF MORAL CHARACTER 247

lent to the organic transmission of certain kinds of modifications
may be brought about.

(/) Is there not some result of the long-drawn-out controversy
on " the inheritance of acquired characters," if we are thereby
freed from indulging in false hopes, but are forced to the convic-
tion that " nurture " is more important than ever ? Although
what is " acquired " may not be inherited, what is not inherited
may be acquired. Thus we are led to direct our energies even
more strenuously to the business of re-impressing desirable
modifications, and therefore to developing our functions and
environments in the direction of progress.

It may be, however, that our methods must change with the
change in our expectations. For though we can by modification
directly influence the individual, and in some measure even
control the expression of his inheritance, it is not through modi-
fications that we can hope directly to influence posterity. Man
is a slowly reproducing, slowly varying organism. What is
above all precious is the conservation of good stock. No number
of veneering modifications — superficial screens of organic defects
—can atone for allowing a deterioration of the germinal in-
heritance to diffuse itself or accumulate. For progress which is
really organic — for progress, that is, in our natural inheritance —
we must wait, or rather work, patiently. The quest after
Eutopias and Eutechnics must be associated with an enthusiasm
for Eugenics.

Inheritance of Moral Character. — In the development of
" character," much depends upon early nurture, education, and
surrounding influences generally, but how the individual reacts
to these must largely depend on his inheritance. Truly the
individual himself makes his own character, but he does so
by his habitual adjustment of his (hereditarily determined)
constitution to surrounding influences. Nurture supplies the
stimulus for the expression of the moral inheritance, and how
far the inheritance can express itself is limited by the nurture-

248 TRANSMISSION OF ACQUIRED CHARACTERS

stimuli available just as surely as the result of nurture is con-
ditioned by the hereditarily-determined nature on which it
operates. It may be urged that character, being a product of
habitual modes of feeling, thinking, and acting, cannot be spoken
of as inherited, but bodily character is also a product dependent
upon vital experience. It seems to us as idle to deny that some
children are " born good " or " born bad," as it is to deny that
some children are born strong and others weak, some energetic
and others " tired " or " old." It may be difficult to tell how
far the apparently hereditary goodness or badness of disposition
is due to the nutritive influences of the mother, both before and
after birth, and we must leave it to the reader's experience and
observation to decide whether we are right or wrong in our
opinion that quite apart from maternal nutritive influence there
is a genuine inheritance of kindly dispositions, strong sympathy,
good-humour, and good-will. The further difficulty that the
really organic character may be half-concealed by nurture-
effects, or inhibited by the external heritage of custom and
tradition, seems less serious, for the selfishness of an acquired
altruism is as familiar as honour among thieves.

It is entirely useless to boggle over the difficulty that we are
unable to conceive how dispositions for good or ill lie implicit
within the protoplasmic unit in which the individual life begins.
The fact is undoubted that the initiatives of moral character are
in some degree transmissible, though from the nature of the case\
the influences of education, example, environment, and the like
are here more potent than in regard to structural features. We '
cannot make a silk purse out of a sow's ear, though the plasticity
of character under nurture is a fact which gives us all hope.
Explain it we cannot, but the transmission of the raw material
of character is a fact, and we must still say with Sir Thomas
Browne : " Bless not thyself that thou wert born in Athens ;
but, among thy multiplied acknowledgments, lift up one hand
to heaven that thou wert born of honest parents, that modesty,

CONCLUSION 249

humility, and veracity lay in the same egg, and came into the
world with thee."

The study of inheritance leaves a fatalistic impression in many
minds, and to some extent this is justified. We cannot get away
from our inheritance. As the poet Heine said half bitterly, half
laughingly, " A man should be very careful in the selection of his
parents." On the other hand, although the organism changes
slowly in its heritable organisation, it is very modifiable indi-
vidually ; and this is man's particular secret — to correct his
internal organic inheritance by what we may call his external
heritage of material and spiritual influences.

CONCLUSION

If there is little or no scientific warrant for our being other
than extremely sceptical at present as to the inheritance of
acquired characters — or better, the transmission of modifications —
this scepticism lends greater importance than ever, on the one hand,
to a good " nature," to secure which is the business of careful mating ;
and, on the other hand, to a good " nurture," to secure which for our
children is one of our most obvious and binding duties : the hope-
fulness of the task resting especially upon the fact that, unlike the
beasts that perish, man has a lasting external heritage, capable of
endless modification for the better, a heritage of ideas and ideals,
embodied in prose and verse, in statue and painting, in cathedral
and university, in tradition and convention, and above all in
society itself.

CHAPTER VIII

HEREDITY AND DISEASE
" Naturam expellas furca, tamen usque recurbet." — Horace.

§ I. Health and Disease.

§ 2. Misunderstandings in regard to the "Inheritance" of
Disease.

§ 3. Are Acquired Diseases transmissible?

§ 4. Can a Disease be transmitted ?

§ 5. Predispositions to Disease.

§ 6. Particular Cases.

§ 7. Defects, Multiplicities, Malformations, and other Ab-
normalities.

§ 8. Some Provisional Propositions.

§ 9. Immunity.
§ 10. Note on Chromosomes in Man.
§ 11. Anticipation and Intensification of Disease.
§ 12. Practical Considerations.

§ 1. Health and Disease

What is Disease ? — The distinction between health and disease
is relative to an ideal — the maximum efficiency and well-being of
the organism under given conditions ; and pathology, the science
of deranged function or disturbed metabolism — deranged or dis-
turbed in comparison with what we call " normal " — is, strictly
speaking, part of physiology, the science of all vital activity.
What we call " normal " in one animal — e.g. a bird's mode of ex-
cretion— is called " diseased " in another ; what is normal at one

250

WHAT IS DISEASE ? 251

period of life — e.g. the breaking down of tissue in a chrysalid —
may be a disease at another period ; what is normal in one
part of the body — e.g. proliferation of cells — may be a morbid
growth in another region. Disease is a relative concept and
does not admit of strict definition.

Our point here is indeed a familiar one, for the tritest of
quotations remind us of the kinship between genius and madness,
or of the resemblance between the lunatic, the lover, and the
poet. As a matter of fact, Ziegler remarks, genius, talent, and
mental derangement do sometimes occur in one family. The
useful glutinous threads of mucus with which the male stickleback
fastens together his nest of seaweed are remarkable renal secre-
tions which, if we did not know their utility, would almost
certainly be regarded as the symptoms of a kidney disease.
Whether we take the changes in the adult salmon when
fasting in freshwater, or the dissolution of the blowfly's maggot
as it passes into the pupa state, or the condition of the tadpole
as it loses its tail and becomes a miniature frog, or the necrosis
at the base of a stag's antlers before they fall off, we have to
deal with processes which, though now normal occurrences in
the cases cited, would in other cases spell disease.

A great authority puts the point tersely : " Disease is a
state of a living organism, a balance of function more unstable
than that which we call ' health ' ; its causes may be imported,
or the system may ' rock ' from some implicit defect, but
the disease itself is a perturbation which contains no elements
essentially different from those of health, but elements pre-
sented in a different and less useful order " (T. Clifford Allbutt,
System of Medicine, 1896, vol. i. p. xxxii).

Optimism of Pathology. — It does not seem possible to find
any criterion which will serve in all cases to differentiate a
new variation making for increased efficiency from another
which makes for disease. Experience lends security to the
judgment of the physician or the breeder in a large number of

25 2 HEREDITY AND DISEASE

cases, but it is probable, as Virchow has maintained, that some
new beginnings which are now — looking backward— regarded
as normal steps in progressive evolution would at the outset
have been claimed by the pathologist as hints of fresh disease.
Leaving microbic and acquired diseases out of account, we
may safely say that various processes of hypertrophy and
atrophy which are associated with disease in a well-finished
organism like man are, as it were, recrudescences of important
steps in past evolution. The persistence of germinal activity
in a patch of cells may give rise to a tumour, but is it not, as
it were, an echo of the power that lower animals have of regener-
ating lost parts ? So it may be that some of the cerebral varia-
tions which we call for convenience "nervous diseases" are
attempts at progress.

Diseases due to Innate Predispositions and to Acquired
Modifications. — From the biologist's point of view diseases are
of two sorts : (i) they are abnormal or deranged processes,
which have their roots in germinal peculiarities or defects
(variations, to start with), which express themselves in the
body to a greater or less degree according to the conditions
of nurture ; or (2) they are abnormal or deranged processes
which have been directly induced in the body by acquired
modifications — i.e. as the results of unnatural surroundings or
habits, including the intrusion of parasites. Often, moreover,
an inborn predisposition to some deranged function may be
exaggerated by extrinsic stimuli, as in the case of gout,* or when
a phthisical tendency is aggravated by the intrusion and
multiplication of the tubercle bacillus. That is to say, deranged
processes which are primarily due to germinal variation often
afford opportunity for equally serious disturbances which
must be referred to exogenous modifications. A rheumatic
tendency may be fatally aggravated by inappropriate nutrition.

* It is now suggested, however, that gout is due to the toxic effect of
some germ or germs.

DISEASE IN ANIMALS AND IN MAN 253

Disease more Frequent in Man than in Animals. — Diseases
occur among wild animals, but, so far as we can judge, they are
very rare. They are certainly rare when compared with the
frequent diseases of mankind. Why is this ? One reason,
probably, is that natural selection has a grip on wild life that
man has refused to allow it to have over him. Elimination is
keener and the wild race is healthier. Animals born diseased
are killed off before they can reproduce. To parasites they
adjust themselves, or become immune. Another reason is that
wild animals live " more natural " lives, and that the stimuli
provoking disease are therefore fewer. A third reason, perhaps,
is that man is relatively younger than most wild races, and,
therefore, with more idiosyncrasies. Fourthly, it seems that
where epidemics occur among wild animals, they are almost
invariably due to human interference. (See Ray Lankester's
Kingdom of Man, 1907, p. 32.)

It should also be recognised that man has created around
himself a social heritage which often evolves quickly, hurrying
and pressing its creator, who cannot always keep pace with
it. This is a frequent condition of mental disorder. More
generally, we may venture to say that many human diseases,
especially of a nervous sort, seem in part due to the fact that
the germ-plasm is not varying quickly enough to keep pace with
the changes in environment — physical, biological, psychical, and
social. We try to adjust ourselves to these by a panoply of
modifications, and this business of adjustment is a strain
that provokes disease.

As the physiological and the pathological are really but
two aspects of the general problem of vital activity, it is mainly
for practical reasons that we have ventured to devote a special
chapter to the facts of inheritance in connection with disease.
Apart from practical interests, it will be seen that, though the
available facts in regard to disease do not lead us to any
novel considerations which are not illustrated in normal cases,

254 HEREDITY AND DISEASE

they throw some useful side-lights on the general problems of
heredity.

§ 2. Misunderstandings in regard to the " Inheritance "

of Disease

As with the transmissibility of acquired characters, so with
the transmissibility of the ills our flesh is heir to, we have to
face a number of current misunderstandings, which in many
cases obscure the real facts. The long series of transmissible
diseased conditions which Prosper Lucas, for instance, gave
in 1847, wm" not Pass muster to-day. It includes many cases
which are outside the rubric of inheritance altogether. A more
critical study, particularly of recent years, has led physicians as
well as biologists to define a number of distinctions between
real and apparent inheritance. Thus, to take a simple instance,
it seems a confusion of thought to speak of the inheritance of
any microbic disease.

1. Reappearance not equivalent to Inheritance — The
reappearance of a diseased condition in successive generations
does not prove that it has been transmitted, or even that it is
transmissible. The Alpine plants which Nageli brought to the
botanical garden at Munich were much modified in their new
environment, and their descendants were similarly modified.
The unusual characters reappeared generation after generation,
but experiment showed that the reappearance was not due
to inheritance, but was due to the re-impression of similar
modifications on each successive crop. So it is with many
diseased states which reappear generation after generation,
not because they have been transmitted, but because of the
persistence of the unhealthy stimuli in function or in environ-
ment which originally evoked them. Collier's lung is a modi-
ficational result ; it reappears in generations of colliers, but
there is no warrant for regarding it as heritable.

PRE-NATAL INFECTION 25$

2. Pre-natal Infection is not Inheritance. — Even when a
child is born with symptoms or definite expressions of a disease
which one or both of its parents exhibited, it does not follow
that the disease was part of the inheritance. If the disease is
microbic, it is never in the strict sense inherited. It may be
acquired by infection through the mother during the foetal
period. This may be illustrated by the rather rare occurrence
of congenital tuberculosis and by some cases of congenital
syphilis. No one who thinks clearly can maintain that these
diseases are in the strict sense heritable.

The unborn offspring may be directly inoculated in utero
with the germs of certain contagious diseases affecting the mother,
and this in spite of the fact that the placenta is a wonderfully
perfect filter. " Diseases of the contagious type seem to differ
in the facility with which they are transmitted by this means.
Thus, in the case of anthrax and tuberculosis, the infection of
the foetus through the mother occurs only very rarely, while we
know that in that of syphilis the liability is extreme " (Hamilton,
1900, p. 290). It is said that a foetus in utero may take small-pox
from the mother ; but this is contagion, not inheritance. Syphilitic
symptoms may appear in the new-born — microbes from the father
or from the mother have passed into the child ; but this is contagion,
not inheritance. Some say this is an academic distinction without
a difference, but to fail to make the distinction means confusion
of thought.

3. Inheritance of a Predisposition to a Disease is not In-
heritance of the Disease. — In many cases it seems possible and
useful to draw a distinction between the inheritance of a definite
disease and the inheritance of a constitutional predisposition
towards it. Thus, since tuberculosis is a bacterial disease,
since relatively few children are born tuberculous, and since
the disease attacks very unequally those who are equally exposed
to the same external conditions of infection, it seems probable
that what is really inherited is a constitutional peculiarity
(arising originally as a germinal variation), which expresses

256 HEREDITY AND DISEASE

itself, for instance, in " vulnerability of the protective epithelia,"
— in fact, in a deteriorated power of resistance to the tubercle
bacillus.

In the same way, to take a case provisionally non-bacterial,
it seems probable that gout is not, as such, transmissible, but
that what is inherited is a constitutional peculiarity (arising
originally as a germinal variation), which expresses itself in an
altered mode of eliminating nitrogenous waste — a constitutional
vice which may be exacerbated by excess of food and alcohol.

4. Acquired and Innate Abnormal Conditions should be
distinguished. — Closely similar abnormal states of the body
may arise in two different ways, and their heritability will
differ with the mode of origin. If the abnormal condition is
inborn in the strict sense — i.e. if it is the expression of a con-
stitutional peculiarity arising originally as a germinal variation —
the probability of transmission is often great. But if the ab-
normal condition has been induced adventitiously by external
influences (including food, drink, poisons, etc.), then the proba-
bility of transmission is slight. The distinction is a real one,
but it is not always readily drawn in actual practice.

Thus the difficulty of distinguishing inborn deafness from
exogenous or adventitious deafness — the result, for instance, of
various infectious diseases, — may, perhaps, explain a curious pecu-
liarity in E. A. Fay's statistics (3,078 marriages, 6,782 children).
The percentage of deaf children in families where both parents
were deaf was 8-458, while in families where only one parent was deaf
the percentage was larger — namely, 9-856. There seems something
wrong here, and the explanation may be that there are two quite
different phenomena slumped under the title deafness — viz. innate
or idiopathic deafness, and acquired or exogenous deafness.

As the case appears instructive, let us pursue it further. Where
both parents were believed to be congenitaliy deaf the percentage
of deaf children was 25-931 ; where one parent was deaf congenitaliy
and the other adventitiously, it was 6-538 ; where both parents were
adventitiously deaf, it was only 2-326. Where one parent was
congenitaliy deaf and the other normal, 11*932 per cent, of the

INNATE AND ACQUIRED DISEASE 257

children were deaf ; where one parent was adventitiously deaf
and the other normal, the percentage was 2-244. In short, there is
no evidence that adventitious deafness is heritable at alL

It may be noted further that Fay's statistics show that deafness
among the relatives of the parents increases very markedly the
likelihood of there being deaf children ; and they also seem to show
that consanguineous marriages greatly increase the probability of
the inheritance of deafness, or of constitutional conditions, e.g.
lymphoid exaggeration, such as naturally lead to deafness. This
is what would be expected from the fact that an individual in-
heritance is a mosaic of ancestral contributions.

The position we venture to maintain is expressed in the
following sentences : — " As inherited (on the part of the off-
spring) or transmitted (on the part of the parents), Biology
includes only those characters or their physical bases which
were contained in the germ-plasm of the parental sex-cells "
(Martius, 1905, p. 11). Similarly, Virchow says : " What
operates on the germ after the fusion of the sex-nuclei, modi-
fying the embryo, or even inducing an actual deviation in the
development, cannot be spoken of as inherited. It belongs
to the category of early acquired deviations, which are therefore
frequently congenital." This pronouncement is the more re-
markable since Virchow believed in the inheritance of acquired
characters.

Is the Distinction between Innate Disease and Acquired
Disease Practicable ? — It is true that the distinction between
an " innate " predisposition to a disease and an acquired
disease " looks better on paper than by the bedside." This
is simply an instance of what we continually find, that the
" abstract " theoretical concepts of science are not always
readily applicable to the intricacies and subtleties of nature.
And yet the distinction is quite legitimate and thoroughly
sound and useful in the present state of our knowledge. We
cannot object to the utility of abstracting an " organism "
from its " environment," although we know that a living

17

253 HEREDITY AND DISEASE

creature is inseparable from surroundings of some sort ; and we
must not object to the distinction between innate (or idiopathic)
diseases and acquired diseases because we know that the innate
disease must have an evocative environmental stimulus, and
that an acquired disease necessarily involves some organismal
susceptibility.

What, then, is the distinction ? It is the old distinction between
a variation and a modification. An innate disease presupposes
some germinal variation to start with, some germinal peculiarity
to continue with. It is there, whether it finds expression or not.
If it does not find any appropriate nurture, it will not express
itself in development, but neither will the normal process of
thinking find expression without the appropriate liberating
stimuli. If an indispensable process, the structural rudiment
of which is a component part of the normal inheritance, finds no
nurture, the organism of course dies. If a dispensable process,
such as an innate disease — the structural rudiment of which
is also part of the inheritance — finds no nurture, the organism
may of course survive if otherwise normal ; but the rudiment of
the disease may simply lie latent, and may be expressed in the
next generation. Eventually, whether it finds expression or
not, it may die away altogether, just as useful variations seem
sometimes to disappear. This might be called the racial cure
of disease.

An acquired disease is exogenous, not endogenous, in origin.
It arises, apart from any particular innate predisposition, as the
direct result of inappropriate nurture (in the widest sense) ;
of unnatural function, over-function, or lack of function ; and
of intruding parasites — e.g. bacteria.

But there are two complications — (i) An acquired disease
may operate in an organism which has an innate bias to disease
— e.g. when a tubercle bacillus infects a phthisical constitution.
(2) A diseased condition may be the result of premature or
local arrests of development, or of excess of development, or of

INNATE AND ACQUIRED DISEASE 259

disturbance of the time-relations of the developing organism :
and this may be due (a) to an intrinsic weakness or dispro-
portion in some components of the complex mosaic of inheritance,
in which case it is likely to be transmitted ; or (b) to some dis-
turbance of the nutritive and other conditions during ante-natal
life, in which case it is not likely to be transmitted.

To sum up in the words of a well-known pathologist, " the
term ' acquired ' should be applied only to what arises in the
individual life-time — from the period of development onwards
— under the influence of external conditions ; and never to
what arises, as we say, spontaneously — that is, from rudiments
already present in the germ " (Ernst Ziegler, 1886, p. 13).

All discussion about "congenital," " pregenital," and "post-
genital " heredity or inheritance is writing on the sand — mere
verbiage and confusion of thought. The inheritance is the organisa-
tion of the fertilised ovum — nothing less, nothing more. That the
developing offspring may be infected or poisoned at an earlier
or later stage, before birth or after birth, has nothing to do with
inheritance. The word " congenital " is properly used to denote
what is manifested by the offspring at birth ; the " congenital "
character may be hereditary — i.e. due to the parental germ-cells —
or it may have been acquired in ante-natal life. But the word is
also used by many to imply an innate constitutional character
which is part of the inheritance in contrast to a character which
has been adventitiously acquired. Therefore, as far as possible
(without undue purism or pedantry), the word should be dropped
altogether.

§ 3. Are Acquired Diseases transmissible?

It seems certain that diseased conditions may arise from
germinal variations appropriately stimulated, as in gout, rheu-
matism,* obesity, and insanity ; it seems equally certain that
diseased conditions may be induced from without by peculiarities

* Even if gout and rheumatism (in its acute form) be complicated by
the presence of specific microbes, we may regard the microbe as the
appropriate stimulus to an idiopathic predisposition.

26o HEREDITY AND DISEASE

of function and environment, including, of course, food and drink.
Without there being any observable hereditary predisposition,
a man may acquire cirrhosis of the liver, neurasthenia, cardiac
hypertrophy, and so on through a long list. That a man may
be invaded by microbes without being in any way peculiarly
susceptible to them, or that he may be poisoned in a score
of ways without there being any constitutional weakness to
blame, seems certain. But are such acquired diseases in any
sense transmissible ? It seems to us that the answer should
be in the negative, but the general reasons for this answer
must be sought in the previous chapter — that dealing with the
transmissibility of acquired characters in general.

No one can suppose that microbic diseases acquired by the
parent can be transmitted to the offspring, though there may
be ante-natal infection, and though the offspring may be pre-
judiced by the fact that the parents had the disease. If the
maternal constitution is seriously affected, it is probable enough
that the child may be born weakly, or imperfectly developed,
or even poisoned. In other words, the embryo is disadvan-
tageously modified by deficient or abnormal ante-natal nurture.
If the parental constitution is seriously affected it is possible
that the germ-cells may be likewise affected. This is most
(ikely in the case of the ova with their relatively larger cytoplasm
or formative cell-substance. In other words, there may be
a transmission of secondary effects of microbic disease. The
same will apply to any case where it can be definitely said that
the parental body is saturated with poisons or toxins. But to
admit this is very different from admitting that a specific modi-
fication of the parent's body can be transmitted to the offspring.
Yet some who should know better persist in calling this " a
distinction without a difference."

Leprosy. — In a leprosy district the children of lepers may
exhibit the disease, but this may mean nothing more than that
they were exposed to the endemic conditions, whatever they may

ARE ACQUIRED DISEASES TRANSMISSIBLE? 261

be, which cause the disease, or that they caught the contagion, if
the disease is contagious, as many believe. " It is quite certain,"
Mr. Jonathan Hutchinson says, " that the children of lepers,
born out of leper districts — in England or the United States,
for example — never inherit it."

Gout. — Because gout sometimes sets in after a particular
course of diet, some have attempted to regard it as an acquired
character, just as Herbert Spencer regarded short-sightedness
and a liability to consumption as acquired characters. But
there is no warrant for such interpretations. In all three cases
we have to do with innate germinal qualities which find various
degrees of expression according to the conditions of nurture.
There is no reason to believe that the expressions of goutiness in
a father can specifically affect the germ-cells in such a fashion
that the son thereby becomes gouty. Moreover, in many cases the
son who becomes gouty was born before his father became gouty.
What, then, is meant by the " heritability of gout" ? The cases
of gout " running in a family " are too numerous to allow us
to take refuge in the suggestion that a germinal variation which
was expressed as goutiness in the father occurs de novo in the
offspring. All that can be said at present is that the predis-
position to gout is an inborn character, which, like any other,
may be transmitted. Even if gout turns out to be definitely
microbic, the general argument will not be seriously affected.

Albuminuria. — There seems to be such a thing as constitutional
albuminuria, and a predisposition to it seems to be heritable. This
means that a defect or peculiarity in the filtering apparatus of the
kidney arises as a germinal variation, and is handed on from genera-
tion to generation. Under conditions which may mean nothing
to normal subjects, the inborn peculiarity may find expression in
the active disease of albuminuria. As in the case of gout, a con-
stitutional tendency to albuminuria is very transmissible, but the
disease must not be called " acquired " simply because particular
external conditions of life seem to supply the liberating stimuli
which lead to its expression. Where the albuminuria is transitory

262 HEREDITY AND DISEASE

and of modificational origin, where it is really an acquired condition,
there is no warrant for believing that it is transmissible.

Other Cases. — It proves nothing to cite instances of myopia
appearing in adolescence and reappearing in the early life of the
offspring ; of neuroses manifested after an accidental shock in the
parent, but patent from the first in the child ; of rupture manifesting
itself in the parent after an abnormal strain, and occurring without
apparently adequate cause in the next generation, — and so on. It
is always possible, and indeed reasonable, to answer that we have
in such cases to deal with an inherited germinal predisposition.

Cardiac hypertrophy due to over-work is in a sense a diseased
condition, though from a wider point of view it may be said that
the organism is here, as always, doing its best in the way of adaptive
response to novel conditions. But is there any warrant for sup-
posing that cardiac hypertrophy in a father will induce cardiac
hypertrophy or even a tendency to it in his son ? " Of course,
the constitution that made the father liable to hypertrophy would
also make the child liable, but this is inheritance of a constitutional
(non-acquired) character — a thing no one disputes " (Dr. Leslie
Mackenzie, Scot. Med. Surg. Journ. vi., 1900, p. 324).

Those who accept the concept of a germ-plasm of unimagin-
able intricacy, persistent with remarkable dynamic inertia from
generation to generation, changing and yet stable, oscillating in
parts and yet on the whole "breeding true," will not lightly
assume that modifications in the body can bring about a specific
change of structure in the germ-plasm. It is possible that
profound bodily changes, such as some acquired diseases effect,
may shake the kaleidoscope and provoke a change to a new
position of organic equilibrium; but it does not seem likely.
On the other hand, important bodily modifications, e.g. serious
derangements due to infectious diseases, may effect a change
in the vigour (functioning-power, growing-power, developing-
power, resisting-power) of particular elements in the inheritance.
And this admission is probably enough to cover all the well-
authenticated cases of inborn changes in the offspring of parents
who had acquired serious diseases.

NERVOUS DISEASES 263

Nervous Diseases. — In regard to so-called "acquired nervous
diseases," I venture to quote again from the late Professor
D. J. Hamilton (1900, p. 299): " Have we crucial evidence to
show that a mental disease may be excited through external
agencies, as, for instance, by the abuse of alcohol in a person
free from any ancestral taint, and that this disease so excited
can be transmitted through several generations ? My own
impression is that we have not. ... So far as I am personally
informed, I feel that, in mental derangement, and in excess of
perhaps any other form of disease, we have to do with an
inherited peculiarity or variation — a variation which may have
occurred in a far-back ancestor and lain dormant for many
generations, but which inevitably manifests itself under con-
ditions of unusual external stimulation, and which is in no
respect bound up etiologically with or necessitated by this
stimulus. The substratum which underlies the mental pecu-
liarity is allied to that underlying the predisposition to tuber-
culosis or gout, and, probably, is referable to a fault in metabolism
excited, it may be, by an inherent bias towards degeneration in
the nerve-cells of the brain, and this is eminently hereditary."

The general verdict of those experts who admit the validity
of the distinction between endogenous germinal variations
and exogenous somatic modifications may be thus summed up :
(1) externally induced nervous disorders (apart from the
results of wounds and wholesale poisoning of the system) are
extremely rare in persons free from ancestral taint ; (2) here-
ditary transmission in such cases is quite unproved, if we discount
cases where the whole system of the parent (including the
germ-cells) is poisoned by alcohol, opium, or the like.

In short, whenever a disease has been acquired, when there
is no specific predisposition towards it, when it is in biological
terminology modiftcational, it seems unlikely that there will
be any specific hereditary effect on the offspring. The most that
can be admitted is that very virulent acquired disease may in

264 HEREDITY AND DISEASE

a general way poison or weaken the germ-cells along with the
whole body, or that in the case of a mammalian mother the
foetus may be poisoned or weakened through the placental
circulation.

It must be noted, however, that many medical authorities
do not in the least agree with the position which we have stated.
Thus Mr. Jonathan Hutchinson says : " Without venturing to
do more than mention the Weismann logomachy, which has
recently disturbed the creeds of some biologists, I will take
permission to avow my belief that with the sperm and germ
supplied by parents there may pass to the offspring tendencies
to the reproduction of all that these parents had acquired up
to the date of the sexual congress. By the term ' acquired '
is meant all that has been received by modification of vital
processes, not what has been imposed or taken away by external
violence." We must refer to the chapter on the transmissibility
of acquired characters for our answer to this opinion.

Experimental Evidence. — It is sometimes said that the famous
experiments of Brown-Sequard showed conclusively that artificially
induced " guinea-pig epilepsy " is transmissible. But a scrutiny
of the case, such as we have given in the previous chapter, leaves
us reluctant to base an argument on Brown-Sequard's results.

The only other cases which seem relevant are those which have
to do with artificially induced immunity. By injections of serum
and the like — the details do not concern us — it is possible to render
an organism immune, e.g. to diphtheria. Are the offspring thereby
rendered hereditarily immune ? No case is known where the
offspring of an immunised father showed any " anti-bodies " in
the blood or any hint of immunity. There is no convincing evidence
of transmission of immunity from the male parent.

It is known, however, that the offspring of an artificially im-
munised mammalian mother (guinea-pig, rabbit, etc.) may exhibit
immunity. But this probably means that the " anti-bodies,"
agglutinins, precipitins, or whatever they may be called, passed
via. the placenta from the maternal to the foetal blood. But this
has nothing to do with inheritance.

TRANSMISSION OF DISEASE 265

§ 4. Can a Disease be transmitted ?

This is not a gratuitous question. Perhaps it is best answered
in the negative !

" A disease," says Prof. Martius (1905, p. 14), " is not an
entity nor a character, but a process — an abnormal process
injurious to the organism, which is set a-going by a causa externa
and runs its course in some part of the body." The process
is not transmitted, but the potentiality of it is involved in some
peculiarity in the organisation of the germ-plasm. " In the
sense in which the word ' inherited ' is used by biology, there
are no inherited diseases." They may be a-going before the
offspring is born, but they are not as such inherited.

As the authority quoted says, the objector will doubtless at
once bring forward the case of haemophilia, which is markedly
heritable. But haemophilia is not a disease. Does not the
subject get along fairly well until he receives a wound ? There
may be some weakness in the walls of his blood-vessels which
makes them peculiarly vulnerable, there may be some obscure
peculiarity in his blood which prevents it coagulating, so that
bleeding even from a slight wound may be very persistent. But
there is no disease, if we mean by disease an abnormal process.
What is inherited is a peculiarity of the vascular system ; or
perhaps we should put it negatively, and say that some part
of the normal inheritance (some "determinant," in Weismann's
phrase) is absent in those who show haemophilia.

Some inborn peculiarity of the nervous system, originating as
a germinal variation, may under appropriate conditions of
stimulus or lack of stimulus manifest itself as a disease, such
as some forms of paralysis. Some inborn peculiarity of the
muscular system, originating as a germinal variation, may
under appropriate conditions of stimulus or lack of stimulus
manifest itself as a disease — such as progressive muscular
atrophy. Similarly, some inborn peculiarity of the alimentary

266 HEREDITY AND DISEASE

tract — a variation not in itself a disease (e.g. simple gastric
achylia) — may in appropriate conditions give rise to disease.
Similarly, phthisis is not as such inherited ; what is inherited
is a predisposition to caseous degeneration of tissue and allied
pathological processes.

Thus, though it may appear pedantic, and though it will
probably be misunderstood, we are inclined from the biological
standpoint to agree with the authority quoted above, that
" there are no inherited diseases."

§ 5. Predispositions to Disease

Up to this point we have argued that mere reappearance
of a disease does not imply that it is inherited ; that infection or
poisoning before birth is quite different from inheritance ; that
microbic diseases should never be spoken of as heritable ; that
there is no warrant for believing in the transmissibility of ac-
quired diseases; and that, if disease means a process , the in-
heritance of predispositions to disease is a more accurate phrase
than the inheritance of disease.

But is not this inherited " predisposition " something " mystical,"
suggestive of the " horologity " of clocks ?

It may be mysterious, but it is not " mystical." We may
not be able to picture it or define it, but it is like any other
germinal potentiality, except that it happens to be prejudicial
to the organism. It implies something out of gear in the proto-
plasmic machinery.

Physically considered, life depends on an ordered sequence
of constructive and disruptive chemical processes, and the
organism is from the outset predisposed, let us say " geared,"
to perform these in a certain routine. But the gearings are from
the beginning very delicately adjusted : a slight initial difference
may mean a life-long friction ; a slight germinal bias may deter-
mine the trend of the whole life.

Among the pathological predispositions of major importance

PREDISPOSITIONS TO DISEASE 267

in human life we may mention those which result in abnormal
nervous processes, in rheumatism with its many forms, in gout,
in obesity, in tendency to stone or gravel, in asthma, and so on.
The reappearance of these diseases in varying degrees is certain,
but what is really transmitted is the original germinal irregularity
of gearing. No one can explain what this irregularity precisely
is, but it is a common experience, baffling to the physician,
that if it is adroitly checked in its outcrop in one direction it
may manifest itself in another. He may cure the disease,
but he cannot reconstitute his patient. Hydra-headed, the
predisposition will show itself in polymorphic guise.

Some men are immune to certain diseases — e.g. to scarlet fever ;
all men are immune to fowl-cholera and many other animal
diseases. This immunity is mysterious, but it is not " mystical."
Of recent years we have begun to understand it, to measure it.
And "predisposition" is the other side of immunity.

It is probable that some " predispositions " are much more
definite than others. Thus, haemophilia may be due to a re-
trogressive variation comparable to albinism ; some particular
item in the normal inheritance has been suppressed or kept
latent. But the predisposition to tuberculosis is probably much
less definite, and due to a more general disturbance of the
" protoplasmic gearing," which finds multiple expressions both
structural and functional.

The predisposition to gout is well known to be hereditary.
It is probably what may be called a general constitutional
predisposition, involving a derangement of the normal meta-
bolism. For while it perhaps finds its primary expression in
peculiarities of the digestive and excretory organs, it may affect
practically every tissue in the body. Its expression may be
accelerated by luxurious living and laziness, but, given the
predisposition, it may manifest itself in those who live very
carefully and take plenty of exercise. That is the peculiar
hardship of it !

268 HEREDITY AND DISEASE

It may seem unsatisfactory to refer the origin of constitutional
diseases, such as insanity and obesity, to a germinal predis-
position— i.e. to the terra ignota of the fertilised egg-cell. But
no other course is at present open. We are only doing in regard
to diseases what we must do in regard to all variations. The
little that can be safely said of their causes has already been
said in Chapter III. Variability is one of the fundamental
properties of the living organism, and the germ-cells are potential
organisms. In their relation to the body which is their mortal
vehicle, and in their own history, there is ample opportunity for
variations to arise, and among these variations we must rank
predispositions to disease. In short, such predispositions form
part of the puzzle of individuality.

If we take a peculiarity like colour-blindness we know
practically nothing in regard to its origin. It is not known
to be associated with any structural defect of the eye ; it is
certainly not acquired ; it arises in a certain percentage of
the population, usually in males ; it is a good example of a
germinal variation which is exceedingly heritable. In the same
way, passing to disease, we cannot tell what a predisposition
to diabetes insipidus precisely means ; we know it in its
expressions in the body, but its origin is as obscure as that of
colour-blindness ; it is a germinal variation which is exceedingly
heritable.

To illustrate the matter further, we may point out that the
inheritance of physiological " idiosyncrasies " which do not
express themselves as diseases is well established. Thus, the
inability to digest the proteids of eggs and milk may be a heritable
" family idiosyncrasy." It seems quite analogous to those
idiosyncrasies which under appropriate conditions manifest
themselves as diseases. A tendency to excessive " freckling "
seems to be hereditary ; it implies an inborn imperfection in
the skin; under appropriate stimulation it may express itself
as Kaposi's disease. Scores of similar cases are well known,

SECONDARY EFFECTS OF DISEASE 269

and they seem to throw a useful light on what is usually called
" the inheritance of disease."

Inherited and Independent Variations. — It is hardly neces-
sary to point out that the occurrence of a particular predisposition
— whether it be to gout, to diabetes, or only to " freckling "
— may be interpreted either as the outcome of an inherited
germinal variation, or as an independent fresh variation similar
to one which occurred in ancestors, just as the occurrence of
great musical or mathematical talent may be interpreted either
as inherited peculiarity or as fresh variation. The facts seem
to show that certain variations have great staying-power through-
out generations, and also that nature often repeats herself.
Each case must be interpreted in terms of what is known of the
lineage.

Inheritance of Secondary Effects of Disease. — In many cases
it seems legitimate, perhaps necessary, to suppose that a disease
in a parent may have a secondary effect on the germinal material,
and may prompt germinal variations which find expression
during the development of the offspring as diseases.

In some forms of rheumatism there is what may be called a
poisoning — an auto-intoxication — of the living body with its
own waste-products — e.g. urates ; in some forms of bacterial
disease, as the popular phrase " blood-poisoning " suggests, the
same result is brought about by the waste-products or by-
products of the intruding microbes ; and it seems certain that
an equally thorough poisoning may be brought about by the
intemperate use of alcohol, ether, opium, etc. Even water-
drinkers may be in certain areas the victims of lead-poisoning,
for which they cannot reproach themselves. Experts may
differ as to the most accurate way of expressing the facts, but
it is certain that a man may thoroughly poison himself, for a
time at least, with alcohol, opium, tobacco, or the like. Organ
after organ may be injuriously affected ; the blood, the urine,
even the sweat will tell the tale, as it were in protest ; and even

270 HEREDITY AND DISEASE

on a priori grounds we should expect the reproductive organs
— apart as they are in some ways from the everyday life — to
be affected by the widespread disturbance of nutritive meta-
bolism.

Weismann has suggested that the oscillations of nutrition
in the body prompt variations in the germ-plasm. Diseases may
cause profound changes in the nutritive stream, and those
particularly constant forms of whirlpool which we call the
germ-cells, which repeat themselves and propagate themselves,
generation after generation, age after age, may as the results
of bodily disease exhibit variations. Stable as the germ-plasm
must be supposed to be, we cannot conceive of it as an unrelated
entity. We believe that this interpretation covers many of the
cases which are called " inheritance of disease."
, It must also be remembered that while the chromatin of
the nucleus is almost certainly the real vehicle of the hereditary
qualities, the germ-cells also include some extra-nuclear
cytoplasm which may be affected in a general way by somatic
changes. The ovum, in particular, has a relatively large mass
of cytoplasm — its general cell-substance — which is the pre-
liminary building-material of the embryo. It is cutting it
too fine to say that what affects the cytoplasm of the egg is
not part of the inheritance, since that is really hidden in the
penetralia of the nucleus. The egg-cell is a unity, an individuality,
a miniature organism, and anything in it (except, of course,
another living creature — namely, a microbe) is at any rate a
close annexe of the hereditary vehicle in the nucleus.

It is experimentally certain that germ-cells are markedly
susceptible to toxins of various kinds, such as alcohol, nicotin,
and hydrocyanic acid, and that abnormal developments result.
Therefore, since man}' diseases produce toxins in the body,
these may affect the germ-cells prejudicially, and thus there
may be an inheritance of the secondary effects of disease.

What comes practically to the same thing for the individual

SECONDARY EFFECTS OF DISEASE 271

offspring, though it is theoretically different, may result if the
toxins in the maternal body affect not the ova, but the developing
embryo. They may saturate through the placenta and disturb
the normal course of development. This would be an ante-natal
modification, and we should not expect its consequences to
extend beyond the immediate offspring, unless the same detri-
mental conditions persisted in subsequent generations.

Illustrations. — " Assume that the last egg of a fowl dying from
tuberculosis is fertile. Weismann would admit— every one would
— that the chick is likely not to be full-grown and robust. It will
fail of ' nutrition,' of a full capacity for regeneration, and of normal
resistiveness to environment (terms which require fuller considera-
tion). It would appear, then, that this chick has an idiopathic [say,
innate] susceptibility to all and sundry, or at least to several,
diseases. It is mere slackness to call that heredity in disease.
It is equally apt to be variation ; the chick turning out to be epileptic,
or deformed, or liable to cholera. That is all that Weismann con-
tends for. The disease has not bred itself " (Dr. George Wilson,
Scot. Med. Surg. Journ. vi. 1900, p. 321).

Martius puts this problem. Two brothers have the same medium
predisposition to tuberculosis ; both take measles. During con-
valescence one (A) becomes definitely tuberculous as the result
of exposure ; the other (B) has his predisposition increased but
resists tubercle-infection. Both marry normal wives and have
children. Now, will the children of A have a worse inheritance
than the children of B ? There seems no reason to answer in the
affirmative, unless it can be shown that the toxins, etc., engendered
by the progress of the disease so saturate through the whole system
that the germ-cells also are specifically affected and thus have
their predisposition exaggerated. This seems very improbable.
But it is possible that when a disease goes far the germ-cells may
be in a general way prejudicially affected. And if they are rendered
in a general way less vigorous, there is some likelihood that the
disorganisation of the germinal machinery may go further.

It is interesting to inquire whether, in cured cases of phthisis
and the like, the protective substances naturally produced, e.g.
the ' tulase ' of Behring, might not even lessen the heritable
predisposition of the ovum towards the disease in question.

272 HEREDITY AND DISEASE

§ 6. Particular Cases

Colour-blindness. — This peculiar condition may recur for many-
generations, but it has an interesting peculiarity. It is usually
restricted to the male members, yet a colour-blind man seems
never to have a colour-blind son unless the peculiarity was also
in his wife's family. Colour-blindness is transmitted from father
to grandson through unaffected daughters.

Short-sightedness. — It is generally admitted that short-
sightedness is due to an inborn peculiarity in the structure of
the eye, occurring in various degrees. In itself it can hardly
be called a disease in the strict sense, and conditions of life are
conceivable in which it might even be advantageous. The
innate peculiarity may become exaggerated and complicated
when the eyes are forced to function in a way to which they
are ill adapted, and acquired " myopic " modifications may
be superadded to what was there by inheritance. Sometimes
these may even lead to an actually diseased condition. But
though the innate peculiarity may be exaggerated and
complicated by the addition of acquired modifications,
there is no evidence that these can be transmitted. What
is transmitted is a structural peculiarity which began as a
germinal variation, and that this is very liable to be trans-
mitted one does not require to go to Germany to see.
It is said that short-sightedness occurs, though rarely, among
wild races.

Bleeding.v— A hemorrhagic tendency or liability to bleeding
is well known to be heritable, but it finds expression only in
males. A case given by Klebs and cited by Sir William Turner
(1889) is instructive in showing how the tendency, though
transmitted through daughters (and therefore part of their
inheritance), finds expression only in the males, and in
illustrating first a diffusion, and then a waning of the peculi-

PARTICULAR CASES 273

arity. The black letters indicate the affected subjects or
" bleeders."

M

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M M

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F F M M M F M

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Alcoholism. — There is practical unanimity among physicians
that the abuse of alcohol is prejudicial to the race as well as
to the individual, but there is considerable difference of opinion
as to the theoretical interpretation of the observed facts. As the
subject has been very frequently discussed, we shall restrict
ourselves to a brief survey.

(1) It is certain that the habit of using large quantities of
alcohol is prejudicial to health, " poisons the system," and
becomes a pathogenic factor. What constitutes abuse varies,
of course, with the individual and his conditions of life. There
seems to be little utility in labelling alcohol a " poison," though
it is a poison in large doses. Arsenic is a poison to man, yet
Gautier seems to prove that the presence of minute quantities
of arsenic in various organs of the body is a condition of health,
Both as regards arsenic and alcohol, it is the amount and the
frequency of the doses that tell.

(2) It is not to be expected that the particular modifications
which the parent acquired through abuse of alcohol will be
transmitted as such to his offspring. There is no secure evidence
of this. The father may acquire cirrhosis of the liver, the child
may be epileptic. There seems to be no authentic instance
of anything like transmission of cirrhosis of the liver from a
drunken father to his son. That a drunken son may also acquire
cirrhosis proves nothing.

18

274 HEREDITY AND DISEASE

(3) In interpreting the dismal records of the families of drunken
parents it is a mistake to attribute the whole result to the here-
ditary influence of alcoholism. It is necessary to make allowances
for cases in which the offspring " have been in the vineyard too."
They may be affected through the mother before and after birth,
by becoming early accustomed to doses of alcohol, and obviously
by suggestion and imitation, and also by the persistence of the
superorganic conditions which " drove the parents to drink."
The resultants of these factors may augment the inherited bias
or the inherited germinal defect. Where there has been no
direct inheritance the nurture-results may simulate the results of
transmission.

The Ostiak forces vodka down his child's throat, and the
same happens nearer home. "Whisky-babies " occur in Merrie
England. But the mischief may begin further back ; even
before birth the mother may poison her child.

Fere and others have described the disturbing effects which
followed injections of small quantities of alcohol into the develop-
ing egg of the fowl. Mairet, quoted by Debierre, found that
the offspring of an artificially intoxicated bitch by a sound
dog showed " alcoholic degeneration " and soon died.

(4) Just as upbringing in an environment of intemperance
may bring about results which simulate direct inheritance, so
it should, we think, be frankly and responsibly recognised that
there is an occupational factor in the persistence of excessive
alcoholic habits. From certain occupations — dreary, unwhole-
some, underpaid, and what not — relief is sought in alcoholic
stimulants. As long as these conditions persist they are likely
to prompt successive generations to similar expedients, and this
must be borne in mind when we try to estimate how much
of so-called alcoholic degeneration is strictly speaking due to
inheritance.

(5) It is certain that a tendency to intemperance is often
associated with other expressions of bodily or mental instability,

ALCOHOLISM 275

but it is difficult to determine whether the alcoholism causes the
instability, or whether the instability causes the alcoholism, or
whether, as seems most likely, both are expressions of some
germinal defect.

Prof. F. W. Mott concludes that " Alcohol is responsible for a
large number of admissions to asylums." But " how far it acts
as the efficient cause of insanity, and how far it is only a co-
efficient or coincident in relation to antisocial conduct in an
individual potentially insane, rendering such a person certifiable,
it is difficult to gauge until more accurate and scientific data are
forthcoming" (1911).

And again : " Coincidence and cause may thus be confused,
for a lapse from moderation to intemperance may be the first
recognisable sign of the mental breakdown. Especially is this
the case with the involutional psychoses occurring at the
climacteric period in women ; also men and women between fifty
and sixty who suffer from melancholia, and at the same time are
the subjects of artero-sclerosis. Again, general paralytics and
cases of adolescent insanity may take to drink. There can be
no doubt that neurasthenics, hysterics, epileptics, imbeciles, de-
generates, eccentiics, and potential lunatics — all those, indeed,
with an inherent narrow margin of highest control — possess a
marked intolerance to the effects of alcohol, and the failure to
discriminate between what is the result of alcoholism and what
is innate and due to inheritance has been the cause of much
confusion " (1911). And again this authority sums up, " Alcohol
is a powerful coefficient, but not of itself the main cause, in the
production of insanity, except in the rather infrequent cases of
alcoholic dementia."

(6) It is certain that parental alcoholism and instability
(taking the two together) are often associated with alcoholism
and degeneracy in the offspring, but this may depend on the
inheritance of a " general controlling determinant " responsible
for both alcoholism and instability. It is difficult to prove that

276 HEREDITY AND DISEASE

parental alcoholism considered, if that be possible, by itself has
a hereditary influence on the offspring.

Many facts point to the conclusion that what the intemperate
member of an intemperate lineage inherits is the weakness which
led the parent to become alcoholic. It does not matter whether
we call this a lack of will-power or a neuropathic or psychopathic
tendency. It is a heritable constitutional defect, as is clearly
illustrated in cases where the parent did not acquire the alcoholic
habit until after he had ceased having children. Let us quote
two great authorities. Dr. T. S. Clouston observes, " It was not
the craving for alcohol that was inherited, but a general psycho-
pathic constitution in which the alcoholic stimulus is an undue
stimulus, and the mental control deficient." Prof. F. W. Mott
writes, " An inherited weak will-power and lack of moral sense
may be transmitted, whereby the individual is more susceptible
to temptation and imitation, and in this way environment plays
an all-important part."

(7) Dr. Archdall Reid has elaborated with great ability the
interesting thesis that alcoholism promotes temperance. The
most temperate races are those that have been habituated to the
use of alcohol for the longest time, during which those who have
an abnormal tendency to intemperance or an abnormal suscepti-
bility to alcohol have been weeded out, leaving a more controlled
and more resistent stock. He points out how intemperate families
rapidly work themselves out.

(8) In this connection reference should be made to a very inter-
esting point raised by Prof. F. W. Mott, How is it that a chronic
alcoholic often has offspring mentally and physically sound ?
It is probable that we have here an instance of the stability of
the germ-plasm in spite of even violent environmental assaults.
It is probable also that we have to distinguish between men who
become alcoholic through deep-seated constitutional defect and
those who have not this excuse. What Mott says is this : "The
question is wrapped up in the causes which lead a man or woman

ALCOHOLISM

277

to drink, and my observations and adduced facts seem to show
that a man who can drink continually for numbers of years, and
keep out of a lunatic asylum, a prison, or a hospital, must have
possessed an inherent stable mental organisation, and he in a
measure transmits this, the virility of the stock remaining potent
in spite of the ruinous habit he has acquired, although it is
probable that his offspring would have been stronger and fitter
had he been a temperate man. Drunkenness in successive genera-
tions would, I believe, undoubtedly lower the virility, and mental
and physical degeneracy of the stock would result " (1911).

(9) It is certain that abuse of alcohol is prejudicial to the
race by lessening in more ways than one the nutritive capacity
of mothers. Thus, to refer to one aspect only, the conclusion of
Prof. G. von Bunge's investigation of over 2,000 families is that
the increasing incapacity of mothers to nurse their children
is referable to chronic alcoholic poisoning continued for genera-
tions (Die zunehmende Unfdhigkeit der Frauen, ihre Kinder zu
stillen, 5th edition, Munich, 1907).

(10) The predisposition which facilitated the hyper-alcoholic
habit in the parent is transmitted. There may be intra-uterine
intoxication of the unborn child if the mother is a drunkard.
The tradition in favour of the abuse of alcohol may persist.
The conditions of nurture may also tend to induce the alcoholic
habit in the offspring ; but there is more. Much evidence points
to the conclusion that the germ-cells may (in cases of extreme
alcoholism) be prejudicially affected along with the body of
the victim. As it is often only the father who is alcoholic, it
follows that the poisoning influence, whether of the alcohol
itself or of by-products resulting from the nutritive disturb-
ances which its abuse provokes, may effect the germ-cells as
such. " This direct deterioration of the germ is a pathogenic
factor of the first rank " (Martius, 1905, p. 23). For if the
germ-cells are affected the offspring will also be affected.

(11) There is some experimental and some general physio-

278 HEREDITY AND DISEASE

logical evidence that alcoholic poisoning may prejudicially affect
the germ-cells, but it is more difficult than most people think
to substantiate this from human cases. Thus in the case of
intemperate mothers we have to allow for the deranged nutrition
as well as for the poisoning, and for the poisoning of the embryo
through the placenta as well as for a possible direct deterioration
of the germ.

(12) There is some evidence that deterioration in the offspring,
as marked by epilepsy, some forms of insanity, lack of control,
feeble-mindedness, deaf-mutism and stunted growth, is apt
to be intensified and to appear earlier if the parents are alcoholic.

Nervous Diseases. — That the nervous system is particularly
liable to disease is well known, and various reasons have been
assigned for this. (1) Nervous organs are of all organs the
most intricate in their complexity, and nerve-cells are the most
highly differentiated cells. But a high degree of complexity
involves greater instability, greater liability to accident. A
free-wheel bicycle with two or three grades of gearing is a finer
mechanism than, let us say, the old-fashioned high bicycle,
where even the complexity of a chain was avoided ; but there
is in the increased excellence the inevitable disadvantage of
a greater range of possibility " for something going wrong."
(2) Nervous organs have a very limited power of regeneration
after injury. There is no increase in the number of our nerve-
cells after we are born, and reports of cases of regeneration of
nerve-cells after injury are few and far between as regards
backboned animals. (3) Characters of recent origin tend to
be more unstable than those of ancient date, and the differentia-
tion of man's brain is relatively recent compared with that
of his food-canal. Prof. Adami (1901, p. 1319) refers to the
discovery made by James Ross of Manchester that " when there
is progressive atrophy of the cells in the cortex of the brain,
the first motor-cells to show signs of that atrophy are those
governing the muscles which differentiate man from other

NERVOUS DISEASES 279

animals— namely, the opponens muscles of the hand." (4) Hamil-
ton suggests, inter alia (1900, p. 298), that the "germ-track
followed in the ontogeny of the nerve-cells is very short, far
shorter than in the case of many other cells throughout the
body, and hence a state of maturity is reached at a comparatively
early period, with an inclination to premature decay." (5) It
may also be noted that, especially as regards his nervous system,
the so-called civilised man takes liberties of unnatural function
and unnatural environment, which often tax the plasticity of
protoplasm beyond the limits of endurance, marvellously wide
as these are, and allow inborn weaknesses to find dire expression.
For these and other reasons, then, the nervous system of man
is peculiarly liable to disease.

Weaknesses, abnormal peculiarities, and actual diseases of
the nervous system are not only very common, but they appear
to be peculiarly persistent in family histories. In old days
it was often remarked that generation after generation of a
particular family might be " possessed of the devil " ; and
there were families of " sorcerers " and " witches " who turned
their hereditary neuroses to account. So now we speak of the
neuropathic family.

It is generally admitted that lack of control, morbid idiosyn-
crasies, subjection to delusions, monomania, hysteria, epilepsy,
chorea, locomotor ataxy, extreme passionateness, homicidal
and suicidal mania, insanity and imbecility, tend to reappear
generation after generation with appalling regularity. It is
often said that about one-fourth of those who are confined in
lunatic asylums have had some more or less insane not-remote
ancestor.

In regard to this very difficult question we wish simply to
make three remarks : (1) in many cases what, the facts suggest
is the inheritance of a general, not a specific, predisposition ;
(2) on the other hand, there are some instances of apparently
very precise and specific inheritance, as if some very definite

28o HEREDITY AND DISEASE

" blot on the brain " was transmitted from generation to
generation ; and (3) that there seems to be little warrant for
believing in the transmission of a nervous disorder of exogenous
origin.

(1) In most cases the facts seem to suggest that what is
inherited and transmitted is a general predisposition to some
dislocation or derangement of the nervous system. If such
a dislocation or derangement occur in a case where we can
exclude the probability of its being due to any infection, intoxi-
cation, or lesion of external origin, we must refer it to some
initial defect or disturbance in the organisation of the germ.
As such, it is likely enough to be transmitted, whether it be
hysteria or epilepsy, melancholia or idiocy ; but it does not by
any means follow that it must be transmitted, or that, if trans-
mitted, it will have in the offspring the form it took in the
parent. In fact, the frequency with which the expression
changes almost forces us to conclude that what is inherited is
something general, not specific. Another reason for this con-
clusion is to be found in the fact that the nervous disorder is
so often associated with some more general constitutional dis-
turbance. Thus the association of hysteria, epilepsy, chorea,
etc., with rheumatism is well known. In such cases it is probably
more accurate to speak of the inheritance of a constitutional
vice, a derangement of metabolism, and to avoid expressions
which suggest that there is, to begin with, anything definitely
wrong with the cerebral machinery. In the third place, it is
instructive to note that the cerebral equipment may work well
for years of ordinary life, and yet break down hopelessly in
face of some extraordinary excitement or some constitutional
crisis (puberty, parturition, menopause, etc.), which again
suggests the inheritance of general weakness rather than the
inheritance of specific disease.

The fact that predispositions to nervous diseases so often
change in particular expression from generation to generation

NERVOUS DISEASES 281

points to the position which many hold, which is well argued
for by Rohde (1895), that what is really inherited is a constitu-
tional peculiarity (arising originally as a germinal variation)
which may express itself in general neurasthenia, easy ex-
haustibility, deficient control, etc., or — under sufficient provo-
cation— in some specific form of acute neurosis. After a careful
survey Rohde concludes that the only nervous disorders which
are transmissible are those which have a germinal origin ; and
another authority, Dr. T. S. Clouston, says, " A neurotic
heredity is seen to resolve itself into general morbid tendencies
rather than direct proclivities to special diseases." What is
inherited is a predisposition, not a disease ; and, fortunately,
the predisposition may never realise itself.

What we have just said does not imply that persistent nerve-
fatigue and neurasthenia in parents may not favour the outcrop
of neurosis in the offspring, for the abnormal nervous condition
in the parent may, through nutritive disturbances, affect the
germ-plasm in a generally deleterious way (as Weismann expressly
says), and the development of the nervous system of the
unborn child may be affected disadvantageously by the abnormal
condition of an over-fatigued mother.

It is exceedingly probable that many neuroses are due to
primary defects in the development of some of the nerve-centres
or of the cells that compose these. Thus, a weakening in the
developmental power of certain rudiments in the inheritance,
which might well arise as a germinal variation, and which might
by hypothesis be inherited, would account for the recurrence
of certain forms of nervous disease generation after generation,
e.g. for a similar breakdown at adolescence or in senescence.

(2) On the other hand, there are some cases — a small
minority — which suggest that a specific predisposition may be
heritable. Thus, some of the records of inherited nervous
disorders disclose an appalling exactness in their mode of
expression, though it is probable that this is due in part to

282 HEREDITY AND DISEASE

suggestion. The point may be illustrated with reference to
suicidal mania.

Debierre (1897, p. 19) cites a case, reported by Macca-
bruni, of a suicide's family. Out of seven, three made
away with themselves ; a fourth, who was assassinated, left
a child who committed suicide. But the tragedy of the in-
heritance of a suicidal tendency is increased by the fact that
it may manifest itself in the offspring at precisely the same
age and in precisely the same way as it did in the parent.

" A monomaniac in the prime of life, Moreau de Tours reports,
was seized with melancholia and drowned himself ; his son, in
good health, rich, the father of two well-endowed children,
drowned himself at the same age." In another case a man
who had met with a disappointment tried to drown himself,
was rescued, but afterwards accomplished his design. It was
found that his father and one of his brothers had committed
suicide at the same age and in the same manner.

It should, we think, be borne in mind that the outcrop of a
morbid hereditary tendency at the same age — often a critical
age — in father, son, and grandson, may not be any more mys-
terious than that they should begin to shave at the same age.
Nor should we exaggerate the tragedy of similar suicides by
forgetting that the methods available are not very numerous.
Originality is as rare in suicide as in other actions. Thirdly,
we should remember the dire influence of suggestion : secret
brooding over the nature of the father's death has doubtless in
many cases added weight to the hereditary burden.

(3) In regard to the transmissibility of nervous disorders of
exogenous origin — i.e. traceable to some external shock or wound —
it may be enough to quote the deliberate conclusion of an expert
pathologist : "I can find no facts which prove that an acquired
disorder of the nervous system can be transmitted to the off-
spring " (E. Ziegler, 1886, p. 30). Where a nervous breakdown
followed a shock^ a wound^ or an illness such as pneumonia,

MICROBIC DISEASES 283

and reappeared in the offspring, it is probable that there was
behind the provocative stimulus an inborn predisposition,
and that the latter alone is transmitted. The case of alcoholism
has been discussed separately.

Microbic Diseases. — In the strict sense there can be no in-
heritance of microbic diseases, for a microbe cannot form part
of the organisation of the germ-plasm. " No specific infective
disease is hereditary, if we use the term ' heredity ' in the sense
which Darwin and the biologists have given to it. If it appear
congenitally it is simply communicated to the foetus by infection "
(A. A. Kanthack, in Allbutt's System of Medicine, vol. i. p. 555).
Let us take two concrete cases — tuberculosis and syphilis.

Tuberculosis. — As this familiar disease, in its many forms, is
always associated with the presence of a specific microbe, the
tubercle bacillus, it is not in itself transmissible. What is trans-
mitted is a predisposition making infection easy, a vulnerability of
epithelial surfaces, a weakness in the power of resisting and dealing
with the invading microbes. As Debierre puts it, " On ne nait pas
tuberculeux, on nait tuberculisable."

Theoretically, it matters little when or where infection occurs,
but the various possibilities are of practical interest.

( 1 ) It seems very unhkely that the spermatozoon is ever the bearer
of the tubercle bacillus. Out of sixteen guinea-pigs inoculated with
the sperm of tubercular males, six became tubercular, according
to Landouzy and Martin, but many have repeated this experiment
with negative results. Landouzy, quoted by Debierre, gives the case
of a phthisical officer who married a wife without any hereditary
taint in that direction. The five children all died of tubercular
disease ; but, of course, this may have been due to post-natal
infection.

(2) Similarly, it seems very unlikely that the ovum is ever the
bearer of the tubercle bacillus.

(3) In a few cases there is direct evidence that the mother may
infect her unborn offspring, the bacillus passing through the placenta.
In rabbits and guinea-pigs and some other animals this ante-natal
infection has been demonstrated ; but it is interesting to notice
that while tuberculosis is extremely common in cows (sometimes,

284 HEREDITY AND DISEASE

it is said, in 16 per cent.), the young calf is very rarely tubercular.
Leclerc found only five cases out of 400,000. As to man, only about
a dozen instances of congenital tuberculosis were admitted by an
expert as securely established in 1905, and Dr. R. Schluter's ex-
ceedingly careful scrutiny (1905) of alleged cases of congenital
tubercle in human infants led him to the conclusion that for prac-
tical purposes the possibility of ante-natal infection might be in
this case disregarded. Thus, Prof. D. J. Hamilton writes : " With
extremely few exceptions — so few that they may almost be
neglected — children are not born tubercular even of tubercular
mothers, nor are the young of animals born tubercular under like
conditions " (1900, p. 293). Even if the mother have genital
tuberculosis, specific contamination of the unborn child seems rare,
and there is no proof that genital tuberculosis in the father has any
specific effect on his offspring.

(4) In all ordinary cases, then, the infection with tubercle bacillus
occurs after birth, and in many cases long after.

The fact that tubercular disease may be a shadow over a family
history for generations is doubtless mainly due to an inheritance of
what began as a truly germinal or blastogenic variation, which is
only a biological way of expressing what the physician means by
" a particular predisposition," " a tubercular temperament," " a
diathesis," and so on. To discuss what the particular weakness
precisely is does not fall within our province ; Prof. Hamilton
says, " Most likely the particular vulnerability resides in the epi-
thelial protective coverings of the body being too little resistant, too
easily stimulated by external agencies, too readily penetrated by the
parasite of the disease " (1900, p. 294). " In support of this
assertion are to be taken into account certain epithelial manifesta-
tions which accompany the tubercular habit — namely, the very
dark or very light degree of colour of the hair, the overgrowth of
hair in the bushy eyebrows and long eyelashes, and, lastly, the
occurrence of a lanugo-like overgrowth in tubercular children
along the spine and over the legs. To my mind, these all point
to an anomaly of the epithelial type which is peculiar to the tuber-
cular habit of body " (Hamilton, 1900, p. 295).

If it be the case that the tubercle bacillus usually gains access,
even to the lungs, mainly by the digestive tract, and almost entirely
through the intestine, and may penetrate into the large lymph
channels without any apparent lesion, we have still perhaps to do

TUBERCULOSIS 285

with epithelial vulnerability, and in any case, which is all that
our argument requires, with a general constitutional peculiarity
or germinal variation.

For the benefit of those who are not satisfied with referring the
hereditary predisposition to a germinal variation (though beyond
this vagueness it is hardly safe at present for any biologist to ven-
ture), we wish to quote again from the late Prof. Hamilton's address
on " Heredity in Disease," which marked a distinct step in the
discussion of the subject.

" Where has the inherited strain come from ? What is its
ancestral history ? Can it be generated by vicious surroundings ?
I question whether it can. No doubt, once in the blood, the par-
ticular habit may be fostered by every external agent which tends
to deteriorate the natural powers of resistance. But will such
external agencies tend to produce a particular colour of hair, a
certain narrowness of chest, tallness of stature, and other peculiar-
ities which are distinctive of the tubercular constitution ? My
conviction is that they will not, and that we must go much further
back in the history of the human race to get at the explanation
of the matter. My own impression is that these features are the
lineal descendants of a variation which took place far back in
our history, that the variation has occurred irrespective of sur-
roundings or external agencies, and that its influence has been
propagated in the descendants ever since. It may be a variation
which is common to many races, but one which apparently is
intensely hereditary " (1900, pp. 295-6). It should be noted,
however, that this way of looking at the facts is not unanimously
accepted. Some experts will hardly admit the inheritance of even
the tubercular diathesis as a thing more to be remarked than the
disposition to typhoid or diphtheria. The tubercle bacillus is very
parasitic, and may bide its time for years, slowly producing, even
from a single infected gland, all the appearances of the tubercular
type. Moreover it should be remembered that (a) open-air animals
rarely suffer from tuberculosis, but suffer at once when confined ;
(b) that well-to-do, well-nourished people are much less liable than
the poor and ill-fed ; and (c) that phthisis is commonest where
overcrowding is greatest, and lessens as hygiene improves.

In any case, the distinction between the inheritance of a predis-
position to a disease and the inheritance of the disease is far from
being a quibble about words, as some prejudiced writers still declare.

286 HEREDITY AND DISEASE

This is evident from the successful results of modern preventive
medical practice in regard to consumption.

Statistics showing that in one sanatorium 35 per cent, of the
tubercular cases belonged to tubercular families, in another 38
per cent., and so on, are not of great theoretical interest. The
reappearance is due, in the first place, to the inheritance of the
constitutional predisposition — i.e. of a bodily soil very open to the
entrance of the weed, very suitable for its culture, very weak in
the power of resisting its ravaging growth. The reappearance is
due in the second place to the too common persistence of functional
and environmental conditions favourable both to infection and to
the enfeeblement which means defeat. It is enough to allude to
the lack of fresh air and exercise. It is an old story, told in many
forms and very true, that one boy of a tubercular family went to
sea and alone escaped the doom which befell his brothers and
sisters. Nor are cases unknown where a return in imagined security
to the old home in the town, and to the sedentary life of a clerk,
has resulted in belated but fatal infection. In the third place, we
have to bear in mind the likelihood of one member of a family
infecting another with the tubercle bacillus.

But besides the transmission of a constitutional vulnerability,
besides the rare occurrence of ante-natal infection, besides the
likelihood of household infection, besides the persistence of con-
ditions of life which favour the disease — are there any other factors ?
There are probably two others. On the one hand, a seriously
tubercular mother may be unable adequately to nourish her offspring
before and after birth, and the ill-nourished offspring becomes the
more readily the prey of disease. On the other hand, it seems
likely that the bodily disturbances induced by tubercular disease
in the parents may prejudicially affect the vigour of the germ-cells
themselves, and thus lead to the production of inferior offspring.

Syphilis. — As this disease appears to be due to a specific microbe,
its reappearance in the offspring of syphilitic parents is not strictly
a fact of inheritance. The father may infect his offspring without
the mother being affected, and it is possible that the microbe may
enter the ovum with the spermatozoon. The father may affect
his offspring indirectly by first infecting the mother — that is, the
microbe may pass through the placenta into the child. In certain
cases — e.g. when conception occurs soon after the date of the primary
disease — the probabilities of the offspring being infected are great.

MALFORMATIONS AND THE LIKE 287

though there is always some uncertainty. Of twins, one may be
infected and the other not. But the chances are so many that a
patently syphilitic father will have syphilitic or in some way deterio-
rated children, that the marriage of a patently syphilitic subject
can only be called a crime — the more heinous since the disease in
the offspring is often more serious than in the parent. It seems,
furthermore, certain in the case of this disease that, apart from
the specific ante-natal infection of offspring, the toxins produced
by the microbes in the body of the parent or parents may induce
general disturbance or debility of constitution in the germ-cells,
and thus result in inferior offspring.

§ 7. Dejects, Multiplicities, Malformations, and other

A bnormalities

For convenience, though we are here passing away from disease,
we may include in this chapter a few references to the inheritance
of abnormalities in the wide sense.

(a) Defects. — There are many cases on record where an absence
or deficiency of a particular structure has persisted for several
generations. Some of these minus variations have been utilised
by man as the origin of new domesticated breeds. It is enough
to mention hornless cattle — e.g. Polled Angus ; earless sheep —
e.g. of Syria and China ; tail-less cats — e.g. of Japan and the
Isle of Man ; short-tailed dogs and pigs. Such cases must be
distinguished from others quite different in nature, where a part
is absent through mechanical constriction during development,
and then, of course, no inheritance is to be looked for.

Albinism or absence of pigment is frequently inherited in
man.

Sir William Turner gives a rather striking case where a shorten-
ing or imperfect growth of the metacarpal bone of the ring-finger
of the left hand " was traceable throughout six generations,
and perhaps even in a seventh, and was, as a rule, transmitted
alternately from the males to the females of the family."

In a family in Pennsylvania described by Farabee many of the

288 HEREDITY AND DISEASE

members had all their fingers and toes two-jointed like the thumb
and big toe. The normal members had normal children, even
in the case of a first-cousin marriage. The abnormal members
married normal individuals, and the fourteen families bred in
this way contained 33 normals and 36 abnormals — a close
approach to equality. The abnormals are indicated by capitals.

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I I I I I I II I I I I I I J J

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I I I I I I I

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M/F»iM/FFMF fmtnYtn tnftnMmYY FMM/mm mfY mU

Along with defects of parts we may include imperfections due
to an arrest of the normal course of development at certain stages,
perhaps through inadequacy of nutrition, perhaps because of
what we must vaguely call " deficient developmental vigour."
Thus, hare-lip is practically the persistence of a normally transient
condition, and cleft palate is in the same category. Hutchinson
has recorded hare-lip in ten members of a family of twenty.

Inhibitions or disturbances during ante-natal life are believed
to result in various other abnormalities, such as cleft-palate,
cervical fistulae (persistence of traces of visceral clefts), spina
bifida, certain peculiarities of the eyes and teeth, and so on.
These abnormalities occasionally recur repeatedly in a family
tree, but it seems probable that what is really inherited is a
deficiency in " developmental vigour," accentuated by nutritive
defects on the part of the mothers during the period of gestation.

(b) Multiplicities. — As with defects, so in regard to multi-
plicities. Polydactylism has been known to recur through six

MULTIPLICITIES AND MALFORMATIONS 28^

generations of a human family. Bedart records quadruple
polydactylism of hands and feet through three generations of a
Perigord family (C. R. Soc. Biol. Paris, 9th series, vol. iv. 1892,
p. 367). Lucas cites a case of a Spanish family which included
forty instances of polydactylism, and Pliny tells of similarly dis-
tinguished families in ancient Rome. Hereditary polydact3'lism
is well known in cats.

(c) Malformations of Parts. — There are records showing the
hereditary recurrence of abnormalities in dentition, in the eyes,

Fig. 28. — Half-lop rabbit, an abnormal variation, which by artificial
selection has become a stable breed. (From Darwin.)

in the hands (e.g. webbed fingers), in the feet (e.g. club-foot)
— indeed, in most parts of the body ; but in most cases the
likelihood of transmission does not seem to be great.

(d) Pre-natal Influences resulting in Mutilations, Multipli-
cations, etc.— Recent embryological experiments have shown
incontestably that certain types of monstrosity can be readily
induced artificially by subjecting the developing ovum to shak-
ings, alterations of temperature, injections of various stuffs,
and so on ; and although the experiments relate mainly to birds,

19

290 HEREDITY AND DISEASE

amphibians, fishes, and lower animals, there is some evidence
that analogous factors may occasionally operate in mammals.
Thus, the pressure of amniotic strands may divide the rudiment
of a limb into two or may cause a mutilation. All such cases
are equivalent to accidents in after-life ; they are in no way ex-
pressions of the inheritance, and there is no evidence to show
that they have any effect upon the inheritance.

" The Hapsburg lower lip or the large nose of Orleans is truly
an item in the inheritance, but the occasional absence of an arm
(due to a constriction of the rudiment by a strand of the amnion)
is an intra-uterine acquisition ; it is congenital, but it is not
inherited " (Martius, 1905, p. 14).

It has sometimes been remarked that certain families show
a hereditary tendency to have wens (" small cystic or encysted
tumours ") on the head and upper parts of the body. The nature
of the growth, its inconstant position, and the time at which it
appears (usually about middle age) show that we should not
speak of the inheritance of a wen, but rather of the inheritance
of some skin-weakness.

§ 8. Some Provisional Propositions

I. Abnormal Peculiarities may find Expression in One Sex
only. — (a) Most of man's defects and predispositions to disease are
transmitted to both sexes (equally or unequally) through suc-
cessive generations. Polydactylism and some forms of cataract,
Huntingdon's chorea and diabetes insipidus, may be given as
instances. The liability of the sexes is in some cases very
unequal ; thus exophthalmic goitre is rare in males.

(b) In some other cases, such as albinism, there is a marked
tendency to skip a generation or even two generations, but as in
the first group both sexes are liable to be affected. Albinism is
Well known to be a recessive character, and we can readily under-
stand, as Mott points out, how the marriage of two apparently

PROVISIONAL PROPOSITIONS 291

quite normal individuals, e.g. cousins, each having albinism
latent or recessive in the germ-plasm, may result in one or more
of the offspring being albinos (Medical Chronicle, 1911, p. 75).

(c) In a third group the disease finds expression in one sex only
(the males), but may be transmitted by the apparently unaffected
other sex. Thus haemophilia — a chronic liability to excessive
bleeding — is almost always, if not (according to Bulloch and
Fildes) always, confined to males. It is partly associated with
weakness in the walls of the blood-vessels, and partly with a lack
of coagulating power in the blood. The disease passes from an
affected father through an unaffected daughter to a grandson.
For some unknown physiological reason it does not find expres-
sion in the female sex, unless, perhaps, in some disguised form.

Colour-blindness or Daltonism has been recorded (Horner) through
the males only of seven generations, and it is usually confined to
males. Dejerine cites a pedigree (fide Appenzeller) in which all the
males had a kind of cataract through four generations. Mott men-
tions as other cases of abnormal conditions generally restricted to
the males, " pseudo-hypertrophic paralysis " and " hereditary optic
neuritis or optic atrophy."

Edward Lambert, born in 1717, is said to have been covered with
" spines." His six children showed the same peculiarity, which
began to be manifest from the sixth to the ninth month after birth.
One of his children grew up and handed on the peculiarity to another
generation. Indeed, it is said to have persisted for five generations,
and in the males only, — unilateral transmission. (See Phil. Trans.
1755; Prichard, History of Mankind, 1851.)

2. The Expression of Disease-inheritance may change
from Generation to Generation.—" Diseased organisms are apt
to breed disease, but not always, though sometimes, their own
disease." This cautious statement seems to be well borne out
by the facts.

Hannot (Arch. gen. de Medecine, 1895) gives the following
illustrations. A typical gouty subject, with his joints hampered
by accumulations of urates, may beget a son as gouty as himself,

292 HEREDITY AND DISEASE

or it may be that the son is asthmatic. An alcoholic patient
may have an epileptic child. A tubercular mother may have
a child with Pott's disease. A man infected with syphilis may
have a son afflicted with general paralysis. In regard to the
last case, it may be, as has been recently suggested, that even
general paralysis has its associated micro-organism, which finds
a suitable soil in syphilised tissues. It is probable, at all events,
that syphilis is one of the predisposing causes of general paralysis.

It is easj' to add to these illustrations. " An inheritance from
a parent who has suffered from psoriasis may possibly be trans-
mitted as ichthyosis, or some form of chronic eczema or lichen "
(Hutchinson, 1896, p. 66). A man with tabes may beget a
child with epilepsy. An eye defect, such as microphthalmia,
may be represented in the offspring by quite a different ab-
normality. Perhaps the best examples of change of outcrop are
furnished by nervous disorders. Convulsions in one generation
may be represented by hysteria in the next, or hyperesthesia
by mania, or insanity by epilepsy, and so on.

As Prof. F. W, Mott says, speaking from a wide knowledge of
nervous diseases : " It is not necessarily insanity that is inherited,
but a neuropathic tendency in the stock which manifests itself
in many forms, e.g. epilepsy, asthma, migraine, chorea, diabetes,
exophthalmic goitre, neurasthenia, eccentricity, hysteria, crimin-
ality, fanaticism, suicide, genius of a certain type, and insanity "
(1911, p. 80).

It may appear for a moment that these illustrations prove too
much, suggesting as they do that the inheritance of morbid pre-
dispositions is very inconstant. But it must be noted, first, that
there are even more abundant instances of diseased predispositions
breeding true, and second, that they bear out what has been already
emphasised, that in most cases what is inherited is rather an abnormal
metabolism than a specific disease.

It is doubtful whether we are warranted in speaking of the " trans-
mutation of disease," for this phrase seems to suggest that a par-
ticular kind of process may change inhereditary transport into another

CHANGE IN EXPRESSION OF DISEASE 293

particular kind of process. It is probable that some diseased
conditions which get different names are fundamentally the same ;
it is their expression only that changes in response to the conditions
of nurture and environment.

The facts of what is often called " transmutation of disease "
suggest that what is inherited is sometimes a very general
peculiarity, which finds this or that expression in relation to the
conditions of the body — a very variable soil — and according to
the liberating stimuli which are available, such as the diet,
climate, and other conditions of life.

It is well known in medicine that a predisposition or diathesis
may express itself in half a dozen different ways — being poly-
morphic, as it is said — though there may be one way or two ways
which, being most frequent, may be called " diagnostic " or " dis-
tinctive." Thus, the tubercular tendency has several different
ways of expressing itself, probably depending mainly on the nature
of the nutritive and other environmental influences.

But if the same disease may find different expression in, let
us say, three brothers, it is not surprising that the disease of
a parent may take a different, though analogous, form in the
offspring, and perhaps a third form in the grandchildren. It may
be intensified, or weakened, or directed on new lines, the change
depending, so far as we can see, partly on the amphimixis or
duality of the inheritance, and partly on the external conditions.
Thus, if both parents have a markedly phthisical tendency, the
probability is that there will be in the offspring a more pronounced
similar predisposition than if one of the parents had belonged
to an untainted stock ; or, again, apart from amphimixis, a
thorough change in habits and surroundings may at least greatly
inhibit the phthisical outcrop in the offspring.

There is probably a very simple reason why a hereditary ten-
dency to nervous disease should have different expressions in
successive generations, and it is this : that many if not most
abnormal neuroses — e.g. epilepsy and insanity— emerge during

294 HEREDITY AND DISEASE

the period of development, and are due to defects or arrests in
the development, ultimately traceable to deficient nutrition
of the tissues, or to a lack of vigour in the germinal material
to begin with. What is inherited is this general tendency to
debility, and it is for the environmental influences to determine
the precise lines of least resistance.

3. Some Predispositions to Disease are much more herit-
able than others. — Statistics seem to prove what a general
outlook suggests, that some predispositions to disease are much
more likely to have hereditary re-expression than others. But
the cautious student will bear in mind two saving clauses :
(1) that non-expression does not necessarily imply non-inherit-
ance, for a morbid character often skips a generation, or more
than one ; and (2) that recurrence does not necessarily imply
inheritance, for a particular predisposition may crop up de novo,
or, in other words, the fountain of variation may repeat itself.
No one will go the length of supposing that the rheumatic
tendency has not originated afresh over and over again, or
of tracing the whole burden of rheumatic disease back to man's
pre-human ancestry because rheumatism occurs in monkeys. As
already noted, acute rheumatism is probably microbic.

A few illustrations of the variable probabilities of transmission
must suffice. In one long family history, gout is said to have
persisted for four centuries. Out of 523 gouty subjects, 309
had a family taint (about 60 %) ; out of 156 cases, 140 had a
family taint (about 90 %) ; various sets of cases show percentages
varying from 50 to 100. Out of 104 cases of diabetes mellitus,
22 had a family taint (about 20 %). In one long family history,
dealing with about 400 members, there were 26 cases of haemo-
philia ; in another, dealing with 100 members, there were 17
"bleeders."

Out of 901 admissions to an asylum, 477 had insane relatives ;
out of 321 cases of epilepsy, 105 had a family taint (about 35 %) ;
out of 208 cases of hysteria, 165 had a family taint (about 80 %).

UNCERTAINTIES IN INHERITANCE 295

Various specialists on mental disorders have found reason to
believe in hereditary transmission in from 25 to 85 per cent, of
their patients, the diversity being doubtless in part due to the
great variety of nervous diseases.

4. Many Uncertainties in Inheritance. — It is seldom possible
to say that a predisposition to a disease expressed in a parent
must be transmitted to the offspring. A predisposition to a
disease is rarely a sharp and definite character, such as we are
familiar with in " varieties " (of a species), which so frequently
breed true. It often means simply a slight disturbance of what
we may call the " gearing " of an organism — a slight derange-
ment of the normal sequences of the metabolism. It is an
unstable fluctuating variation. It is usually admitted that there
are families hereditarily predestined to be gouty or rheumatic,
but is any expert on either disease willing to stake his reputation
on the prediction that a particular gouty father is sure to have
gouty children ?

It is not difficult to understand why individual prediction as
to the inheritance of predispositions to certain diseases is im-
possible. An individual inheritance is a mosaic of parental and
ancestral contributions. The reduction of these in the process
of maturation, the possibilities of fresh permutations and com-
binations in amphimixis, the variability of the germ-plasm under
the influence of nutritive oscillations in the blood-stream, the
probable occurrence of some sort of intra-germinal struggle
among the hereditary items (all living and self-assertive), the
importance of nurture in favouring the expression of one char-
acter and hindering that of another — the whole circumstances
of the case, in short, are so complex that prediction for individuals
is out of the question, no matter how certain we may be as to the
average result in 1,000 cases. A predisposition to a disease has
to run the gauntlet just like a predisposition to mathematical
insight or musical talent. There is just this difference, that
predispositions to disease are commoner, that they often occur

296 HEREDITY AND DISEASE

in many ancestors of a given child, and that the chances of
their being transmitted are therefore greater.

Even when there is reason to believe that an offspring has inherited
a predisposition to a particular disease, it does not necessarily
follow that this item in the inheritance must be expressed in develop-
ment.

5. Predispositions may dwindle away, — There is, at least,
some evidence to show that hereditary tendencies to particular
diseases may dwindle until it becomes almost permissible to say
that they have been eradicated.

The biological interpretation of this is twofold : (1) that
interbreeding with an untainted stock may result in an over-
powering of the vicious tendency or in a re-habilitation of the
normal ; and (2) that in the course of selection the more severely
tainted tend to die out, thus leaving the race relatively stronger.
The biological caution is that we must not infer from non-
reappearance," or from non-expression — e.g. in healthier con-
ditions of function and environment — that the evil tendency has
ceased to be inherited. Prof. Hamilton says (1900, p. 301),
" My firm conviction is that if a vicious line be introduced it
may die out, and probably does in most cases die out by inter-
breeding with a series of pure stocks ; but that no reliance can
be placed upon its not recurring atavistically, it may be, genera-
tions after."

§ 9. Immunity

Immunity to a disease may be inborn or acquired. It may
be acquired in various ways : by having the disease and surviving
it — thus, recovery from smallpox usually confers an immunity
which lasts for years ; by being inoculated with the modified
virus of the disease — thus, vaccination confers immunity from
small-pox ; by being inoculated with a very minute quantity. of
the virus ; by being inoculated with the metabolic products or
toxins of the microbes • by having injections of the blood or

IMMUNITY 297

serum of another artificially immunised organism ; and even
by ingesting the microbes or their products. (See A. A. Kan-
thack, Allbutt's System of Medicine, vol. i., article " Infection.")

It seems that artificial immunity depends on processes within
the body which make the tissues able to destroy intruding
bacteria and to rob their products of their fatal potency. It
seems that specific anti- toxins are formed which immunise
the body to specific infection.

An acquired specific immunity may be transferred from
a mother to her offspring through the placenta, but this is
not in the strict sense inheritance. Ehrlich and others have
shown experimentally that rabbits and the like may be born
immune if the mother has been artificially rendered immune ;
and it has been asserted that in mankind the foetus may
become, through the mother, immune to smallpox.

In support of the view that those who are infected with a
plague and survive can transmit relative immunity to their
offspring, attention is called to the fact that epidemics have
their day and cease to be. But this admits of another inter-
pretation— the plague eliminates the most susceptible and
leaves the race in this way more resistent. What is transmitted
is the inborn power of resistance — which may be enhanced by
the selective process, especially if the plague is very severe
and lasts a long time. It is to be feared that there is very little
evidence of the transmission of acquired immunity — to smallpox,
for instance !

What is to be made of the alleged fact that two of the commonest
infective diseases in Britain— namely, scarlet fever and measles-
are much less virulent than they used to be ? According to a
skilful pathologist, Dr. William Russell, " this is almost certainly
to be attributed, not to an attenuation of the virus, or to im-
proved treatment, but to a measure of immunity acquired by a
population whose progenitors for generations have passed
through the ordeal of these infections."

298 HEREDITY AND DISEASE

Of course, this is a very difficult question, in regard to which
no one would wish to dogmatise. But one must not too readily
assume that the correct interpretation is the hereditary trans-
mission of acquired immunity, (i) It is possible that the
micro-organisms concerned are evolving in the direction of
attenuated virulence. (2) Much may be due to improved
treatment. Thus " measles " may not be really milder, but
simply better treated. (3) The result may be in part due to an
elimination of the most susceptible, which leaves the race as a
whole more resistent. (4) There may be a quite independent
widespread variational change in the direction of more resisting
power to these two diseases — a germinal variational change
quite apart from the ordeal of infection. (5) The power of
resistance may be improved by diet ; thus measles is often most
acute in out-of-work winters, and least acute when the nutritive
conditions are good. (6) The severe so-called " types " of cer-
tain diseases were probably " mixed infections." Nowadays
there is perhaps a greater number of " pure infections." (7)
Some of the more virulent germs are probably being stamped
out. There is no proof that the germ of the now somewhat rare
scarlatina maligna is the same race as that of the common
scarlatina simplex. (8) It is possible that the mothers may
through the placenta confer their own acquired immunity on
their offspring.

Natural immunity is a well-known inborn peculiarity, some-
times racial, sometimes personal, and manifested in various
degrees. Negroes are relatively immune to yellow fever and
ague ; Algerian sheep are relatively immune to anthrax ; certain
individuals appear to enjoy peculiar immunity in the midst of
epidemics.

It is generally believed that racial immunity has been gradually
wrought out in the course of natural selection. Germinal varia-
tions in the direction of immunity enable their possessors to
survive ; the survivors transmit their refractory -constitution ; the

CHROMOSOMES IN MAN 299

most susceptible are persistently weeded out : and thus, if the
infection persists long enough as a common mode of elimination,
a race may become relatively immune. No one doubts the
heritability of natural immunity, though there is still great
uncertainty as to what the mechanism of immunity is.

§ 10. Note on Chromosomes in Man

In a very interesting paper, Prof. H. E. Ziegler has illustrated
the modern doctrine of the material basis of inheritance with
particular reference to man ("Die Chromosomen-Theorie der
Vererbung in ihrer Anwendung auf den Menschen." Archiv.
fur Rassen- und Gesellschafts-Biologie, iii., 1906, pp. 797-812).

Let us take two parents, P1 S and P1 ? ; in each body-cell there
are 24 chromosomes, and in each mature germ-cell there are
12 chromosomes. Thus the fertilised ovum has again 24, and in
each cell of the offspring (F1) there are 12 chromosomes of paternal
origin (from P1 c?) and 12 of maternal origin (from P1 ?).

In the mature sperm-cell or egg-cell of the parent (P1 <$ or
P1 $) there are 12 chromosomes, but it does not necessarily
follow that 6 of these must be from a grandfather (P2 S), and

6 from a grandmother (P2 ?). Why not ? Simply because in
the reduction of chromosomes from 24 to 12, which occurs in
maturation, it does not necessarily follow that the parental (P2)
contributions are retained in equal number. The total number
12 always results, but it may be made up of 5 from P2 S and

7 from P2 ? , or of 8 from P2 <$ and 4 from P2 ? , and so on.
Suppose the mature sperm-cell had 9 from P2 S and 3 from P2 ? ,
then, as far as the paternal inheritance goes, we should expect
the offspring (Fl) to be very like its grandfather.

The chances are that the grand-paternal and grand-maternal
contributions in any mature germ-cell will approximate to
equality, but the numerous possibilities enable us to see one
reason at least why there is often great diversity in a family.

300 HEREDITY AND DISEASE

If we suppose that the chromosomes are all of equal value, there
is always a theoretical possibility in a human family of 169
different combinations of the grandparental contributions. It
is a well-known fact that certain predispositions to disease may
be seen in two or three children in a household and be quite
absent from other two or three.

The chromosomes of the four grandparents (P2) are made up
of contributions from eight great-grandparents (P3), and if the
reduction processes were always quite regular, the 24 chromo-
somes in a fertilised egg-cell should contain in the 12 of paternal
origin, 6 grand-paternal and 6 grand-maternal ; and either of these
groups of six should contain 3 great-grand-paternal and 3 great-
grand-maternal contributions. But if the reduction-processes
do not exhibit this improbable regularity, we may look for a
great variety of possible mosaic arrangements, — as indeed we
find. If we accept the chromosome theory, we can readily
understand how an innate defect or morbid predisposition in,
let us say, a grandfather, may be sifted out of the lineage ;
and similarly for a virtue !

The business becomes more complicated when we notice that
in a number of cases there are differences in the size of the
individual chromosomes ; it may be that particular characters
are bound up with particular chromosomes, and are not repre-
sented even by analogous items in others. Thus a particular
predisposition to disease in a particular organ may be embodied
in a particular chromosome, which might be thus conceivably
sifted out of the lineage altogether. In man, however, the
chromosomes are approximately of equal size.

Ziegler supposes that in man each chromosome has the same
value and influence, that each is capable of influencing the whole
organism, and that they differ only inasmuch as they are derived
from different ancestors, and thus embody diverse hereditary
tendencies.

The chromosomes of an individual usually represent eight

CHROMOSOMES IN MAN 301

families, and it is therefore likely that every one has some chro-
mosomes with a predisposition to some disease, such as phthisis,
or gout, or diabetes, or " nerves." A mosaic made up of con-
tributions from eight families can hardly avoid some such taint.
But the important point, Prof. Ziegler continues, is this, — what
numerical proportion do the tainted chromosomes bear to the
untainted ? If 3 out of the 24 have a diabetic taint, this will
mean much less than if there were 12 tainted. It follows that
taint on both sides of the house is particularly dangerous.
Ziegler gives the following illustrative schema.

Father — with marked taint, inherited from his father
and mother, as shown by the dark chromo-
somes— 13 out of 24.

000 © e o ••• • © o ooe © • o e o 0 000

T, 11 f«. OO® ©•©•©• ©00

Three mature sperm-cells www^-^www ^ ^

showing three different ■{ 6. O O O ©O© ©00 000

combinations

U'o

000 000 000

Mother— normal, though with a latent taint, inherited
from her mother, as shown by the dark
chromosomes — 4 out of 24.

000 000 000 000 oo© • © o 000 000

^, . u [ d. 00© ©©©000 OOO

Three mature egcr-cells

showing thiee different -'. e. O © © 000 000 OOO
combinations 000 000 000 000

U

It is evident that the child resulting from a x d would have
a badly tainted inheritance, that another resulting from ex/
would have a good inheritance, and that another from c x e
would be in the same position as the mother, and so on.

The practical importance of this very theoretical inquiry is
great, for we have here a suggestion of the way in which taints
may fall out of a lineage. A tainted determinant may be liter-
ally lost in the course of the reducing divisions of the germ-cells,

302 HEREDITY AND DISEASE

or it may be counteracted in amphimixis by stronger healthy
determinants from the other germ-cell.

§ II. Anticipation and Intensification in Disease

Careful work in recent years has brought into prominence a
very interesting and important tendency in certain diseased
conditions. The unsoundness becomes in successive generations
intensified and antedated. This has been called " the law of an-
ticipation." According to Nettleship, anticipation in hereditary
disease means the manifestation of the morbid change at an
earlier age in each successor, either in members of each succeed-
ing generation as a whole or in successively born children of one
parentage.

Thus a particular morbidity like the diabetic tendency may
come on earlier and earlier in successive generations. Thus, too,
a mentally degenerate stock may show earlier and earlier collapse,
e.g. by lack of resisting power to tubercle.

As to the theory of this anticipation, various suggestions have
been made. Dr. Nettleship says : " Anticipation or antedating
of onset or of completion in a family might be taken to show the
transmission of an acquired character. But it may probably be
explained as well or better by assuming certain defects, taints,
or vices of the system, say of the blood, are not only hereditary
in the true or germinal sense, but able to produce toxic agents
in the embryo which have an evil influence upon all its cells, and
thus so lower their power of resistance that the innate hereditary
factor has freer play and is likely to manifest itself earlier. There
may also be toxic agents in the embryo that have no relation
to the hereditary vice but yet may and probably do act in a
similar manner as excitants of the disease." That is to say, more
roughly, a general and progressive degeneracy may give a specific
morbidity more and earlier opportunity.

Another authority who has done much to disclose the facts of

PRACTICAL CONSIDERATIONS 303

" anticipation," Prof. F. W. Mott, looks at the problem in another
way. He supposes that unsound determinants in the germ-cells
may be attracted to one another, " and as it were coalesce or
crystallise out," thus causing the disease to appear in a more
intense form and at an earlier age.

In the present state of our knowledge it is impossible to be
otherwise than vague in regard to these things. It may be
useful, however, to recall Weismann's subtle conception of the
struggle of determinants within the germ-plasm, which he sup-
poses may account for alleged cases of definite variation in a given
direction. Perhaps this " anticipation " is of the nature of a
definite variation, though as it happens in a fatal direction — a
facilis descensus Averni.

But the practical importance of the fact of anticipation is
obvious. It is one of Nature's many devices to eliminate unfit-
ness, to sift out the unsound members of the stock. The diseased
condition is pushed further and further back, even to birth, or
even before it !

§ 12. Practical Considerations

A medical authority (quoted by F. Martius, 1905) goes the
length of saying, " For the practitioner the concept of heredity
is quite useless, and he should not deal with it at all. What
is wrought out during the life of the individual can be dealt with.
What is due to the parents is unalterable.^

This is an extreme expression of the practical pessimism which
many feel. We cannot choose our parents ; we cannot refuse
our legacy.

But this extreme pessimism is unwarranted. The fact is that,
if " the inheritance of disease " really occurred to the extent and
in the manner many medical writers assume with so much convic-
tion, the human race would have been extinct long ago, or in any
case we could not now have the broad and strong stream of

304 HEREDITY AND DISEASE

healthfulness which, in spite of all disease, still surges around
us.

Let us look for a little at the more hopeful aspects of the
question :

(i) As regards microbic diseases, a predisposition to which may
be inherited, the progress of hygiene and preventive medicine
tends increasingly to diminish the risks of infection or of fatal
infection.

(2) There is some reason to believe that, in regard to some
microbic diseases, a relative constitutional immunity is in
process of evolution.

(3) There is no scientific warrant for believing that acquired
diseases — i.e. those arising as modifications from without, to
which there is no specific predisposition — are as such trans-
missible. By liberating toxins and the like in the body, or by
depressing the general nutrition, acquired diseases may pre-
judicially affect the germ-cells, and therefore the offspring. But
this is more remediable than specific changes in the germ-plasm.

Our view of the harm done by an ill-considered widespread
belief in the transmissibility of modificational or exogenous
diseases has been well expressed by one of the keenest workers
in the Public Health service : " The nightmare of the specific
inheritance of acquired diseases overloads the spontaneity of
life, paralyses the will, and hampers the preventive service in its
efforts to improve the environment. Weismannism exalts the
social inheritances, which, as the great organs of selection, consti-
tute the basis of preventive medicine" (W. Leslie Mackenzie).

(4) In regard to constitutional diseases, it seems on the whole
that " the inheritance of predispositions to particular diseases "
is a more accurate description of the facts that the common
phrase, " the inheritance of disease." There is no doubt that
many predispositions to particular constitutional diseases are
inherited. What have we to set against thic ? We must
recognise that every item in an inheritance requires an ap-

PRACTICAL CONSIDERATIONS 305

propriate nurture if it is to be expressed, or expressed fully,
in development. This nurture is to some extent in our hands.
An organism with a predisposition to a constitutional disease
— let us say albuminuria, asthma, gout, diabetes, or some ner-
vous disorder — is obviously handicapped, more or less terribly
according to the strength of the predisposition. There is a
struggle for existence. But in this struggle a most momentous
factor is nurture in the widest sense — the conditions of function
and environment. If these favour the morbid elements in
the inheritance, the organism has to fight a battle with two
fronts, which is seldom hopeful. But if well-adapted conditions
of life be secured, and secured early, there is always considerable
hope that nurture may inhibit the full expression of the un-
desirable elements in the inherited nature.

(5) It seems to us that even expert writers have sometimes
exaggerated the necessity of the persistence of constitutional
taints and defects ; for as it is well known that a highly advan-
tageous variation may fail to persist, why may not this be
equally true of one that is highly disadvantageous ? A " retro-
grade variety " — that is, one which has lost one of the charac-
teristics of the parent species — may arise in our garden and
breed true. Why may not something analogous occur in a
peculiarly vulnerable stock ? Why may not the vulnerability,
the disadvantageous predisposition, disappear ? Apart from
natural selection, sexual selection, and the like, it may be that
the subtle process of germinal selection is sometimes able prac-
tically to eradicate an abnormal or morbid peculiarity.

(6) Crosses between wheat-plants immune to rust and others
susceptible to rust yield hybrids which are all susceptible. But
if these hybrids be inbred, the progeny are partly susceptible
and partly immune, and all those that are immune breed true.
If it is thus possible among plants " to get a pure thing out of
an impure " — it may be that for domestic animals and for man
himself the purification of a tainted stock is not a chimera.

20

3o6 HEREDITY AND DISEASE

(7) As to the diffusion of disease by the intermarriage of
badly tainted with relatively healthy families, we have this in
our own hands, and we need not whine over it. The basis
of preferential mating is not unalterable ; in fact, we know that
it sways hither and thither from age to age. Possible marriages
are every day prohibited or refrained from for the absurdest of
reasons ; there is no reason why they should not be prohibited
or refrained from for the best of reasons — the welfare of our race.

By the education of conscience on a scientific basis there is
already arising a wholesome prejudice against the marriage and
especially the intermarriage of subjects in whom there is a
strong hereditary bias to certain diseases — such as epilepsy and
diabetes, to take two very different instances. Is it Utopian
to hope that this will extend with increasing knowledge, and
that the ethical consciousness of the average man will come
more and more to include in its varied content " a feeling of
responsibility for the healthfulness of succeeding generations " ?

The argument always used against deliberate preferential
mating on a eugenic basis is that our ignorance is immense.
And this must be frankly admitted. Yet there are some things
that we do know. We know that " the manifestly syphilitic
subject who marries before he is thoroughly and definitely cured
commits a crime, not only because there is the possibility —
indeed, the probability — that he infects his wife, but also because
he deliberately [vcraussichtlich] begets syphilitic children. . . .
The Eugenic office of the future, which will have to test applicants
for a marriage-licence, not merely juristically or socially, but
also biologically and medically, to decide as to their fitness for
legitimate reproduction, will have no difficulty in refusing per-
mission to uncured syphiliticus and incurable drunkards, and
perhaps also to those who are patently tubercular " (freely
translated from Martius, 1905, p. 24).

That the best general constitutions should be mated is the
first rule of good breeding.

PRACTICAL CONSIDERATIONS 307

That a markedly good constitution should not be paired with
a markedly bad one is a second rule — a disregard of which
means wanton wastage.

A third rule is that a person exhibiting a bias towards a specific
disease should not marry another with the same bias. A man
with a very marked phthisical tendency, if he marries at all,
should not marry a woman whose family history is known to
show many phthisical subjects.

In other words, every possible care should be taken of a
relatively sound stock. The careless tainting of a good stock
is a social crime. Every reasonable precaution should be taken
to prevent a badly tainted stock from diffusing itself.

(8) Besides the advance of preventive medicine, the spreading
enthusiasm for health, the awakening of a eugenic conscience,
the suggestions as to " marriage-licences " and other forms of
social selection, all making for the greater healthfulness of the
human breed, we have, of course, to remember that our race has
not got beyond the scope of natural selection, much as we try
to evade it.

In the course of natural selection, keenest during the early
years of life, the most tainted and the least immune or resistent
tend continually to be " weeded out," and the standard of fitness
is thus kept from falling rapidly. When predispositions to
specific diseases accumulate (e.g. by in-breeding of similars), a
non-viable, sometimes a non-reproductive, type arises, and —
disappears for ever. Rotten twigs are always falling off the
tree of life. There is a continual irrecoverable precipitation of
incapables, who thus cease to muddy the stream.

But while this is true, every one is aware that man is so con-
stituted that he cannot submit to Nature's winnowing. For
reasons that go to the very foundations of our social frame-work,
we can neither act as Spartan eliminators ourselves nor allow
Nature to have her way. That this does not prevent us from
being perhaps more cruel than either, is to be gravely feared, but,

308 HEREDITY AND DISEASE

in any case, the fact is that we consistently try to conserve lives
which natural selection would eliminate. This may be for social
reasons necessary, but it cannot be regarded with satisfaction
unless it is associated with positive selection of the fitter types.

It has often been said that modern hygiene, in tending to
eliminate our eliminators — the microbes — is destroying a most
valuable selective agency which has helped to make our race
what it is.

It is difficult to find justification for the enthusiastic confidence
which some seem to have in the value of microbes as eliminators.
Which microbe ? Surely not that of plague, which strikes in-
differently, and is no more discriminatively selective than an
earthquake. Surely not that of typhus, which used to kill weak
and strong alike. Surely not that of typhoid, which may strike
anyone, and does not confer more than a passing immunity.
And so on through a long list.

It would perhaps be a subtler and more convincing line of
argument to say that, throughout the ages, man has been select-
ing the microbes, lessening their virulence, in a sense taming
them — sometimes to death — as his phagocytes were strengthened
by more suitable food, or as his "Opsonic Index" improved,
again perhaps in relation to food. As the body increases in its
power of holding out — and this is demonstrably modifiable — it
can prolong the contest with intruding microbes with more and
more hope of ultimate victory.

In any case, whether microbes have been important and
valuable selective agents or not, it is a sad confession on the
part of the " paragon of animals " if he cannot discover other
selective agencies — more discriminating, let us hope — to take
the place of disease germs.

At present, we can only indicate that the future of our race
depends on Eugenics (in some form or other), combined with
the simultaneous evolution of Eittechnics and Entopias. " Brave
words," of course ; but surely not " Utopian " I

CHAPTER IX

STATISTICAL STUDY OF INHERITANCE

" L'hybride est une mosa'que vivante." — Naudin.

The law of frequency of error " would have been personified by the
Greeks, and deified if they had known of it." — Francis Galton.

§ I. Statistical and Physiological Inquiries.

§ 2. Historical Note.

§3-/1 Hint of the Statistical Mode of Procedure.

§ 4. Filial Regression.

§ 5. Law of Ancestral Inheritance.

§ 6. Criticisms of Gallon's Law.

§ 7. Illustration of Results reached by Statistical Study.

§ 1. Statistical and Physiological Inquiries

When we study complex phenomena, such as the weather, we
usually follow two methods. On the one hand, we may collect
a multitude of observations — e.g., as to the rainfall in different
localities and at different times of year — and try from a careful
scrutiny of these to make some general induction, which will
show the inherent orderliness of sequences, even in such an
apparently disorderly complex as the weather. On the other
hand, we may give our attention to the actual mechanism of
certain occurrences — e.g., heavy rain with westerly winds and
low barometric pressure — and seek to show how certain con-
ditions are necessarily followed by certain results. In so doing,
we fall back on the general laws of physics, and we may be

309

3io STATISTICAL STUDY OF INHERITANCE

greatly assisted by crucial experiments — e.g., on the role of
atmospheric dust in connection with the precipitation of water
vapour.

Similarly, in regard to the complex facts of inheritance we
may pursue the same two methods. We may collect statistics
as to the resemblances and differences — e.g. as regards stature,
colour of eyes, intellectual ability, in successive generations —
and try to arrive at some general induction, which will show
the inherent orderliness even in a domain where occurrences
seem at first sight as capricious as those of weather. On the
other hand, we may focus our attention on the detailed course
of events in particular cases — we may inquire, for instance, into
the behaviour of the germ-cells before, during, and after ferti-
lisation— and try to understand how certain conditions are
necessarily followed by certain results. In so doing, we fall
back on the general laws of biology, and we are greatly assisted
by crucial experiments.

It is the aim of this chapter to illustrate what has been done
by following the statistical method of inquiry into the facts of
inheritance, and to state some of the inductions which have
rewarded this mode of procedure. As the subject is not an easy
one, and as it has been recently discussed by modern masters
like Francis Galton and Karl Pearson, and in expository works
such as Dr. H. M. Vernon's Variation in Plants and Animals
(London, 1903), and Mr. R. H. Lock's Variation, Heredity, and
Evolution (London, 1906), we shall confine ourselves to a brief
sketch.

When we have to study results that depend upon numerous
complicated conditions, the statistical method is of special
service. Not that it can ever tell us how the conditions lead up
to the results, but it will tell us what regularity there is in the
occurrence of the results, and by displaying some unexpected
correlation between certain antecedents and certain results, it
may put us on the track of discovering the mechanism that

APPLICATION OF STATISTICAL METHODS 311

connects them. Thus, while every one knows that the stature
attained by a thousand young men depends upon a multitude
of dimensions of different parts of the body, and that these
dimensions depend on numerous conditions, of which food is one,
climate another, and parentage a third, we owe it to statistical
methods that we are able to say definitely what relation
the average height of these thousand sons bears to the average
height of their fathers, that we are able to say, furthermore,
that their stature depends more on the stature of their fathers
than on that of their mothers. Thus we get a solid foundation
for further inquiries of a deeper sort.

Again, to take another illustration, we know enough in regard
to the results of four thousand throws of approximately sym-
metrical dice, to be able to say dogmatically, in regard to the
quite divergent results of four thousand throws of other dice,
that the latter must have been loaded. Similarly, as our know-
ledge of the laws of random sampling grows, we become able to
detect when Nature's dice are loaded.

It should be clearly understood that the generalisation
" Like begets like " may be much truer for the race at any
given time than for any one relation of parents and offspring.
Processes of selection in many forms tend to prune off pecu-
liarities— operating even before birth, operating in very early-
stages of independent life, and never ceasing to operate — and
thus one generation of a race may be very like the preceding
generation, although in cases of individual heredity there may
be marked differences between offspring and their parents. In
short, it is very important to realise the distinction between
individual heredity and race-heredity. The statistical study of
inheritance enables us to do this.

§ 2. Historical Note

In order to appreciate the statistical point of view and the
general ideas underlying its methods, the reader is advised to

3i2 STATISTICAL STUDY OF INHERITANCE

read chapter xii. of J. T. Merz's invaluable History of European
Thought in the Nineteenth Century (vol. ii., 1903, pp. 548-626).
He traces the development of methods — e.g. : the investigations
of Gauss and Laplace on the theory of error ; he gives examples
of their application — e.g. the kinetic theory of gases ; and he
shows how Quetelet was practically the first to apply statistical
methods to human problems in his celebrated work Sur V Homme et
le Developpement de ses Facultes, ou Essai de Physique sociale (1823) .

But " the first who seems to have fully grasped the Darwinian
problem from this (statistical) point of view is Mr. Francis
Galton, who in a series of papers, and notably in his well-known
works on Hereditary Genius (1869), and on Natural Inheritance
(1889), made a beginning in the statistical treatment of the
phenomena of Variation." " Mr. Galton's application of the
theory of error to the facts of distribution and variation
enabled him to bring method and order into such questions
raised by the Darwinian theory as natural selection, regression,
reversion to ancestral types, extinction of families, effect of
bias in marriage, mixture of inheritance, latent elements, and
generally to prepare the ground for the combined labours of
the naturalist and the statistician " (Merz, p. 618).

Among those who have followed Mr. Galton's lead the most
prominent and progressive worker is Prof. Karl Pearson, who
has published numerous important mathematical contributions
to the theory of evolution in the Transactions and Pro-
ceedings of the Royal Society since 1893, and in his journal
Biometrika. The reader who is not prepared for much
mathematics should consult the second edition of Pearson's
Grammar of Science. See also his Chances of Death and other
Studies in Evolution (2 vols., 1897).

§ 3. A Hint of the Statistical Mode of Procedure

Some idea of the mode of procedure in dealing statistically
with the facts of inheritance may be got from the following

STATISTICAL METHODS 313

statement by an experienced statistician, Mr. G. Udny Yule
(1902, p. 196) :

" A series of measurements is made of some one variable
character, e.g. a length, in parents and in their offspring, noting
the individual families (the more the better) and not merely
measuring the first generation as a whole and then their offspring
as a whole. From these measurements an equation is derived,
giving, as nearly as may be, the mean character of the offspring
in terms of the character of the parent. Supposing X to be the
character in the parent, Y the mean character in the offspring,
then the simplest form of such equation is :.

Y=A 4- BX,

where A is a dimension of the same order as X or Y, and B is a
number that will vary from case to case. We have for instance,
from the data collected by Mr. Galton for inheritance of stature
in man, reduced by Prof. Pearson, the equation relating mean
stature of sons and stature of father :

Y =31-10 + '45 X,

i.e. the mean stature of sons is 31 #i inches, together with nine-
twentieths of the stature of the father (also in inches, of course).
The father's stature is thus some guide to the stature of his
offspring ; it enables us to form a closer estimate of their stature
than we could from a mere knowledge of the mean characters
of the race, and we may therefore say that stature is an inherited
character. The sons do diverge from the race-mean in the
same direction as their parent. Quite generally, the statistician
speaks of a character as inherited whenever the number or
" constant " B is greater than zero ; if it does not differ sensibly
from zero the character is held to be non-heritable, quite apart
from the question whether the mean is more or less constant
from one generation to the next, a consideration which does
not affect the conception of individual heredity."

314 STATISTICAL STUDY OF INHERITANCE

§ 4. Filial Regression

It has often been remarked that the children of extraordinarily
gifted parents are sometimes very ordinary individuals, and that
the children of under-average parents sometimes turn out sur-
prisingly well, both physically and mentally. Every one who
has looked into the facts of inheritance in greater detail, and
has compared the average of qualities in successive generations,
has noticed in a general way that there is a tendency to sustain
the same average level from generation to generation. Even
the older inquirers, like Lucas, called attention to the fact that
extraordinary qualities in families tend to wane away, as if
there were some mysterious succession-tax levied on marked
deviations from the average, whether in the way of excellence or
of defect. But we owe to Mr. Francis Galton's careful statistical
work the generalisation known as the Law of Filial Regression,
which has replaced a vague impression by a definite formula.
He has defined and measured that tendency towards mediocrity
- — that tendency to approximate to the mean or average of the
stock, which is expressed by the term Filial Regression. We
may notice at the outset that this has nothing to do with
reversion or with degeneration, that it works upwards as well
as downwards, forwards as well as backwards.

The data which Gaiton utilised were chiefly the Records of
Family Faculties, obtained from about one hundred and fifty
families, and dealing especially with stature, eye-colour, temper,
artistic faculty, and some forms of disease. These were supple-
mented by measurements at Galton's anthropometric laboratory,
and by observations on sweet peas and to some extent on moths.

Most trustworthy, however, were the data procured in regard
to stature, which, as Gaiton points out, is a quality with many
advantages as a subject of investigation. It is nearly constant
during mature life, it is readily and frequently measured with
accuracy, and it does not seem to be of appreciable moment in

FILIAL REGRESSION 315

sexual selection. Its variability, though small, is nearly normal ;
that is to say, the normal curve of the frequency of error nearly
fits the distribution in many cases.

As the subject is by no means easy to those unaccustomed
to statistical inquiry, and as we cannot within our limits explain
the methods which Galton followed, it may be most profitable to
give a few illustrative quotations from Natural Inheritance (1889).

" If the word ' peculiarity ' be used to signify the difference
between the amount of any faculty possessed by a man, and
the average of that possessed by the population at large, then the
law of Regression may be described as follows. Each peculiarity
in a man is shared by his kinsmen, but on the average in a less
degree. It is reduced to a definite fraction of its amount, quite
independently of what its amount might.be. The fraction differs
in different orders of kinship, becoming smaller as they are
more remote " (p. 194).

In the population with which Galton dealt the level of medi-
ocrity in height was 68 J inches (without shoes). The law or
fact of regression which the statistics revealed was that the
deviation of the sons from the mean of the population (P) is, on
the average, equal to one-third of the deviation of the parent
from P, and in the same direction. If P ± D = stature of
the parent, then P =fc ^D = stature of the son. In these in-
quiries it is convenient to use the concept of a mid-parent, whose
stature is half-way between the stature of the father and the
" transmuted stature " of the mother, the last phrase meaning
practically the stature that the mother would have if she were
not female, i.e. an additional inch for every foot.

" However paradoxical it may appear at first sight, it is
theoretically a necessary fact, and one that is clearly confirmed
by observation, that the stature of the adult offspring must on
the whole be more mediocre than the stature of their parents,
that is to say, more near to the mean or mid of the general
population " (p. 95).

316 STATISTICAL STUDY OF INHERITANCE

While Galton's clearest results were obtained from data as
to stature, the general conclusion was confirmed in regard to
eye-colour, artistic faculty, and other qualities. There seems
no reason to doubt the general occurrence of regression towards
mediocrity, though it is doubtless modified in regard to char-
acters which are subject to keen selection, either natural or
sexual.

" The law of regression tells heavily against the full hereditary
transmission of any gift. Only a few out of many children
would be likely to differ from mediocrity so widely as their
mid-parent, and still fewer would differ as widely as the more
exceptional of the two parents. The more bountifully a parent
is gifted by nature, the more rare will be his good fortune if he
begets a son who is as richly endowed as himself, and still more
so if he has a son who is endowed yet more largely. But the
law is even-handed ; it levies an equal succession-tax on the
transmission of badness as of goodness. If it discourages the
extravagant hopes of a gifted parent that his children will
inherit all his powers, it no less discountenances extravagant
fears that they will inherit all his weakness and disease " (p. 106).

" It must be clearly understood that there is nothing in these
statements to invalidate the general doctrine that the children of
a gifted pair are much more likely to be gifted than the children
of a mediocre pair. They merely express the fact that the
ablest of all the children of a few gifted pairs is not likely to be
as gifted as the ablest of all the children of a very great many
mediocre pairs" (p. 106).

Nor must the fact of regression be supposed to affect the
general value of a good stock or the general disadvantage of
a bad one. Two gifted members of a poor stock may be person-
ally equivalent to two ordinary members of a good stock, but
" the children of the former will tend to regress ; those of the
latter will not " (p. 198).

Let us give a concrete illustration from Prof. Karl Pearson's

FILIAL REGRESSLON 317

Grammar of Science (1900, p. 454). " Fathers of a given height
have not sons all of a given height, but an array of sons of a
mean height different from that of the father and nearer to the
mean height of sons in general. Thus take fathers of stature
72 inches, the mean height of their sons is 70" '8, or we have a
regression towards the mean of the general population. On
the other hand, fathers with a mean height of 66 inches give
a group of sons of mean height 68"-3, or they have progressed
towards the mean of the general population of sons. The
father with a great excess of the character contributes sons with
an excess, but a less excess of it ; the father with a great defect
of the character contributes sons with a defect, but less defect
of it. The general result is a sensible stability of type and
variation from generation to generation."

The quotations which we have given make the general idea
of regression quite clear ; for the detailed evidence and for
further elaboration we must refer to the works of Galton and
Pearson.

It is necessary, however, to ask what this statistically
established fact of filial regression really means biologically.

Interpretation of Regression. — The facts of regression
are expressed as a whole in the striking statistical resemblance
between successive generations of a people. There is a continual
tendency to sustain the specific average. It can hardly be
denied that the similarity is in part the result of similar con-
ditions, e.g., of selection, but this hardly applies to the proportions
persisting between tall and short, dark and fair, and so on.
That it is not due to completeness of inheritance is obvious,
for " the large do not always beget the large, nor the small
the small " ; the children do not in any precise way repeat the
qualities of their parents. (Galton, 1889, pp. 1 and 116.) On
what then does this regression depend ?

Galton suggests two different reasons for the occurrence of
regression (pp. 104, 105). The first is connected with his idea

318 STATISTICAL STUDY OF INHERITANCE

of the stability of type, and may be thus expressed. This word
" type " has for its central idea the existence of a limited
number of recurrent forms — forms which have attained a con-
siderable degree of organic stability. A deviation from the
type may mean the attainment of a new position of organic
equilibrium, and many " sports " are said to be very stable ;
but it may also mean a position of instability from which a
regression to the old equilibrium is what might be expected.
Just as certain kinds of cells have very definite dimensions,
doubtless dependent in part on the optimum adjustment between
the volume and the surface, so many animals have a very definite
limit of growth, which doubtless represents a condition of con-
stitutional equilibrium. Where this is the case, it is easy to
understand that marked deviations in the direction of giants
or in the direction of dwarfs would tend to be unstable. Their
offspring may tend to regress to the position of stability simply
because it is the physiological optimum in given conditions.
The regulative phenomena in development would tend to secure
the regression, in the same mysterious way as they secure the
development of a perfect larva from a mutilated embryo. In
the particular case of human stature, a deviation of a few inches
may be quite immaterial, but it is easy to think of organisms
in which the proportions of the various bodily dimensions are
very important.

The other reason which Galton gives for the occurrence of
regression is found in what may be called the fact of mosaic
inheritance. The child inherits partly from its parents, partly
from its ancestry. " In every population that intermarries
freely, when the genealogy of any man is traced far backwards,
his ancestry will be found to consist of such varied elements
that they are indistinguishable from a sample taken at haphazard
from the general population. The mid-stature M of the remote
ancestry of such a man will become identical with P [the mean
of the present population] ; in other words, it will be mediocre."

FILIAL REGRESSION 319

" To put the same conclusion in another form, the most pro-
bable value of the deviation from P, of his mid-ancestors in
any remote generation, is zero " (p. 105).

Pearson interprets Filial Regression in similar terms. " A
man is not only the product of his father, but of all his past
ancestry, and unless very careful selection has taken place
the mean of that ancestry is probably not far from that of the
general population. In the tenth generation a man has [theo-
retically] 1024 tenth great-grandparents. He is eventually
the product of a population of this size, and their mean can
hardly differ from that of the general population. It is the
heavy weight of this mediocre ancestry which causes the son of
an exceptional father to regress towards the general population
mean ; it is the balance of this sturdy commonplaceness which
enables the son of a degenerate father to escape the whole burden
of the parental ill. Among mankind we trust largely for our
exceptional men to extreme variations occurring among the
commonplace, but if we could remove the drag of the mediocre
element in ancestry, were it only for a few generations, we
should sensibly eliminate regression or create a stock of excep-
tional men. This is precisely what is done by the breeder in
selecting and isolating a stock until it is established." {Grammar
of Science, 1900, p. 456.)

Prediction. — When we know the heights of a thousand fathers
of a given stock, and the heights of their sons, and the mean
height of the general population, we have a basis for constructing
a " regression equation," which may be used to calculate the
probable stature of the son of any father. But this prediction
maybe wide of the mark, since exceptional individual variability
often occurs. What will not be wide of the mark, however, is a
prediction as to the average height of the sons of a group of, say,
fifty fathers. If the formula [stature of son = 38""45 + '446
. x stature of father] be applied to fifty English middle-class
fathers of the same height^ it will be found that their sons have

320 STATISTICAL STUDY OF INHERITANCE

an average height differing but little from that indicated by the
formula. In regard to all these statistical conclusions, it must
be carefully borne in mind that they cannot be applied to indi-
vidual cases. " Of the individual we can assert nothing as certain,
only state the probable. The individual varies owing to the vari-
ability of the gametes, and we know nothing of the particular
gametes which fused to give the stirp, of which he is the product.
All we know in heredity is what degree of resemblance there is
on the average. . . . The statistician dealing with heredity is
like the physicist dealing with the atom — he can say little or
nothing of the individual, his knowledge is of the group containing
great numbers." (Pearson, op. cit., p. 457.)

Regression and Correlation. — As the term regression, used by
Galton to describe the extent to which an average son is more like
the mean of the stock than his father is, has been often misunder-
stood to imply something in the nature of a " throwback," it is
probably desirable to get rid of it and to substitute for it the tech-
nical term correlation, which expresses the extent to which a son
approximates nearer to his father than to the average of the stock.

The term " regression " which Mr. Galton introduced into
biometry is not really a biological term. As the late Prof.
Weldon pointed out in an interesting lecture, there may be
regression between two different sets of results of dice-throwing
if the second set of results is in some way, but not entirely,
dependent upon the first. He protested against regarding
regression "as a peculiar property of living things, by virtue
of which variations are diminished in intensity during their
transmission from parent to child, and the species is kept true
to its type " (1906, p. 107).

If a set of fathers deviate, in respect to some character, a certain
amount from the general mode of the whole population, their
sons will, in respect to the same character, vary about a mode
which is between the paternal deviation and the mode of the
whole population. This is filial regression.

FILIAL REGRESSION 32 1

Now, the amount of the regression affords a useful measure
of the intensity of the inheritance. If the regression is slight,
it means that the intensity of the inheritance is high ; if the
regression is considerable, it means that the intensity of the in-
heritance is low. The ratio between the deviation of sons in
general and the deviation of their fathers in general in respect
to a given character gives a measure of the intensity of inheritance
for that character, and is called the " coefficient of correlation."
A simple and very clear account of the way of obtaining a " co-
efficient of correlation " will be found in Doncaster's Heredity,
1910, chap. iv.

The correlations worked out by Pearson and others for a
number of characters in plants, animals, and man, vary between
0-42 and 0-52, which means that on the average the offspring
deviate from the mean of the general population about half as
much as the parent.

' It seems likely that in cases where the mating of parents
is not determined to any serious extent by their likeness or
unlikeness in the character discussed, the regression of children
on parents has a value very nearly the same, and very nearly
equal to i, for a large series of characters, mental as well as
physical, in human beings, and for a large series of characters
in the higher animals, at all events, if not in animals generally "
(Weldon, 1906, p. 108).

Summary. — Many individual organisms differ markedly from
the mean of the stock or race to which they belong. In some
character or characters they are extraordinary individuals.
What is the chief conclusion in regard to the offspring of these
individuals ? It is that they are, on an average, more mediocre
than their parents.

As Mr. Yule puts it, " This phenomenon of the relapse of the
offspring from the parental type towards mediocrity is termed
regression. Regression and not constancy of type is, for the
statistician, the fundamental phenomenon of heredity and the

21

322 STATISTICAL STUDY OF INHERITANCE.

prime fact to be explained by any physical theory " (1902,
p. 197).

It is explained on the general assumption that an inheritance
is a mosaic made up of contributions from a complex of ancestors
which when traced say to a tenth generation back correspond
to an average sample of the stock in question.

Note on Reduction of Ancestors. — To appreciate the possible
complexity of our mosaic inheritance we must recall the number
of our ancestors. We have two parents, four grandparents, eight
great-grandparents, about sixteen great-great-grandparents, and
so on. "If," as Prof. Milnes Marshall said, " we allow three
generations to a century, there will have been twenty-five since the
Norman Invasion, and a man may be descended not merely from
one ancestor who came over in 1066, but directly and equally
from over sixteen million ancestors who lived at or about that
date." But on these theoretical lines the existence of one man
to-day would involve the existence of nearly seventy thousand
millions of millions of ancestors at the commencement of the
Christian era. Which is absurd. What the theoretical scheme
fails to take account of is the frequent occurrence of close inter-
marriage— of cousins for instance. When we are dealing with a
large group of families, we find individual ancestors figuring in
different genealogical trees.

Brooks {Science, 1895, p. 121) points out that if the population
of a given district had for ten generations married first cousins
the total ancestry of each person would be only thirty-eight, instead
of the theoretical possible 2046. " An investigation into the
ancestry of three persons, not nearly related, living on an island
on the Atlantic coast where the records are complete for seven and
eight generations, shows that the ancestry of each of the three
averages only 382 persons" (Cope, 1896, p. 460).

The problem of reduction in the number of ancestors has been
very carefully discussed by genealogists like Prof. Lorenz and
Dr. F. T. Richter. We must be content to take one example.
Theoretically, Kaiser Wilhelm II. might have had in the direct line
the number of ancestors indicated in the upper row on the next
page ; the second row indicates the number actually known, on to
the twelfth generation ; the third row gives the number of those

REDUCTION OF ANCESTORS 323

possible ancestors of whose existence there is deficient record ;
and the fourth row gives the probable total.

Generations I. II. III. IV. V. VI. VII. VIII. IX. X. XI. XII.

(1) Theoreti-^ „ , , „ , _
cal number ) 2 4 8 16 32 64 128 256 512 1024 2048 4096

(2) Actual \

number > 2 4 8 14 24 44 74 1 1 1 162 200 225 275
known. J

(3) Inadequately known. 5 15 56 117 258

(4) Probable total. 116 177 256 342 533

§ 5. Law of Ancestral Inheritance

I In all ordinary cases of reproduction the offspring has a
strictly dual or bi-parental inheritance. Whether the inheritance
be blended, particulate, or exclusive in its expression, it is made
up, to begin with, of equal contributions from the two parents.
Obviously, however, if the concept of the continuity of the
germ-plasm be correct, the contribution from the father is
made up of contributions from his two parents, and the contri-
bution from the mother is made up of contributions from her
two parents. And so on backwards. Thus we reach the idea,
so often referred to in this volume, that an individual inheritance
is a mosaic of ancestral contributions. Incidental corroborations
of this fruitful idea are familiar to all — e.g. in the re-expression
of trivial details which were characteristic features of, say, the
grandfather or the great-grandmother. To Mr. Galton's careful
statistical work, however, we owe a generalisation which formu-
lates the share which the various ancestors have on an average
in the inheritance of any individual organism. This is the Law
of Ancestral Inheritance.

Galton's Statement of his Law. — Mr. Galton based his
generalisation on data as to stature and other qualities in man
and as to coat-colour in Basset hounds. His law is as follows :
" The two parents between them contribute on the average one-
half of each inherited faculty, each of them contributing one-
quarter of it. The four grandparents contribute between them

324 STATISTICAL STUDY OF INHERITANCE

one-quarter, or each of them one-sixteenth ; and so on, the
sum of the series i + J + i + xr + ■•••» being equal to i, as it
should be. It is a property of this infinite series that each term
is equal to the sum of all those that follow : thus | = 4 + i + iV
+ • • • •, \ = i + tV + • • • •> and so on. The prepotencies or
sub-potencies of particular ancestors, in any given pedigree, are
eliminated by a law that deals only with average contributions,
and the varying prepotencies of sex in respect to different quali-
ties are also presumably eliminated." Thus an inheritance is
not merely dual, but through the parents it is multiple, and
the average contributions made by grandparents, great-grand-
parents, etc., are definite, and diminish in a precise ratio according
to the remoteness of the ancestors.

The idea of diminution according to remoteness of ancestry
may be made more concrete by looking at some of the tables in
Galton's Hereditary Genius (1869). Thus 100 eminent men
have about 31 eminent fathers, 17 eminent grandfathers, and 3
eminent great-grandfathers.

Diagrammatic Expression. — The proportions contributed on
an average by the parents, grandparents, great-grandparents, etc.,
may be seen at a glance from a diagram (on the opposite page)
which we have borrowed from one of Mr. Galton's papers.

Pearson's Statement of Galton's Law. — Prof. Karl Pearson
states Galton's law in the following form : " Each parent con-
tributes on an average one-quarter or (o'5)2, each grandparent
one-sixteenth or (0'5)4, and so on ; the occupier of each ancestral
place in the «th degree, whatever be the value of 11, contributes
(o#5)2n of the heritage." He calls attention to the extreme
importance of the law, for " if Darwinism be the true view of
evolution — i.e. if we are to describe evolution by natural selection
combined with heredity — then the law which gives us definitely
and concisely the type of the offspring in terms of the ancestral
peculiarities is at once the foundation-stone of biology and the
basis upon which heredity becomes an exact branch of science "

GALTON' S LAW

325

9

2 I

§1§§mBW

1 20

■v> HW22

1

W i

":_:::::::HB|nG

=^J28

«i B — ffti

ral

Fig. 29. — Diagram illustrating Galton's Law of Ancestral Inheritance.
(After Galton.) The figure was originally due to Mr. A. J. Meston
(The Horseman, Chicago, Dec. 28, 1897).

" The area of the square diagram represents the total heritage of any particular form or
faculty that is bequeathed to any particular individual. It is divided into subsidiary squares,
each bearing distinctive numbers, which severally refer to different ancestors. The size of
these subsidiary squares shows the average proportion of the total heritage derived from the
corresponding ancestors. . . . The subject of the pedigree may be called :. Thenceforward
whatever be the distincUve number of an ancestor, which we will call n, the number of its
sire is 211, and that of its dam is 2»-f-i. All male numbers in the pedigree are therefore even
and all female numbers are odd. To take an example— 2 is the sire of 1, and 3 is the dam
of 1 ; 6 is the sire of 3 and 7 is the dam of 3. Or, working backwards, 14 is a male who is
mated to 15 ; their offspring is 7, a female, who is mated to 6 ; their offspring is 3, a female,
who is mated to 2, and their offspring is i, the subject. . . . The numbered squares could
be continued indefinitely ; in this small diagram they cease with the fourth generation,
which contributes a 16th part of the totai heritage, therefore the whole of the more distant
ancestry, comprised in the blank column, contributes one-sixteenth also" (Galton, 1898).

326 STATISTICAL STUDY OF INHERITANCE

(Grammar of Science, 1900, p. 479). Elsewhere he says : ' The
law of ancestral heredity is likely to prove one of the most brilliant
of Mr. Galton's discoveries ; it is highly probable that it is the
simple descriptive statement which brings into a single focus all
the complex lines of hereditary influence. If Darwinian evolution
be natural selection combined with heredity, then the single
statement which embraces the whole field of heredity must
prove almost as epoch-making to the biologist as the law of
gravitation to the astronomer."

Prof. Karl Pearson has himself given a statement of the law of
ancestral inheritance somewhat different from Galton's, but his
methods and general results are practically the same. The
following quotation (1903a, p. 215) is useful :

" Taking our stand, then, on the observed fact that a know-
ledge neither of parents nor of the whole ancestry will enable
us to predict with certainty in a variety of important cases the
character of the individual offspring, we ask : What is the correct
method of dealing with the problem of heredity in such cases ?
The causes A, B, C, D, E, . . . which we have as yet succeeded
in isolating and defining are not always followed by the effect
X, but by any one of the effects U, V, W, X, Y. We are, there-
fore, not dealing with causation but correlation, and there is,
therefore, only one method of procedure possible ; we must
collect statistics of the frequency with which U, V, W, X, Y, Z,
respectively follow on A, B, C, D, E. . . . From these statistics
we know the most probable result of the causes A, B, C, D, E,
and the frequency of each deviation from this most probable
result. The recognition that in the existing state of our know-
ledge the true method of approaching the problem of heredity
is from the statistical side, and that the most that we can hope
at present to do is to give the probable character of the offspring
of a given ancestry, is one of the great services of Francis Galton
to biometry."

Pearson has worked out the average correlation between off-

LAW OF ANCESTRAL INHERITANCE 327

spring and their parents, their grandparents, and so on backwards,
He finds that the correlation between offspring and parent is
about o"5, between offspring and grandparent 0-33, between off-
spring and great-grandparent 0"22. These figures indicate the
degree of resemblance, in respect of a character measured, between
offspring and an ancestor of each generation. From these he
has worked out the average ancestral contributions, and he has
been led to conclude that the series 0*6244, 0-1988, 0-0630, etc.,
is more accurate than Galton's series 0-5, 0*25, 0-125, erc-

Summary. — Galton formulated his Law of Ancestral In-
heritance as follows : " The two parents contribute between
them on the average one-half or (0-5) of the total heritage of
the offspring ; the four grandparents, one-quarter, or (o-5)2 ;
the eight great-grandparents, one-eighth, or (o-5)3, and so on.
Thus the sum of the ancestral contributions is expressed by the
series [(0-5) -f (o-5)2 -f (o-5)3, etc.], which, being equal to 1,
accounts for the whole heritage " (1897, p. 402).

But it is quite legitimate to accept the general idea of this
Law without accepting the fixity of the fractions of partial in-
heritance which it expresses.

Mr. G. Udny Yule states the law of ancestral heredity in the
most general way possible when he says : " This law, that the
mean character of the offspring can be calculated with the more
exactness, the more extensive our knowledge of the corresponding
characters of the ancestry, may be termed the Law of Ancestral
Heredity " (1902, p. 202).

Prof. Weldon (1902) states the law of ancestral inheritance in
the following terms : " The degree to which a parental character
affects offspring depends not only upon its development in the
individual parent, but on its degree of development in the ancestors
of that parent." Mr. Yule suggests that, instead of the word
" affects," which to some extent implies a direct physical in-
fluence, it would be more accurate to read " serves as a basis
for estimating the character of,"

328 STATISTICAL STUDY OF INHERITANCE

In a later paper Prof. Weldon discussed the validity of Galton's
Law, and wrote as follows : —

"... The results so far achieved make it probable that Mr.
Galton's original prediction will be verified for the large class of
cases to which he intended it to apply, and that the influence
of the different generations of ancestors, as measured by the
regression coefficients between these and existing individuals,
will be found to diminish with the remoteness of the ancestors,
according to the terms of a simple geometric series, which is
sensibly the same at least for all those characters among the
higher animals which have been properly examined " (Weldon,
1906, p. 108).

§ 6. Criticisms of Galton's Law

Since the importance of the law is great, we must devote
some attention to certain criticisms which have been made.
It goes without saying that those who wish to criticise the basis
on which the generalisation is founded must consult the original
documents, referred to in the bibliography.

It must be borne in mind that the Law of Ancestral Inheritance
is a statistical conclusion dealing with what is true on an average
for a large number of cases. To say that we know of particular
cases where it does not hold — where, for instance, the amount of
resemblance between an individual and his paternal grandfather
is far greater than is represented by the theoretical fraction
— is no argument against the induction. It is like saying that
the statistics showing the percentage of deaths in cases of scarlet
fever must be wrong because we know of large families which
were visited by the disease without a single fatal result !

It may be urged against the crispness of Galton's Law, (1) that
the hereditary relation is a complex affair ; (2) that most or-
ganic qualities, and the amounts of resemblance in successive

LAW OF ANCESTRAL INHERITANCE 329

generations, can seldom be measured with the accuracy possible
in the case of a quality like stature ; and (3) that the actual
quota of any character which forms part of a heritage is some-
thing different from the expression which that, quota finds in
development — for the expression depends in part on the con-
ditions of nurture. For these and similar reasons it may seem
suspicious that the fractions indicating the average contributions
of parents, grandparents, great-grandparents, etc., should be
representable in such a simple series as \ -f- \ -f \ -f- . . . .

The general answer is, of course, that when the data are
large enough, the irregularities of result due to particular pecu-
liarities, such as a highly prepotent great-grandfather, are
smoothed out.

While Galton sometimes spoke of his law in its physiological
aspect, there can be no doubt that it regarded it in the main as a
statistical description, dealing with average inheritances, and
applying to masses rather than to the component individuals
considered separately. Thus he distinctly says (1897, p. 402) :
" The neglect of individual prepotencies is justified in a law that
avowedly relates to average results."

Darbishire has tried by means of a diagram to clear up the
prevalent confusion which opposes statistical and physiological
formulae. In the figure there is a diagrammatic representa-
tion of four successive generations ; a1, bx, x1 ; a2, b2, x2, etc.,
represent adult individuals of these generations ; a1, /31, w1 ; a2,
[i2, (o2, etc., represent the germ-cells produced by those individuals.
Now the statistical formulation contents itself with keeping
above the line A — B, and deals with the successive generations
as generations, stating the relation of hereditary resemblance
which subsists between them. But the physiological inter-
pretation seeks to penetrate below the line A — B, and seeks to
show by a theory of germinal contributions how it is that a>
gives rise to a1, which may be more or less different, how a2
gives rise to rt3, which again may be more or less different,

330 STATISTICAL STUDY OF INHERITANCE

To bring out the contrast between statistical and physiological
conclusions, Darbishire refers to the familiar riddle " Why do
white sheep eat more than black ones ? " with its answer " Because
there are more of them." " When you ask the riddle you do not say
that you are not referring to individual white and black sheep,
but the man of whom the riddle is asked invariably thinks you are "
— with interesting consequences. " If he is a biologist he may
be trying to think of some physiological explanation of the fact,

B

(oT.

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gj

i

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J?5

tf\

A

@

®

1 j

.

,^

} j

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■

i j

5 i

';

.-■•■'"."•••'"
..-••*■

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i >

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j i
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,

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f

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i-

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Fig. 30. — Diagram to illustrate the difference between statistical and
physiological formulation. (After Darbishire.)

in connection possibly with the well-established relation between
pigmentation and the getting rid of waste products." " In the
answer he is told that the amount eaten by the sum-total of white
sheep as compared with that eaten by the sum-total of black sheep
is the subject under discussion."

" If the antithesis between truths about masses, and truths about
individuals, which constitutes the point in this riddle, were more
widely and more clearly perceived than it is to-day, there would
no longer be that confusion in the minds of most biologists which
prevents them seeing the profound difference that exists between
a physiological law like Mendel's, which is true of units, and a
statistical one like the Law of Ancestral Inheritance, which is true
of masses. All intending students of heredity should be asked this
riddle ; and if they cannot detect the fallacy in it they should be
declared unfit for their intended task."

While it is a confusion of thought to oppose a statistical con-
clusion and a physiological interpretation, it cannot be denied
that the Galtonian and the Mendelian views of heredity are
not yet in harmony.

Anticipating the next chapter, let us take a typical Mendelian
case. A pea of a tall race is crossed with one of a dwarf race ; the

STATISTICAL AND PHYSIOLOGICAL LAWS 331

offspring are all tall. Tallness is the " dominant " character and
dwarf ness is " recessive." The tall hybrids are allowed to self-
fertilise, which comes to the same thing as inbreeding, and the
next generation show 75 per cent, tails and 25 per cent, dwarfs.
The dwarfs if self-fertilised will produce only dwarfs ; they are
pure from all taint of tallness. Of the tails, one-third will
produce only tails when self-fertilised ; they are pure from all
taint of dwarfness. The remaining two-thirds will produce
when self-fertilised the same proportion of 75 per cent, tails and
25 per cent, dwarfs. They may be called impure tails. Thus
the result is:

Tall x Dwarfs = Parents

I
All Tails = Hybrid offspring
J-

25% Pure Tails + 50% Impure -f 25% Pure = Next generation

Tails Dwarfs

As we shall afterwards see, the result thus briefly summarised
— a statement of fact — was interpreted by Mendel in terms of a
simple theory, viz. that the germ-cells of the hybrid offspring
are segregated into two sets of pure gametes, each set bearing
in potentia one of the contrasted characters of the original tall
and dwarf parents. If so, the chances of fertilisation must give
the result indicated. Now the point is, that when we are dealing
with Mendelian unit characters, such as tallness and dwarfness
in peas, Galton's law is not relevant, whereas when we are dealing
with non-Mendelian characters, such as the ordinary fluctuations
of stature in mankind, Galton's law is a valuable statistical
description.

What are called Mendelian phenomena are illustrated when
the parents differ in sharply defined contrasted characters
which cannot blend or compromise, and the extension of ex-
periment will doubtless go on increasing our knowledge of these
unit characters and their behaviour. The formulation will

332 STATISTICAL STUDY OF INHERITANCE

remain whether the theory of the segregation of pure gametes
be confirmed or not. In other cases, however, the Galtonian for-
mulation seems the only one applicable, and here the need is to
work out — perhaps along the lines of Weismann's germinal
selection of determinants — a conceivable physiological inter-
pretation.

We must refer the reader to Mr. Yule's discussion (1902)
of the supposed antagonism between Mendelian and Galtonian
conceptions — a discussion which leads this expert to conclude
" that Mendel's Laws and the Law of Ancestral Heredity are
not necessarily contradictory statements, one or other of which
must be mythical in character, but are perfectly consistent
the one with the other, and may quite well form parts of one
homogeneous theory of heredity."

§ 7. Illustration of Results readied by Statistical Study

While we can neither explain the methods nor summarise the
arguments, it may be permissible to cite some of the results
reached by the statistical study of inheritance, always bearing
in mind the caution that the validity of a statistical result, like
the validity of any other scientific result, depends on the value
of the data. The world of organisms is very large and hetero-
geneous, and results that hold good for certain forms of life may
not be true of others.

It has been shown statistically that in the human race the
father is prepotent in the matter of stature, and this for offspring
of both sexes (Pearson).

It has been shown statistically that a subtle quality like
fertility is a heritable quality, and more detailed statements
can be made — e.g. that the woman inherits fertility equally
through the male and female lines.

The immediate practical bearing of some of these researches

ILLUSTRATION OF STATISTICAL RESULTS 333

is evident. Thus Messrs. Rommel & Philipps (1906) have shown
in regard to Poland China hogs : (1) that there has been an
increase of '48 in the size of litter in the twenty years between
1882 and 1902, and (2) that the size of litter is a character trans-
mitted from mother to daughter. " It would appear proved that,
by judicious selection for breeding purposes of sows from large
litters, the average for the breed may be increased."

Prof. Karl Pearson has been led by rigorous statistical methods
to statements like the following : —

" If selection were to act upon our 5' 9" Englishmen, and
the 6' among them were the type best fitted to survive, then
with fairly stringent selection it would not take more than six
generations to produce a type sensibly 6' high, and this type
would be permanently established even if selection ceased. . . .
Our determination of the quantitative strength of heredity is
thus seen to give values quite intense enough to produce
rapid and permanent changes of type, when selection is
stringent."

Prof. Pearson has worked out the following case. Suppose
the mean height of a population be 5 ft. 8 in., that a start is
made with individuals 6 ft. 2 in., and that for successive genera-
tions individuals of this height are selected as parents. It is
calculated that in the first generation the offspring would show
o-62 of the particular quality selected (h), viz. 6 in. of deviation
above the general mean height. It is calculated that after two
generations the offspring will show oSzh, after three generations
0-89//, and so on up to o-g2h. Thus by persistent selection
an array of individuals would result, almost all of whom were
over six feet in height.

But if at a given generation the artificial selection of tall
parents stops, and the tall array is left to inbreed, there will be
a gradual sinking back towards the mean height of the population.

The importance of definite conclusions of this kind can hardly
be overestimated.

334 STATISTICAL STUDY OF INHERITANCE

" Looked at from the social standpoint, we see how exceptional
families, by careful marriages, can within even a few generations
obtain an exceptional stock, and how directly this suggests
assortive mating as a moral duty for the highly endowed. On
the other hand, the exceptionally degenerate isolated in the
slums of our modern cities can easily produce permanent stock
also : a stock which no change of environment will permanently
elevate, and which nothing but mixture with better blood will
improve. But this is an improvement of the bad by a social
waste of the better. We do not want to eliminate bad stock
by watering it with good, but by placing it under conditions
where it is relatively or absolutely infertile " (Pearson, Grammar
of Science, p. 486).

By statistical methods Pearson has reached the interesting
conclusion that while blended inheritance illustrates regression,
it is to cases of exclusive inheritance that we should look for
reversion {i.e. the reappearance of a character which occurred
in a definite ancestor). In exclusive inheritance, in which the
offspring inherits the full character of either parent, and does
not blend the two, the law of ancestral inheritance in the strict
sense ceases to hold, for it presupposes a blend. Thus eye-
colour in man rarely, if ever, blends, and it is in regard to such
characters that we should look for reversion.

By statistical methods Pearson has sought to ascertain how
far the inheritance of the duration of life extends, and has reached
the important conclusion that in a large percentage of cases
there is evidence in the death-rate that discriminate selection is
at work. It is no longer possible to say of natural selection,
as Lord Salisbury did in 1894, that " no man, so far as we know,
has seen it at work." " It is at work, and at work among civilised
men, where intra-group struggle — i.e. autogeneric selection —
is largely suspended, with an intensity of a most substantial
kind. Of the existence of natural selection there can be no
doubt ; we require careful experiments and observation to indicate

ILLUSTRATION OF STATISTICAL RESULTS 335

the rapidity of its action. In a few years we may hope no
longer to hear natural selection spoken of as hypothetical,
but rather to listen to a statement of its quantitative measure
for various organisms under divers environments " (Grammar
of Science, p. 500).

CHAPTER X

EXPERIMENTAL STUDY OF INHERITANCE

As regards Mendel's Law, " The experiments which led to this advance
in knowledge are worthy to rank with those that laid the foundation
of the atomic laws of chemistry." — Bateson.

" The breeding-pen is to us what the test-tube is to the chemist — an
instrument whereby we examine the nature of our organisms and deter-
mine empirically their genetic properties." — Bateson.

" That Hurst can predict. the difference between the result of mating
two pairs of rabbits externally identical, by means of a knowledge of
the difference between their gametic constitutions acquired by previous
breeding from them, constitutes, it seems to us, the longest stride the
study of heredity has made for some time past." — Nature, lxxi. 1905, p. 315.

§ i. Mendel's Discoveries.

§ 2. Theoretical Interpretation.

§ 3. Corroborations.

§ 4. Illustrations of Mendelian Inheritance.

§ 5. Mendel's Discovery in Relation to Other Conclusions

§ 6. Practical Importance of Mendel's Discovery.

§ 7. Other Experiments on Heredity.

§ 8. Consanguinity.

§ 1. Mendel's Discoveries

In 1866 Gregor Johann Mendel,* Abbot of Briinn, published
what some regard as one of the greatest of biological discoveries.
After many years of patient experimenting, chiefly with the

* Gregor Johann Mendel was born in 1822, the son of well-to-do peasants
in Austrian Silesia. He became a priest in 1847, and studied physics

336

MENDEL'S EXPERIMENTS 337

edible pea, he reached a very important conclusion in regard to
the inbreeding of hybrids, which is often briefly referred to as
"Mendel's Law." His publication was practically buried in the
Proceedings of ilie Natural History Society of Briinn ; those
who knew of it, as Nageli for instance did, failed to realise its
importance : in fact, Mendel's epoch-making work was lost
sight of amid the enthusiasm and controversy which the pro-
mulgation of Darwinism (1858) had evoked. Mendel's Law
seems to have been rediscovered independently in 1900 by
the botanists De Vries, Correns, and Tschermak; and to Mr.
Bateson we owe much, not only for his recognition of the
far-reaching importance of the abbot's work, but also for a
notable series of experiments in which he has confirmed and
extended it.

Mendel's Experiments. — What Mendel sought to discover was
the law of inheritance in hybrid varieties, and he selected for
experiment the edible pea (Pisum sativum). The trial plants,
he says, must possess constant differentiating characters, and
must admit of easy artificial pollination ; the hybrids of the
plants must be readily fertile, and readily protectable from the
influence of foreign pollen. These conditions were afforded by
peas, and twenty-two varieties or subspecies of pea were selected,
which remained constant during the eight years of the experi-
ments. Whether they are called species, or subspecies, or
varieties, is a matter of convenience ; the names Pisum quad-
ratum, P. saccharatum, P. umbellatum, etc., do in any case repre-
sent groups of similar individuals which breed true inter se. It

and natural science at Vienna from 185 1 to 1853. Thence he returned
to his cloister and became a teacher in the Realschule at Briinn. It was
his hobby to make hybridisation experiments with peas and other plants
in the garden of the monastery, of which he eventually became abbot.
Apart from two papers, one dealing with peas and a shorter one with
hawkweeds, and some meteorological observations, he does not seem to
have published much. But what he did publish, if small in quantity,
was large in quality. He died in 1884.

22

338 EXPERIMENTAL STUDY OF INHERITANCE

should be noted that these peas have the particular advantage,
for experimental purposes, that they are habitually self-fertilised
— in North Europe, at least.

In studying the different forms of peas, Mendel found that
there were seven differentiating characters which could be
relied on :

i. The form of the ripe seeds, whether roundish, with shallow
wrinkles or none, or angular and deeply wrinkled ;

2. The colour of the reserve material in the cotyledons — pale

yellow, bright yellow, orange, or green ;

3. The colour of the seed-coats, whether white, as in most peas

with white flowers, or grey, grey-brown, leather brown,
with or without violet spots, and so on ;

4. The form of the ripe pods, whether simply inflated, or

constricted, or wrinkled ;

5. The colour of the unripe pods, whether light or dark green,

or vividly yellow, this colour being correlated with that
of stalk, leaf-veins, and blossoms ;

6. The position of the flowers, whether axial or terminal ; and

7. The length of the stem, whether tall or dwarfish.

Mendel's Results : The Law of Dominance. — Having defined
the differentiating characteristics of the varieties, Mendel pro-
ceeded to make crosses between these, investigating one character
at a time. Thus, pollen from a pea of the round-seeded variety
was transferred to the stigma of a pea of the angular-seeded
variety, the stamens of the artificially pollinated flower being,
of course, removed before they were ripe. The same was done
all along the line.

What was the result in the hybrid or cross-bred offspring ?
It was found that they showed one of each pair of contrasted
characters, to the total, or almost total, exclusion of the other.
No intermediate forms appeared.

Mendel called the character that prevailed dominant, and
the character that was suppressed, or apparently suppressed,
recessive. And the first big result was that crosses between a
plant with the dominant character and a plant with the recessive

B

MENDEL'S LAW
F.g.31.

Fio. 31.— Peas showing Mendel's Law.

A, Pod of yellow-seeded (dominant) parent ; B. Pod of green-seeded (recessive) parent ;
C, Pod of hybrid offspring— all with yellow seeds (F1) ; D, Pod showing the splitting up of
the next self-fertilised generation (Fa) into yellow seeded and green-seeded.

[Facing p. 389

THE LAW OF DOMINANCE 339

character yielded offspring all resembling the dominant parent
as regards the character in question. Let us for shortness
call the parents D and R, and the first result may be expressed
thus : D x R = D.

It must be carefully noted that the complete dominance
which Mendel observed has been shown in other cases to be
the exception rather than the rule. Thus a cross between a
"Chinese" primula with wavy crenated petals and a "star"
primula with flat simply notched petals is intermediate between
the two parents ; and yet, as the next generation shows, the
case is one of Mendelian inheritance.

In many cases the hybrid, while on the whole dominant, may
show some influence of the recessive character, but not nearly
enough to warrant us in speaking of a blend. Thus, when white
(dominant) Leghorn poultry are crossed with brown (recessive)
Leghorn, most of the offspring have some " ticks " of colour.
When these are inbred they produce a quarter brown (extracted
recessives) and three-quarters pure white or white with a few
ticks. The dominance is not quite perfect.

The Law of Splitting or Segregation. — In the next generation
the cross-bred plants (products of D and R, or R and D, but
all apparently like D) were allowed to fertilise themselves, with
the result that their offspring exhibited the two original forms,
on the average three dominants to one recessive. Out of 1,064
plants, 787 were tall, 277 were dwarfs.

When these recessive dwarfs were allowed to fertilise them-
selves they gave rise to recessives only, for any number of genera-
tions. The recessive character bred true.

When the dominants, on the other hand, were allowed to
fertilise themselves, one-third of them produced "pure" domi-
nants, which in subsequent generations gave rise to dominants
only ; and two-thirds of them produced once again the
characteristic mixture of dominants and recessives in the
proportiqn of 3:1.

*

340 EXPERIMENTAL STUDY OF INHERITANCE

The general results may be expressed in the following
scheme : —

D?xRc? or R?xDc?

Parent-forms (P1).

D(R)

Hybrid-offspring (F1).

3D

i R . Generation of inbred hybrids
(F*).

i D

D

2 D(R)

3D

i R R . (F3)

D

i D

D

+ 2 D(R)

3D

iR

iD + 2D(R)

I
D D D

R

R R . (F<)

R R . (F5)

The result of the hybridisation is a generation (F1) like the
dominant parent. They may be represented by the symbol
D(R), for they carry with them the possibility of having off-
spring with the recessive character ; that is to say, the recessive
character remains latent in the inheritance.

When these D(R)s are inbred (self- fertilised, in the case of
peas) they have offspring (F2), some of which resemble the re-
cessive parent, while others resemble the dominant parent, and
these occur in the proportion of 1:3. When those resembling
the recessive parent are inbred, they breed true —i.e. they give
rise to a line of pure recessives. Those resembling the dominant
parent are all apparently alike, but their subsequent history shows
that they may be divided into a set which breed true to the

FlG. 32. — Diagram, photographed from draughtsmen, to illustrate
Mendel's Law.

First line (P) a black dominant and a white recessive. Second line (F'l the hybrid
offspring D(R), the black patent, the white latent below. Third line (F2) one " pure"
black, two "impure "blacks, and one "pure" white, iDD + 2D(R) + iRR. Fourth line pure
extracted dominant to the extreme left, pure extracted recessive to the extreme right ;
in the middle, as usual, iDD+2D(R) + iRR.

[Facing p. 340.

DOMINANCE AND SEGREGATION ILLUSTRATED 341

dominant type and a set which behave like the first generation
of hybrids — i.e. they go on splitting up into dominant-like forms
and pure recessives. These two sets occur in the proportions
of 1 : 2.

A Case of Peas. — Let us consider a concrete case. Peas with
rounded seeds were crossed with peas having angular wrinkled
seeds. In the offspring the character of roundness was dominant ;
the angular wrinkled character had disappeared or receded. It
was not lost, as the next generation showed.

The hybrid offspring, all with rounded sesds, were allowed
to self-fertilise. In their progeny roundish seeds and angular
wrinkled seeds occurred in the proportions of 3 : 1. Here were
the recessives again, and when they were allowed to self- fertilise
they produced pure recessives only, with angular wrinkled seeds.

The dominants, however, were not all pure dominants, for
when they were allowed to self-fertilise they produced one-third
pure dominants and two-thirds "impure " dominants, the latter
being distinguished by the fact that in their offspring recessives
reappeared in the proportion of one recessive to three dominants.

The outstanding facts, taking the case of yellow-seeded and
green-seeded peas, may be thus summarised : —

Parental Yellow-seeded "pure" Green-seeded "pure"

Generation (Pi) plant (dominant) plant (recessive)

First Filial (Hybrid) All the offspring were yellow-seeded.

Generation (Fi) Self-fertilised they yielded

Second Filial (inbred) Yellows Yellows Greens

Generation (F2) (pure type) (impure type) (pure type)

Third Filial (inbred) Yellows Yellows Yellows Greens Greens
Generation (F3) (pure type) (pure) (impure) (pure) (pure type)

Thus intercrossing of forms with contrasted characters
results not in transitional blinds, but in the dominance of one

j42 EXPERIMENTAL STUDY OF INHERITANCE

character and the recession of another. Self-fertilisation (the
extreme of inbreeding) of the hybrids results in a number of pure
recessives and a number of dominants in the proportion 1:3;
some of these dominants (one-third) are pure, and produce only
dominants ; some (two-thirds) are apparently pure, but produce
dominants and recessives in the old proportion, 3 : 1.

A Case of Mice. — Let us take a concrete case from among
animals. A grey house-mouse is crossed with a white mouse ;
the offspring are all grey. Greyness is dominant ; albinism is
recessive.

The grey hybrids are inbred ; their offspring are grey and white
in the proportion 3:1. If these whites are inbred they show
themselves " pure," for they produce whites only for subsequent
generations. But when the greys are inbred they show them-
selves of two kinds, for one-third of them produce only greys,
which go on producing greys; while the other two-thirds, ap-
parently the same, produce both greys and whites. And so it
goes on.

(Pi)

G(W)

(F1)

1 G

2GXW)

1 w

1

(F2)

iG

2 G(W)

1 W

W

(F3)

Summary. — In his exceedingly clear exposition of Mendelism
(1905) Mr. R. C. Punnett states the result thus : " Wherever
there occurs a pair of differentiating characters-, of which one is
dominant to the other, three possibilities exist : there are
recessives which always breed true to the recessive character ;

D

i)

(2)®. ®0)

DD

0(4)

0M

RR

DIAGRAM OF MEN DELS LAW PARTICULARLY AS
ILLUSTRATED IN PROF. CORRENSS CROSSING OF
MIRABILIS JALAPA ROSEA AND MIRABILIS JALAPA ALBA.

Fig. 33— Diagram showing Mendelian inheritance in Mirabilis jalapa.

D, deep rose parent, Mirabilis jalapa rosea: the thick vertical stroke indicates dominance
of the deep rose-colour. R, White parent, Mirabilis jalapa alba ; the thin horizontal stroke
indicates recessiveness of the white colour. F1 Hybrid offspring, light rose D(R). The
dominance of the rose was incomplete. G, Germ-cells hypothetically segregated into pure
deep rose and pure white ; their possible fertilisations indicated by arrows. The male
cells are to the right, the female to the left. The fertilisation of two" homozygotes "or similar
germ-cells indicated by the arrow ' 1 • yields ill in the next generation F — extracted pure
dominant: the fertilisation of two "homozygotes" indicated by the arrow (4 1 yields (4) in
the next generation F2— extracted pure recessive. The fertilisation of " heterozygotes "
indicated by the arrows (2 and 8) yield (2 and 3) in the next generation Fa- impure domi-
nants, which being inbred (self-fertilised (split up in the next generation Fs into deep rose,
light rose, and white as before, in the proportions 1:2:1. Note also that 1 in the generation
F* yields a pure dominant 1* in the third generation F* ; and that 4 in F- yields a pure
recessive 4* in the third generation F".

[Facing p. 343

SCHEMATIC REPRESENTATION MENDEL S I A IV 343

there are dominants which breed true to the dominant character,
and are therefore pure ; and thirdly, there are dominants which
may be called impure, and which on self-fertilisation (or in-breed-
ing, where the sexes are separate) give both dominant and re-
cessive forms in the fixed proportion of three of the former to one
of the latter."

Schematic Representation of Mendel's Law. — Following Mr.
Punnett's suggestion, with slight modifications, we may use
the symbols P1, P2, P3 for the parental, grandparental, and great-
grandparental generations ; F1 for the first filial (hybrid) genera-
tions ; F2, F3, F4 for the subsequent inbred generations. The
symbol D(R) means a dominant with the recessive character
unexpressed, but potentially present ; DD or RR means pure
" extracted " dominants or recessives — i.e. those pure forms
which are sifted out from the inbreeding of " impure " dominants.

. P3 — great-grandparental generation.
. P2 — grandparental generation.
. P1 — parental generation.

D(R) . . F1 — first filial (hybrid) generation.

1

1

D

1

R

1
D

1
R

1

1
D

1
R

1 DD 2 D(R) 1 RR . F2— second filial

"Extracted " pure Impure dominants. Pure recessives. (inbred) generation,

dominants.

I I I I

DD 1 DD 2D(R) 1 RR RR . F3— third generation.

I I I

DD DDiDD 2D(R) iRR RR RR . F*— fourth generation.

§ 2. Theoretical Interpretation

Mendel was not content with formulating his results in a
law ; he advanced a theoretical interpretation which is at once
ingenious and simple.

344 EXPERIMENTAL STUDY OF INHERITANCE

Let us take the case of pea-plants with the quality of tallness
or dwarfness, of round seeds or angular seeds, of coloured seed-
coats or white seed-coats, of yellow or green cotyledons, or of

® ®\ 0

s ® ® ®

® ®;(D 1, 0

Fig. 35. — Diagram illustrating segregation of germ-cells.

D', dominant parent, its ancestry — D-, D1 ; Rl, recessive parent, its ancestry— R% R» ;

G and G, germ-cells ; Z, the zygote or fertilised egg-cell ; enclosed in the dotted line S S,

the somatic cells of the developing body ; G1 two germ-cells, one with a dominant character
and one with a recessive character ; dominance is indicated by the strong vertical stroke :
recessiveness (latent in the body S S) is indicated by the light horizontal stroke.

THEORETICAL INTERPRETATION 345

purple or white flowers (in each case, the dominant character has
been named hrst). Let us assume that these are pure-bred
varieties, well-established and breeding true, the tall form always
producing tall offspring, the dwarf form always producing dwarf
offspring, and so on. Let us also assume that the germ-cells
contain material representatives of these " unit characters " —
tallness, dwarfness, rounded seeds, angular seeds, yellow cotyle-
dons or green cotyledons, purple flowers or white flowers.

The egg-cell of the tall pea is normally fertilised by a pollen-
grain from the same pea, and the fertilised egg-cell develops into
an embryo which becomes a tall pea. As the varieties breed
true we assume that the only quality affecting dimensions which
the germ-cells bear (in expressible strength, at least) is the quality
of tallness.

But let us now take the case of a tall pea pollinated from
a dwarf pea. The offspring become tall peas — the parent with
the dominant character is prepotent. But the fertilised egg-
cells which gave rise to these tall peas must have contained not
only representative primary constituents corresponding to the
quality of tallness ; but also representative primary constituents
corresponding to the quality of dwarfness. This quality of
dwarfness is not expressed in development, but it must be
present, as subsequent generations show ; for when the egg- cells
of the hybrids are self- fertilised they develop into offspring
partly tall and partly dwarf. What Mendel suggested was that
the hybrid produces in equal numbers two kinds of germ-cells
(two kinds of egg- cells or two kinds of pollen-grains) — that there
is in the developing reproductive organ a segregation of germ-
cells into two equal camps, one camp with the potential " fac-
tor " of tallness, the other camp with the potential " factor "
of dwarfness. If there be six ovules, three have in their egg-
cell the primary constituent or factor corresponding to tallness,
and three contain the primary constituent or factor of dwarf-
ness. Each of these is pollinated by a pollen-grain, which, by

346 EXPERIMENTAL STUDY OF INHERITANCE

hypothesis, contains the potential quality of tallness or of
dwarfness ; and if the two kinds of pollen-grains are present
in equal numbers, each ovule has an equal chance of being
fertilised by a pollen-grain with a potential quality of tallness
or by a pollen-grain with a potential quality of dwarfness.
Therefore the result must be a set of offspring partly dominant
and partly recessive, in the proportions of 3 : 1.
A schema will make the theory obvious :

D (tetf)?

I

Egg-cell . ft)

x R (dwarf) <$

© . Male-cell.

Fertilised egg-cell 0

The mature egg-cells consist
of two sets ; half with the
potential quality " tallness,"
half with the potential quality
" dwarfness."

The result must be —

CD

0

fe
©

©

0

CD
©

©

©
©
©

This develops into an organism,
whose body-cells express the
quality " tallness " (D). The
germ-cells of the organism
segregate into two sets.

The mature male cells also
consist of two sets, with the
potential quality of " tallness"
or of " dwarfness." What are
the chances of fertilisation ?

®o©e©9©©

i.e. 2 with the quality of tallness ;

4 with the qualities of tallness and dwarfness ;
2 with the quality of dwarfness.

In other words —

2D + 4D(R) + 2R;

or more generally —

»Dt 2 «D(R) + »R

But as the D(R) offspring are not distinguishable from the D offspring,
until further breeding shows that they carry the recessive character
in latent form, the proportion is —

3 dominants to 1 recessive.

Thus, Mendel assumed that in the hybrid D(R) — between a
parent with a dominant character D and a parent with a homo-
logous recess ve character R — the germ-cells segregate into two

D

u

./

"/ \

M v \

0 o

-v u-

o

1 J \ j ( ) { }

MENDEL'S LAW

" G2

Fig. 34 — Diagram illustrating Mendelian segregation of germ cells.

D, dominant parent: R, recessive parent : F1, hybrid offspring, the recessive character
latent ; GA the germ-cells of F', supposed to be segregated in two camps, green and yellow,
with dominant and recessive characters. The arrows indicate possibilities of fertilisation
Two greens may combine, producing pure dominant offspring — to the left. Two yellows
may combine, producing pure recessive offspring — to the right. Green and yellow may
combine, as at the start, yielding impure dominants— green enclosing yellow. G ; this line
indicates the kind of germ cells produced by the second generation F*.

I Facing p. 347

SEGREGATION OF GERM-CELLS 347

camps, one half containing the dominant character in potentia (d),
and the other half containing the recessive character (r). This
occurs in both males and females, so that when inbreeding
takes place the possibilities are expressible thus :

D(R) produces (50 with (d) 50 with (d)\ D(R) produces 100

100 egg-cells \ 50 with (r) 50 with (r)J sperm-cells

(1) 25 egg-cells (d) fertilised by 25 sperm-cells (d) = 25 fertilised gametes (d).
(2)25 „ (d) „ „ „ (r)=25 „ „ (dr).

(3)25 „ (r) „ „ „ (d)=2S „ „ (dr).

(4) 25 „ (r) „ „ „ (r) =25 „ „ (r).

To sum up, 25 (d) developing into 25 pure D.
50 (dr) „ „ 50 D(R).

25 (r) „ „ 25 pure R.

Bateson has proposed the useful term homozygous for individuals
in which two like characters have met together (the pure do-
minants and pure recessives), and heterozygous for individuals
in which unlike characters have met (the impure dominants).

The Presence and Absence Theory.— One of the root-ideas of
Mendelism is that the inheritance includes numerous distinct and
independently heritable unit-factors. In certain cases Mendel
found that these factors occurred in contrasted or alternative
pairs, of such a nature that only one member of any one pair
can be carried by a germ-cell. The contrasted characters to
which the factors give rise are technically called " allelomorphs,"
and, as we have seen, one is called dominant and the other
recessive. These can be distinguished by their behaviour in
breeding, but we do not know what the exact nature of the
contrast between dominance or recessiveness may be.

In this connection Bateson has proposed a modification of the
Mendelian conception which may be called "the presence and
absence theory." "It is possible to express all Mendelian
phenomena in terms of a simpler system, according to which
the allelomorphism may be represented as consisting essentially
not in the presence of separate factors for the dominant and
for the recessive characters, but in the presence of something
constituting the dominant character which is absent from the

348 EXPERIMENTAL STUDY OF INHERITANCE

recessive gametes." A black guinea-pig is dominant over an
albino guinea-pig, all the offspring being black ; it is probable
that this should be expressed by saying (with Castle) that "a
distinctive something of the black parent dominates a corre-
sponding nothing of the white parent." But it is especially
when we pass beyond such simple cases that the advantages
of the presence-and-absence conception over the original Men-
delian contrast are seen.

It will, of course, be clearly understood that the facts of
Mendelian inheritance remain secure, though the interpretation
of what is meant by . dominance or of segregation itself may
have to undergo modification. Thus we may refer to Dr.
Berry Hart's independent interpretation (1909) (which Men-
dclians will not accept) of admitted Mendelian phenomena.

Mendel's Theory summarised. — Mendel discovered an im-
portant set of facts, and he also suggested a theoretical inter-
pretation— the theory of gametic segregation. As Mr. Bateson
says, " The essential part of the discovery is the evidence that
the germ-cells or gametes produced by cross-bred organisms
may in respect of given characters be of the pure parental types,
and consequently incapable of transmitting the opposite char-
acter ; that when such pure similar gametes of opposite sexes
are united in fertilisation, the individuals so formed and their
posterity are free from all taint of the cross ; that there may
be, in short, perfect or almost perfect discontinuity between
these germs in respect of one of each pair of opposite characters."

§ 3. Corroborations

Impure Dominants bred with Pure Types.—In the typical
cases discussed above, a hybrid form D(R) — an impure dominant
■ — is supposed to be self-fertilised or inbred. The results are accord-
ing to the formula 1 DD (pure or extracted dominants) -f- 2 D(R)
(impure dominants) -f- 1 RR (pure or extracted recessives).

CORROBORATIONS OF MENDEL'S LAW 349

But let us suppose the impure dominant or dominant-recessive
D(R) to be bred with a pure type — e.g. RR (extracted recessive)
(in technical phrase, a heterozygote unites with a homozygote).
The impure dominant has, by hypothesis, equal numbers of
two kinds of germ-cell — let us say, of egg-cell. The pure type
has only one kind of germ-cell — let us say, of sperm-cell. The
chances of fertilisation should be as follows :

11 CD + m Qi: egg-cells of impure dominant ;
' « 0 + « 0- ■ sperm-cells of pure recessive :

The result will be

n ova Q fertilised by n sperms 0 = n offspring $
n ova © fertilised by n sperms Q = n offspring 0

That is to say, equal numbers of impure dominants and pure recessives.

"This is what actually happens on crossing a fowl having
a single comb (RR) with one having a heterozygous ' rose
comb.' "

Or let us suppose the impure dominant D(R) to be bred with
a pure dominant DD :

)j ® -(- ji 0 egg-cells of impure dominant ;
n CD + w CD sperm-cells of pure dominant :
The result will be n ® + « CD equal numbers of impure dominants and
pure dominants.

" Here again experiment has borne out theory." Therefore,
as Mr. Punnett says, " the generalisation known as the principle
of gametic segregation may be regarded as firmly established
on the phenomena exhibited by plants and animals when strains
are crossed which possess pairs of differentiating characters."

Case of Paired Dominants and Paired Recessives. — A
beautiful experiment was made by crossing a variety of pea with
Round seeds and Yellow albumen (a pair of dominant characters)
with another variety with angular seeds and green albumen (a
pair of recessive characters). The result was offspring all like
the dominant parent. These hybrids were inbred, and the
results were some Round and Yellow, some Round and green, some

350 EXPERIMENTAL STUDY OF INHERITANCE

angular and Yellow, some angular and green. (The dominants
are represented by italics and capitals.)

RY

(i) ioo RY

(2) 100 Rg

(3) 100 aY

(4) 100 ag

which in inbreeding unite with four similar
kinds ........

RY(ag)

Suppose the germ-cells segregate into the four possible kinds (say
100 of each) :

RY 100

Rg 100

aY 100

ag 100

What are the possible combinations (it being understood that

form and colour represent a pair of characters — i.e. RR, Ra, etc.,

are impossible).

(2)
25Rgx 25RY=25RY(g)

25 Rg x 25 Rg = 25 Rg

25 Rg x 25 aY = 25 RY (ag)

25 Rg x 25 ag = 25 Rg (a)

(0
25 RY x 25 RY = 25 RY

25 RY x 25 Rg = 25 RY (g)

25 RY x 25 aY = 25 RY (a)

25 RY x 25 ag = 25 RY (ag)

= 100 RY

(3)

25 aY x 25 RY = 25 RY (a)
25 aY x 25 Eg = 25 RY (ag)
25 aY x 25 aY = 25 aY
25 aY x 25 ag = 25 aY (g)

= 50 RY + 50 Rg

(4)

25 ag x 25 RY - 25 RY (ag)
25 ag x 25 Rg = 25 Rg (a)
25 ag x 25 aY = 25 aY(g)
25 ag x 25 ag = 25 ag

= 25 RY + 25 Rg +
25 aY + 25 ag

= 50 RY + 50 aY

The characters in brackets may be disregarded, since they behave as
recessives to their correspondents. Thus the total is —

225 RY + 75 Rg + 75 aY + 25 ag
or 9 RY + 3 Rg + 3 aY -r 1 ag

This actually corresponds with results obtained.

In illustration of the crossing of forms with two pairs of con-
trasted characters, let us take one worked out by Toyama,
concerning two races of silk-moths. The one had white unstriped
caterpillars and yellow cocoons ; the other had banded cater-
pillars and white cocoons. Yellow is dominant over white, and
striped over unstriped. Thus all the hybrids (F) had striped

CORROBORATIONS OF MENDEL'S LAW 351

caterpillars with yellow cocoons. The germ-cells of the hybrids
are, according to hypothesis, of four kinds, which may be repre-
sented by the letters (Y = yellow ; y = white ; G = striped ;
g = unstriped) YG, Yg, yG, yg.

Now, the possible combinations of these in fertilisation are :

YG with YG = YG = Yellow Striped

„ „ Yg = YG = Yellow Striped

„ „ yG = YG = Yellow Striped

,, ,, yg = YG = Yellow Striped

Yg with YG = YG = Yellow Striped
,, ,, Yg = Yg = Yellow unstriped
„ „ yG = YG = Yellow Striped
.. » yg = Yg = Yellow unstriped

yG with YG = YG = Yellow Striped

„ „ Yg = YG = Yellow Striped

n ,, yG = yG = white Striped

». .. yg *= yG = white Striped

yg with YG = YG = Yellow Striped
,, „ Yg = Yg = Yellow unstriped
t) n yG = yG = white Striped
>> " yg = yg = white unstriped
9 Yellow Striped + 3 Yellow unstriped + 3 white Striped -f- 1 white
unstriped.

Toyama's actual results show a very close approximation to
the theoretically to be expected results :

Yellow Striped 6,383 individuals, 56-38%
Yellow unstriped 2,099 ,, 18*53%

white vStriped 2,147 »» 18-96%

white unstriped 691 „ 61%

The proportion 9 : 3 : 3 : 1 in 16 is called the normal Mendelian
ratio for a " dihybrid cross," where two pairs of contrasted
unit-characters are implicated. In each of the four groups
making up the 16 there is one individual homozygous, con-
taining units all similar — viz. YG x YG (one of the 9 yellow
striped forms), Yg x Yg (one of the 3 yellow unstriped), yG x yG
(one of the white striped), and yg x yg (the single pure recessive
white unstriped). Any of these four, if mated with an individual
like itself, will breed true — a point of great practical importance.

352 EXPERIMENTAL STUDY OF INHERITANCE

In books that deal with Mendelism in particular (see Biblio-
graphy) the reader will find an account of further complications,
e.g. when the parents differ in three pairs of contrasted unit-
characters. These more complicated cases are of great interest
to the breeder or cultivator who wishes to know how to combine
various excellences in a type that will breed true.

Blue Andalusian Fowls. — When black and white fowls are
crossed there sometimes results a blue or Andalusian fowl " with
a minute patchwork of black and white." When these are
inbred they produce 25% black, 50% blue, and 25% white with
black splashes. This splitting-up is characteristically Mendelian,
but what gives rise to the " blue " feature is obscure.

The ingenious Mendelian interpretation in the case of the An-
dalusian fowl is that the black and the splashed white are the
pure breeds, and that the blue Andalusian is a peculiar mongrel.
We must refer to Mr. Punnett's essay on Mendelism for the
interesting theoretical working out of the case, which is exceed-
ingly instructive, since it shows that Mendelian interpretation
is feasible even when the hybrid (the Andalusian) is quite distinct
from either parent (black or splashed white).

Yellow Mice. — Somewhat similar is the much-discussed case
of yellow mice. The yellow is dominant over all other colours,
but it is itself quite unfixable. No pure or homozygous yellows
can be obtained. When two yellows are mated, two-thirds of
the offspring are yellow and one-third some other colour. It
has been suggested that the fertilisations which give pure yellow
do occur, but that they come to nothing for some unknown
reason. Another case, worked out by Baur, is that of a so-called
"golden" snapdragon, which is also unfixable. It produces
when self-pollinated two-thirds golden offspring and one-third
green. And here there is some evidence of the existence of a
few feeble entirely yellow seedlings which are not viable.

Compound Allelomorphs. — A differentiating unit character
capable of replacing another or of being replaced by another

Fig. 36. — Combs of Fowls.

A. Simple serrated comb ; B, Tea comb ; C, Rose comb.

[Facingp. 353;

COMPOUND ALLELOMORPHS 353

is technically called a simple allelomorph. But there are other
differentiating characters which seem to consist of several
components capable &i being isolated and of entering into new
combinations. These are called compound allelomorphs.

Thus, to take Mr. Punnett's example, the " walnut " comb
of Malay fowls — broad, flattened, corrugated like half a walnut,
and with small bristle-like feathers posteriorly — becomes, as it
were, a compound allelomorph. " This is shown by the fact
that it may be synthesised from pure rose and pure pea. It
behaves as a dominant to rose, pea, and single * combs. In
a zygote formed by the union of walnut with rose or pea the
walnut character is stable, and such heterozygotes form an equal
number of gametes bearing the walnut, and either the rose or
the pea allelomorphs. In other words, the compound allelo-
morph is stable in the presence of certain presumed simple
allelomorphs. When, however, the zygote is formed by the
union of walnut with single, the compound allelomorph would
appear to undergo partial disintegration with the formation of
walnut, rose, pea, and single allelomorphs in equal proportions.
The zygote formed by the union of walnut with single is, so far
as we at present know, precisely similar to that produced by
the meeting of rose and pea " (Punnett, 1905, p. 40).

Sometimes pairs of characters go inextricably together, so
that the breeder has not as yet been able to break their correla-
tion. Thus, violet colour and hairiness in Leucoja go together,
and so do whiteness and baldness in the same flower.

Some very difficult cases are known where the inbred hybrids
have progeny some of which resemble one or both of the original
parent types, while others resemble quite different types. Thus the
Stanley variety of Lathynis odoralus, crossed with the Giant White

* A high serrated " single " comb is familiar in Leghorns, etc. ; a flattened
papillated " rose " comb with a posterior pike is seen in Wyandottes, etc. ;
a low " pea " comb, with three well-marked ridges, the median slightly-
higher than the other two, is characteristic of Indian game-fowl.

23

354 EXPERIMENTAL STUDY OF INHERITANCE

variety, yields Giant Purple, which, when inbred, has as progeny
Giant White, Giant Purple, Mars, Her Majesty, and a new form.

Mr. Bateson interprets this kind of phenomenon as due to
the analysis of a composite character into several sub- characters,
while others suppose that latent characters from previous pedigree
are liberated by a departure from the usual routine of inbreeding.

Correns has investigated the interesting case of Mirabilis jalapa.
The white variety, alba, crossed with the yellow variety, gilva,
yields a hybrid with rose flowers and red streaks. When this
is inbred the progeny include forms with white, red, rose, yellow,
yellowish flowers, with or without various kinds of streaks.

In his important work of 1909, Mendel's Principles of Heredity,
Professor Bateson wrote : "Of the various cases alleged as
exceptional, or declared to be incompatible with Mendelian
principles, few have any authenticity. . . . The progress of
research has gone steadily to show that facts of heredity which
at first seemed hopelessly complicated can be represented in
terms of a strict Mendelian system." On the other hand, we
find an experimenter like Professor W. L. Tower declaring
(1910) that " in the attempt to preserve the letter of the law
of Mendelian theory of unit characters with segregation in
gametogenesis, a host of hypotheses have been developed in
order to save the original theory."

§ 4. Illustrations of Mendelian Inheritance

How far has Mendel's Experience been confirmed ? — There
has been confirmatory work by Correns (on peas, maize, and
garden-stock), by Tschermak (on peas), by De Vries (on maize,
etc.), by Bateson and his collaborators (on a large variety of
organisms), by Darbishire (on mice), by Hurst (on rabbits), by
Toyama (on silk-moths), by Davenport (on poultry), and so on.
There are some difficulties and not a few discrepancies, but, as
Bateson says, " the truth of the law enunciated by Mendel is

CONFIRMATIONS OF MENDEL'S LAWS 355

now established for a large number of cases of most dissimilar
characters."

In experimenting with Lychnis, Atropa, and Datura, Bateson
and Saunders found that the phenomena conformed with Mendel's
law " with considerable accuracy, and no exceptions that do
not appear to be merely fortuitous were discovered. In the
case of Matthiola (garden stock), the phenomena are much
more complex. There are simple cases which follow Mendelian
principles, but others of various kinds which apparently do
not. The latter cases fall into fairly definite groups, but
their nature is obscure."

In experiments with poultry, the phenomena of dominance
and recession were detected ; interbreeding of the hybrid
offspring resulted in a mixed progeny, " some presenting the
dominant, others the recessive character, in proportions following
Mendel's Law with fair consistency, though in certain cases dis-
turbing factors are to be suspected."

The general result, so far, is that Mendel's Law has received
confirmation in a number of very dissimilar cases.

Dominant and Recessive Characters. — Let us first of all
collect a number of instances of contrasted characters which
behave in relation to one another as dominants and recessives.

Dominant.

Recessive.

Tallness.

Dwarfness.

Pisum

Round seeds

Wrinkled seeds.

sativum

Coloured seed-coats.

White seed-coats.

Yellow albumen in coty-

Green albumen in cotyle-

ledons.

dons.

Purple flowers.

White flowers.

Sweet pea.

Tall ordinary form.

Dwarf or " Cupid " variety.

Coloured.

White.

Stocks.

Coloured.

White.

Wheat and barley.

Beardless.

Bearded.

Later ripening Rivett

Early ripening Polish wheat

wheat

Non immune to " rust."

Immune to " rust."

#

356 EXPERIMENTAL STUDY OF INHERITANCE

Dominant.

Recessive.

Maize.

" Starch " seed.

" Sugar " seed.

Nettles (Urtica

pilulifera and

U. dodartii).

Serrate leaf margin.

Entire leaf margin.

Mirabilis jalapa

and M. rosea.

Rose colour.

Other colours.

Mice.

Coloured coat.

Albino coat.

Normal.

" Waltzing " variety.

Rabbits.

Coloured coat.

Albino coat.

Angora fur.

Short fur.

Poultry.

" Rose " comb of Ham-

High serrated " single "

burghs and Wyandottes.

comb of Leghorns and
Andalusians.

Cattle.

Hornlessness.

Horns.

Snails.

Bandless shell.

Banded shell.

Other Instances in Plants. — As is well known, there are
two almost equally common forms of wild primrose : (A) thrum-
types, with short styles and with anthers at the top of the
corolla-tube ; and (B) pin-types, with long styles and with anthers
half way down the tube. The thrum-type is dominant over
the pin-type.

The original species of Chinese primrose (Primula sinensis)
has a palmate leaf. About i860 a sport arose (from seed) which
had a pinnate or " fern " leaf. The palmate form is dominant,
and the fern leaf is recessive.

The deformed " Snapdragon " variety of sweet pea behaves
as a recessive to the normal type.

The 2-row barley has certain lateral flowers which are ex-
clusively staminate ; in 6-row barley all the flowers are staminate
and pistillate, and all set seed. Mr. Biffen crossed these forms,
and found that the more negative character was dominant.
The offspring were 2-rowed.

Maize.— When the common or starchy round-seeded maize
is crossed with the wrinkled-seeded sugar-maize, the round

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CONFIRMATIONS OF MENDEL'S LAWS 357

starchy character dominates. When an egg-cell of the wrinkled
sugar-maize stock is fertilised by a pollen-cell of the round
starchy stock, the result is a round seed with starchy endosperm
If this seed is sown, it becomes a plant which, on self-fertilisation,
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D(R)

ii iiiuftii #6

DP Df> DD DIRJ DM RR DO D(R) DM RR RR RR

/ ■'■•■■ /

Fig. 38. — Diagram showing Mendelian phenomena in nettles. (By

permission of Prof. Correns.)

P', leaves of the two parents ; D, Urtica pilulifera ; R, Urtica dodartii ; F', leaf of the
progeny, D(R), the serrated type being dominant; F2, leaves of the hybrid's offspring ; iDD
+ 2D(R) + iRR ; F', leaves of the next generation ; DD, pure extracted dominants ; RR,
pure extracted recessives ; D(R), impure dominants.

maize ; the round seeds yield two " impure rounds " to one
" pure round." Correns has observed a very interesting case
in which two pairs of contrasted characters are implicated.

One variety, Zea mays alba, which has smooth white seeds,
was crossed with another variety, Zea mays coeruleodidcis,
which lists wrinkled blue seeds, The hybrids (F1) had smooth

358 EXPERIMENTAL STUDY OF INHERITANCE

blue seeds, one character of each parent being dominant, and
one character of each parent being recessive. The hybrids were
inbred, and the progeny (F2) showed four combinations — smooth
blue, smooth white, wrinkled blue, and wrinkled white (the
dominant characters are italicised).

In the next generation (F3), the wrinkled white, inbred, yielded
wrinkled white — a case of extracted recessives breeding true.
The smooth whites and wrinkled blues, inbred, yielded partly
forms like themselves and partly wrinkled white. The smooth
blues, inbred, yielded the same combinations as in F2.

A finer corroboration of Mendelism could hardly be wished.

Nettles. — Correns crossed two " species " of stinging-nettle,
Urlica pilulifera L. and U. dodartii L., which resemble one
another except as regards leaf-margin, strongly dentate in the
former, almost entire in the latter. The hybrid offspring (F1)
have all dentate leaves like the male or the female parent, as
the case may be. The dentate character is absolutely dominant.
The inbred '(self-fertilised) hybrids produce offspring (F2) of two
kinds, with dentate and with entire margins, on an average in
the Mendelian proportion, 3:1.

Immunity to Rust in Wheat. — Some kinds of wheat are very
susceptible to the fungoid disease known as " rust " ; others
are immune. The quality of immunity to rust is recessive
to the quality of predisposition to rust.

" When an immune and a non-immune strain are crossed
together the resulting hybrids are all susceptible to ' rust.'
On self-fertilisation such hybrids produce seed from which
appear dominant ' rusts ' and recessive immune plants in the
expected r^tio of 3 : 1. From this simple experiment the
phrase ' resistance to disease ' has acquired a more precise
significance, and the wide field of research here opened up in
this connection promises results of the utmost practical as well
as theoretical importance. To the question, ' Who can bring
a clean thing out of an unclean ? ' we are beginning to find an

Fig. 39. — Mendelian phenomena in wheat. (After R. H. Biffen.)

A, Stand-up wheat ; B, Bearded wheat ; C, The hybrid, showing that the beardless
condition is dominant over the bearded.

[Facing p. 358.

CONFIRMATIONS OF MENDEL S LAWS 359

answer, nor is the answer the same as that once given by Job "
(R. C. Punnett, 1905, p. 18).

Silkworms. — Toyama paired Siamese silkmoths, with yellow
or with white cocoons; the offspring produced only yellow
cocoons. When the hybrids were inbred, the result was two
sets, one producing white cocoons, 'he other producing yellow
cocoons, and the proportion was ^endelian — 25*037 white and
74-96 yellow. The whites bred true ; the yellows when inbred
showed themselves to be pure dominants or " yellows " and
dominant- ecessives — i.e. splitting up again into yellows and
whites in the usual proportion. More intricate experiments
confirmed this general result.

It must be noted, however, that Coutagne has made much
more elaborate experiments with different results, which in many
cases cannot be interpreted on the Mendelian theory. Thus he
found (1) that the hybrid forms were sometimes blends of
the parents and different from both ; (2) that in other cases
the brood included some like one parent in a particular
character, some like the other parent, and some intermediate ;
and (3) that in other cases the individuals showed no fusion
of characters, but resembled one or other parent. It is likely
that the discrepancy may be explained as due to considerable
diversity of origin in the domesticated races of silkworm, so
that, while they breed true when left to themselves, a dis-
turbance of the usual routine leads to the liberation of latent
characters.

Lina lapponica. — Miss McCracken has made a fine study of
the hereditary relations in this Californian beetle, which occurs
in two types, spotted (dominant) and black (recessive). They
are always crossing in natural conditions, but there are no
intermediates, and it is easy by isolation to rear a" pure " spotted
race and a " pure " black race. When spotted forms are paired
they may produce only spotted progeny — a case of extracted
dominants. In other cases, however, they yield spotted and

360 EXPERIMENTAL STUDY OF INHERITANCE

black forms (1,021 spotted, 345 black), i.e. in the Mendelian
proportion of 3 : 1 — a case of dominant-recessives inbred.

Snails. — Lang paired " pure " five-banded forms of the
common or garden snail, Helix hortensis, with bandless forms
from bandless colonies. The young of the first generation were
all bandless, the banded character being recessive. When these
were paired the offspring were bandless and banded in the
Mendelian ratio, 3:1. Further experiments confirmed this,
not only as regards bands, but also as regards colour (yellow
or red), size, and the form of the umbilicus. It may be said,
therefore, that common snails [Helix hortensis and Helix nemoralis)
illustrate Mendelian inheritance.

Poultry. — Numerous breeding experiments with poultry
have been made by Bateson, Bateson and Punnett, Hurst,
Davenport, and others, many of which show Mendelian pheno-
mena with great clearness, while others are strangely conflicting.
One of the reasons for the complicated results is evidently to be
found in the difficulty of securing thoroughly " pure " breeds,
for many that breed true as long as they are inbred tend to
liberate latent characters when the ordinary course of breeding
is departed from.

Hurst contrasts the following characters, which usually show
themselves dominants and recessives ; but it has to be admitted
that the dominance — always complete f)r some characters —
is for others frequently, or even always incomplete — i.e. showing
traces of the corresponding recessives.

Dominant Characters. Recessive Characters.

Rose comb. Leaf comb, single comb.

White plumage. Black plumage, buff plumage.

Extra toes. Normal toes.

Feathered shanks. Bare shanks.

Crested head. Uncrested head.

Brown eggs. White eggs.

Broodiness. Non-broodiness.

Davenport's copiously illustrated work is also of great interest.
He shows in case after case that the character dominant in the

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CONFIRMATIONS OF MEND ELS LAWS 361

first hybrids is more or less influenced by the recessive character.
Polish fowls with a large hernia of the brain on the top of the
head were paired with Minorcas with normal heads. The
hybrids showed no hernia, but most of them showed a frontal
prominence. When the hybrids were inbred the hernia oc-
curred in 23-5% — a close approximation to the theoretical 25%.

Single-combed black Minorcas were crossed with white-crested
black Polish fowls with a very small bifid comb. The hybrids
had combs single in front, split behind. When the hybrids were
inbred there resulted in a total of 101 offspring, 297% with
single combs (like Minorcas), 46*5% with Y-shaped combs, and
23*8% with no combs or only papillae (like the Polish forms).
Here, again, the result is in a general way Mendelian, but the
Y-like comb is a complication.

Pigeons. — R. Staples-Browne crossed -a web-footed pigeon (an
occasional discontinuous variation) with a normal form, and got
six normal young. In other words, the web-foot character is
recessive to the normal foot character. The hybrids were
inbred, and in one case produced nine with normal feet and
three with webbed-feet — a Mendelian splitting-up But from
another pair of hybrids seventeen normal offspring resulted.
Thus, the illustration of Mendelian inheritance is inconclusive.
Besides, the numbers were too small.

We have noticed elsewhere that crossing different breeds
of pigeons often results in forms which more or less resemble
the reputed original ancestor, the wild rock dove; in other
words, reversions occur. Often, however, the results seem quite
anomalous, which is probably due to the number of latent
characters which different races of pigeons appear to carry.

Mice. — Mendelian phenomena have been carefully studied
in mice. Thus, when a grey mouse is paired with an albino, the
hybrid offspring are always grey. When these are inbred, they
yield greys and albinos, approximately in the proportion 3 : i,
Thus Cuenot obtained 198 greyr and 72 albinos.

362 EXPERIMENTAL STUDY OF INHERITANCE

Darbishire has obtained many results which harmonise well
with Mendelian theory, while others require some ingenuity if
they are to be fitted in with this interpretation. As a good case
we may cite one where the inbreeding of pigmented mice —
derived from crossing pigmented and albino individuals — yielded
159 pigmented young and 55 albinos (53-5 being the theoretical
anticipation). When similar hybrids were paired with pure
albinos, they yielded 69 pigmented and 69 albino forms, pre-
cisely as the theory would lead us to expect :

D R

D(R)
D(R)

ID + 2D(R) + iR
x

2R

D(R) R

Cuenot crossed an albino AG (with latent grey) with an
albino AB (with latent black), and obtained albinos (AGAB).
He crossed a black mouse CB with an albino AY (with latent
yellow), and obtained }^ellow mice (CBAY). He then paired
AGAB (albino) with CBAY (yellow) and obtained 151 young—
81 albinos, 34 yellow, 20 black, 16 grey ; the theoretical an-
ticipation being — 76 albinos, 38 yellow, 19 black, 19 grey.
This is an exceedingly striking and convincing case.

Waltzing Mice. — The mice of this interesting Japanese breed
have among other peculiarities the habit of waltzing round in
circles. When waltzing mice are crossed with normal mice,
their abnormal quality behaves as a recessive.

Guinea-pigs. — If a black guinea-pig of pure race be crossed
with a white one the offspring will be all white, and if these are
mated with each other the recessive white character reappears

MEN DELI AN INHERITANCE IN RABBITS 363

on the average in one in four of their offspring. These whites
mated with each other produce only white offspring, while the
black are as usual of two kinds, pure blacks and impure blacks.
Similarly, as Professor Castle has shown, a rough coat is dominant
over a smooth coat, and a short coat over a long coat.

Rabbits. — Hurst paired white Angora rabbits (with pink eyes
and silky hair) with " Belgian hare " rabbits (with pigmented
skin, dark eyes, and short yellow fur). The hybrids were pig-
mented like the " Belgian hares," but the fur was grey like that
of the wild rabbit. These hybrids were inbred, and 14 distinct
types resulted — an apparent " epidemic of variation " to which
Mendel's theory has supplied the clue, for four pairs of contrasted
characters are involved in the hybrid inbreeding — namely, short
hair versus long hair, pigmented coat versus albinos, grey versus
black coat, uniform versus marked coat (Dutch marking latent
in the albinos), and the 14 distinct types illustrate the possible
combinations.

As regards short hair versus long hair, Hurst found that when
the short-coated hybrids were inbred they produced short-haired
forms like the Belgian hare grandparent, and long-haired forms
like the Angora grandparent. Out of 70 which reached the age of
two months or more, 53 were short-haired and 17 long-haired — a
close approximation to the Mendelian anticipation, 52*5 : ly^.
Similarly, as regards pigmented coat versus albino, the hybrids,
when inbred, yielded 132 pigmented and 39 albino forms — a
close approximation to the Mendelian expectation, 129 : 43 ;
and so on.

Cats. — There are some interesting results as to colour (Don-
caster). Thus, " pure " orange ? crossed by " pure " black 6
gives tortoiseshell females and yellow males, but black ? crossed
by orange S gives black males or females, tortoiseshell females,
and orange males. It seems that orange usually dominates over
black in males, while in females the orange (for some unknown
reason) is less dominant and tortoiseshell results. Male tortoise-

364 EXPERIMENTAL STUDY OF INHERITANCE

shell cats are very rare. In this case, the results are complicated
by some peculiarity wrapped up with " sex."

When a male tortoiseshell is paired with a female tortoiseshell
the kittens are tortoiseshell, orange, and black — which is what
Mendelian theory would lead us to expect.

^Man. — The evidence of Mendelian phenomena in man is as
yet very scanty. It appears that the condition known as brachy-
dactylism, where the fingers are all thumbs with two joints
instead of three, is dominant over the normal. In five genera-
tions chronicled by Farabee about half of the offspring were
of the abnormal type, though the marriages were apparently
always with unrelated normal individuals. Moreover, no normal
member of the lineage is known to have transmitted the ab-
normality. Another good case has been recently discussed by
Drink water.

Of great interest also is Mr. Nettleship's account of the des-
cendants of one Jean Nougaret (born 1637), wno was afflicted
with " night-blindness " — a condition apparently due to loss of
the visual purple. It seems to behave like a unit character.
There are records of over 2,000 individuals ; and the night-
blindness is dominant over normal eyesight. The notable point
is that during two and a half centuries no normal member of the
lineage who married another normal, whether related or not,
ever transmitted the disease.

Human eye-colour affords another illustration. It is largely
determined by the presence or absence of two distinct layers of
pigment. " In the true blue eye onty one of these pigmentary
layers is visibly present, the posterior purple pigment of the
choroid, which, being reflected through the fibrous structure of
the iris, produces the blue colour. In the absence or partial
absence of this pigment the eye appears to be " pink," as in
albinos. In the ordinary brown eye two layers of pigment are
present, for in addition to the posterior purple laj^er there is
also an anterior brown layer, in front of the iris. Major C. C,

MEN DELI AN INHERITANCE IN MAN 365

Hurst found that the eye with two layers of visible pigment
(duplex) is dominant and the eye with one layer of visible
pigment (simplex) recessive. Or, putting it in another way,
the presence of the brown front layer is dominant to its absence.
Practically the same conclusion was reached independently by
Professor and Mrs. Davenport.

The Davenports and Major Hurst have also brought forward
some evidence illustrating in typical Caucasians the dominance
of dark to fair skins, their segregation in the same family, and the
apparent purity of the extracted fair individuals. Hurst also
gives evidence that " fiery red " hair behaves as a recessive to
brown, and that the musical sense or temperament is also reces-
sive. It seems as if an individual is non-musical owing to the
presence of an inhibitory factor preventing the expression of the
musical temperament which is potentially present in every one
(Hurst, 1912).

It would be interesting to have precise information as to the
progeny of Eurasians who intermarry, for here the original
hybrids result from the mixture of two very distinct races.

§ 5. Mendel's Discovery in Relation to Other Conclusions

Conception of the Organism. — A keen critic has pointed out
that the Darwinian or Selectionist theory of evolution is obviously
a projection on nature of anthropomorphic ideas partly due to the
keen competition of the industrial age, partly due to a temporary
pressure of over-population, partly due to the process by which
mechanical devices, such as spinning and weaving machinery
on the one hand and bicycles on the other, are improved by the
addition of one patent after another. Taking the last point,
the critic asks if we can seriously believe that organisms have
evolved by piecemeal variation and selection of particular parts,
comparable to improvements now in the gear, again in the
steering, and again in the chain of the bicycle ? Is it not one

366 EXPERIMENTAL STUDY OF INHERITANCE

of the clearest and surest facts about an organism that it is a
unity ? It lives as a unity, does it not evolve as a unity ?

We cannot here enter into a discussion of the alleged anthro-
pomorphism or sociomorphism of what we flatter ourselves by
calling " pure science." That is a very interesting thesis, and
worthy of much discussion. But we wish to refer for a moment
to the idea of the " piecemeal patenting theory " of evolution,
since it seems to us that the facts brought to light by Mendel
and the Mendelians are sufficient to show that there is some truth
in this way of looking at the organism.

It has been shown that some organisms have clear-cut, we
may almost say crisp, unit characters, which behave in inherit-
ance as if they were independent constituents, being transmissible
en bloc and in their entirety— not blending with analogous
characters, but remaining quite distinct, and developing in
absolute intactness and exclusiveness or not at all.

The Mendelian facts, as Bateson says, lead us to regard the
organism as " a complex of characters, of which some at least
are dissociable and are capable of being replaced by others. . . .
We thus reach the conception of unit characters, which may
be rearranged in the formation of the reproductive cells. It
is hardly too much to say that the experiments which led to
this advance in knowledge are worthy to rank with those that
laid the foundation of the atomic laws of chemistry."

Weismann has not paid much attention to Mendel's Law,
because he regards the basis of facts as still insufficiently broad,
and because he sees so many discrepancies in the experimental
results ; but it may be pointed out that the general idea of in-
dependently heritable unit characters is not inconsistent with,
but rather corroborates Weismann's picture of an inheritance as
composed of numerous sets of determinants or primary constitu-
ents, each corresponding to an independently variable and
heritable structure. It is quite possible that the germ-cells of
the hybrids of two distinctively contrasted parents do not separate

A NEW VIEW OF EVOLUTION 367

into two sets bearing " pure " dominant determinants and
" pure " recessive determinants, but that the practical " purity "
is wrought out by a process of germinal selection.

However this may be, the facts of Mendelism lead us to a
renewed confidence in the relative independence of unit char-
acters. It looks as if a unit character sometimes behaves like
a radicle in chemistry ; it can be replaced en bloc by another,
but it cannot compromise with that other. " The outlook,"
as Bateson says, " is not very different from that which opened
in chemistry when definiteness began to be perceived in the
laws of chemical combination."

A New Yiew of Evolution. — As is well known, Darwin believed
that specific differences and adaptations were slowly brought
about by the consistent selection of small continuous variations
in a profitable direction. He did, indeed, recognise that large
discontinuous variations may suddenly arise, as in the case of
the short-legged Ancon sheep. He could not, however, lay
stress upon such occurrences, believing as he did that they were
of rare occurrence, and therefore very liable to be swamped by
intercrossing with the normal forms.

Over and over again, both before and after Darwin, naturalists
had suggested that sudden emergences of new structures with
no small degree of completeness, brusque transitions from one
position of organic equilibrium to another, might be of evolution-
ary importance. We need only mention Etienne Geoffroy
Saint-Hilaire and Francis Galton. But the difficulty always
was, that these discontinuous variations seemed to be of rare
occurrence, and liable to be swamped.

In 1894 Bateson showed in his Materials for the Study of
Variation that discontinuity in variation was a fairly common
phenomenon, and might, therefore, have played in the past an
important role in the origin of species (see Chapter III.).

Similarly, Hugo de Vries showed in most convincing detail
that sudden discontinuous variations or mutations not infre-

368 EXPERIMENTAL STUDY OF INHERITANCE

quently occur among plants and give rise to true-breeding
varieties (see Chapter III.).

Now it is evident that, if Mendel's Law applies in such cases,
the mutation, once present, is not likely to be lost or swamped
by inbreeding with the normal types. Thus, through Mendel's
discovery we are led to a new view of organic evolution, in which
we attach less importance to the minute fluctuations on which
Darwin relied, and more importance to mutations or saltatory
variations.

Light thrown on Variation. — Mendelian experimentation hat.
thrown light on at least some kinds of variation. In connection
with the colours of flowers and of the coats of mammals, it has
been shown that varieties may arise by the loss or modification
of unit-characters. Thus in th£ case of a rabbit, some colour-
factor may drop out altogether, giving albinos, or the pattern-
factor of the individual hairs may drop out, giving a mingling
of pigment which appears black, or the factor for black may
drop out, giving brown and cinnamon varieties. But what
does this " dropping out " mean ? Prof. Castle answers : " Loss
of a unit-character might easily come about by an irregular
cell-division in which the material basis of a character failed to
split, as normally. . . . On the other hand, a modified condition
of a unit-character might possibly result from unequal division
of the material basis of a character, so that one of the cell-
products would transmit the character in weakened intensity,
the other in increased intensity " (1911, p. 86).

Some Mendelians would also admit the idea that a unit-
character may lose some of its " potency," some of its power of
" dominating " or of asserting itself in development — just as
it might on Weismann's theory of germinal selection. We have
already referred to the not infrequent imperfection of the domin-
ance of a dominant character. It may be that this is due to a
weakening of the potency of a particular unit-character ; it may
be that something must be allowed for the condition of the

MENDELISM AND SELECTION 369

entire germ-cell at the time of fertilisation. Prof. W. L. Tower,
in a series of important experiments testing the influence of
altered environmental conditions on the breeding of potato-
beetles (Leptinotarsa), found that the conditions surrounding
and incident upon the germ-cell at the time of fertilisation may
be to a very considerable extent responsible for the determina-
tion of the dominant character in the cross and largely responsible
for the variability of such characters (1910, p. 332).

Mendelian experiments give us a vivid impression of the pos-
sibilities of variation. Crossing two races of silk-moth, one
with striped caterpillars and yellow cocoons, the other with
unstriped caterpillars and white cocoons, yields in the hybrid
generation (F1) only forms with striped caterpillars and yellow
cocoons, these being the two dominant characters. But the
inbreeding of the hybrids yields in the next (F2) generation,
four different combinations which we may briefly allude to as —
Yellow Striped, Yellow unstriped, white Striped, and white
unstriped. But if there had been 10 unit-characters instead
of four, there would have been a theoretical possibility of 1,024
combinations. In short, Mendelism enables us to understand
the origin of that kind of variation which consists of permutations
and combinations of already existing qualities.

Mendelism in Relation to Selection. — The facts of Mendelism
are in several ways important in relation to natural selection : —
(1) The facts warrant us in believing in the possibility of the
particular evolution of unit characters while the rest of the
organism remains stable. (2) When a variation is, through
inherent stability or through inbreeding, prepotent — i.e. when
its possessors breed true inter se — we can understand how it is
that even crossing with variants having an antagonistic character
need not imply any diminution of the dominance of the character
in question. The inbreeding of the hybrids simply results in
the sifting out of the pure parental types. (3) Suppose Mendelian
phenomena to occur in a series of generations, and suppose

24

370 EXPERIMENTAL STUDY OF INHERITANCE

that natural selection favours the possessor of the dominant
character, they will ex hypothesi prevail as elimination proceeds.
But it should also be noted that, apart from selection, the
possessors of the dominant character will be in a gradually
increasing majority, since extracted dominants and dominant-
recessives (practically indistinguishable as far as natural selection
goes) are always to recessives in the proportion of 3 : 1.

In the beautiful case of the two nettles given by Correns, the
plants with entire leaf-margins are markedly more susceptible
to fungoid attacks than those with dentate margins, so that in
the course of time in certain conditions the former race would
tend to be eliminated by natural selection ; but it is also handi-
capped by the hereditary conditions, since three dominants are
always being produced to one recessive.

Swamping Effects of Intercrossing. — A well-known objection
to Darwinism, first clearly stated by Prof. Fleeming Jenkin,
is that variations of small amount and sparse occurrence would
tend to be swamped by intercrossing before they had time
to accumulate and gain stability. In artificial selection the
breeder takes measures to prevent this " swamping-out," by
deliberately pairing similar or suitable forms together, or by
deliberately removing undesirable forms ; but what, in nature,
corresponds to the breeder ?

Various answers are possible : — (1) It may be that similar
variations occur in many individuals at once and many times
over. (2) It may be that the variations which really count
in evolution are not small individual fluctuations, but discon-
tinuous variations. (3) It may be that many variations are
not from the first unstable, but express changes of organic
equilibrium which come to stay if they get a chance at all. (4)
There are numerous conditions in nature — summed up in the
concept " isolation " — e.g. geographical barriers, differences in
habit, psychical likes and dislikes — which tend to prevent free
intercrossing between sections of a species. Similar forms

MEN DELI SM AND ANCESTRAL INHERITANCE 371

may pair, and, in various ways, assortative mating may come
about naturally. And whenever inbreeding sets in prepotency
develops — i.e. peculiarities, even if trivial, gain great staying-
power in inheritance. (5) But even more important are the
facts disclosed by Mendel and his school, that crossing does not
tend to swamp new features, for if the hybrids be inbred there is
a persistent segregation of the parental type. A new mutant
crossed with a related form of contrasted character may be
dominant or recessive in the immediate hybrid (F1), but in
either case, if the hybrids are inbred, it will reappear in pure
form in the next generation (F2), and so forth. There is, how-
ever, no warrant for the common belief that hybridisation in
itself gives rise to new races.

Mendelism in Relation to Ancestral Inheritance. — It may
be that the conception of ancestral inheritance and the conception
of segregate parental inheritance apply to different sets of cases.

1. At one extreme we may perhaps place cases of sterility,
where the fertilised egg-cell fails to develop, owing perhaps to
mutual incompatibility between the paternal and maternal
contributions. " The sterility of distinct species when crossed
is probably due to the confusion and disruption of the systems
of forces in the pronuclei of the germ-cells by antagonising
ancestral stimuli " (Dendy, 1903).

2. It is possible that in some cases where a spermatozoon
enters an egg it fulfils one of its functions— acting as a liberating
stimulus prompting the egg to develop — and yet does not fulfil
its other function of contributing half of the inheritance. It is
possible that it is sometimes only the egg-nucleus which develops.
This possibility is suggested by some of the results of experi-
mental embryology — e.g. that an egg may develop with only a
sperm-nucleus (merogony), or with only its own nucleus (artificial
parthenogenesis).

3. Dendy suggests that those remarkable abnormal insects
(see Darwin, Variation of Animals and Plants under Domestica-

372 EXPERIMENTAL STUDY OF INHERITANCE

Hon, vol. ii. p. 394), in which one-half or one-quarter of the body
is like that of the male and the other half or three-quarters like
that of the female, may be due to an inadequate blending of the
male and female nuclei. "They may separate completely at
the first or at some subsequent division of the segmentation
nucleus, and thereafter each may control a certain fraction of
the developing organism, yielding a lop-sided result."

4. The maternal and paternal contributions may remain
together in the development of the body, though one is dominant,
but they may be dissociated in the formation of the germ-cells,
so that two sets of germ-cells result (Mendelian inheritance).

5. The maternal and paternal contributions may find equal
expression in development, and through them ancestral con-
tributions may also find realisation (Galtonian inheritance).

There should not, of course, be any opposition between Men-
delian and Galtonian formulae, for that is a confusion of thought,
to obviate which we have sharply separated the statistical from
the experimental study of inheritance. They are correlated,
and ultimately they will be seen in complete harmony, as different
aspects of the same phenomena. But it is simply muddle-headed-
ness which can find any opposition between a statistical formula
applicable to averages of successive generations breeding freely,
and a physiological formula applicable to particular sets of
cases where parents with contrasted dominant and recessive
characters are crossed and their hybrid offspring are inbred.
We may refer to the admirable essay by Darbishire (1906).

§ 6. Practical Importance of Mendel's Discovery

As Mendel's discovery is extended it is bound to have a great
influence on the breeding of animals and the cultivation of
plants. Wherever it is applicable it will afford a solid basis
for action, enabling the breeder to reach his desired result more
surely, more rapidly, and more economically. The case we have

PRACTICAL IMPORTANCE OF MENDEL'S LAW 373

mentioned of the varieties of wheat susceptible and immune
to " rust " is in itself very suggestive.

A case like that of the Andalusian fowls shows how immediate
may be the practical utility of Mendelism. The pairing of two
Andalusians yields only six Andalusians to the dozen ; the crossing
of a black and a white yields twelve Andalusians to the dozen.

The impossibility of fixing the Andalusian characters into
a stable race is simply due to the fact that the Andalusians are
hybrids, and the same is probably true of cases like the sugar-
beet, where selection seems to have ceased to produce further
improvement.

The breeder who wishes to get a stable and pure strain rapidly,
has obviously a clue in the behaviour of the extracted recessives
and the extracted dominants of the F2 generation. There are
many similar practical applications of Mendelian results.

Inbreeding. — Breeders who have with carefulness evolved a
fine herd are often very loath to introduce fresh blood, even when
they suspect that they are approaching the limits of safe in-
breeding. But if Mendelism applies to the organisms bred,
then it does not seem as if the introduction of fresh blood need
affect the purity of the stock. A cross is effected to secure
reinvigoration ; when the results of the cross are inbred, forms
like the original parent will reappear.

Old-established form. " Fresh blood."
A B

If A be dominant, A(B) ; or B(A), if B be dominant,
x x

A A

n A + n A(B) n A + «B(A)

Or if A(B) be inbred the result will be n A + 2 n A(B) + n B.
Or if B(A) be inbred the result will be n A + 211 B(A) + n B.
There is obviously no theoretical danger of losing A,

374 EXPERIMENTAL STUDY OF INHERITANCE

No one can, of course, at present say that these "simple
equations " will apply to the introduction of fresh blood into
a herd of cattle, but the time has come for more daring ex-
periment on Mendelian lines. It might obviously happen that
the " fresh blood " (B) introduced was quite incompatible with
the pure-bred (A), and the progeny was an undesirable freak.
But do not such casualties happen under the present instinctive
or empirical regime followed by most breeders ?

§ 7. Other Experiments on Heredity

Our survey of cases must be supplemented by reference to
the works of Bateson, T. H. Morgan, Punnett, De Vries, and
others ; but we have said enough to show,— (1) that Mendelian
phenomena are well illustrated in certain cases — e.g. peas, mice,
rabbits, poultry, snails ; (2) that in other cases, while there are
clear Mendelian phenomena according to some observers, dis-
crepant results have been reached by others — e.g. silk-moths ;
(3) that in other cases, while there are hints of Mendelian pheno-
mena, the results cannot be readily interpreted in conformity
with Mendelism — e.g. pigeons.

It seems to us that the results depend in part on whether
there are or are not sufficiently well-marked contrasted unit
characters in the two parents. When the differences between
the two original parent-types are not crisply definable in terms
of contrasted unit characters, the conditions of Mendelian in-
heritance are not afforded, and we have to fall back upon the
old-fashioned description of the inheritance as " blended " or
" particulate " or " reversionary," and so forth.

It must be clearly noted that Mendelian phenomena are not
known except in certain cases of hybridisation. They chiefly
occur in the inbreeding of the hybrid progeny of two well-j
marked varieties or " elementary species," We do not know!

OTHER EXPERIMENTS ON HEREDITY 375

how far they may be found to apply in the breeding of pure
strains.

No one can say at present that Mendelian inheritance is con-
spicuously illustrated in what we know of inheritance in horses,
dogs, and man. Yet, it is quite conceivable that Mendelian
inheritance may be demonstrated in horses, dogs, and man — in
cases where the parents do not contain a medley of latent strains,
but are sharply contrasted with one another in respect to one
or more unit characters. The danger is of trying to universalize
the Mendelian formula, and some of the attempts that have
been made to give a Mendelian interpretation to discrepant
facts seem to us very far-fetched. On the face of it, when we
remember all the possible variables, it seems very unlikely
that there should be only one mode of inheritance — and that
Mendelian.

There is, we think, much reason to believe that in some cases
the unit characters are represented in the germ-plasm by deter-
minants which are very stable in themselves, which must be
everything or nothing in the hypothetical struggle antecedent
to and associated with development, whose expression will not
blend with, or even allow of the expression of contrasted analo-
gous determinants. There is, we think, equal reason to believe
that in other cases the unit-characters are not so " exclusive,"
but may combine with analogous unit-characters to form a blend
or a particulate mosaic.

As was to be expected, the discoveries of the Mendelian
experimenters have raised problems in solving others, and
various elaborations have been found necessary in order to
bring or keep certain phenomena within the scope of Mendelian
interpretation. It may be that some of the subtleties of for-
mulation are necessary and transitional ; it may be that some
of the difficulties are due to over-stretching the Mendelian
concepts.

One of the active experimenters has given expression to this.

376 EXPERIMENTAL STUDY OF INHERITANCE

" From the simple conditions discovered by Mendel there has
arisen through the work of the last decade an array of observa-
tions tending to show that the Mendelian phenomenon is not
in many instances as distinct and simple as one might wish, and
at present diverse kinds of variability in the behaviour of char-
acters are described and attributed, in some instances, to several
different kinds of latency, to gametic coupling, to variable
potency, to variable dominance, and so on. The situation
essentially is this, that as investigation has progressed it has
been discovered that not one, but a host of determining factors
(I use the word factor as meaning something that makes possible
a given result, with no idea expressed or implied as to the nature
of this factor) are operative in the production of alternative
inheritance ; and in the attempt to preserve the letter of the
law of Mendelian theory of unit-characters with segregation in
gametogenesis, a host of hypotheses have been developed in
order to save the original theory " (W. L. Tower, igio).

Tower's Experiments. — Great interest attaches to the ex-
periments of Prof. W. L. Tower (1910) on crossing species of
potato-beetle (Leptinotarsa). In these experiments the chief
variables were the conditions surrounding and incident upon
the germ-cells at the time of fertilisation, and it was found that
changes in the external conditions (temperature, humidity, etc.)
are associated with changes in the alternative (Mendelian)
inheritance. He succeeded " in creating a series of behaviours
in which the same characters are dominant to the complete
exclusion of others ; dominant to a lesser degree, or in which
there is a complete blend between the two in the F1 generation,
or the appearance of both parental types in F1 and both breed
true."

The question of dominance, according to these experiments,
is not entirely dependent on the constitution of the germ-cells,
it is partly dependent on the external conditions operative on
the germ-cells at the time. In short, conditions external

TOWER'S EXPERIMENTS

377

to a cross are important factors in determining the results
thereof.

Tower's general conception of the attributes of the organism
appears to us to have a wholesome breadth and elasticity
such as the present state of knowledge demands. He recognises
the following facts as regards organic constitution :

' i. That there is in organisms a form basis, relatively un-
alterable as regards symmetry, pattern and arrangement of parts.

" 2. That there are in organisms an array of attributes capable
of variation, but blending in heredity, forming blends and inter-
mediates.

" 3. That there are in organisms an array of attributes which
can exist only in a definite state of stability — they are either
there or not there.

" 4. That there are in organisms characters that by crossing
can be replaced by other more or less similar but different
characters."

Johannsen's Experiments on Pure Lines.— Experiments by
Nilsson and others at Svalof in Sweden have shown that the
progeny of an isolated particularly good ear of barley may all
exhibit the parental characters, and that their progeny in turn
breed true. If a single plant exhibits a desired result it is
shortest and surest to work from it alone, without going on for
years selecting also the nearest approximations. We owe to
the Danish botanist Professor Johannsen an elaboration of this
idea in a series of very important experiments, carried out
with unsurpassed patience and precision. He calls all the de-
scendants of a single individual in a self-fertilising race a "pure
line." An apparently homogeneous race or " population " is
a congeries of pure lines. Given an isolated pure line, the
cultivator can get no more out of it ; there are plus and minus
"fluctuations," but even with selection there is always a return
to the average. Similarly, in a population, which is made up
of a congeries of pure lines, selection cannot do more than

378 EXPERIMENTAL STUDY OF INHERITANCE

isolate the best pure lines, it cannot get beyond the extremes
which the included pure lines illustrate.

One of Johannsen's main results was to show that a pure line
is very constant from generation to generation. There are
many individual differences, but these do not tend to be repro-
duced in the offspring. They appear to be modifications due to
peculiarities in the " nurture " of individuals. There do not
appear to be the numerous germinal variations which are so
often postulated. If a big hereditary change occurs, it comes

210

I5S

Ni 2

MMaaa««

flflflOfl

OO0M

flOflOM

690

+

OOOflOOGOM

X

Fig. 40a. — Pure Lines in Paramcecium. (From Jennings.)

"PURE LINES" 379

about by a " mutation " in the pure line, not by selection among
the individual differences.

Jennings has illustrated the significance of " pure lines " in
reference to the slipper-animalcule (Paramoccium). The figure
shows a population made up of eight pure lines, each of which
is marked by a certain range of size. The line x — x indicates
the mean of the populate ; the crosses indicate the means of
the several lines. If a giant be isolated from the first line,
its progeny, kept in the same conditions, keep up the characters
of that line. The large-sized stock thus arising can hardly be
called the result of selection from the population in question ; it
is the result of the isolation of an individual of a particular
pure line. And the other point is, that no amount of selection
will get anything out of the isolation beyond the limits of the
pure line from which it came.

The conclusion that experiments on " pure lines " suggest
is one towards which many lines of modern experimentation
point, namely that in certain sets of cases the variations
that count are mutations, not fluctuations. By a germinal
stride a most excellent ear of wheat is formed ; there is
more to be got out of that single ear than out of years of
selection of smaller fluctuations. The fact that, although plus
and minus fluctuations occur in the pure line, selection can
make nothing more of them, seems to show that these small
fluctuations are often not transmitted. It is probable indeed
that many of them are not germinal variations at all, but ac-
quired modifications due to diversities of nurture. There is a
question, however, which must not be left out of account, namely,
whether the individual new departures, which are often far
from abrupt or startlingly discontinuous, may not be the outcrop
of the long-continued selection of forms showing small fluctua-
tions. This is, indeed, suggested by the conclusion of some
investigators, though not of De Vries, that a mutation is a
stride in the same direction as that of the majority of the flue-

380 EXPERIMENTAL STUDY OF INHERITANCE

tuations. And it may be that small fluctuations, not in them-
selves demonstrably heritable, may, in the course of generations
of consistent selection, be summed up in heritable mutations.
This would not exclude another possibility that mutations are
due to deeply saturating environmental influence.

Hybridisation in General. — It is not desirable to attempt to
draw any definite line between the various kinds of crossings —
which may all be arranged on an inclined plane — for they differ
simply in the degree of difference between the two parents. We
may conveniently use the word " hybridisation " (cross-breeding,
outbreeding, exogamy) whenever there is a marked difference
between the two parents. The cases may be arranged on an
inclined plane.

Different genera.
Different species.
Different subspecies.
Different breeds.
. . Mutants.
Variants.
Apparently identical forms.
Self-fertilisation (autogamy).
Parthenogenesis.

Examples. — Individuals belonging to different genera — e.g.
domestic fowl andpheasant, sea-urchins, different genera of orchids.

Individuals belonging to different species — e.g. capercaillie
and black grouse, carrion crow and hooded crow, different
species of Saturnia, different species of Medicago.

Individuals belonging to different subspecies — e.g. maize.

Individuals belonging to different breeds — e.g. poultry,
Short-horn and Aberdeenshire Angus cattle, Clydesdale and Shire
horses, silkmoths.

Individuals belonging to different " varieties " which have
not risen to the stability of " breeds " — e.g. wheat susceptible
and immune to rust.

Hybridisation of Distinct Species. — The conception of species

VARIED RESULTS OF CROSSING 381

is confessedly quite relative — it is a term of convenience when we
wish to include under one title all the members of a group of
individuals who resemble one another in certain characteristics.
A species is often simply a segment of a curve of closely related
forms. It is a statistical conception, and as there is no abso-
lute constancy in specific characters, as one species melts into
another, with which it is connected by intermediate varieties,
by frequent or casual variations, we have to confess that it is
a human device, the validity of which varies greatly according
to our knowledge or ignorance of the forms in question. A specific
name is sometimes, when we are very ignorant, as unmeaning as
the name of a constellation in the starry heavens. But it is
equally convenient.

At the same time, since science is systematised common sense,
it is usually admitted — oftener, perhaps, as a pious opinion, than
as a practice — that the characters on account of which a naturalist
gives a specific name to a group of similar individuals should be
more marked than those which distinguish the members of any one
family, should show a relative constancy from generation to genera-
tion, and should be associated with reproductive pecidiarities which
tend to restrict the range of mutual fertility to the members of the
proposed species (see the author's Outlines of Zoology, 5th ed.,
1910, pp. 14-16).

The popular impression that crosses between " distinct
species " are rare is erroneous ; for, apart from the familiar
mules, fertile pairing is known between lion and tiger, dog and
jackal, wild and domestic cat, brown bear and polar bear,
American bison and European wild ox, horse and zebra, hare
and rabbit, duck and goose, canary and finch, thrush and black-
bird, capercaillie and blackcock, carrion crow and hooded crow,
pheasant and fowl, and the list soon becomes very long if we
include backboneless animals and plants (see Evolution of Sex,
revised ed., 1901, p. 163).

The popular impression that fertile crosses between " distinct

382 EXPERIMENTAL STUDY OF INHERITANCE

species " result invariably in sterile offspring is also erroneous ;
for the hybrids of American bison and European wild ox, of
Indian humped cattle and domesticated ox, of common goose
and Chinese goose, of common duck and pintail duck, of different
kinds of pheasants, and many more are certainly fertile.

At the same time, it seems safe to say that the likelihood
of successful crossing and of the fertility of the hybrid offspring
is in inverse proportion to the distinctness of the species crossed.
It seems also safe to say that the characters of species-hybrids
do not conform to any general formula. They may be a blend
of the parental characters, they may be exclusive or particulate,
they may be reversionary — i.e. allowing expression of long-latent
ancestral characters — or they may be novel and peculiar.

On the whole, the crossing of distinct species, while it may be
interesting physiologically, does not seem to have much interest
for the evolutionist. It does now and then occur in nature, but
it seems to be a mere by-play of little phylogenetic importance
— unless perhaps in very early days, of which we know nothing.
Diverse Results of Hybridising. — An inheritance is such a
complex integrate of items that no one can hope to predict
the result of mingling two more or less distinct inheritances.
We have two organisms, A and B, which can be crossed and
produce offspring : but, before the germ-cells of A and B are
ready for union, they have undergone a process of maturation
which may definitely affect the burden of hereditary qualities of
which each germ-cell is the vehicle ; by the process of amphi-
mixis or fertilisation a new integrate or zygote is formed —
the fertilised egg-cell — and in this integration the inheritance
may be affected by permutations and combinations, mutual
adjustments and new states of equilibrium, victories and defeats
of particular items, of all which we have no actual knowledge.
In the process of development, if there are several different sets
of primary constituents representative of a future structure — an
hypothesis from which we can see no escape— then the result

RESULTS OF HYBRIDISATION 383

may in part depend on the struggles and interactions of these
in the course of development ; for, as we have often said, it does
not follow that everything represented in the inheritance finds
expression in development. Finally, it must be remembered
that the process of development implies interaction between
the inheritance and an appropriate environment, and that since
this appropriate environment is variable (within limits of the
embryo's viability) the result may again be modified by minor
peculiarities of nurture. It is, therefore, plain that prediction
as to individual results of crossing is out of the question.

The Mendelian theory has thrown light on the variability
which has often been remarked when crosses have been effected.
Cross-breds are produced and inbred, and new forms appear in
their progeny. The Mendelians contend, in Mr. Bateson's
words, that " in all the cases which have been properly examined
these new forms are created by simple re-combination of characters
brought in by the original parents."

Summary. — There are several well-known results of hybridisa-
tion :

1. The hybrids may be an intermediate blend of the parental

characters, as in mulattos, finch and canary, carrion
crow and hooded crow, and in many plants. —

AB

A x B yields

2. The hybrids may show a particulate juxtaposition with-

out a blend of the parental characters, as in piebald
animals, or in the cross between male Lady Amherst
pheasant and female golden pheasant, —

A x B yields A + B

J 2

3. The hybrids may resemble an ancestral form, whose

characters have not been recently patent, as in many
crossings of pigeons, red-eyed albino house-mouse and

384 EXPERIMENTAL STUDY OF INHERITANCE

Japanese waltzing mouse (with progeny like wild
mouse), white Angora rabbit and Belgian hare rabbit
(with progeny like wild rabbit), —
A x B yields r (AB)

4. The hybrids may be quite different from either parent,

" with a character of their own " — e.g. Andalusian

fowl, —

A x B yields C

Fig. 41.— Varieties of Wheat. (After R. H. Biffen.)

A, Rivet; B, Polish; C, The hybrid Rivet X Polish, intermediate in laxness and
glume length between its parents.

5. The hybrids may exhibit the (dominant) characters of
one parent, the (recessive) characters of the other
parent remaining latent ; this is the first step in Men-
delian inheritance, —

A x B yields A(B)

RESULTS OF HYBRIDISATION 385

It has been stated in some cases, — (a) that the hybrid shows
more of the character of that parent which is phyletically older
or more securely established— see e.g. some of the results of Stand-
fuss ; (b) that the hybrid shows more of the character of that
parent whose gametes were relatively more mature at the time
of fertilisation—^, some of the results of Vernon. Other
generalisations have been ventured, but all require to be
revised in the light of what we now know of Mendelian
phenomena.

It remains to be seen how far the known cases of blended,
exclusive, and particulate inheritance are interpretable as forms
of Mendelian or alternative inheritance, and there are many
who suspect that the result will be the great extension of the
Mendelian interpretation. Until we have wider knowledge of
unit characters and of their alternative inheritance we must
retain the descriptive terms- — blended, exclusive, and particulate.
In many cases where there is a pairing of closely similar organisms,
the most striking fact is the uniformity of the inheritance —
which we might describe as continuous.

It may seem strange, at first sight, that there can be any
question of bringing a " blend " within the Mendelian category.
But where we have to deal with a multiplicity of independent
characters, some dominant on the paternal side, some on the
maternal side, the impression that there is blending in the off-
spring may readily arise, and still more when we come to the
mixtures in the next (Fa) generation.

What is called particulate inheritance may be due to the
alternative inheritance of the elements of a patchwork of char-
acteristics, which, as Galton said, " are usually transmitted in
aggregates, considerable groups being derived from the same
progenitor." He went on to say : " Skin-colour is a good
example of what I call blended inheritance. It need be none
the less ' particulate ' in its origin, but the result may be
regarded as a fine mosaic too minute for its elements to be

25

386 EXPERIMENTAL STUDY OF INHERITANCE

distinguished in a general view " (Natural Inheritance, 1889,
chap. ii.).

Sometimes, as in mules, the hybrid offspring are sterile. This
may show itself (1) in atrophy of the reproductive organs, (2) in
abnormalities in the reproductive ducts ; or (3) in more obscure
conditions in regard to which we can only shroud our ignorance
with the words, " constitutional incapacity."

§ 8. Consanguinity

Consanguinity. — In many peoples — Jewish and Mohammedan,
Indian and Roman — laws against the marriage of near kin go
back to remote antiquity, but it seems probable that the basis
of these was social rather than biological. In other peoples —
Persian, Phoenician, Arab, and even Greek — consanguineous
marriages were permitted and sometimes encouraged. The
idea that the marriage of near kin is a cause of degeneracy seems
to be relatively modern, and is probably based in large measure
on the observed degeneracy in closely intermarried noble
families. In certain closely inbred communities, moreover, a
large percentage of deaf-mutes and weak-minded has been often
observed. But it is not difficult to find counter-instances — e.g.
in the Norfolk Islanders and in the people of Batz on the lower
Loire — where close inbreeding has not been followed by ill-effects.
Mr. George H. Darwin has made out a strong case in support of
the position that consanguineous marriages are not in themselves
causes of degeneration or of diminished fertility.

Biologically it seems certain that close inbreeding can go far
without any ill effects, but further in some types than others.
Many plants, such as garden-peas, wheat, and oats are habitually
self-fertilising ; the same is true of a few of the hermaphrodite
animals, e.g. the parasitic flukes and tape-worms. But these
are cases which have become adapted to this sort of (auto-

CONSANGUINITY 387

gamous) reproduction — the extreme of in-breeding. The prac-
tically important inquiry is in regard to the limits of profitable
in-breeding among types which are normally cross-breeders or
exogamous.

Darwin's Conclusions. — Charles Darwin devoted much at-
tention to the question of inbreeding (see especially his Animals
and Plants under Domestication), and his conclusions were :
(1) " The consequences of close interbreeding carried on for too
long a time are, as is generally believed, loss of size, consti-
tutional vigour, and fertility, sometimes accompanied by a
tendency to malformation " ; (2) " The evil effects from close
interbreeding are difficult to detect, for they accumulate slowly
and differ much in degree in different species, whilst the good
effects which almost invariably follow a cross are from the first
manifest " ; (3) " It should however be clearly understood that
the advantage of close interbreeding, as far as the retention of
character is concerned, is indisputable, and often outweighs the
evil of a slight loss of constitutional vigour."

Experiments. — Weismann inbred mice for twenty-nine genera-
tions, and his assistant Von Guaita continued the inbreeding
for seven more generations. The general result was a notable
reduction of fertility — about 30%.

Ritzema-Bos inbred rats for thirty generations ; for the first
four years (twenty generations) there was almost no reduction of
fertility, but in the following generations there was very marked
decrease of fertility, increase of mortality, and decrease of size.
But there was no disease or abnormality, such as other experi-
menters— e.g. Crampe — have observed. It goes without saying
that if there is a diseased stock, or rather a stock with an here-
ditary predisposition to disease to start with, then the evil
results of inbreeding will soon be evident. But the point is,
what will happen if the stock be healthy ?

Extensive experiments by Castle and others on the inbreeding
of the pomace-fly, Drosophila ampelophila, led to the general

388 EXPERIMENTAL STUDY OF INHERITANCE

result that " inbreeding probably reduces very slightly the
productiveness of Drosophila, but the productiveness may be
fully maintained under constant inbreeding (brother and sister)
if selection be made from the more productive families."

Castle (191 1 ) also reports that a polydactylous race of guinea-
pigs all descended from one individual remained exceedingly
vigorous for ten years and then showed no hint of diminishing
fertility.

While it seems certain that prolonged and close inbreeding
may afford opportunity for an inherent taint to show itself, to
spread, and to accumulate, it is not the consanguinity that is
to blame for the taint. The same consequences would probably
result if matings took place among unrelated organisms with the
same kind of taint. The idea that there can be any objection
to the marriage of two healthy cousins who fall in love with
one another is preposterous.

Some variations are from the first so stable that their per-
sistence is certain without any precautions of inbreeding. But,
in other cases, it appears to be the experience of breeders that
a period of inbreeding, with elimination of any " weeds " that
may crop up, serves to fix characters, developing " prepotency "
in regard to the desired qualities. Crossing may then be resorted
to without any fear of the excellence being lost, and with the
expectation of an increased stimulus to vigour.

It seems well established that some stable and important
breeds of cattle — e.g. polled Angus — have arisen under conditions
involving in the early stages extremely close inbreeding, and it
is well known in horse-breeding that very valuable results have
been reached by using the same stallion repeatedly on successive
generations.

Thus, if we take the pedigree of the short-horn bull " Courtier,"
calved January 6th, 1896, owned by the Iowa Agricultural
College, we find from the tabulation given by Mr. R. W. Barclay
that " Champion of England " (17526) appears in the pedigree

CONSANGUINITY

389

over twenty-five times, and " on both sides of the house." We
find another famous bull, " Roan Gauntlet " (45276), functioning
over and over again in the lineage. Let us take, for instance,
the pedigree of the paternal grandfather of " Courtier " (see

P- 390)-

39o EXPERIMENTAL STUDY OF INHERITANCE

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CHAPTER XI

HISTORY OF THEORIES OF HEREDITY AND INHERITANCE

" Like leaves on trees the race of man is found,
Now green in youth, now withering on the ground ;
Another race the following spring supplies,
They fall successive and successive rise."

Iliad {Pope's Translation).

[The same may be said of the succession of theories of heredity, but,
in both cases, there is a persistent living tree, to whose growth all the
leaves contribute.]

§ i. What is required of Theories of Heredity and

Inheritance.
§ 2. The Old Theories of Heredity.
§ 3. Theories of Pangenesis.
§ 4. Theory of Genetic or Germinal Continuity,

§ 1. What is required of Theories of Heredity and Inheritance

The main object of a theory of heredity is to express in as
simple terms as possible the nature of the genetic relation which
binds generations together, and to interpret the facts of inheri-
tance in terms of this relation.

The Uniqueness of the Germ -Cells. — The first and chief pro-
blem is to account for the material basis of heredity — i.e. in all
ordinary cases, for the germ-cells. What is their origin and
history ? what relation have they to the parental body which
bears them, from which they are liberated ? what relation have
they to the germ-cells of the body into which they develop ? Or,
more generally, in what way are they peculiar ? how do they

39i

392 HISTORY OF THEORIES

differ from ordinary cells ? to what do they owe their unique
reproductive power ? In short, what enables them to develop
into organisms like the parent-organisms ? To these questions
it is possible to give a satisfactory answer.

The Architecture of Inheritance. — The second problem is
of a different nature, and much more difficult. In some way,
every one must admit, the germ-cells or gametes are potential
organisms. Without any aid except that afforded by an
appropriate environment, they can ' develop into complete
organisms. In some way, the organism, the inheritance, lies
in fosse in the germ-cells. Can we form any image of this ?
Can we construct any hypothetical scheme of the manner in which
the inheritance is organised within the germ-cells ? Chemists
frame hypothetical conceptions regarding the structure of
chemical molecules, and judge of the validity of these by their
usefulness in formulating the changes which the molecules under-
go in certain conditions ; physicists make similar mental pictures
— imaginary models — of the constitution of atoms and so on.
Can biologists do the same in regard to the material basis of
inheritance ?

This is the fundamental problem of inheritance, and it can
only be approached indirectly. The organisation can never be
seen or verified ; all the complexities in germ-cells which micro-
scopic analysis reveals are not more than the rough outlines of
the real edifice — the edifice which the scientific imagination
must build. But the speculative construction is not left to
irresponsible fancy ; it must be such that it corresponds to and
enables us to formulate the visible and measurable facts of
inheritance, and the processes of development. It must be
harmonious with the large generalisations of inheritance, such
as Mendel's law or Galton's law ; it must also be harmonious
with every peculiar phenomenon, such as resemblance to a
remote ancestor.

Theory of Development.— A careful study of the history

THEORY OF DEVELOPMENT 393

of the germ-cells enables us to form a general theory of heredity
enables us to understand how the germ-cells have their peculiar
reproducing power.

A consideration of the facts of inheritance, both general and
special, enables us to form a general theory of inheritance — i.e. a
speculative thought-model of what the architecture of the
germinal material may or must be.

But it is also necessary to try to form some picture of what
occurs during development. The inheritance is in some way
expressed, the potentialities are realised, the legacy is cashed —
can we form any image of what occurs ? As before, our image
may not be actually what occurs, but it must not contradict
anything that occurs, and, more positively, it must help us to
formulate what occurs. This is the business of the theory of
development.

Other Theories are involved. — The result of development is
always an organism more or less like the parent, but the com-
pleteness of hereditary resemblance is usually affected by the
occurrence of variations, sometimes minute and quantitative,
sometimes large and qualitative. It is evident, therefore, that
theories of heredity, inheritance, and development must be
supplemented by a theory of variation.

Nor is it possible to abstract the theory of heredity and in-
heritance from the theory of growth, reproduction, and sex;
from the theory of environmental and functional influences which
we sum up in the term " nurture " ; from the theory of the corre-
lation of psychical and corporeal life ; and from the general theory
of organic evolution in which all biological theories are combined.

But while we recognise that abstraction of particular problems
is merely a device to facilitate clear thinking, and by no means
without the counterbalancing dangers which all abstraction
involves, we propose in this chapter to restrict our attention
to the theories of heredity and inheritance, and to give a
general historical retrospect.

394 HISTORY OF THEORIES

It cannot be said that this historical retrospect leads us to
any complete and satisfactory interpretation of all the puzzling
facts which are covered by the word " heredity," but it will
indicate some of the main attempts which have been made, and
which of these are most promising. We must still recognise the
justice of Herbert Spencer's words :

" A positive explanation of heredity is not to be expected
in the present state of biology. We can look for nothing beyond
a simplification of the problem, and a reduction of it to the same
category with certain other problems which also admit of hypo-
thetical solution only. If an hypothesis which certain other
wide-spread phenomena have already thrust upon us can be
shown to render the phenomena of heredity more intelligible than
they at present seem, we shall have reason to entertain it." *

§ 2. The Old Theories of Heredity

There have been many attempts at theories of heredity and
inheritance, but it is not profitable to say much about the earlier
ones, most of which were theological or metaphysical rather
than scientific. It will be seen, however, that shrewd enough
ideas are sometimes hidden in the old theories, whose phraseology
no longer appeals to the scientific mind.

(a) Theological Theories. — In olden times the idea was
prevalent that the germ of a new human life was at conception
possessed by a spirit, which thereafter became responsible for
development. As it is not so very long ago (1760 or later) that
even digestion was explained as the work of a spirit, it need not
surprise us that development was relegated to a similar unverifi-
able efficiency. Sometimes the spirit was, so to speak, of second-
hand origin, having previously belonged to some ancestor or
to some animal. The idea of successive reincarnations has had
many expressions in the West as well as in the East.

* Herbert Spencer, Principles of Biology, vol. i. (1st ed. 1863).

OLD THEORIES OF HEREDITY 395

So far as the idea persists in the minds of civilised men, it
is so much purified and sublimed that, if it does not appeal to
the student of science as what he would call true, it is at least
such that he cannot wisely call it false. For we believe in mosaic
or ancestral inheritance, and though we know that this has
a definite material basis, we have no warrant for denying that
this has also its metakinetic or spiritual aspect. In any case,
there is more than a metaphor in such phrases as " the hand of
the past," or " the beast in the man."

(b) " Metaphysical " Theories. — For a time, especially in the
latter half of the eighteenth century, it was the custom to appeal
to vires formativce, " hereditary tendencies," and " principles
of heredity," by aid of which the germ was supposed to grow
into the likeness of its parents. It was in part the old story of
explaining the working of the clock by " the principle of horo-
logity," and in part a pedantic way of saying " We don't know."

Nor need we sneer at our predecessors in this respect, for the
tendency to resort to verbal explanations is hardly to be driven
from even the scientific mind except by severe intellectual as-
ceticism. And in so far as it expresses a respectful ignorance, a
consciousness of the complexity of the problem, an awareness
that we have still to use x (the power of life) in our biological
equations, such " metaphysical " mist is perhaps preferable to
the frost of a materialism which blasts the buds of wonder and
gives an illusory clearness to the vision.

Although William Harvey (1578-1657), working " in the
harness of Aristotle," maintained that " all animals are in some
sort produced from eggs," he at the same time believed in spon-
taneous generation as firmly as his master did. Although he
maintained that the living creature begins in an apparently
simple primordium in which " no part of the future offspring
exists de facto, but all parts inhere in potential he was quite
unable to suggest or give any scientific account of the primor-
dium and its powers of development. IJe was forced to fall

396 HISTORY OF THEORIES

back on a metaphysical conception of inheritance and develop-
ment. " Not only is there a soul or vital principle present in
the vegetative part, but even before this there is inherent mind,
foresight, and understanding, which, from the very commence-
ment to the being and perfect formation of the chick, dispose and
order and take up all things requisite, moulding them in the new
being, with consummate art, into the form and likeness of its
parents."

(c) " Preformationist " Theories. — During the seventeenth
and eighteenth centuries, and even within the limits of the nine-
teenth, a theory of inheritance and development prevailed,
according to which the germ (either the ovum or the sperm),
contained a miniature organism, pre-formed though invisible,
which only required to be unfolded (" evolved ") in order to
become the future animal.

Moreover, the egg of a fowl contained not only a micro-organism
or miniature model of the chick, but likewise, in increasing
minuteness, similar models of future generations. Thus the rash
theorists pointed out that Mother Eve must have included
1,543,657 — or, according to another computation, 200,000 million
— homunculi ; and, what was still more rash, they figured, the
miniature homunculus which lay within the sperm. The "ovists,"
who held that the ovum contained the miniature, did battle
with the " animalculists," who supported the claims of the
sperm ; but both schools agreed in the general idea, that
microcosm lay within microcosm, germ within germ, like the
leaves within a bud awaiting successive unfolding, or like an
infinite juggler's-box, to the "evolution " of which there was no
end.

A thoroughgoing representative of the preformationist school
was Charles Bonnet (1720-93), who discovered the partheno-
genesis of green-flies, and made many important observations
on polyps and worms, but after the failure of his eyesight became
more exclusively a speculative thinker. He pondered over

PRE FORM A TIONIST THE ORIES

397

generation and development, and ended by almost denying
them both. He assumed "as a fundamental principle, that
nothing is generated, and that what we call generation is but the
simple development of what pre-existed under an invisible form,
and more or less different from that which becomes manifest
to our senses." In the same way, the renowned physiologist,
Albrecht von Haller, said " Esgibt kein Werden " (" There is no
becoming ") ; and it became the fashion to declare that all
development was an illusion — only an unfolding or evolutio. In
contrast to Harvey's conclusion, " The first concrement of the
future body grows, gradually divides, and is distinguished into
parts ; not all at once, but some produced after the others, each
emerging in its order," Haller wrote, " No part of the body is
made from another ; all are created at once."

To the main conception of preformation and unfolding, two
subsidiary hypotheses were added : (i) that of emboitement,
according to which the germ contains the preformation not of
one organism only, but of successive generations ; and (ii), that
germs occurred scattered throughout the organism, capable
of developing into buds, of replacing lost parts, and so forth —
neither of them ideas to be laughed at, though their particular
expression was necessarily erroneous.

The long-lived theory, variously termed the " preformation
theory," the " theory of evolutio," the " mystical hypothesis,"
the theory of "emboitement" or " Einschachtelung," or "die
Skatulationstheorie," seemed to get its deathblow from Wolff's
demonstration (1759) of " epigenesis," or the gradual develop-
ment of obvious complexity from an apparently simple rudiment.
We say " seemed," because the theory, as theories will, persisted
long after the deathblow was given. Moreover, though Wolff
demonstrated in the chick that gradual becoming which we call
development, he had no way of accounting for the uniqueness
of the germ-cells, and had to fall back on the postulate of a
vis corporis essentialis.

398 HISTORY OF THEORIES

Every one allows that the concrete expressions of the prefor-
mationist doctrine were crude and false. No microscope, how-
ever powerful, will show a miniature model of the future organism
lying within either egg or sperm. But, as Huxley pointed out,
the preformationists were obviously right in insisting that the
future organism must indeed be materially implicit within
the germ ; and they were also right in supposing that the germ
involved the rudiment not only of the organism into which it
grew, but of the next generation as well. But the preformation-
ists themselves had not and could not have any understanding
of the two elements of truth which we can now read into their
theories, and which are at present expressed in modern rehabili-
tations, (i) in the " evolutionist" conception of inheritance and
development, and (ii) in the conception of germinal continuity.
It is a mistake to think that either of these is in any direct
way affiliated to the preformationist doctrine.

The preformationists stocked the germ with some sort of
preformed model, quite unverifiable as they thought of it, and
thus made development easy by reducing it to mere unfolding ;
but they could not account for the preformation.

Yet their antagonists were equally unsatisfactory, for as one
of the most scholarly of embryologists, Prof. C. O. Whitman,
has said, " Aristotle, Harvey, Wolff, and Blumenbach all tra-
versed the same problem, and landed in the same pitfall. They
all faced the question of preformation, and discovering no natural
way by which the germ could come ready-made, they insisted
that the germ must start anew every time and from the pit of
material homogeneity, acquiring everything under the guidance
of hyperphysical agencies, assisted by the accident of external
conditions."

It was, indeed, a deadlock until concrete investigation dis-
closed the origin of the germ-cells with their heritage of organi-
sation, until the actual nature of the genetic linkage between
successive generations was disclosed.

THEORIES OF PANGENESIS 399

§ 3. Theories of Pangenesis

Passing from theological, metaphysical, and mystical inter-
pretations, we come to a whole series of theories, which are in
varying degrees scientific, and may be fairly enough described
by the general designation pangenetic. They all have this in
common, that they seek to explain the uniqueness of the germ-
cell by regarding it as a centre of contributions from different
parts of the organism.

Early Forms. — We need not delay over the earlier and vaguer
forms of this supposition. At such different epochs as are sug-
gested by the names of Democritus and Hippocrates, Paracelsus
and Maupertuis, incipient theories of pangenesis — prophecies of
Darwin's — were suggested. Thus, Democritus maintained that
the " seed " of animals was elaborated by contributions from
all parts of the body, and that the constituent parts reproduced
in development the organs and parts from which they had
originated. Two millennia later, Buffon, of whose speculation
Darwin appears at first to have been unaware, again conceived
of the germs as mingled extracts from all parts of the body, or
as collections of samples from the various organs. If such were
indeed the case, Buffon and his predecessors saw no further
difficulty, for each contributed sample produced in the
development of the embryo a structure like its parental origin.
Bonnet (1776) was another who suggested the possibility of
molecules passing from the organs of the body to build up
the germ.

Spencer's Theory of Physiological Units. — In 1861, the
physiologist Brucke emphasised the usefulness of assuming the
existence of biological units (Elemen(arorganismen) ranking
between the molecule and the cell. In July, 1863, Herbert
Spencer adopted a somewhat similar hypothesis of " physiological
units," lower in degree than the visible cell-units, but more
complex than the chemical molecules. As there is much in his

400

HISTORY OF THEORIES

argument which seems useful to-day, we give a brief summary
(see Principles of Biology (ist ed.), vol. i. p. 181 et seq.).

In the growth of an embryo from apparent simplicity to
obvious complexity, in the regeneration of lost parts, in the
regrowth of a whole by a part, the living substance arranges
itself in definite form as some not-living substances do when
crystallising out of a solution. In restating the fact, Spencer
supposes that certain units composing the living substance possess
" polarity," like the chemical units in crystallisation, meaning
by " polarity " the unexplained power of definite arrangement.
The units cannot be the chemical molecules of albumen and the
like, for these do not show the particular kind of differentiation
seen in growth ; nor can the units be the cells, for the differen-
tiation in question may be seen within the limits of a single cell.

" There seems no alternative but to suppose that the chemical
units combine into units immensely more complex than them-
selves, complex as they are ; and that in each organism, the
physiological units produced by this further compounding of
highly compound atoms have a more or less distinctive character.
We must conclude that, in each case, some slight difference of
composition in these units, leading to some slight difference in
their mutual play of forces, produces a difference in the form
which the aggregate of them assumes."

After the judicious sentences quoted on page 398, Spencer goes
on to say : " The applicability of any method of interpretation
to two different but allied classes of facts is evidence of its truth.
The power which organisms display of reproducing lost parts,
we saw to be inexplicable except on the assumption that the
units of which any organism is built have an innate tendency
to arrange themselves into the shape of that organism. We
inferred that these units must be the possessors of special polari-
ties, resulting from their special structures ; and that by the
mutual play of their polarities they are compelled to take the
form of the species to which they belong." This is illustrated

SPENCER'S PHYSIOLOGICAL UNITS 401

by reference to the way in which pieces of a Begonia-leaf will
reproduce the whole plant. " The assumption to which we
seem driven by the ensemble of the evidence, is that sperm-
cells and germ-cells [better, egg-cells] are essentially nothing
more than vehicles, in which are contained small groups of the
physiological units in a fit state for obeying their proclivity
towards the structural arrangement of the species they belong
to." If the likeness of offspring to parents is thus determined,
it becomes manifest, a priori, that besides the transmission of
generic and specific peculiarities, there will be a transmission of
those individual peculiarities which, arising without assignable
causes, are classed as " spontaneous." So far, in our quotations,
there is no distinct suggestion of the central idea of pangenesis
nor of the transmissibility of modifications.

But Spencer goes on to say : " That changes of structure caused
by changes of action must also be transmitted, however obscurely,
from one generation to another, appears to be a deduction from
first principles — or if not a specific deduction, still, a general
implication. . . . The units and the aggregate must act and
react on each other. The forces exercised by each unit on the
aggregate, and by the aggregate on each unit, must ever tend
towards a balance. If nothing prevents, the units will mould
the aggregate into a form in equilibrium with their pre-existing
polarities. If, contrariwise, the aggregate is made by incident
actions to take a new form, its forces must tend to re-mould
the units into harmony with this new form ; and to say that the
physiological units are in any degree so remoulded as to bring
their polar forces towards equilibrium with the forces of the
modified aggregate, is to say that when separated in the shape
of reproductive centres, these units will tend to build themselves
up into an aggregate modified in the same direction" (p. 256).
That is to say, representative physiological units of the body
congregate in vehicles which we call ova and spermatozoa,
carryii g wi h them, on their journey to form a new generation,

26

402 HISTORY OF THEORIES

some definite and representative results of the modifications
acquired by the parental body.

The physiological units may be compared to a band of
travellers who found a settlement, who build houses and arrange
many matters according to their " character," " tendency,"
" individuality," " polarity " — phrase it as one will. In course
of time their constructed aggregate is modified by circumstances,
by incident forces of war, want, weather, and the like, and the
characters of the units are also modified ; subsequently, some
of them gather into " reproductive centres," which establish
new aggregates, largely after the likeness of the first, and yet
modified by the experiences endured.

On a 'priori grounds, this view seems not without plausibility,
but Spencer's theory had to yield before the fact of germinal
continuity.

Darwin's Theory of Pangenesis. — The best-known theory
of this class is, of course, the " provisional hypothesis of pan-
genesis " suggested by Darwin in his Variation of Animals and
Plants under Domestication (1868). The chief suggestions of
this theory are well known to be as follows :

(1) Every cell of the body, not too highly differentiated,

throws off characteristic gemmules ;

(2) These multiply by fission, retaining their characteristics ;

(3) They become specially concentrated in the reproductive

elements in both sexes ;

(4) In development the gemmules unite with others like them-

selves, and grow into cells like those from which they

were originally given off, or they may remain latent

during development even through several generations.

We do not know whether Mr. Darwin had seriously considered

Mr. Herbert Spencer's hypothesis of " physiological units," but,

as Prof. Ray Lankester points out, the hypotheses might be

called complementary. " The persistence of the same material

gemmule and the vast increase in the number of gemmules,

DARWIN'S HYPOTHESIS 403

and consequently of material bulk, make a material theory
difficult. Modified force-centres, becoming further modified in
each generation, such as Mr. Spencer's physiological units,
might be made to fit in with Mr. Darwin's hypothesis in other
respects " (Ray Lankester, 1870, p. 32). " In fact, in place of
the theory of emission from the constituent cells of an organism
of material gemmules which circulate through the system and
affect every living cell, and accumulate in sperm-cells and
germ-cells, we may substitute the theory of transmission of
force, the two theories standing to one another in the same
relation as the emission and undulatory theories of light."

But we fear that this suggestion has only prophetic value, for
we are not yet in biology in a position to utilise ideas of " modified
force-centres " or " transmission of force." We must creep
along with the slippery clue " metabolism " in our fingers !

One impression, however, we must emphasise — namely, that
for the time Darwin's " provisional hypothesis of pangenesis "
had all the merits of a warrantable scientific hypothesis, and
had the marks of that insight of genius which the illustrious
author was wont to deny in his humble conviction that " it's
dogged as does it."

" Mr. Darwin wished to picture to himself, and to enable others
to picture to themselves, a process which would account for
(that is, hold together and explain) not merely the simpler facts
of hereditary transmission, but those very curious though abun-
dant cases in which a character is transmitted in a latent form,
and at last reappears after many generations, such cases being
known as ' atavism,' or ' reversion ' ; and again, those cases of
latent transmission in which characteristics special to the male
are transmitted to the male offspring through the female parent
without being manifest in her ; and yet again, the appearance
at a particular period of life of characters inherited and
remaining latent in the young organism." *

* E. Ray Lankester, 1890, p. 279 ; Nature, July 15th, 1876.

404 HISTORY OF THEORIES

Jager's Theory. — The next theory— Jager's— is difficult to
summarise, partly because of its technical character, partly
because the author does not appear to be quite consistent in his
statement of it at different times. The main points, under the
present section, are as follows :

(i) Each organ and tissue contains, along with the molecules
of its albumen, a specific " scent-stuff " (Duft- und
Wiirzestoff).

(2) In hunger and similar experience the albumen liberates

the " scent-stuff," which penetrates through the body as
fatty acids, ethers, etc.

(3) These are specially attracted to the reproductive cells,

which, when mature, are thus specialised by the reception

of scent-stuff, and have in their protoplasm vires forma-

tivce enough to reproduce a new organism like the

parent.

It will be seen later on that this hypothesis of chemical

pangenesis is not the most important contribution made by

Jager to the theory of heredity.

Galton's Modified Theory of Pangenesis. — From experi-
ments on the transfusion of blood, Mr. Francis Galton was led
to conclude that " the doctrine of pangenesis, pure and simple,
is incorrect." But he did more than urge serious objections
against Darwin's theory ; he formulated one of his own, to
which ; with the exception of Prof. Herdman, subsequent in-
vestigators do not appear to have attached sufficient import-
ance. The very important part of Galton's theory will be
discussed in its proper place ; it is not included in the series of
pangenetic hypotheses. Galton is, in fact, one of the numerous
biologists who have suggested the continuity of the germinal
protoplasm. He is included at this stage, however, because
he admitted as a subsidiary hypothesis a limited amount of
pangenesis. To account for those cases which suggest that
characters acquired by the individual parent are " faintly

THEORIES OF PANGENESIS 405

heritable," Galton supposed that " each cell may throw off a few
germs that find their way into the circulation, and have thereby
a chance of occasionally finding their way to the sexual elements
and of becoming naturalised among them." This part of his
theory is obviously a cautious admission of limited pangenesis
to account for a number of puzzling cases.

Brooks' Theory. — In 1883, in his valuable work entitled The
Law of Heredity, Prof. W. K. Brooks gave full expression to
a modification of Darwin's view of pangenesis. The main
positions, which are here relevant, may be summarised as follows,
almost in the author's words :

(1) The male and female cells are specialised in different

directions ; their union gives variability.

(2) The ovum is a cell which has gradually acquired a compli-

cated organisation, and which contains material particles
of some kind to correspond to each of the hereditary
characteristics of the species.

(3) The ovum reproducing its like, as other cells, gives rise

not only to the divergent cells of the organism, but also
to cells like itself.

(4) Each cell of the body has the power to throw off minute

germs. When, through a change in its environment, its
functions are disturbed, and its conditions of life become
unfavourable, it throws off small particles which are
the germs or gemmules of this particular cell.

(5) These germs may be carried to all parts of the body. They

may penetrate to an ovarian ovum or to a bud, but the
male cell has gradually acquired, as its especial and
distinctive function, a peculiar power to gather and
store up germs.

(6) In fertilisation each gemmule unites with that particle of

the ovum which is destined to give rise in the offspring
to the cell which corresponds to the one which produced
the gemmule, or else it unites with a closely related

4o6 HISTORY OF THEORIES

particle, destined to give rise to a closely related cell.
Such a cell will be a hybrid, tending to vary.

(7) As the ovarian ova of the offspring share, by direct in-

heritance, all the properties of the fertilised ovum, the
organisms to which they give rise will tend to vary in
the same way.

(8) A cell which has thus varied will continue to throw off

gemmules, and thus to transmit variability to the corre-
sponding part in the bodies of successive generations of
descendants until a favourable variation is seized upon
by natural selection.

(9) As the ovum which produced this selected organism will

transmit the same variation to its ovarian ova by direct
inheritance, the characteristic will be established as
specific, and transmitted henceforth without gemmules.
The above theory, being important, has been stated at some
length. Apart from the suggestion of variation as due to sexual
intermingling, with which Weismann has made us more familiar —
apart, too, from the suggestion of germinal continuity, the credit
of which Brooks shares — there are several important points to
be emphasised in the modification proposed. It is in unwonted
and, abnormal conditions that the cells of the body throw off
gemmules. The male elements are the special centres of their
accumulation ; the female it is that keeps up the general resem-
blance between offspring and parent.

It is not proposed to enter into criticism of pangenetic theories.
The best criticism is found in that abandonment of special
hypotheses which more recent advances have rendered possible.
It has often been urged that the hypothesis of pangenesis involves
not one but many suppositions — that it is just as difficult to
understand why a gemmule should reproduce a cell like its own
origin as to understand the entire problem, and so on. Detailed
criticism will be found in the works of Galton, Ribot, Brooks,
Herdman, Plarre, and others. It is enough for us to emphasise

THEORY OF GERMINAL CONTINUITY 407

the comparative gratuitousness of any special theory whatever,
a paradox which is explained in the succeeding section.

Apart from the fact that the pangenetic hypothesis is not in
harmony with the results of experiments (e.g. on the transfusion
of blood), or with what we know of the physiology of cells, it
may be pointed out that the facts of inheritance are not such
as might be expected if pangenesis were an actual occurrence.
If it were, we should look for a frequent recurrence of, or for
some specific hereditary influence from, exogenous morbid
conditions, especially those associated with marked structural
changes — for instance, injuries to the brain and spinal cord,
cirrhosis of liver and kidney, cirrhotic induration of the lungs
from dust inhalation. In fact, after a short series of generations
the number of healthy subjects would be reduced to a
minimum (Ziegler, 1886, p. 19).

§ 4. Theory of Genetic or Germinal Continuity

Owen. — As far back as 1849, Owen pointed out in his paper
on parthenogenesis that in the developing germ it was possible
to distinguish between cells which became much changed to
form the body, and cells which remained little changed and
formed the reproductive organs. This was probably the earliest
distinct suggestion of the modern theory of germinal continuity.

Haeckel. — In 1866, in his classic Generelle Morphologie,
Haeckel emphasised the simple and yet fundamental fact of the
material continuity of offspring and parent. In an historical note
upon the distinction between the " personal " and " germinal "
parts of an organism, Rauber states that the distinction was
proposed by Haeckel in 1874, and by himself in 1879.

Jager. — Jager stated the doctrine of germinal continuity
very clearly and concisely at an early date: "Through a great
series of generations the germinal protoplasm retains its specific
properties, dividing in every reproduction into an ontogenetic

408 HISTORY OF THEORIES

portion, out of which the individual is built up, and a phylo-
genetic portion which is reserved to form the reproductive
material of the mature offspring. This reservation of the
phylogenetic material I described as the continuity of the germ
protoplasm. . . . Encapsuled in the ontogenetic material, the
phylogenetic protoplasm is sheltered from external influences,
and retains its specific and embryonic characters."

Brooks. — Brooks notes that, in papers published in 1876
and 1877, he had also suggested the notion of germinal continuity,
and the conception is clearly expressed in his work already
quoted : " The ovum gives rise to the divergent cells of the
organism, but also to cells like itself. The ovarian ova of the
offspring are these latter cells, or their direct unmodified de-
scendants. The ovarian ova of the offspring share by direct
inheritance all the properties of the fertilised ovum."

Galton. — The important theory of Galton now requires
notice. Two preliminary notes are requisite. Galton was
extremely doubtful in regard to the genuine inheritance of
acquired characters. It was to account for the possible faint
inheritance of some of these that he still admitted, as a subsidiary
hypothesis, a limited amount of pangenesis. In the second
place, it is needful to notice at the outset Galton's term " stirp,"
which he uses to express the sum-total of the germs, gemmules,
or organic units of some kind, which are to be found in the
newly fertilised ovum.

(1) Only some of the germs within the stirp attain develop-

ment in the cells of the " body." It is the dominant
germs which so develop.

(2) The residual germs and their progeny form the sexual

elements or buds. The part of the stirp developed into
the " body " is almost sterile. The continuity is kept
up by the undeveloped residual portion.

(3) The direct descent is not between body and body, but

between stirp and stirp. " The stirp of the child may

THEORY OF GERMINAL CONTINUITY 4<>9

be considered to have descended directly from a part of

the stirps of each of its parents, but then the personal

structure of the child is no more than an imperfect

representation of his own stirp, and the personal structure

of each of the parents is no more than an imperfect

representation of each of their own stirps."

Here it will be seen that there is a definite expression of the

notion that the germinal cells of the offspring are in very direct

continuity with those of the parents. The antithesis between

the " soma " and the chain of germ-cells is emphasised.

Nussbaum. — The history must also include Nussbaum, who
called emphatic attention to the very early differentiation and
isolation of the sex-elements to be observed in some cases. The
theory both of Jager and of Nussbaum is that of a continuity
of germinal cells. The theory of Weismann is more strictly
that of the continuity of germinal protoplasm. The position of
Jager and Nussbaum may first be summarised more definitely:

(1) At an early stage in the embryo, the future reproductive

cells of the organism are distinguishable from those
which are forming the body.

(2) The latter develop in manifold variety, and lose almost

all likeness to the mother germ.

(3) The former — the reproductive rudiments — are not im-

plicated in the differentiation of the " soma," remain
virtually unchanged, and continue the protoplasmic
tradition unaltered.

(4) The sex-cells of the offspring being thus continuous with

the parental sex-cells which gave rise to itself, they will

in turn develop into similar products.
Now this fact of continuity of reproductive elements is
obviously most satisfactory. If a fertilised egg-cell has certain
characters, x, y, z, it develops into an organism in which these
characters x, y, z are expressed ; but, at the same time, the
future reproductive cells are early set apart, retaining the

410

HISTORY OF THEORIES

characters x, y, z in all their entirety, to start a new organism
again with the same capital. Balbiani, who was not influenced
by theoretical considerations, observed in Chironomus that the
future reproductive cells were isolated before even the blastoderm
was completed ; that is to say, before almost any differentiation
had occurred, a portion of the unspecialised ovum was insulated
to continue the constancy of the species.

In this aspect the reproductive cells form a continuous chain,
and the reproduction of like is as natural and necessary as it
was in the Protozoa. No special theory is required. Similar
conditions produce similar results. Unfortunately, however,
a serious difficulty besets this easy theory. Such an early appear-
ance and insulation of the reproductive cells, continuous with
the very ovum itself, does indeed occur, and where it does the
problem of heredity is simple. Early origin of special germ-cells,
distinguished from those of the general " body," has been ob-
served in some " worm-types " (leeches, Sagitta, threadworms,
many Polyzoa) and in some Arthropods (Moina and Cyclops
among crustaceans, not a few insects, Phalangidae among
spiders), while indications of the same early separation are not
wanting in a number of other organisms. But it must be dis-
tinctly allowed that in most cases it is only after differentiation
is relatively advanced that the future reproductive cells make
their appearance. Thus we have to pass from the few cases as
yet known of the continuity of the germinal cells, to the more
general fact of the " continuity of the germ-plasma."

Weismann's Theory. — Weismann, like the previous investi-
gators, had reached his conclusion independently. In the fact
of continuity between the reproductive elements of generations,
the solution of likeness must be found. But a direct chain
of cellular continuity can only be said to exist in a few cases.
The solution which is proposed for the majority of cases is
as follows :

(i) "In each development a portion of the specific germinal

CONTINUITY OF THE GERM-PLASM 4"

plasma (Keimplasma), which the parental ovum con-
tains, is not used up in the formation of the offspring,
but is reserved unchanged for the formation of the
germinal cells of the following generation."

(2) What is actually continuous is the germ-plasm — " of

definite chemical and special molecular constitution."
A continuity of germinal cells is now rare ; a continuity
of intact germinal plasma is constant.

(3) This germ-plasm has its seat in the nucleus, is extremely

complex in structure, but has nevertheless an extreme
power of persistence and enormous powers of growth.

(4) " The germ-substance proper must be looked for in the

chromatin of the nucleus of the germ-cell, and more
precisely still in those ids or chromosomes which we
conceive of as containing the primary constituents of a
complete organism. Such ids in larger or smaller
numbers make up the whole germ-plasm of a germ-cell,
and each id in its turn consists of primary constituents
or determinants, i.e. of vital units, each of which
determines the origin and development of a particular
part of the organism."

(5) "The splitting up of the substance of the ovum into a

somatic part, which directs the development of the
individual, and a propagative part, which reaches the
germ-cells and there remains inactive, and later gives
rise to the succeeding generation, constitutes the theory
of the continuity of the germ-plasm which I first stated in
a work which appeared in the year 1885 " (1904, vol. i.
p. 411).

CHAPTER XII

HEREDITY AND DEVELOPMENT

*' To think that heredity will build up organic beings without mechanical
means is a piece of unscientific mysticism." — Wilhelm His. But would
even an omniscience of mechanical means explain the facts ?

§ i. Theories of Development

§ 2. Weismann's Theory of the Germ- Plasm

§ 3. Note on Rival Theories

§ 4. Weismann's Theory of Germinal Selection

§ 1. Theories of Development

The Secret of Development. — In his forty-ninth exercitation
on the " efficient cause of the chicken," Harvey (1578-1657),
quaintly expressed his bewilderment before the baffling problem
of development. " Although it be a known thing subscribed
by all, that the fcetus assumes its original and birth from the
male and female, and consequently that the egge is produced
by the cock and henne, and the chicken out of the egge, yet
neither the schools of physicians nor Aristotle's discerning brain
have disclosed the manner how the cock and its seed doth
mint and coine the chicken out of the egge." How much nearer
a disclosure are we to-day ? The visible sequences in the
process of development are in many cases familiar, the external
conditions of development are in many cases well known, and
we have a little insight in regard to what is called the mechanics
of development ; but, on the whole, we have to confess that we

A12

THE SECRET OF DEVELOPMENT 413

do not know the secret of development, which is part of the
larger secret of life itself.

No doubt the process of development may be considered for
certain analytical purposes as an orderly sequence of chemical and
physical events. The developing embryo is the arena of intricate
processes of chemical construction and disruption, of physical
attractions and repulsions ; but the characteristic feature of
the whole business is, that it is co-ordinated, regulated and
adaptive in a manner for which it seems at present, to say the
least, very difficult to suggest any analogue in inanimate nature.
For this reason not a few embryologists, such as Driesch, believe
themselves warranted in frankly postulating a vitalistic factor —
an Aristotelian " Entelechy."

An Outline of what is known. — We know that the germ-cells,
and their nuclei more particularly, form the physical basis of
inheritance ; that there is a genetic continuity between the
fertilised egg-cells which gave rise to the parents and those
which gave rise to their offspring and those of their offspring ;
that fertilisation implies an intimate and orderly union of two
individualities, condensed and integrated for the time being
in the ovum and spermatozoon ; that the sperm acts as a libera-
ting stimulus on the ovum, as well as being the bearer of the
paternal half of the inheritance and of a peculiar little body,
(the centrosome), that plays an important part in the subse-
quent division of the fertilised egg-cell ; that the mode of all
development is by division of nuclei and the integration of
the living matter into unit areas or cells, each presided over by
a nucleus ; that differentiation comes about very gradually —
the obviously complex slowly arising out of the apparently
simple ; that paternal and maternal characteristics are dis-
tributed in exact equality by the nuclear or cellular divisions,
and that the paternal and maternal contributions usually form
the warp and woof of the web which we call the organism, and
persist in the germ-cells thereof, though the expression or realisa-

414 HEREDITY AND DEVELOPMENT

tion of the bi-parental heritage varies greatly in each individual
case ; that the parental heritages include ancestral contributions
which may be expressed in development or may lie latent ; that
normal development implies an appropriate environment, and
that, during the development, there are subtle interactions
between the growing organism and this environment, and
between the different constituents of the growing organism ;
that the development is in certain aspects like the building-up
of a mosaic out of many independently heritable and variable
parts, and that it is in other aspects the expression of an
integrated unity, with subtle correlations between the parts,
and with remarkable regulative processes working towards an
unconsciously predetermined end ; that in a general way the
individual development of organs progresses from stage to
stage in a manner which suggests a recapitulation of the steps
in racial evolution ; that many items in the inheritance, pre-
sumed to be present because of their re-expression in subsequent
generations, may lie latent and find no realisation in the in-
dividual development ; that minute peculiarities of an ancestor
may be handed on from generation to generation, although other
peculiarities of that ancestor find no expression ; that the
offspring of two parents differing in regard to some well-defined
character may all resemble one parent as regards that character ;
that the inbred offspring of these hybrids may have offspring
divisible into two groups, one group resembling the one ancestor
and the other group resembling the other ancestor ; that in
other cases the expressed inheritance seems as if it were a mosaic
of ancestral contributions from parents, grandparents, great-
grandparents in a diminishing geometrical ratio according to
the remoteness of the ancestors : and we know much more
than all this !

A Glimpse of our Ignorance. — On the other hand, we have
still to confess our inability to solve the old problems : How
are the characteristics of the organism potentially contained

EV0LUT10 AND EPIGENESIS 415

within the germ-cells ? how do they gradually find expression in
development ? what is the nature of the compelling necessity
that mints and coins the chick out of a drop of living matter ?
what is the regulative principle that secures the order and
progress which, by devious and often circuitous paths, results
in the fully- formed organism ?

The solution is still far off, and perhaps we shall never get
beyond saying that a germ-cell has the power of developing,
just as a crystal has the power of growing. But this need not
hinder us from trying imaginatively to formulate what takes
place, for it is largely through these provisional hypotheses that
research is provoked and facts are won.

It may be said that there are two main ways of considering
the fundamental problem of " individual becoming " which
embryology raises, and as these are analogous to the theories
of " Epigenesis " and " Evolutio " which were so much dis-
cussed in the seventeenth and eighteenth centuries, the same
catch-words may be retained.

The Old Evolutio and Epigenesis. — Without going into
the details of an often-repeated story, we may recall how
men, like Bonnet (1720-93) and Haller (1708-77), maintained
the preformation of the organism and all its parts within the
germ. The egg, Bonnet said, contained tres en petit the ele-
ments of all the organic parts. " Es gibt kein Werden," Haller
said ("There is no becoming"). Those of this preformationist
school regarded the apparent new formation of organs during
development as an illusion ; what occurs is only an unfolding
[evolutio) of a preformed miniature. How the germ came to have
this preformed miniature, they could not tell.

On the other hand, Caspar Friedrich Wolff (1733-94) was
the pioneer of another school, in maintaining the reality of
what he saw — a gradual differentiation from apparent simplicity
to obvious complexity. The various organs of the developing
embryo make their appearance successively and gradually,

4i6 HEREDITY AND DEVELOPMENT

and are to be seen being foimed. There is no " evolutio " ;
there is new formation or " epigenesis." But how a germ that
seems to start anew every time " from the pit of material homo-
geneity " can develop as it does, the upholders of epigenesis
could not tell.

In fact, the preformationists and the believers in epigenesis
came to a dead-lock, and both schools usually fell back on the
assumption of hyperphysical agencies. Until the genetic or
germinal continuity which links generation to generation was
realised, there could be no real progress in theories of develop-
ment.

The New Evolutio and Epigenesis. — With the growth of
embryology the whole venue has changed, and it would be
misreading history to say that students of development are
still facing the dilemma expressed in the opposition between the
eighteenth-century schools of evolutio and epigenesis. Yet there
is to-day an analogous antagonism, which we must briefly discuss.

The Mosaic Evolutio Theory. — On one view it is supposed
that the germ-cell has an architectural organisation, prede-
termined before development begins, and that development is
in part a " histogenetic sundering " of the pre-existing germinal
mosaic. Some authorities have suggested that the predeter-
mination is in the organisation of the egg- cytoplasm— the
central idea of the theory of " organogenetic germinal areas "
which His elaborated in 1874. This theory may find support
in experiments such as those of Roux on the frog ovum, in
which one of the first two cleavage-cells was punctured, and
its intact neighbour developed into a one-sided embryo ; though
the edge is taken off this case by the observation of Hertwig
that in other conditions the intact blastomere may develop into
a complete embryo of half the normal size. T. H. Morgan has
shown that if the ova experimented with are kept stationary
the result observed by Roux is likely to be seen, while if they
are allowed free movement in the water the result observed

MOSAIC THEORY 417

by Hertwig is likely to be seen. The theory may find support
in the experiments of Morgan and Driesch on Ctenophore ova,
where a defect in the cytoplasm (not involving the nucleus)
is often followed by a modified cleavage and a defective embryo,
as if the architecture had been seriously injured ; but it may be
opposed by Delage's experiments on merogony, where a small
(and non-nucleated) fragment of a sea-urchin's egg may be
fertilised and give rise to a complete larva. In some cases like
the last it seems impossible to maintain that different parts
of the egg are predetermined in relation to particular structures,
and the same conclusion is suggested by Wilson's experiments
on the lancelet ovum, where an isolated blastomere of the
four-cell stage develops into a complete larva. In other cases,
however, it seems as if the egg had a fixed and set architecture,
which cannot be damaged without affecting the embryo. In
certain cases there is proof that the egg contains pre-formed,
and even pre-localised, organ-forming substances, and the re-
moval of a small part may be followed by the absence of a
definite structure, should development go on. In some cases it
seems clear that the old view of the ovum as homogeneous and
isotropic must give way before experimental evidence of hetero-
geneity.

The " Preformation " mainly Nuclear. — But the researches
of Kolliker, Strasburger, Hertwig, and others led to a transfer-
ence of attention from the cytoplasm of the germ-cell to the
nucleus. From the importance of the nucleus in metabolism, in
the regeneration of Protozoon fragments, in maturation, in fertili-
sation, and in cleavage, it was argued — most forcibly, perhaps, by
Weismann — that the nucleus must be the bearer of the heritable
qualities. Meanwhile, many were recognising the value of
Nageli's conception (1884) of a specific idioplasm — a complex sub-
stance which, in its molecular organisation and in the metabolism
it induces, is different for each species. Weismann developed
this in his theory of the germ-plasm, which he regarded as wholly

27

4i8 HEREDITY AND DEVELOPMENT

resident in the chromosomes of the nucleus. Thus, the locality
of the pre-established organisation was shifted from the cyto-
plasm to the nucleus, but it is not inconsistent with this to
suppose that the essential mosaic or organisation within the
chromosomes of the nucleus may induce a secondary mosaic or
localisation in the building material which the general substance
of the egg-cell affords. It need hardly be pointed out that the
organisation or architecture which is thought of in such cases is
something infinitely finer than the microscopically visible
(reticular or alveolar) structure which all living matter exhibits.

What is Distinctive in Development? — Unicellular organ-
isms divide and redivide rapidly ; it is their normal mode
of multiplication. The germ-cells of multicellular animals do
the same in the early chapters of their history. The fertilised
egg-cell does the same ; but the daughter-cells or blastomeres
cohere to form an embryo, just as the daughter-cells into which
some Protozoa divide also cohere to form a " colony." For a
time there is no growth among the cleavage-cells into which
the fertilised ovum divides, so that an embryo of sixty- four cells or
more is no larger than the undivided egg. This, again, is paral-
leled by cases of spore-formation in Protozoa, where many
divisions occur in a short time and within the limited space of
the mother-cell. In some cases the young embryo shows a large
number of nuclei derived from the division of the fertilised
nucleus of the egg-cell, while the cell-substance is slow in being
segregated around the nuclei into unit-areas or cells. This,
again, is paralleled by some multinucleate Protozoa.

Thus the really distinctive fact in development is the
progressive differentiation. The daughter-cells do not remain
homogeneous ; some start a lineage of nerve-cells, others
a lineage of digestive cells, and so on. In a gradual, orderly
fashion the apparently simple gives rise to the obviously com-
plex, and throughout the process there are striking phenomena
of adjustment to temporary conditions, of " self-differentiation "

DIFFERENTIAL DIVISION 419

on the one hand and mutual influence on the other, and of
integrated " regulation " throughout. We are so familiar with
the orderly succession of events that we hardly realise the
marvel of it, until we play some trick with the developing egg,
introducing disorder, and see how equilibrium and normality
are restored. Thus the one-sided half-embryo of the frog pro-
ceeds at a certain stage to develop the missing half.

Roux. — Starting from the assumption that the nuclei of the
germ-cells contain a specific idioplasm or architectural sub-
stance (the vehicle of the heritable qualities), and with the
further assumption that this substance is a complex aggregate
of different kinds of particles (the material expressions of the
different sets of qualities), Roux invented the hypothesis of two
kinds of nuclear division, quantitative and qualitative. The
former results in equivalent, the latter in dissimilar nuclei ; the
former is an integral, the latter a differential division. Roux
supposed that the latter mode was characteristic of the early
stages of development, during which the different components
or qualities of the idioplasm are distributed among the blas-
tomeres. Thus it comes about that each blastomere, though
not independent of its neighbours, is endowed with a power
of "self-differentiation" along particular lines defined by its
specific share of the idioplasm.

Weismann. — Similarly, but even more elaborately, as we shall
see, Weismann pictured development as a gradual process of
differential division, distributing the representative particles
or primary constituents of the germ-plasm. While this is
going on there is also a process of quantitative division, which
gives rise to the lineage of future germ-cells bearing the complete
equipment of germ-plasm, and this quantitative division also
occurs amid the qualitative divisions when many cells with
identical characters are produced.

Criticism of Mosaic Theories. — These mosaic theories of
development have been criticised from many sides. Thus it is

420 HEREDITY AND DEVELOPMENT

pointed out that no visible phenomena of nuclear division
suggest that the partition may be qualitative ; on the contrary,
that the whole elaborate process of nuclear division seems
adapted to secure the exact equivalence of the two daughter-
nuclei. It may be, however, that while there is always a general
equivalence, in the sense, for instance, that the large nuclear
bodies or chromosomes are accurately split, and that each
daughter-cell gets the same number, there may be at the same
time a more intricate qualitativeness in the division. Again,
the critics have brought forward some of the results of experi-
mental embryology which seem at first sight to tell against the
hypothesis of differential division, especially where one of the
first two or first four blastomeres is seen to form a complete
and normal embryo, or where under artificial conditions (of
pressure, etc.) certain cells develop into tissues which in normal
conditions are formed by quite different cells. To explain these
and other difficulties — e.g. regenerative phenomena — various in-
genious sub-hypotheses have been invented. It seems highly
probable that the distribution of particular characters (if it be
a reality at all) occurs sooner in some developing eggs than in
others ; in other words, that the cells of some embryos are " set "
and defined at an earlier date than those of other embryos.

Non-Mosaic Theories.- — All embryologists agree that a germ-
cell has a specific organisation, but many will not admit that
it is necessary or usefid to people the nucleus with a large body
of representative particles, ready to distribute themselves and
work upon the virgin soil which the protoplasm affords. All
agree that there is gradual differentiation of cells as development
proceeds, but many will not admit that it is necessary or useful to
think of this in terms of a distribution of representative particles
from the original depot in the nucleus of the fertilised egg-cell.

Oscar Hertwig may be named as a prominent representative
of those who give the facts of development an interpretation
somewhat different from that suggested by Roux and Weismann,

N ON -MOSAIC THEORIES 421

We may suppose that, from the youngest ovarian ovum onwards,
the nucleus exerts a " control " upon the surrounding cytoplasm,
whether by the migration of " pangens " (De Vries), or of
specific formative substances (Sachs), or of enzymes, or by a
propagation of molecular movements (Nageli). In some such
way — varying greatly in degree in different cases — the nucleus
prepares in the cytoplasm of the egg a framework for its future
operations. This may be so slightly pre-established that from
a minute fragment of the egg a complete larva may be reared
(as in sea-urchins), or so well established that if a part of the
unsegmented egg be removed the remainder forms a defective
larva (ctenophore).

The nucleus of the fertilised egg-cell divides into equivalent
halves, but these find themselves in more or less different territory,
as the result of the preparatory framework which the nucleus,
before division, had established in the cytoplasm. In technical
language, the nuclei, though equivalent, find themselves in a
not altogether isotropic medium.

The dividing nuclei, as Driesch and Boveri suggest, are differ-
ently stimulated to expression in the different areas of the hetero-
geneous cytoplasm, and they thus call forth new differentiations
in these, in ever-increasing complexity of action and reaction.

If the initial cytoplasmic differentiation was slight, the first
steps in differentiation will be correspondingly slight, and in these
cases an isolated cleavage-cell or blastomere may still form a
complete embryo, as in the lancelet. If the initial cytoplasmic
differentiation was more pronounced, an isolated blastomere may
not be able to do more than form a partial embryo ; the "setting "
of the cytoplasm may be too strong to be overcome even by the
completely equipped blastomere-nucleus.

Thus we reach the idea, expressed, for instance, by Driesch,
that " the relative position of a blastomere in the whole deter-
mines in general what develops from it ; if its position be changed,
it gives rise to something different ; in other words, its prospective

422 HEREDITY AND DEVELOPMENT

value is a function of its position.'' But the " position" has a
more than merely topographical connotation ; it means, as Prof.
E. B. Wilson says, " the physiological relation of the blastomere
to the inherited organisation of which it forms a part."

But, here again, even when we recognise as fully as we can
(a) the importance of the initial inherited organisation, (b) the
influence of segment upon segment as development proceeds,
and (c) the continually operative influence of the normal en-
vironmental stimuli, we have still to confess that the process
of development remains very mysterious.

The Antithesis of the Two Views. — The student who is not yet
clear as to the antithesis of the two views of development outlined
above should read Dr. Chalmers Mitchell's admirably lucid intro-
duction to his translation of Prof. Oscar Hertwig's Biological Pro-
blem of To-day (London, 1896). It concludes with the following
contrast : " Hertwig says that all the cells of the epiblast, hypo-
blast, mesoblast, and of the later derivatives of these primary
layers receive identical portions of germ-plasm by means of doubling
[quantitative or integral (erbgleich)] divisions. The different
positions, relations to each other and to the whole organism, and
to the environment in the widest sense of the term, cause different
sides of the capacities of the cells to be developed ; but they retain
in a latent form all the capacities of the species. Weismann says
that the nuclear divisions are differentiating [qualitative (erb-
ungleich)], and that the microcosms of the germ-plasm, in accord-
ance with their inherited architecture, gradually liberate different
kinds of determinants into the different cells, and that, therefore,
the essential cause of the specialisation of the organism was con-
tained from the beginning in the germ-plasm."

That differentiation may occur at very early stages is certain ;
that it has potentially occurred, although there is no visible evidence
of it, is also certain ; it seems to us difficult to interpret this without
the hypothesis of differential division.

At the 2-cell or 4-cell stage of the development of the egg 0$ the
sea-urchin, the cells are equipotential, for an isolated blastonvere
(even at the 8-cell stage) may develop into a complete larva (Driesch).

But a little later, when invagination has occurred, when two
germinal layers are established, the cells are no longer equipotential-

TWO VIEWS OF DEVELOPMENT 423

They can no longer regenerate complete larvae. Even when several
cells are separated off, they are not able to develop into complete
larvae. They grow into monstrous forms, which soon die. It is
difficult to see why this should be so, unless differential division has
occurred.

An Analogy.— A well-organised body of colonists reaches
a new land, which they will develop. Soon after they land
they distribute themselves in bands, according to their bent,
as hunters, shepherds, fishers, farmers, miners, and so forth.
As they possess the new land more and more fully, they segregate
more and more, dividing into increasingly specialised bands ;
and as these find themselves in appropriate areas they settle
down, and they stamp the areas with their particular character.
Here a farm arises and there a factory, here a sheep-ranch
and there a store, here a mine and there a fishing village. We
can quite well understand that certain interpreters or historians
would lay emphasis on the fact that, as the emigrant bands
journeyed, they segregated persistently into smaller and more
specialised groups, according to the old-established — indeed,
hereditary — predispositions or qualities of the members com-
posing the bands. This is a far-off image of the mosaic theory
of development with its hypothesis of differential divisions. {

On the other hand, we may imagine another well-organised
body of colonists reaching another new land, which they will
develop. They have a complex organisation with many po-
tentialities, and they work best together. It cannot be said
that some are preformed to be hunters, others to be shepherds,
others to be fishers, others to be farmers, others to be miners,
and so on. They begin by marking out the surrounding area
into localities, and into each locality a representative band of
emigrants proceeds to journey. They divide into homogeneous
bands, each with a full representation of the capacities of the
original body of colonists. But as they spread they are neces-
sarily influenced by the area in which they find themselves,

424 HEREDITY AND DEVELOPMENT

and by their relations to neighbouring bands, and gradually
they, too, differentiate into distinctive kinds of settlements.
We can quite well understand that certain interpreters or his-
torians would lay emphasis on the fact that, as the originally
similar bands of colonists journeyed, they became differentiated
in response to the varied environmental conditions and in
relation to their neighbours. Their prospective value at any
moment is " a function of their locality." This is a far-off
image of the non-mosaic theory of development. It is surely
conceivable that both interpretations are correct.

Summary. — According to the mosaic theory, the main mode
of differentiation is qualitative nuclear division, which sifts out
the various items of the mosaic (the representative particles
or primary constituents) into different cells. Thus, if the fer-
tilised ovum had the qualities or potential qualities abcxyz, its
first four daughter- cells or blastomeres might have the qualities
abcxyz, abxyz, abcxy, and abcxz. What each cell becomes is prim-
arily determined by the particular contingent of representative
particles which possesses it.

According to the non-mosaic theory, the division of the
nucleus is always quantitative — i.e. without any sifting out of
particular potentialities — and differentiation is due to the varied
relations in which the nuclei find themselves. The prospective
value of an embryonic cell, Driesch said, is " a function of its
location." Each of the early cells is supposed to have a com-
plete set of specific characteristics in potentia ; but some of these
remain latent, while others become active, this being determined
by the relations of the cell to the whole system of which it forms
a part.

Thus, while the two views agree in attributing to the essential
germinal material a specific organisation corresponding to the
hereditary qualities, they differ in their picture of what dif-
ferentiation implies, the mosaic theory relying on the hypothesis
of qualitative division which segregates representative particles,

THEORY OF THE GERM-PLASM 425

the non-mosaic theory denying qualitative division and em-
phasising the importance of environmental interaction in the
widest sense.

It must be carefully noted that as far as the eye can see,
there is in the development of the embryo only one kind of cell-
division, which involves a visibly accurate longitudinal halving
of each of the chromosomes of the nucleus. Therefore if there
is any qualitative or differential division it must be of a subtler
sort.

§ 2. Weismann's Theory of the Germ-plasm

No one has done more to further the scientific study of here-
dity than Prof. August Weismann, of Freiburg, although his
work has been on different lines from that of the statistical
school which we particularly associate with the names of Mr.
Francis Galton and Prof. Karl Pearson, or from that of the
experimental school which we particularly associate with the
names of Gregor Mendel and Mr. Bateson. In general we
may say that Weismann has thought out a theory of heredity,
coherent with a theory of development and a theory of evolution,
which has inspired much research and has commanded the
admiration of his most resolute opponents. He has done for
the study of heredity what Dalton with his atomic theory
did for chemistry, and though his theory will doubtless be
developed, as Dalton's has been, it seems unlikely that the
fundamental ideas of Weismannism will be discredited in the
future evolution of biology.

As Weismann's interpretations have gone on growing as
facts accumulated and as his insight increased, they present
difficulty to those who have not followed their development,
and it is therefore necessary to present a brief statement of
Weismannism as developed, for instance, in The Evolution
Theory (1904).

426 HEREDITY AND DEVELOPMENT

The Material Basis of Inheritance. — It seems that the
botanist Nageli was the first to point out that the material basis
on which the hereditary tendencies depend must be a minimal
quantity of substance. The inheritance from the father and
from the mother is potentially equal ; the vehicle of this in-
heritance is in the germ-cells ; the mass of a spermatozoon
may be only 1001000th part of the mass of the ovum which
it fertilises ; in one respect the two sex-cells are equivalent
— they have the same number of stable readily stainable
bodies or chromosomes in their respective nuclei ; the number
of these bodies is constant for each species, except that the
number in the mature sex- cells is half that found in the ordinary
cells of the body ; the chromosomes play an obviously important
part in the intermingling or amphimixis which occurs in fer-
tilisation and in the subsequent divisions of the fertilised egg :
for these and other reasons, Weismann concluded in 1885, ^
Strasburger and O. Hertwig did about the same time, that
the hereditary substance is in the chromosomes of the nucleus of
ihi germ-cell.

Microscopic vivisection experiments on Protozoa — e.g. the
trumpet animalcule, Stentorshow that a fragment of a cell
with a portion of nucleus will live on and reconstruct an entire
organism, whereas a portion without nucleus, though it lives for
a time, is unable to assimilate or recuperate its losses and soon
dies. " It is in the nucleus, therefore, that we have to look for
the substance which stamps the material of the cell-body with
a particular form and organisation— namely, the form and organi-
sation of its ancestors." It goes without saying that the sex-cell
is a unity, a minute organism, that its cell-protoplasm (in the
case of the ovum at least) represents the building-material
(trophoplasm), in which alone the hereditary substance (idio-
plasm) can unfold its wonderful powers ; but it must be remem-
bered that even a non-nucleated fragment of an ovum may
develop (into a larva at least) if it be fertilised — i.e. supplied

THEORY OF THE GERM-PLASM 427

with a sperm-nucleus. Everything points to the conclusion
that there is a definite hereditary material, and that it is in part
at least bound up with the chromosomes of the nuclei of the
paternal and maternal germ-cells.

The Germ-plasm mainly Nuclear. — No one can doubt
that a germ-cell is a unity, that it represents a " cell-firm," that
its virtue is dependent on the interaction of nucleoplasm, cyto-
plasm, and centrosome, or that the substance of the egg is the
actual building-material out of which the embryo is constructed.
And yet, there are many facts which compel us to conclude that
the basis of inheritance is essentially bound up with the chromo-
somes of the nucleus. Repeating, in part, what we have said
in Chapter II., we may note the following facts :

1. In some cases almost the whole cytoplasmic differentiation
of the spermatozoon — namely, the locomotor apparatus — is left
ou'.side the ovum, and what enters is the head, which is almost
purely chromatin-material, plus the minute mid-body or centrosome,
which functions as a dynamic centre in division.

2. The chromatin-bodies or chromosomes have a constant number
for each species, except that in the mature sex-cells the number is
half the normal, i.e. half the number found in the body-cells.

3. In nuclear division the chromosomes are longitudinally split,
and are in various ways so distributed that each of the daughter-
ceils into which a mother-cell divides receives a precisely equivalent
quota of chromosomes.

4. In many cases it is certain that the chromosomes of the sperma-
tozoon entering the ovum are precisely equivalent in number to
those which the mature ovum contains.

5. Throughout the whole world of life, the chromosomes —
whether during the growth, or the maturation, or the amphimixis
of germ-cells — behave in a generally similar manner, though there
are many differences in detail.

Ancestral Plasms.— Assuming that the chromatin substance
cf the nucleus of the germ-cell is the vehicle of the inheritance,
Weismann argued that it " contains not only the primary con-
stituents of a single individual of the species, but also those

428 HEREDITY AND DEVELOPMENT

of several, often even of many, individuals." In fact it is
a mosaic of " ancestral plasms." But what evidence is there
of this ?

A fertilised egg develops into an organism by cell-division.
For a time it is demonstrable that the nucleus of each of the
daughter-cells into which the fertilised egg-cell divides contains
paternal and maternal chromosomes in equal number. Gradu-
ally differentiation sets in, and various kinds of bod}'-cells with
specialised structure and function appear ; but often it is quite
demonstrable that the maternal and paternal contributions are
forming the warp and woof of the organism. While most of the
ever-increasing crowd of embryonic cells undergo differentiation,
some do not, but remain unspecialised, retaining the characters
of the fertilised ovum. From this lineage of unspecialised cells,
as we have explained in Chapter II., the germ-cells of the new
organism arise. By-and-by when the organism becomes mature,
these germ-cells are liberated, and each of them will have, by
hypothesis, chromosomes derived from the original father and
mother. But fertilisation will occur between these liberated
germ-cells and others whose chromosomes are likewise derived
from another father and mother, assuming that the usual
cross-fertilisation occurs. Thus there comes to be an ac-
cumulation of contributions from different ancestors, though
the actual number of visible stainable bodies or chromosomes is
always kept the same. It seems impossible to evade the con-
clusion that the material basis of inheritance is a mosaic of ancestral
plasms.

It is interesting to recall Darwin's memorable saying : " Each
living creature must be looked at as a microcosm — a little
universe, formed of a host of self-propagating organisms, in-
conceivably minute and as numerous as the stars in heaven."
He thought of his hypothetical gemmules as including not
merely the contributions of the immediate parents, but ancestral
items from even remote progenitors.

THEORY OF THE GERM-PLASM 429

As a non-nucleated fragment of egg fertilised by a sperm will
in some cases — e.g. sea-urchins — develop into a normal larva, as
an unfertilised ovum — e.g. of sea-urchin — may under certain
treatment develop into a normal larva, it is obvious that each of
the germ-cells has in its nucleus a complete set of hereditary
qualities.

As a single egg often produces two complete organisms (true
twins), and in some cases — e.g. the parasitic Hymenopteron
Encyrtas— produces a legion of embryos, it is obvious that,
however the hereditary qualities are contained in their
chromatin vehicle, they can be very readily and rapidly multi-
plicated by division ; and every one is aware how many germ-
cells can be produced in a short time by a sexually mature
animal.

It is now well known for a large number of animals and plants
that during the maturation of ovum and spermatozoon the
number of chromosomes is reduced to half the normal number
characteristic of the body-cells of the species, so that the union
of sperm-cell and egg-cell results, not in a doubling of the usual
number of chromosomes (as would be the case were there no re-
duction), but in a restoration of the normal number. It there-
fore follows that a reduction of the number of chromosomes by
a half does not in any way affect the completeness of the heritage.
" The halved hereditary substance still contains the whole mass
of primary constituents."

By following up this line of argument, Weismann was led to the
theoretical conclusion that each of the chromosomes must con-
tain a complete equipment of hereditary constituents, and that
the germ-plasm represented by all the chromosomes in the
germ-cell must include several " complexes of primary con-
stituents," each complex sufficient in itself to form a complete
individual. In other words, the fertilised egg-cell is a mosaic
of " ancestral plasms."

" I call the idioplasm of the germ-cells Germ-plasm, or the

430

HEREDITY AND DEVELOPMENT

Fig. 43. — The relation between
reproductive cells and the
" body." The broken
vertical line to the left
represents a succession of
ova from which " bodies "
are produced. The other
part of the figure indicates
a chain of " bodies," — suc-
cessive generations. For
convenience of the diagram,
the " bodies " are repre-
sented as if larger at each
generation. A sperm fer-
tilising an ovum at the
beginning of each generation
is indicated.

primary-constituent - substance
of the whole organism ; and
the complexes of primary-
constituents necessary to the
production of a complete indi-
vidual I call Ids." [In some
cases these " ids " are probably
the chromosomes, but many
band-like chromosomes (or
" idants ") are visibly com-
pound, consisting of several
ids.] It is through the co-
operation of these ids that the
precise constitution of the indi-
vidual which develops from the
fertilised ovum is determined.

Every one admits that the
germ-cell has a complex or-
ganisation, with the details of
which every year makes us
better acquainted. Every one
admits that the whole sub-
stance of the fertilised ovum
cannot be equally important
as regards inheritance. Every
one admits that small but still
visible units — the ids or the
chromosomes — behave as if
they were of fundamental im-
portance. If we admit that
there is a hereditary substance
at all, the theoretical interpre-
tation begins when we regard
these ids as containing a com-

THEORY OE DETERMINANTS 431

plete set of hereditary qualities, as containing implicitly all the
parts of a perfect animal, as the units in that multiplicate mosaic
which makes up an inheritance.

There is more than a superficial resemblance between this
doctrine and the Buddhistic theory of Karma. As Huxley said,
" the tendency of a germ to develop according to a certain
specific type is its Karma. It is the ' last inheritor and the last
result ' of all the conditions that have affected a line of ancestry
which goes back for millions of years to the time when life first
appeared on the earth. The germ-plasm is the last link in a
once continuous chain extending from the primitive living sub-
stance ; and the characters of the successive species to which it
has given rise are the manifestations of its gradually modified
Karma." (See Evolution and Ethics.)

Determinants. — " I assume," Weismann says, " that the germ-
plasm consists of a large number of different living parts, each
of which stands in a definite relation to particular cells or
kinds of cells in the organism to be developed— that is, they are
' primary constituents ' in the sense that their co-operation in
the production of a particular part of the organism is indispens-
able, the part being determined both as to its existence and its
nature by the predestined particles of the germ-plasm. I there-
fore call these last Determinants, and the parts of the complete
organism which they determine Determinates " (1904, vol. i.

P- 355)-

But how many determinants are to be postulated in any given

case ? Weismann supposes that every independently variable
and independently heritable character is represented in the germ-
plasm by a determinant. A lock of white hair among the dark
s may reappear at the same place for several generations ; it is
difficult to interpret such facts of particulate inheritance except
on the theory that the germ-plasm is built up of a large number
of different determinants.

It may be pointed out that almost all biologists who have

432 HEREDITY AND DEVELOPMENT

tried to form a conception of the ultimate structure of living
matter have been led to the assumption— expressed in very varied
phraseology — of ultimate protoplasmic units which have the
powers of growth and division. It is in no way peculiar to Weis-
mann to imagine biophors and to credit them with the powers
of growing and dividing. This cannot, indeed, be proved, but
many facts point to it. The cell divides, but this is preceded
by the division of the nucleus ; the nucleus divides, but this
involves splitting of the chromosomes ; and the chromosomes
are sometimes visibly composed of still smaller bodies, arranged
like beads on a string. As Prof. E. B. Wilson says (1900, p. 84),
" Our study of nuclear division reveals to us, not a homogeneous
dividing mass, but a descending series of dividing elements,
which, as if seen through an inverted telescope, recede from
the eye almost to the limits of microscopical vision. There is
no reason to place the limit of this series at the point where it
vanishes from view, and we are thus almost irresistibly drived
to the conclusion that the division of the nuclear substance
as a whole must be the result of division on the part of invisible
elements, by the aggregation of which the visible structures
are formed." Moreover, in many cases the cytoplasm or
extra-nuclear part of the cell contains minute bodies or
" plastids "—e.g. chlorophyll corpuscles — which also multiply
by division.

Those who find it difficult to believe in the theory that there
are multiple sets of analogous determinants in the germ-plasm
should consider, for instance, the facts of sex and sexual dimor-
phism. A queen bee lays an unfertilised egg which develops
into a drone or male, which is in many detailed ways different
from the queen, and is primarily different in producing sper-
matozoa, not ova. But since this drone has only a mother, no
father, there must have been in the fertilised ovum which
developed into the mother-bee the potentiality— i.e. the deter-
minants— of male reproductive organs and masculine characters.

THEORY OF DETERMINANTS 433

Yet there was no hint of these in the queen bee herself. They
must have lain as latent elements in her inheritance. In the
case of plant-lice (Aphides) and some water-fleas (Daphnids),
where there is a succession of parthenogenetic females, the
primary constituents of masculine characters must remain latent
for several generations. In some cases — e.g. sea-urchins — the
sexes are so closely alike, even as regards their reproductive
organs, that we may almost say that they differ only in ' physio-
logical gearing,' and that to postulate one army of determinants
is sufficient without complicating matters by postulating at
least two analogous armies. But in the great majority of
cases there is marked dimorphism between the sexes, and, even
in the cases above referred to, it has to be remembered that
the spermatozoon itself is a very complex structure, with
apical piece, head, middle piece, tail, and other minutias,
many of which have no analogue in the ova, and are, indeed,
specially adaptive peculiarities which aid the spermatozoon in
finding the ovum. Thus it is difficult to escape Weismann's
conclusion that both kinds of sexual characters must be
present, some active, some latent, in every germ-cell and in
every organism.

Another good example may be found in wheel-animalcules or
Rotifers, where the primitive germ-cells divide into two kinds of
eggs, externally identical, and yet so different that from the
one kind only females develop, and from the other kind only
males. Neither kind is fertilised. The ova which develop into
females must carry with them determinants corresponding to
masculine characters, though these remain quite latent, for
these females give origin to males as well as females. It may
be that nutritive and other environmental influences deter-
mine whether the determinants corresponding to the female
sex or those corresponding to the male sex become active ;
but the point at present is, that it is difficult to think out

what occurs except on the hypothesis that the germ-plasm

28

434 HEREDITY AND DEVELOPMENT

contains both male and female determinants, analogous but
distinct.

Summary of Weismann's View. — " The germ-substance owes
its marvellous power of development not only to its chemico-
physical constitution, but to the fact that it consists of many
and different kinds of primary constituents — that is, of groups of
vital units equipped with the forces of life, and capable of inter-
posing actively and in a specific manner, but also capable of
remaining latent in a passive state until they are affected
by a liberating stimulus, and on this account able to interpose
successively in development. The germ-cell cannot be merely
a simple organism ; it must be a fabric made up of many
different organisms or units— a microcosm " (1904, vol. i.
p. 402).

A living creature usually takes its origin from a fertilised egg-
cell, from a union of an ovum and a spermatozoon — two dimorphic
germ-cells. These germ-cells are descended by continuous cell-
division from the fertilised ova which gave rise to the two
parents ; they have retained the organisation of those fertilised
ova, and this organisation has its vehicle in the stainable
material of the nuclei — the germ-plasm. This germ-plasm con-
sists of several chromosomes or idants, each of which is made up
of several pieces or ids, each of which (here hypothesis begins),
is supposed to contain all the potentialities — generic, specific,
and individual — of a new organism. Each id is a microcosm j
with an architecture which has been elaborated for ages ; it is
supposed to consist of numerous determinants, one for each part
of the organism that is capable of varying independently or of
being independently expressed during development. Lastly,
each determinant is pictured as consisting of a number of ulti-
mate vital particles or biophors, which are eventually liberated
in the cytoplasm of the various embryonic cells. All these units
of various grades are capable of growth and of multiplication
by division.

MATURATION AND AMPHIMIXIS 435

Summary.

The physical basis of inheritance — the germ-plasm — is in

the chromatin of the nucleus of the germ-cell.
The chromatin takes the form of a definite number of

chromosomes (or idants).
The chromosomes consist of ids, each of which contains a

complete inheritance.
Each id consists of numerous primary constituents or

determinants.
A determinant is usually a group of biophors, the minutest

vital units.
The biophor is an integrate of numerous chemical molecules.

Maturation and Amphimixis. — It is necessary here to inter-
polate a reference (a) to the facts of maturation — the processes
that occur in the immature egg-cells (oocytes) and in the im-
mature sperm-cells (spermatocytes) ; and (b) to the facts of
amphimixis or fertilisation — the intimate and orderly union of
the (reduced) nuclei of the two kinds of sex-cells.

Since the spermatozoon is known to bring into the mature
ovum the same number of chromosomes as the mature ovum
contains in its nucleus, each act of fertilisation would double the
normal number of chromosomes if there were not some process
obviating this. The doubling of the normal number does not
occur, because the mature spermatozoon and the mature ovum
have already undergone a reduction of the number of their
chromosomes to half the normal number.

In various ways, during the divisions of the sperm-cells ante-
cedent to their complete differentiation, and during the process
which is called the maturation of the ovum — the two divisions
which result in the liberation of two polar bodies —the normal
number of chromosomes is reduced by a half. Thus, when
fertilisation occurs, the number of chromosomes is restored to the
normal. This fact has been securely established by the researches
of Van Beneden, Oscar Hertwig, Boveri, Henking, and others.

Reducing Divisions. — Since Van Beneden discovered that

436

HEREDITY AND DEVELOPMENT

each of the two nuclei which unite in fertilisation contains one-
half of the number of chromosomes characteristic of the somatic
cells, though the nuclei of the earlier stages of the germ-cells have
the same number as the somatic cells, it has been plain that a re-
ducing process must occur at some stage, and there is now general
agreement that the reduction takes place in the last two cell-
divisions by which the definitive germ-cells arise — namely, when
the ovarian ovum gives rise to the mature ovum and two or three

Fig. 44. — Diagram of maturation and fertilisation.
of Sex.)

(From Evolution

The upper line shows development of spermatozoa. The lower line shows maturation of
the ovum. The middle line to the right shows fertilisation, a, an amoeboid primitive sex-
cell ; A, ovum, with nucleus or germinal vesicle (>») ; B, ovum, liberating first polar body (/>') ;
C, extrusion of second polar body (p2) ; 1, a mother-sperm-cell or spermatogonium ; 2, 3, balls
of immature spermatozoa, resulting from the division of (1) ; sp., mature spermatozoa; D,
the entrance of a spermatozoon into the ovum ; E, the male and female nuclei sp.n and na,
approach one another.

polar bodies, and when a spermatocyte divides into four sper-
matids or young spermatozoa. The parallelism in the two cases
is very striking, but as O. Hertwig says, " while in the latter case
the products of the division are all used as functional sperma-
tozoa, in the former case one of the products of the egg-mother-
cell becomes the egg, appropriating to itself the entire mass of
the yolk at the cost of the others, which persist in rudimentary
form as polar bodies." The hypothesis of Minot, adopted also by

MATURATION AND AMPHIMIXIS 437

Van Beneden, that each germ-cell is originally hermaphrodite, and
that the maturation processes imply the removal of male qualities
from the ovum and of female qualities from the spermatozoon,
has been abandoned ; and the reducing divisions are recognised
as securing a constancy in the number of chromosomes character-
istic of each species, for without some such preliminary reduction
the number would obviously be doubled at each fertilisation.
That a reduction does really occur in both plants and animals
seems now incontrovertible, but the precise manner of the re-
duction seems to differ considerably in different organisms.

Reduction in Parthenogenetic Ova. — There is an interesting variety
of occurrence.

(a) In ants, bees, and wasps, all the ova (with 2N chromosomes to
start with) undergo reduction, the number of chromosomes being
halved (N). Some ova are unfertilised and these develop into males,
whose cells have therefore half the normal number of chromosomes
(N). There is no reduction of the chromosomes in the making of the
sperms. Thus when a spermatozoon (with N chromosomes) fertilises
a reduced ovum (with N chromosomes) the normal number, 2N, is
restored, and the resulting female retains that number.

(b) In Rotifers and some water-fleas (e.g. Daphnia) parthenogenesis
occurs when the nutritive and other conditions are favourable, and
only females are produced. The ova do not undergo reduction, but
retain the normal number of chromosomes (2N). In unfavourable
conditions eggs of two sizes are produced, and both undergo reduction.
The small ones, with N chromosomes after reduction, are not fertilised,
and develop into males. The large ones, also with N chromosomes
after reduction, are fertilised, and develop in the 2N condition into
females.

(c) In Aphides or plant-lice, parthenogenesis is the rule in favour-
able conditions ; reduction does not occur, and females are produced.
In unfavourable conditions males appear. Some of the eggs undergo
ordinary reduction (to N), and being raised to the 2N condition by
fertilisation develop into females. Other eggs produced in unfavour-
able conditions undergo a partial reduction to 2N — 1, or 2N — 2,
are not fertilised, and develop into males. In the formation of the
male-cells there are some with N — 1 chromosomes and some with
N chromosomes, but the former degenerate and only the latter
become effective spermatozoa.

438 HEREDITY AND DEVELOPMENT

Minute inquiries have gone so far that it is possible to assert
that in some cases the young germ-cell has an equal number
of paternal and maternal chromosomes. And similar minute
inquiries — which almost baffle us with their intricacy — make
it exceedingly probable that in the reduction divisions maternal
chromosomes separate from paternal chromosomes, and yet not
so thoroughly that all the paternal chromosomes pass into one
cell and all the maternal into another. If this be true, we can
better appreciate the importance of the reduction-divisions which
occur in maturation, for they afford opportunity for new per-
mutations and combinations of hereditary qualities. They do
not originate anything new, but they shuffle the cards, so to speak.

Fertilisation. — Recent work has forcibly suggested that there
are in fertilisation two more or less distinct processes : on the
one hand, the process by which the gametes, bearing the
hereditary characters, unite to form the beginning of a new in-
dividuality ; on the other hand, the process by which the sperma-
tozoon supplies some stimulus, prompting the ovum to divide.
The first aspect is that of amphimixis, believed by many to be
of importance in initiating — and, it may be, also in checking —
variations, but in any case effecting the union of hereditary
qualities contained in the two gametes. The second aspect
is that of mitotic stimulus, believed by some to be afforded by
an enzyme — for which the name of " ovulase " has been suggested
— and by others to be localised in the sperm-centrosome. It is
seen in many cases that equivalent numbers of chromosomes
are contributed by the two nuclei ; it is evident that the ovum
contributes by far the larger quantity of cytoplasm ; it seems
to have been securely demonstrated in some cases that " from
the father comes the centrosome to organise the machinery of
mitotic division by which the egg splits up into the elements
of the tissues, and by which each of these elements receives its
quota of the common heritage of chromatin." " Huxley hit
the mark two-score years ago when he compared the organism

I.

2.

Fig. 42. — Modes of segmentation.

1, Ovum, with little yolk, segments wholly and equally into a ball of cells (blastula),
e.g. sea-urchin ; 2, ovum, with a considerable quantity of yolk [y), segments wholly but un-
equally, e.g. frog ; y.s. larger yolk-laden cells ; 3, ovum, with much yolk \y\ tow-ards lower
pole, segments partially and discoidally, forming blastoderm [bl.),e.g. bird : 4. ovum, with
much central yolk \y), segments partially and peripherally, e.g. crayfish.

[Facing p. 438.

FERTILISATION 439

to a web, of which the warp is derived from the female and the
woof from the male. What has since been gained is the know-
ledge that this web is to be sought in the chromatic substance
of the nuclei, and that the centrosome is the weaver at the
loom " (Wilson, 1896, p. 171). While the ovum-centrosome
of many animals seems to disappear, that introduced by the
spermatozoon divides into two, and around each a system of
rays develops. The sperm-centrosomes migrate to opposite
sides of the segmentation nucleus, and between them appears
the spindle of the first cleavage. It may be hasty to call them
" kinetic centres," but they seem to have an important role in the
division-process.

Let us suppose that a young egg-cell has sixteen chromosomes or

idants, 16A ;
in the course of maturation the number is reduced by a half to

8A;
the mature egg-cell is fertilised by a (reduced) spermatozoon with

eight chromosomes, 8B ;
the fertilised egg-cell has then eight maternal and eight paternal

chromosomes, 8A + 8B ;
the young germ-cell capable of initiating a new generation has the

same ;
in the maturation of this young egg-cell reduction occurs to 4A

+ 4B;
it is fertilised by a sperm of analogous history with 4C 4- 4D ;
the fertilised egg of the second generation has therefore 4A + 4B

+ 4C + 4D ;
similarly, the fertilised egg of the third generation may have 2A

+ 2B + 2C + 2D + 2E + 2F + 2G + 2H ;
similarly, in the fourth generation the chromosomes may be

A+B+C+D+E+F+G+H+I+J+K+L+M+
N + O + P (sixteen all different).

But the number of different chromosomes need not mount up so
rapidly, for some of the paternal chromosomes may be the same as
maternal. Moreover, the reducing division need not leave the
maximum number of different chromosomes. The number sixteen,
by hypothesis characteristic of the species, cannot be exceeded ;

440 HEREDITY AND DEVELOPMENT

but the heterogeneity may spread into the individual chromosomes,
affecting the ids.

Summary. — Put as simply as possible, the case is as follows.

The independently heritable and variable qualities of an organism
are represented in the young germ-cell by a number of material
elements (determinants).

As the young egg-cell ripens it divides in such a way that its
determinants are reduced in number by one-half. Not that it
need lose any particular kind of determinant, corresponding let us
say to the colour of the eye or the colour of the hair, for each kind
of determinant is represented in multiplicate. It loses one-half of
its sets of determinants. The same happens with the ripening
sperm-cell.

When the mature egg-cell is fertilised by the mature sperm-
cell, the number of sets of determinants is once more raised to what
it was in the young cells before maturation. But though the number
of sets is the same as before, the collocation of the sets is not the
same. At any rate, it need not be the same ; for there is an appar-
ently random reduction.

The character of the offspring depends upon the adjustments arrived
at among the different sets of determinants of maternal and paternal
origin.

Hypothesis of Development. — Postulating an equipment of
primary constituents or determinants within the germ-plasm,
Weismann proceeded to elaborate a hypothesis as to the manner
in which these determinants determine the cells or cell-groups
to which they correspond.

The fertilised egg-cell divides and redivides, and at first
the resulting cells (blastomeres) of the embryo are often equiva-
lent to one another. This is demonstrable experimentally,
for if the first four cells of the lancelet's ovum, for instance,
be shaken apart, each goes on developing on its own account
and forms a complete larva. In other cases, the resulting cells
are heterogeneous from the first division onwards ; and, in any
case, they soon become heterogeneous — that is to say, they form
certain parts of the embryo, and these only. In other words,
there must be a distribution of determinants in the course of
segmentation.

THEORY OF DEVELOPMENT 441

But if the various kinds of determinants are to get into ap-
propriate cell-groups, this cannot be a matter of chance. There-
fore, we must further postulate that from the first each deter-
minant has a definite position in relation to its neighbours,
that the germ-plasm is not a mere loose aggregate of deter-
minants, but that it possesses a structure, an architecture, in
which the individual determinants have each their definite place.
It must be borne in mind that the germ-cell is a unity, a potential
organism, and not a heap of hereditary contributions. Weis-
mann supposes that the determinants are kept in relation to
one another by " vital affinities," by internal forces, some ex-
hibition of which is, indeed, demonstrable, as when a chromosome
or ribbon of ids splits into a double ribbon of ids.

But if the mechanism of the distribution of determinants is
by cell- division — one of the features of which is that the chro-
mosomes are halved with minutiose accuracy, so that each of the
two daughter-cells obtains a longitudinal half of each chromo-
some— how does it come about that different determinants pass
into different cells of the embryo ? This difficulty led to the
further hypothesis that, while ids may divide into two identical
halves, they may also divide into two dissimilar halves. Weis-
mann supposed that besides integral (erbgleich) division of the
nucleus, there is also differential (erbungleich) division. The\
reality of this differential division — which many histologists
vigorously dispute — cannot be directly demonstrated any more
than the splitting up of a complex molecule into different mole-
cules can be demonstrated. But in both cases we may infer
the occurrence from the results. It is not a hypothesis, but a
fact, that a cell may divide into two daughter- cells, one of which
goes to form ectoderm, while the other goes to form endoderm,
and this implies some sort of differential division. What in-
ternal forces or vital affinities are concerned we do not know.

If an egg-cell can divide differentially into a primordial
ectoderm-cell and a primordial endoderm-cell, or into a formative

442 HEREDITY AND DEVELOPMENT

cell and a purely nutritive cell, and so on, it seems legitimate
to suppose that corresponding differential divisions on a finer
scale go on in the course of development. The embryonic
cells go on dividing into daughter-cells having dissimilar deve-
lopmental import or prospective value, and " such differential
divisions will continue to occur until the determinant archi-
tecture of the ids is completely analysed or segregated out into
its different kinds of determinants, so that each cell ultimately
contains only one kind of determinant, the one by which its
own particular character is determined. This character, of
course, consists not merely in its morphological structure and
chemical content, but also in its collective physiological capacity,
including its power of division and duration of life " (1904,
vol. i. p. 378).

It goes without saying that development also includes many
integral divisions. Cells are continually producing their like,
especially when there are numerous similar organs or parts in
the organism. It must also be noted that the segregation-
process cannot be pictured unless we suppose that the deter-
minants— being alive — can multiply among themselves, so that
a cell dominated by one kind of determinant may contain a
whole army of determinants of that kind. We must also suppose
that determinants may remain for a long period in an inactive
state, and that it is only when they find themselves in an ap-
propriate environment, largely determined by the cellular
neighbourhood, that liberating stimuli awaken them to their
controlling power.

The Breaking-up of the Determinants. — The segregation
or distribution of the determinants goes on, and each unit-area
or cell of the developing organism becomes the seat of a particular
kind of determinant or of a contingent of these. What then
happens ? Weismann supposes that the determinant, having
attained mature strength and its appropriate environment,
breaks up into the biophors which compose it, and that these

THEOR Y OF DETERMINANTS 443

migrate from the nucleus into the cell-substance. But there
a struggle for food and space must ensue between the proto-
plasmic elements already present and the newcomers, and this
gives rise to a more or less marked modification of the cell-
structure. The biophors need not be supposed to correspond
in advance to particular constituent parts of the cell, such as
muscle elements or chlorophyll corpuscles ; it is more plausible
to suppose that they are the architects of these. Of course,
they must have some definite character, but they need not be
the infinitesimal rudiments of what they form. Many of them
may be regulative, rather than formative. They may be
organisers as well as architects. We need not stint their quali-
ties, for they are alive.

Weismann does not conceive of the determinants as " seed-
grains of the individual characters of the organism " ; they are
" codeterminants of the nature of the part which they in-
fluence." Like colonists entering upon a new territory, they
owe their power to their co-operation. Again, the " character "
of the cell — its size, intimate structure, length of life, and so
forth, is not determined by a number of special determinants
for each feature in the character. " There are only determin-
ants of the whole physiological nature of the cell," and they
work out the character of the cell in co-operation with one
another and with the cell-body into which they have penetrated.

We cannot give a short account of the ingenious elaborations
of the theory of determinants, by the aid of which Weismann
has endeavoured to give a consistent all-round interpretation
of special phenomena, such as budding, fission, regeneration
of lost parts, alternation of generations, dimorphism, poly-
morphism, and so on. He supposes, for instance, that in those
organisms which can multiply by liberating a bud or a fraction
of the bod\' there must be in many of the cells a residual
contingent of determinants — amounting, it may be, to a repre-
sentation of the en tire j, germ-plasm — and that this contingent

If

444 HEREDITY AND DEVELOPMENT

remains latent until special circumstances arise which call it into
activity.

Note on Regeneration. — When half of a highly differen-
tiated Infusorian like Stentor regenerates the missing half, we
suppose that it does so because in each half there are diffusely
distributed " specific units " or " groups of determinants,"
which can in appropriate environment grow into wholes. We
are encouraged to hold this hypothesis since we know that slices
of Stentor a millimetre or less in thickness can re-grow wholes.

We shift the experiment to a slightly higher level, and we find
that fragments of relatively simple multicellular animals, such
as Hydra and Planarians, can grow into entire organisms. We
suppose that the excised groups of cells have among them a
sufficient complement of " specific units " to ensure the de-
velopment of a complete organism.

But as we ascend higher in the scale, we find that while
the earthworm can re-grow a new head or a new tail, a few
median segments cut out of the middle of an earthworm will
soon die. A crab can re-grow a lost limb, but the limb cannot
re-grow a crab. The inference is that as differentiation increases
the diffuse distribution of " complete specific units " ceases, so
that the excised part is no longer a viable fragment. All this
points to the reality of differential cell-division.

If the eye-bearing horn of a snail be cut off, it is regenerated
over and over again, with the complex eye complete. If the eye
of a crab be excised, there is usually regenerated an antenna in-
stead of an eye, but if the optic ganglion is not injured a normal
eye is regenerated. If the front of the eye of a newt or of a
salamander be cut off, a new lens is regenerated. All this points
to the hypothesis that within limits, probably punctuated by
natural selection, the maimed stump or foundation of an im-
portant organ retains in reserve a contingent of units capable of
growing the whole of that organ. Thus, while the distribution of
complete residual specific units or ids becomes more and more

THEORY OF DETERMINANTS 445

restricted, there is a much more useful retention, at spots liable
to injury, of local contingents of " organ-forming units " which
can replace lost parts.

Difficulties. — 1. If definite determinants are distributed in
development as the number of unit-areas or cells increases,
how is it that an isolated group of cells, cut off from a begonia-
leaf, a potato-tuber, a hydra-polyp, a sea-anemone, a simple
worm, may in appropriate conditions grow into an entire organ-
ism ? It must be noted, in the first place, that this capacity
is more or less restricted to relatively simple organisms. In
the second place, the theoretical answer is that in such cases
the cells retain a representation of the whole germ-plasm in an
inactive state, though each one of them is differentiated under
the control of a particular set of determinants.

2. A man has a peculiar " crooked nose " and his son has the
like. Are we to suppose that the inheritance includes " crooked
nose "-determinants ? Weismann would say " emphatically
not." A large number of different kinds of determinants are
concerned in the up-building of the nose, and they work co-
operatively towards a general result. There may be some
slight peculiarity in those that contribute, let us say, to the
cartilage of the nose, and this peculiarity may, in the course of
the co-operative development, lead to a crooked nose as the
result of some inequality of pressure during the early formative
period. The results of experimental embryology show clearly
that the behaviour of particular cells in development is not
absolutely stereotyped j they will do their best, as it were, to
work out a constant result, but if this is interfered with environ-
mentally they will do something else. At the same time, it
is very interesting that abnormal larvae — e.g. the so-called
Lithium-larvae of sea-urchins — have a remarkable power of
righting themselves when they are relieved from the disturbing
influence of the abnormal environment.

Objections to the Theory of Determinants. — Some biologists

446 HEREDITY AND DEVELOPMENT

have objected to Weismann's theory of determinants, because,
as they say, no one has ever seen or can ever hope to see one.
Determinants are scientific fictions and all discussion of them
is in the air. But the same sort of objection may be raised
against the theory of, let us say, the ether. The point is whether
the concept of determinants helps us to interpret visible pheno-
mena. Science works from beginning to end with imaginative
concepts which facilitate description and formulation, and which
are so truly representative of the invisible that we can utilise
them in prediction.

Other biologists, who are aware of the impossibility of
a science without imaginative concepts, object to the theory
of determinants on the ground that they can be done without.
Thus Prof. Yves Delage rejects all determinants, primary con-
stituents, or particules representatives, and will only postulate
a germ-plasm with " an extraordinarily delicate and precise
physico-chemical composition." " There are not," he says,
" in the germinative plasm any distinctive particles repre-
senting the parts of the body or the characters and pro-
perties of the organism " (1903, p. 749). What is there, then ?
According to Delage, the germ-cell contains a number of
characteristic chemical substances — which every one admits —
characteristic of the chief categories of cells ; and its development
is comparable to the flow of a river, now running deep and
again shallow, here forming a waterfall and there an eddy,
but always explicable in terms of action and reaction between
the flowing water and its surroundings. Given the power of
developing (which no one understands), given a characteristic
chemical composition (which every one admits), and given an
appropriate environment (which nobody can deny), and voilct
tout. There is no more need to cumber biology with deter-
minants and biophors than there was to cumber astronomy
with Ptolemaic circles and epicycles.

But even in the apparently simplest cases it seems impossible

OBJECTIONS TO THEORY OF DETERMINANTS 447

to dispense with the concept of " units," or " primary constitu-
ents " or " determinants " or groups of these including all the
specific characters. Take the case of the common Infusorian
Stentor. It seems to be certain that a thin slice, a millimetre
thick, of this unicellular organism may, in appropriate con-
ditions, grow into a complete individual, with vibratile oral
cilia, smaller superficial cilia, a mouth, a long necklace-like
nucleus, three smaller nuclei, a contractile vacuole, internal
contractile fibrils, and so on. Is it possible to think of this
marvellous regeneration of a highly differentiated unity from a
thin slice, without postulating " units " of some sort, which, when
removed from the system as a whole, have yet the power of
reconstituting that system ? (See Weldon, 1905, p. 42.) Simi-
larly, a thin slice of the multicellular Hydra-polyp may, in ap-
propriate conditions, grow into an entire and complete Hydra.
Is it possible to conceive of this apart from the postulate of
diffusely distributed " specific units " ?

Prof. H. E. Ziegler has briefly and temperately stated the
two most frequent objections to the theory of representative
particles.

I. When we try to interpret any result or occurrence we
must refer it to what is known. If we interpret it in terms
of a something invented for the purpose we are simply making
a fictitious hypothesis. When we refer facts of inheritance to
observable processes — e.g. in the chromosomes of the nuclei —
we are making scientific progress ; but when we deduce the
phenomena of inheritance from the behaviour of pangens or
determinants which have been invented we are simply indulging
in verbal speculation. As it appears to us, this is not a just
statement of scientific procedure. The imaginary pangens or
determinants are elements in a notation like the graphic symbols
of chemical molecules : their utility does not depend on any
visible reality ; their validity is tested by the degree in which
they enable us to formulate conceptually what does occur, and

448 HEREDITY AND DEVELOPMENT

to reach forward from this formulation to more precise observa-
tion and experiment. It goes without saying that the moment
the symbolic notation is shown to be inconsistent with demon-
strable facts, it must be thrown overboard and replaced by
another.

2. It is difficult, Ziegler says, to think out clearly what we
mean by a unit-character and by its being represented by a
unit-germinal-constituent, whether pangen or determinant.
Many a quite definite character of an organism depends upon
a multitude of growth-conditions, and to conceive of the char-
acter being represented in the germ by one representative
particle is as difficult as it is to conceive of an infinite number
of representative particles, one for each item in the character.

But it should be noted that Weismann simply assumes as
many determinants in the germ-plasm as there are parts in
the organism capable of independent and transmissible variation.
The fiddling string and bow on a grasshopper's thigh and wing
will have at least one determinant each, but one determinant
may suffice for all the millions of red blood corpuscles in man.
Again, Weismann expressly emphasises his view that " deter-
minants are not seed-grains of individual characters, but co-
determinants of the nature of the parts which they influence.
There are not special determinants of the size of a cell, others
of its specific histological differentiation, and still others of its
duration of life, power of multiplication, and so on ; there are
only determinants of the whole physiological nature of a cell,
on which all these and many other ' characters ' depend." Or
again, " There are no determinants of ' characters,' but only
of parts. The germ-plasm no more contains determinants of
a ' crooked nose ' than it does those of a butterfly's tailed wing ;
but it contains a number of determinants which so control
the whole cell-group in all its successive stages, leading on to
the development of the nose, that ultimately the crooked nose
must result, just as the butterfly's wing, with all its veins, mem-

PERSISTENCE OF THE GERM-PLASM 449

branes, tracheae, glandular cells, scales, pigment deposits, and
pointed tail, arises through the successive interposition of numer-
ous determinants in the course of cell-multiplication."

In any case, whether the idea of representative primary
constituents commends itself to us or not, we must remember
that it is a fact that the organism — unified as it is — is built
up of a very large number of independently variable, inde-
pendently heritable items.

The Persistence of the Germ-plasm. — We have given an
outline of the consistently- thought-out scheme which Weismann
has suggested as an interpretation of development — the dis-
tribution of the determinants, their " maturation," their " libera-
tion," their migration from the nucleus, their dissolution into
biophors, and the manner in which the biophors may control
the area or cell in which they find themselves. But it remains
to inquire how the germ-cells which start the next generation
are constituted. If the building-up of the body involves segre-
gation of the determinant architecture into smaller and smaller
groups, how does the organism produce germ-cells — that is, cells
with intact germ-plasm — with a complete equipment of deter-
minants ? The answer, already given in Chapter II., is that
it does not in the strict sense produce them ; they are there
all the time.

In more detail, Weismann's answer (1885) — the theory of the
continuity of the germ-plasm — is that in the divisions of the
ovum the whole of the germ-plasm is not broken up into deter-
minant groups ; part of it is kept intact and handed on from cell
to cell along a lineage or " germ-track," which may be very
short or very long, until, sooner or later, it stamps a cell as a
primordial germ-cell. In other words, while most of the cells,
derived by division from the fertilised ovum become differenti-
j ated as body-cells, some of the cells retain a quota of intact
germ-plasm, and eventually give rise to recognisable germ-cells.
Body-cells and reproductive cells alike owe their being to the

29

i

450 HEREDITY AND DEVELOPMENT

germ-plasm of the fertilised ovum, and are its lineal descendants ;
but the somatic cells are dominated by particular segregated and
liberated sets of determinants, whereas the germ-cells are those,
or the descendants of those, that retain the complete equipment.
In studying the development of the threadworm of the horse
(Ascaris megalocephala), Boveri found that the two first segmen-
tation-cells both receive the four chromosomes characteristic
of the species; one gives rise to all the body-cells, the other
to all the germ-cells. In the lineage of the former there is a
visible reduction of the chromatin ; in the lineage of the other
there is no such reduction. This is perhaps the clearest of all
cases, and the case of some of the Diptera is almost as clear.
But theoretically it makes no difference how long the " germ-
track " may be, or how long it may be before recognisable
germ-cells are seen in the. developing organism. In some familiar
cases — the alternation of generations in Hydroids — the repro-
ductive cells, as such, are not demonstrable till after the asexual
generation forms a sexual bud ; and yet, even here, we know
some very interesting facts regarding the germ-cell lineage.

§ 3. Note on Rival Theories

Darwin's Theory of Gemmules. — Darwin's provisional theory
of pangenesis suggests, as we have already seen, that particular
cells of the body give off representative gemmules, and that
these are collected in the reproductive cells. When the fertilised
egg-cell divides and redivides, the army of gemmules is contained
in each cell ; but at every stage of development particular kinds of
gemmules are stimulated to activity, and proceed to influence
the area in which the}/ find themselves — an area corresponding
to that from which they were originally given off. As Weismann
points out, this hypothesis requires us to postulate an enormous
number of specific stimuli, distributed through the crowd of

RIVAL THEORIES OF DEVELOPMENT 45*

embryonic cells, which almost amounts to assuming the differen-
tiation which the theory was intended to interpret.

Weismann tries to avoid this difficulty by assuming an auto-
nomic dissolution of the determinant complexes, though he does
not reject the view that the differently related vital areas or
cells in which the determinants find themselves may serve as
liberating stimuli. In a marching army the differently related
localities serve as liberating stimuli to the diverse kinds of men
composing the army ; here the sappers and miners go to work,
there the commissariat erects a depot, in a third place a heliograph
is set up, and so on.

Herbert Spencer's Theory of Physiological Units. — Spencer
postulated " physiological units," ultimate life-bearing elements,
intermediate between the chemical molecules and the cell.
Just as the same kinds and even the same number of atoms may
compose, by different arrangements, numerous quite different
chemical molecules — e.g. in the protein-group — so out of similar
molecules diversely grouped an immense variety of " physio-
logical units " may be evolved, like the variety of patterns in
a kaleidoscope. But for each kind of living creature Spencer
postulated " physiological units " or " constitutional units " of
one kind.

Spencer credited his " constitutional units " with much.

i. They carry within them the traits of the species, and even
some of the traits of the ancestors of the species ; the traits of the
parents, and even some of the traits of their immediate ancestors ;
and the inborn idiosyncrasies of the individual organism itself.

2. They " must be at once in some respects fixed and in other
respects plastic ; while their fundamental traits, expressing the
structure of the type, must be unchangeable, their superficial traits
must admit of modification without much difficulty ; and the
modified traits, expressing variations in the parents and imme-
diate ancestors, though unstable, must be considered as capable o t
becoming stable in course of time."

3. Moreover, " We have to think of these physiological units

452 HEREDITY AND DEVELOPMENT

(or constitutional units, as I would now rename them) as having
such natures that while a minute modification, representing some
small change of local structure, is inoperative on the proclivities
of the units throughout the rest of the system, it becomes operative
in the units which fall into the locality where that change occurs."

4. Furthermore, Spencer supposed " an unceasing circulation
of protoplasm throughout an organism," such that " in the course
of days, weeks, months, years, each portion of protoplasm visits
every part of the body " — a wild assumption. Therefore, " we
must conceive that the complex forces of which each constitutional
unit is the centre, and by which it acts on other units while it is
acted on by them, tend continually to remould each unit into
congruity with the structures around ; superposing on it modifica-
tions answering to the modifications which have risen in these
structures. Whence is to be drawn the corollary that in the course
of time all the circulating units — physiological, or constitutional,
if we prefer so to call them — visit all parts of the organism ; are
severally bearers of traits expressing local modifications ; and that
these units, which are eventually gathered into sperm-cells and
germ-[egg-]cells, also bear those superposed traits."

5. According to Spencer, " sperm-cells and germ-[egg-]cells
are essentially nothing more than vehicles in which are contained
small groups of physiological units in a fit state for obeying their
proclivity towards the structural arrangement of the species they
belong to"; and "if the likeness of offspring to parents is thus
determined, it becomes manifest, a priori, that, besides the trans-
mission of generic and specific peculiarities, there will be a trans-
mission of those individual peculiarities which, arising without
assignable causes, are classed as spontaneous."

We have illustrated Spencer's position at some length because
so many British biologists have recoiled from what they call the
complexity of Weismann's theory. But a little consideration
will show that the protagonist of British biology invented a
system in comparison to which Weismann's is simplicity.

Nor can we close our exposition without recalling how Spencer
confessed that " the actual organising process transcends con-
ception. ... It is not enough to say that we cannot know it;

INTRACELLULAR PANGENESIS 453

we must say that we cannot even conceive it. . . . If even the
ordinary manifestations of the dynamic element in life which
a living body yields from moment to moment are at bottom
incomprehensible, then still more incomprehensible must be
that astonishing manifestation of it which we have in the
initiation and unfolding of a new organism. . . . Thus, all we
can do is to find some way of symbolising the process so as
to enable us most conveniently to generalise its phenomena ;
and the only reason for adopting the hypothesis is that it best
serves this purpose."

But Spencer's hypothesis only serves the purpose because
the constitutional units are gradually invested with the powers
of effective response, co-ordination, and the like which remain
the secret of the organism as a whole — the secret of life, which
many think will never be read until we recognise that it is also
the secret of mind.

De Yries's Theory of Intracellular Pangenesis. — A theory
different from Darwin's and also from Weismann's has been
suggested by Hugo De Vries under the title " Intracellular
Pangenesis." The gist of it may be summed up as follows :

1. Organisms are built up of unit-characters, independently
variable and independently heritable.

2. These unit-characters are represented in potenlia in the
hereditary substance of the nucleus of the germ-cell by definite
bodies (pangens), far too minute to be visible, but together
constituting the chromosomes of the nucleus.

3. The pangens multiply in the idioplasm of the nucleus,
and some of them migrate into the surrounding cytoplasm,
where they become active, dominating it, and giving it a par-
ticular character. But a representative contingent of pangens
always remains in the nucleus and is handed on from cell to
cell by nuclear division. Into each cell as it is formed a fresh
migration of pangens occurs.

Other Suggestions. — It need hardly be said that many other

454 HEREDITY AND DEVELOPMENT

schemes have been suggested with the laudable end of throwing
some light on one of the most familiar facts of life — the develop-
ment of the germ. Thus the illustrious physiologist of Prague,
Ewald Hering, and that acute English thinker, Samuel Butler,
have suggested that development is, as it were, a materialised
recollection of the past ; Ernst Haeckel conceived of develop-
ment as due to the persistence of characteristic and complicated
wave-motions acquired in the past by the organic molecules ;
many others have looked at the matter chemically, " the same
substances and mixtures of substances being reproduced in
similar quantity and quality with regular periodicity."

A scholarly account of these and other suggestions will be
found in Delage's great work on heredity, where every known
view is presented with fairness and lucidity and criticised with
unrivalled acuteness and justice. There also will be found
the finest exposition of the view, which we find ourselves quite
unable to entertain, that it is possible to dispense with any
postulate of " representative particles."

§ 4. Weismann's Theory of Germinal Selection

In 1895-6 Weismann expounded an ingenious hypothesis,
the main idea of which is expressed in the phrase "Germinal
Selection." It is an extension of the biological concept of
"struggle" to the individual items which compose the germ-
plasm — i.e. the inheritance.

Extension of the Struggle-and-Selection Formula. — In
human affairs there is often struggle between different societary
forms — as in war and international commercial competition ; and
no one doubts that this involves a process of selection. This
is often so complex that it must be termed superorganic. An
adumbration of it is seen in the wars of the ants, and in the
competition between a pack of carnivores and a herd of herbivores.
Similarly, within one human societary form there may be

GERMINAL SELECTION 455

struggle between rival organisations and rival institutions, and
no one doubts the reality of an intrasocietary selection. This,
again, is more complex than the ordinary personal or individual
selection.

" Personal " Selection. — Of personal or individual struggle
there are many forms and phases, notably (a) the competition
between fellows of the same kin for food and foothold, which
is not self-regarding only, but for the sake of mates and family
as well ; (b) the opposition between foes of quite different kin —
e.g. between birds of prey and small mammals ; and (c) the
struggle between organisms and the changeful inanimate en:
vironment. Besides these three main forms there are many
special cases, such as the battles between males of the same
species for the possession of females, as in the case of seals and
stags, and the sometimes serious disagreements between mates,
so quaintly illustrated in some spiders. Corresponding to these
different forms of struggle there are different modes of selection
and elimination.

Intra- organismal Selection. — In 1881 Roux introduced the
idea of a struggle of parts within the organism. He pointed out
that functional stimulus tends to strengthen an organ, that
there is a " quantitative self-regulation of an organ according
to the strength of the stimulus supplied to it." It may be
over-compensated for its expenses, and grow, just as the opposite
conditions may lead to atrophy. It is well known that if all
the work of renal excretion be thrown on one kidney, that
organ increases greatly in size, and that if the nerve to a muscle
or gland be cut, that muscle or gland begins to degenerate. If
we pursue this line of thought we begin to realise what is meant
by a struggle of parts within the organism,rand by intra-organ-
ismal selection. Some change occurs in the conditions of nutri-
tive and other stimuli ; there are limitations affecting the
nutritive supply, the amount of available space, and so on ;
and there has to be an internal give and take, a mutual re-

456 HEREDITY AND DEVELOPMENT

adjustment of parts— in fact, a struggle. This is often referred
to as intra-selection or histonal — i.e. tissue — selection.

As Weismann says, " The tissues and the parts of the tissues
have to distribute and arrange themselves so that each comes
to fill the place in which it is most effectively and frequently
affected by its specific stimulus — that is, the stimulus in regard
to which it is superior to other parts ; but these places are also
those the occupation of which by the best reacting parts makes
the whole tissue capable of more effective function, and there-
fore makes its structure the fittest. . . . The cells which as-
similate more rapidly because of the more frequent functional
stimulus increase more rapidly, draw away nourishment from
the more slowly multiplying cells around them, and thus crowd
these out to a greater or less extent " (1904, vol. i. p. 247).

As Weismann points out, it is impossible at present to give
any precise limitation of the respective spheres of personal
and histonal selection. The intra-organismal struggle may
be, so to speak, the internal adjustment necessary towards a
result which the external process of personal selection is bringing
about. " The differentiation of the particular kinds of cells
is an ancient inheritance, and depends upon personal selection ;
but their distribution and arrangement into specially adapted
tissues, so far as there is any plasticity at all, depend upon
histonal selection." The architecture of every organ is implicit
in the germ and must be referred to a long-drawn-out process of
personal selection, but the particular local modifications of the
architecture may be adjusted by the intra-organismal struggle.
And, again, it must be borne in mind that personal selection may
put a full stop at any moment to the achievements of histonal
selection if they affect the viability of the creature as a whole.
A hypertrophied organ may express the organism's internal
endeavour to make the best of a new situation, but it may be
fatal.

In so far as a process of intra-organismal struggle is of normal

GERMINAL SELECTION 457

occurrence in development, where we often see one organ
waxing and another waning, we must regard it as part of the
plan of campaign which is hereditarily predetermined in the
germ-plasm. But since the organism develops in intimate
dependence on a changeful environment, we are prepared for
local modifications of adjustment arising as the results of histonal
selection. Many malformations represent attempts on the
organism's part to solve an insoluble problem forced upon it by
peculiar environmental conditions ; many individual adapta-
tions are wrought out by the modus operandi of histonal selection
in the individual lifetime, and are of real value to the organism
that acquires them. But there is no good reason for believing
that either can be entailed on the offspring.

None the less, it is important that the student of inheritance
should vividly realise the existence of this modus operandi
which Roux called the "struggle of parts within the organism."
For, although we cannot say that it has any direct evolutionary
importance in securing new steps in evolution, and although we
do not understand how it is that parts regulate themselves
appropriately in reference to new conditions of stimulus — for
that is obviously part of the secret of life itself — it is useful
to bear in mind that there is in a real sense a competition among
organs, a struggle of parts, and a warfare among cells. Vivid
illustrations may be found in the histolysis or disruption of
tissue associated with metamorphosis (e.g. in many insects),
in the behaviour of teratogenic growths, in the involutions or
degenerations associated with senility (e.g. in the invasion of
the brain of the aged parrot by hungry " neurophagous " cells) ,
and in the familiar fact that the hypertrophy of one organ may
handicap or even suppress another organ.

In short, the concepts of struggle and selection may be ex-
tended to the parts of the organism.

Struggle between Gametes. — There may be struggle be
tween groups of organisms, struggle between individual organ-

45§ HEREDITY AND DEVELOPMENT

isms, struggle between organisms and their surroundings, and
struggle between parts within the organism — between organs,
tissues, and cells. Can the formula be extended further ?

Before we pass to Weismann's proposal to extend the concept
" struggle " to the determinants within the germ, it may be of
interest to call attention to a form of struggle and selection
which may be interpolated between Roux's histonal selection
and Weismann's germinal selection. Although Weismann does
not seem to favour the idea, it seems to us that there is a real
and important struggle between the germ-cells as such.

i. There is a well-known struggle between potential ova.
In many cases the majority are sacrificed to a minority, which
sometimes literally feed upon their fellows. In the common
freshwater polyp, Hydra, and in a common marine polyp,
Tttbularia, only one egg-cell usually survives out of an originally
numerous sisterhood, reminding one of the combat to the death
which may occur among sister queens in a beehive.

2. There is a kind of struggle between the hundreds of sperma-
tozoa in their race towards the ovum, which only one of them
in normal conditions will fertilise. In the familiar fertilisation
of frog's ova, several spermatozoa may be seen boring their
way through the jelly surrounding the ovum; but after one has
entered the ovum a rapid change in the peripheral protoplasm
seems to shut the door on others. It may well be, allowing
a margin for the purely fortuitous, that the most vigorous,
most sensitive spermatozoa tend to fulfil their particular office
of fertilising the ova, and this will tend to be to the advantage o/y
the species. Again, we are quaintly reminded of the race be-
tween the drone-bees to overtake the queen in her nuptial
flight. Usually, one drone effects sexual union, and all the
rest are futile.

3. There is sometimes, according to Iwanzoff and others,
a struggle between ova and spermatozoa, for young ova may
literally digest intruding sperms. There is also a form of selection

STRUGGLE BETWEEN GAMETES 459

involved in the fact that in some cases there are more ova than
sperms, though the reverse is usually the case. Thus Maupas
has shown that in Rhabditis and some other threadworms only
about a third of the ova can be fertilised ; there are no sperms
left for the other two-thirds produced later.

Many other illustrations might be given, but our point here is
simply this, that a vivid realisation of the visible struggle among
germ-cells or gametes, and the frequently discriminate nature of
the ensuing elimination, may lead us naturally to an appreciation
of germinal selection which deals with the wholly invisible.

Statement of Weismann's Theory. — As we have seen, Weis-
mann pictures the germ-plasm as composed of an army of living
determinants — that is to say, of an aggregate of primary consti-
tuents (or potentialities), of particular parts of the organism.
These particular parts will not arise if their determinants are
absent from the germ-plasm, and we know in some cases — e.g. in
the development of some Ctenophores (usually globular free-
swimming Ccelenterates)— that the abstraction of certain cells
from the embryo means an absence of certain structures from
the adult.

Let us suppose, then, that the physical basis of inheritance is
composed of a multitude of representative vital particles, which
have the capacity of feeding, growing, and multiplying. As
the supply of nutriment necessarily fluctuates continually in the
reproductive organs as a whole, " we may therefore assume
that there are similar irregularities and differences in the
minute and unobservable conditions of the germ-plasm likewise,
and the result must be a slight shifting of the position of
equilibrium as regards size and strength in the determinant
system ; for the less well-nourished determinants will grow
more slowly, will fail to attain to the size and strength of their
neighbours, and will multiply more slowly " (1904, vol. ii. p. 117).

Every one must admit that there are fluctuations in the
nutritive supply of the germ-cells, and to these, according to

460 HEREDITY AND DEVELOPMENT

Weismann, we must refer those individual germinal variations
which form part of the raw material of evolution. But it can
hardly be imagined that all the determinants or hereditary
constituents are equally vigorous, or have equal assimilating
power. Thus, a determinant may become weaker because
there is less food for it, and also because it has less power of
utilising the available food. If a determinant is thus weakened,
its determinate — the structure to which it corresponds — will
also be weakened ; and we call this a germinal variation on the
down-grade. On the other hand, a vigorous determinant with
strong assimilative power will tend to become stronger if it
is well and appropriately fed. Its determinate will be
correspondingly strengthened, and we call this a germinal
variation on the up-grade.

" To the ascending progression there are limits set, not only
by the amount of food which can circulate through the whole
id (a complete system of determinants), but also by the neigh-
bour determinants, which will sooner or later resist the with-
drawal of nourishment from them ; but for the descending
progression there are no limits except total disappearance,
and this is actually reached in cases in which the determinants
are related to a part which has become useless " (Weismann,
1904, vol. ii. p. 118).

" If the germ-plasm be a system of determinants, then
the same laws of struggle for existence in regard to food and
multiplication must hold sway among its parts that obtain
between all systems of vital units — among the biophors which
form the protoplasm of the cell-body, among the cells of a tissue,
among the tissues of an organ, among the organs themselves,
as well as among the individuals of a species and between species
which compete with one another."

When a structure becomes useless in the life of a species,
those individuals who have more of it are no better off than
those who have less of it ; natural selection no longer operates

GERMINAL SELECTION 461

as far as that structure is concerned ; a state of panmixia, as
it is called, sets in ; and the structure in question tends to
dwindle. But this external selection is abetted by the germinal
selection, for when a determinant corresponding to the useless
structure becomes weaker through the intragerminal fluctuations
of nutrition, " it finds itself upon an inclined plane, along which
it glides very slowly but steadily downwards. The determinant
whose assimilative power is weakened by ever so little is con-
tinually being robbed by its neighbours of a part of the nourish-
ment which flows towards it, and must consequently become
further weakened." By hypothesis, personal selection cannot
help it to persist — i.e. cannot favour those individuals in whose
inheritance it is relatively stronger ; therefore, by an internal
struggle and selection, which may be quite real though quite
unverifiable, the determinants of a disused part dwindle away
in the course of many generations. On the other hand, when
personal selection favours the increase of a part — i.e. favours
individuals whose inheritance includes strong determinants of
that part, again the internal struggle will back up the external
sifting. In short, nothing succeeds like success.

The theory helps us to understand the slow dwindling of
useless structures, but it is also applicable to the augmentation
of useful parts. Suppose it be important for humming-birds
to have a longer tongue, and that natural selection favours
variants with longer tongues. Corresponding to the tongue
there are, by hypothesis, in the germ-plasm, several sets of
homologous determinants. (We need not complicate the
argument by recognising that many different kinds of deter-
minants will be required for a complex structure like the tongue.)
There are fluctuations in the food-supply and some tongue-
determinants get the advantage ; they become stronger, they
i exhibit a plus variation, and as they become stronger they
increase in assimilative capacity. They therefore tend to pre-
dominate more and more over other tongue-determinants which

462 HEREDITY AND DEVELOPMENT

may exhibit a minus variation ; and personal selection favour-
ing the birds with longer tongues — i.e. birds in whose inheritance
there is a predominance of tongue- determinants varying in the
plus direction— the direction of variation will remain positive.
In the case of artificial selection the continuance in the plus
direction may go much further and much more rapidly than
in the case of natural selection, for rapid increase of any part
is apt to prejudice the viability of the whole organism, which
in the case of domesticated animals is artificially preserved.
Thus we have the Japanese breed of cocks with feathers six
feet long.

Illustration. — It is admitted by all that in the course of
evolution the hind-limbs of whales have dwindled away and
are now represented simply by vestigial structures. As the
far-back ancestors of the whales of to-day became thoroughly
aquatic and look to swimming with great strokes of the tail,
the hind-limbs became functionless, futile, and actually in the
way. Natural selection would favour those individuals whose
hind-limbs varied in a retrogressive or minus direction ; that
is to say, natural selection would favour those individuals in
whose germ-plasm or inheritance determinants of the hind-limb
varying in a minus direction came to be predominant over
those varying in a plus direction. As the result of persistent
personal selection the determinants varying in a minus direction
would come to be more and more dominant. Weismann's
point is, that when a bias in favour of minus determinants or
short hind-limb determinants was thus established, it would
go on increasing automatically because of germinal selection.
Determinants varying in a plus direction, in the direction of
longer hind-limbs, would be more and more thoroughly van-
quished in the germinal struggle with the more numerous, more
vigorous, perhaps larger determinants varying in the direction
of utility. And after personal selection had ceased to operate
— e.g. when the hind-limbs had quite sunk beneath the surface—

GERMINAL SELECTION 463

the germinal selection would still continue, and thus we can
picture to ourselves a modus operandi whereby the useless
organ would dwindle more and more.

Similarly, every one admits that the huge canines of various
mammals have evolved from relatively small teeth in the same
position. For many generations natural selection would favour
variants with larger canines — i.e. those in whose germ-plasm
or inheritance canine-determinants varying in the direction oi
greater size and strength of teeth were predominant. " The
moment that these come to predominate in the germ-plasm of
the species, at once the tendency. must arise for them to vary
still more strongly in the plus direction, not solely because the
zero-point has been pushed further upwards, but because they
themselves now oppose a relatively more powerful front to
their neighbours — that is, actively absorb more nutriment, and
upon the whole increase in vigour and produce more robust
descendants. From the relative vigour or dynamic status of
the particles of the germ-plasm an ascending line of variation
will thus spontaneously arise, precisely as the facts of evolution
require." Furthermore, if we admit this consideration we
can in some measure understand why the ascending line of
variation often tends to go too far ; and sometimes does go
too far when the check of natural selection is removed by the
artificial conditions of domestication.

Yalue of the Theory. — Weismann emphasises the following,
among other advantages of the theory of germinal selection,
It suggests an interior mechanism which interprets the occur-
rence of definitely directed variations, the occurrence of appro-
priately useful variations at the right place and time, the di-
minution of organs below the level touched by personal selection
or its cessation (panmixia), the occasional exaggeration of organs
beyond the limits of demonstrable utility, the simultaneous
occurrence of many similar variations, and so on.

It must remain a question for personal judgment whether

464 HEREDITY AND DEVELOPMENT

these and other alleged advantages of the theory are real ad-
vantages. Does the theory clarify our conception of inherit-
ance ? and does it suggest experimental work, on which,
after all, we must base our conclusions as to these abstruse
questions ? Do the advantages of the theory outweigh the
difficulties ?

The chief difficulties are (1) in the argument that the struggle
will work out in a discriminate selection, and (2) in the postulate
that a slight advantage gained by a set of determinants will
be able to persist through a long series of cell-divisions, till the
sex-cells of the offspring are again matured.

Objections. — What we have stated above is not more than
an outline of a theory which Weismann has developed with
great subtlety and in great detail, and many objections may
occur to our statement of the theory which are well met in
the author's own presentation.

1. It has been objected that the whole concept of germinal
selection is visionary and unverifiable. The point, however,
is : does this hypothetical construction enable us to interpret
the facts better ? does it harmonise with visible facts ? is it con-
sistent with what we know of the behaviour of observable
living units ? It seems to us that an affirmative answer may
be given. The concept deals with an invisible world, but it
helps us to interpret such facts as the dwindling of useless parts,
the excessive growth of more or less indifferent structures
(such as some of the ornaments of shells), and in general the
frequent definiteness of variation.

2. It may be objected that we can hardly think of invisible
bodies such as determinants struggling for food. But why
not ? Size seems an irrelevant consideration. Cells which are
invisible to the naked eye are seen under the microscope strug-
gling for food. The germ-cells in the ovary of Hydra devour
one another just as really as the embryos of the dog-whelk in
their egg-capsules on the sea-shore, just as really as the locusts

GERMINAL SELECTION 4^5

in a swarm. And if there is competition among cells for food,
why not among the chromosomes within the cell, and why not
among the determinants within the chromosome ?

Yet, is not the supply of food brought by the vascular fluids
of the body always more than sufficient ? Who can tell ?
When we consider, for instance, the enormous ovary of a cod — ■
the familiar cod-roe of the breakfast-table — and its legions of
eggs, can we be sure that the food-supply is always superabund-
ant ? Moreover, it is very improbable that all the hungry
units are equally well-placed ; how much more is there
likely to be inequality within the labyrinth of the ovum-
nucleus, which is a little world in itself ? And again, it by no
means follows that all the food supplied is appropriate, or that
all the homologous determinants are equally able to use it.

As Weismann says, to suppose that food is always super-
abundant " seems to me much the same as if an inhabitant of
the moon, looking at this earth through an excellent telescope
and clearly descrying the city of Berlin, with its thronging crowds
and its railways, bringing in the necessaries of life from every
side, should conclude from this abundant provision that the
greatest superfluity prevailed within the town, and that every
one of its inhabitants had as much to live upon as he could
possibly require " (1904, vol. ii. p. 156).

As an instance of severe criticism by an expert who sees no
utility in these imaginative interpretations, we may quote the
following passage from Prof. T. H. Morgan's Evolution and
; Adaptation (1903, p. 165) : " Weismann has piled up one hypo-
thesis on another as though he could save the integrity of the
theory of natural selection by adding new speculative matter
to it. The most unfortunate feature is that the new speculation

I is skilfully removed from the field of verification, and invisible
germs, whose sole functions are those which Weismann's ima-
gination bestows on them, are brought forward as though they
could supply the deficiencies of Darwin's theory. This is,

30

466 HEREDITY AND DEVELOPMENT

indeed, the old method of the philosophises of nature. . . .
The worst feature of the situation is not so much that Weismann
has advanced new hypotheses unsupported by experimental
evidence, but that the speculation is of such a kind that it is,
from its very nature, unverifiable, and therefore useless."

These are hard words, but it would have been more to the
point to inquire whether Weismann's imaginative picture of
what may go on within the microcosm of the germ-plasm is in
any way contradictory of known biological results. Of course,
the theory is " unsupported by experimental evidence," and
" removed from the field of verification " ; but why it is therefore
" useless " we fail to see. It appears to us quite on the same
plane as many symbolic interpretations in chemistry and physics,
where we say that if we picture atoms and molecules, electrons
and corpuscles, in such and such a way, then we can redescribe
more clearly the observable sequences of conditions and results,
and devise further experiments which will test the adequacy
of our symbols and enable us to improve them. The struggle
of determinants may not be quite as Weismann supposes, but
the idea is a logical extension of the selective process which
occurs at many different levels ; it clarifies our picture
of observable facts, and it stimulates further inquiry.

Summary. — Convinced that the theory of natural selection
in the Darwinian sense required some rehabilitation, dissatisfied
with the assumption of merely " accidental " variations, con-
fronted with evidence of definitely directed variations, Weismann
devised this theory of germinal selection. The personal selection
of the possessors of a plus or minus variation in any part means,
of course, that those org nisms are favoured in which the corre-
sponding determinants within the germ-plasm are varying in a
plus or minus direction. But if there be inequality (in size
and assimilating power) among the homologous determinants,
and if there be fluctuations in the nutritive supply, there, ^may
come about a germinal struggle among the homologous deter-

GERMINAL SELECTION 467

minants. Those that are weaker will tend to become weaker
still, those that are stronger will tend to become stronger still,
and thus germinal selection fosters and strengthens personal
selection. In other words, there is an internal reason for pro-
gressive variation (either plus or minus) in the direction of
utility.

A Suggestion. — If we admit the concept of representative
particles in the germ-plasm, which it seems to us is almost
demanded by the facts of particulate inheritance, by the inde-
pendent variability and heritability of often trivial peculiarities ;
and if we admit the probability of some sort of germinal struggle
among these living units, which seems to us warranted by what
we know of the behaviour of visible living units and by general
biological considerations — then it seems at least interesting to
ask whether we need limit the conception of germinal struggle
to a competition between homologous determinants, as Weismann
always does.

In personal selection, as we have seen, there are three distinct
types of struggle — classified according to the parties involved —

(a) between kindred or homologous organisms, (b) between
organisms which are not akin, and (c) between organisms and
the inanimate environment. Logically, we may look for the
same three modes of struggle in the course of germinal selection.
They might be illustrated (a) by struggle between, say, the
maternal and the paternal, or the parental and the grand-
parental, homologous determinants of a single determinate ;

(b) by struggle between determinants of quite different kinds —
e.g. between determinants of the notochord and the deter-
minants of its more effective substitute, the backbone ; and

(c) by struggle between all or any of the determinants and a
disturbing external influence, such as some toxin in the parent's
blood or lymph, or some change in the osmotic conditions of
the sea-water. Is there any theoretical reason why we should
restrict the concept of germinal struggle, as Weismann does,

468 HEREDITY AND DEVELOPMENT

to competition between homologous determinants in relation
to the fluctuating food-supply ?

Testing the Theory. — The chief objections that have been
brought against the theory of germinal selection are,— (i) that
it is bound up with a particular notation and theory of develop-
ment and evolution — in terms of representative particles or
primary constituents, the determinants, which many regard as
at once unveriiiable and gratuitous ; (2) that it cannot be
objectively verified or directfy tested by experiment, being,
like many other scientific theories, part of an intellectual game
with invisible counters ; and (3) that it is gratuitous, since
the results of evolution can be interpreted without this extension
of the selection-process into the invisible microcosm of the
germ-plasm. In answer to these objections, Weismann's
original essays and later lectures on germinal selection seem
to us quite sufficient, and we must ask the interested reader
to consult the original documents and not to base his verdict
upon a necessarily brief and incomplete presentation of the case.
We offer this commonplace advice because some objectors raise
difficulties which a perusal of the original documents would
have shown to be inept.

The progressive course seems to be to take a set of facts
from different fields, and to see whether the key which Weismann
has given us does or does not fit. We propose, therefore, to
assume the concept of a germinal struggle between primary
constituents (not necessarily homologous determinants), and to
inquire whether Weismann's suggestion has interpretative
value.

I. No one is very willing to predict the hereditary result of
pairing two organisms. Average predictions may be ventured in
regard to the issue of a hundred or a thousand pairings. These
predictions may be Galtonian or Mendelian, and they may be
justified on the average. But individual results continually crop up
which are unpredictable ; and even apart from these valuable

GERMINAL SELECTION 469

generalisations — Galtonian and Menclclian — we are accustomed,
in predicting the issue of crossings, to say that the offspring will
exhibit a blended, or exclusive, or particulate expression of the
parental characters. How often, however, must we not frankly
admit, the individual result seems anomalous ! Now, is not this
result just what we should expect if germinal struggle is a reality ?

2. No phenomenon of inheritance is more familiar than that
of preponderant and exclusive inheritance, where, in regard to the
expression or development of a given character, the offspring follows
one parent preponderantly or exclusively, instead of being merely
a " blend." If we suppose that ovum and spermatozoon have each
a complete organisation of hereditary qualities (as we seem bound
to suppose), and that the fertilised ovum has determinants repre-
senting the character in question from both parents and from the
ancestors of both parents, may we not consistently interpret the
hereditary re-expression of only one set, by supposing that there
is a struggle for expression between the various sets — a struggle
in which the most vigorous have for the time the mastery ?

3. A frequent phenomenon of inheritance is a change in the
direction of preponderance in the successive children of a large
family. Suppose a virile middle-aged father and a much younger
mother : the older children may be markedly paternal in the
expression of their inheritance, the younger children as markedly
of the maternal type. Introduce the conception of germinal
struggle ; suppose it to occur not only in the germ-cell lineage
within the gonads, but in the fertilisation and afterwards ; recall
the fact that the ova tend to be more stable than the spermatozoa,
being formed and to some extent fixed in very early days, whereas
the spermatozoa continue to appear in crop after crop. At first
we picture a victory on the part of the determinants of the relatively
prepotent father ; but gradually, in his post-mature spermatogenesis,
there is a weakening of paternal determinants such that, in fertilisa-
tion, those from the mother have now a better chance of asserting
themselves. Naturally enough, the Benjamin is after the mother's
image and after the father's own heart.

4. A very* young pigeon of hooded or frilled breed is mated with
an old one : the first young are smooth-headed and smooth-breasted,
but those of later broods have the specialised characteristics of the
parents. May this not mean that in the too-young egg-cells the more
recent determinants as to head- and breast-feathers — though in the

470 HEREDITY AND DEVELOPMENT

ascending line through selection — yielded to the old-established
combinations ? After a period of nutrition, however, they were
strong enough to assert themselves. Give them time, Prof. Ewart
says, and they will become so prepotent that they may hand on all
the peculiarities ever when the pigeon is crossed with another breed.

Similarly, the first fertilised almost immature ova of a rabbit,
liberated by an ovulation subsequent to the first serving, result in
offspring which take after the male. In the fertilisational struggle
the paternal determinants have the mastery. If, on the other
hand, a doe is served, not at the right time, but a week or ten
days after, when the next young come they are all exactly like
the mother. The expression of inheritance is after the parent
whose germ-cells were the riper.

These results, Prof. Ewart said, " were altogether different from
Weismann " ; from another point of view they are altogether
illustrative of Weismann's theory of germinal selection.

Conclusion. — If we accept the concept of ancestral plasms —
that is to say, the idea that an inheritance is a mosaic of ancestral
contributions, and that a complete hereditary equipment is
present not merely in dual but in multiple form within the
fertilised egg — then we pass naturally enough to the idea of
a struggle among the hereditary tendencies, which Darwin
indeed suggested — which Weismann, however, has elaborated
into a fascinating hypothesis.

If there are multiple analogous but not identical deter-
minants corresponding to any independently variable and
heritable part of the organism, what is to decide the expression
of these ? It is plain that the organism is not usually a melange
or blend of the ancestral contributions which made up its
inheritance. Must we, then, simply fall back on the general
assumption of a regulative entelechy which determines the deter-
minants ? In other words, perhaps, is the mysterious unity of
the organism, which applies to the fertilised egg-cell as well as
to the full-grown creature, such that it determines, by the very
fact that there is a unified organisation, which determinants

GERMINAL SELECTION 471

shall be in the foreground and find expression, and which shall
remain in the background, and latent ? Or is it enough to
suppose that the cytoplasmic soil — the cell — in which the
analogous determinants find themselves, and environmental
influences in the widest sense, decide which determinants are
to be liberated and to find expression ? Weismann suggests
that we may reach a clearer possible image of occurrences if we
introduce the concept of struggle.

The analogous determinants need not all be of equal strength,
and when they liberate their biophors in the appropriate area
there may be a struggle amongst these ; or long before it comes
to the actual liberation and dissolution of determinants there
may be a struggle between them. They are by hypothesis
living units, feeding, growing, and multiplying, and if there are
inequalities amongst them, as there may well be, since some
are older and others younger and since they have had diverse
histories, then there may be struggle amongst them, and here
too — as in the wider world of nature — the weaker may go to
the wall. Moreover, the analogous determinants need not be
all different from one another ; similars may, so to speak, support
one another in development, while incompatibly different forms
may be in a minority and have little chance of asserting them-
selves. All this is apt to become anthropomorphic speculation,
but then the determinants are alive.

CHAPTER XIII

HEREDITY AND SEX

§ I. Relations between Sex and Inheritance.

§ 2. The Determination of Sex.

§ 3. Different Ways of Attacking the Problem.

§ 4. Classification of the Theories.

§ 5. First Theory : Environment Affects Offspring.

§ 6. Second Theory : Fertilisation is Decisive.

§ 7. Third Theory : Two Kinds of Germ-cells.

§ 8. Fourth Theory : Maleness and Femaleness are Mende-

lian Characters.

§ 9. Fifth Theory : Nurtural Influences Operate on the

Germ-cells through the Parents.

§ 10. Another Way of Looking at the Facts.

§ 11. Conclusion.

§ 1. Relations between Sex and Inheritance

The main question here is : What determines sex ? but there are
some accessory questions.

(a) Whatever " maleness " and " femaleness " may imply
in final analysis, there seems no doubt that a single germ-cell
may contain the potentiality of them both, and of all the mascu-
line and feminine characters as well. The drone-bee has a
mother, but no father, and many other instances are known
of unfertilised eggs developing into males, whose quality of
maleness and masculine characters are handed on through their
daughters to their grandsons.

472

RELATIONS BETWEEN SEX AND INHERITANCE 473

(b) The differences between man and woman, peacock and
peahen, ruff and reeve, stag and hind, lion and lioness, are so
conspicuous and manifold that we are apt to lose sight of the
primary distinction that the male is a sperm-producer and the
female an egg-producer. In the lower reaches of the animal
kingdom the two sexes are often superficially alike; it is as we
ascend the series that the primary differences have all manner
of secondary differences added to them. We hold to the central
thesis of The Evolution of Sex (1889) that there is a deep
constitutional difference between the male and the female
organism — a fundamental difference in metabolic gearing — the
female being relatively more constructive or anabolic, the male
relatively more disruptive or katabolic. This difference in the
organism is an expression of a similar deep initial difference in
the fertilised ova, which determines whether they get on to male
or female lines of development. The getting on to male or female
lines of development determines, late or early, whether the
detailed characters will find a masculine or a feminine expression.

(c) In some cases, notably in insects, the differentiation of
the secondary sex-characters occurs at the same time as the
differentiation of the reproductive organs, and it cannot be said
at present that the latter influence the former. Both may be
simultaneous and independent expressions of the same initial
differences in the fertilised ova.

In other cases, certainly, it is the saturating influence of the
early established maleness or femaleness that determines the
development of detailed parts, and of habits as well as structure.
A castrated pullet may acquire not only the outward structural
features of the opposite sex — cock's comb, wattles, long hackle
and tail feathers, rapidly developing spurs, carriage, etc. — but
the behaviour as well and the pugnacious character. There is
rapidly accumulating evidence of the importance of internal
secretions or hormones which pass from the reproductive organs
and exert a pervasive influence in development. One can argue

474 HEREDITY AND SEX

from an abnormality of an antler to an abnormality of a testis.
If a merino male lamb be castrated the adult is hornless like the
female. We are led to the idea that what is actually inherited
may be in many cases common to the two sexes, but is capable of
masculine or feminine expression according to the liberating
stimuli which activate it.

(d) Of much interest in this connection is the occurrence of what
are called " sex-limited characters." Colour-blindness in man-
kind is a familiar example. It is much commoner in men than
in women. But the colour-blind man with a quite normal wife
does not have colour-blind children. His sons are normal and
his daughters apparently normal ; but the condition is trans-
mitted through the daughters to half their sons.

In Plymouth Rock poultry with alternate light and dark
bars on the feathers, the barred character illustrates sex-limited
inheritance. When a male is crossed with a non-barred breed,
the offspring are all barred, whether male or female. This means
that the male Rock is homozygous, that all his germ-cells bear
the determinant of the barred character. When a female Rock
is crossed with a non-barred breed, the offspring are half-barred
(the males) and half non-barred (the females). This means
that the female Rock is heterozygous as regards barred-ness, that
half of her germ-cells have and half have not the determinant
of the barred character. But there is the further point that in
her germ-cells there is some linkage between male-producing
and the barred character, between female-producing and the
absence of the barred character.

Another instance may be given. When Dorset sheep, horned
in both sexes, are crossed with Shropshire sheep, hornless in both
sexes, horns occur on the male offspring, but not on the female.
The horn-producing character is dominant in the male sex,
recessive in the female. WThen the hybrids are interbred, their
progeny — the F2 generation — include hornless males and horned
females— both breeding true — as well as horned males and

THE DETERMINATION OF SEX 475

hornless females, which may be either pure or impure, homozygous
or heterozygous.

§ 2. The Determination of Sex

The Determination of Sex is one of the great unsolved problems
of Biology. It seems to be peculiarly elusive, but perhaps that
simply means that it is near the central secret of life itself. Over
and over again the solution has slipped through the fingers of
Science just when they seemed to be closing upon it. Perhaps
this means that we have not yet learned how to ask the question
rightly. Perhaps the problem is very complex, with different
answers in different cases ; perhaps the solution is, after all, very
simple.

A Multitude of Theories. — From ancient times a keen interest
has been taken in the question of the determination of the sex of
the offspring, and of the answers that have been proposed it
may well be said that " their name is legion." For many of the
answers are bound up with " theories of sex," which are also
legion. It is quaint to notice that the number of speculations
connected with the nature of sex has been well-nigh doubled
since Drelincourt, in the eighteenth century, brought together
two hundred and sixty-two " groundless hypotheses," and since
Blumenbach caustically remarked that nothing was more certain
than that Drelincourt's own theory formed the two hundred and
sixty-third. Subsequent investigators have at least tried to add
Blumenbach's theory of a fundamental " Bildungstrieb " or
formative impulse to the scrap-heap.

The numerous answers offered to the question : What settles
the sex of the offspring ? might be arranged on an inclined
plane so as to illustrate the progress of natural knowledge. " As
in so many other cases, the problem of the determination of sex
has been looked at in three different ways. For the theologian,
it was enough to say that ' God made male and female.' In
the period of academic metaphysics, still so far from ended, it

^6 HEREDITY AND SEX

was natural to refer to ' inherent properties of maleness and
femaleness ' ; and it is still a popular ' explanation ' to invoke
undefined ' natural tendencies,' to account for the production
of males or females. Thirdly, it has been recognised that the
problem is one for scientific analysis " (Geddes and Thomson,
Evolution of Sex, 1889, revised edition 1901, p. 35).

Even after the problem of the determination of sex was
recognised as one that must be tackled scientifically, or not at
all, the suggestions offered have varied greatly in their con-
sistency of adherence to scientific method. There are still
frequent appeals to " natural tendencies," and these must be
judged, not by their self-explanatory character (for biological
formulae will never be that), but by their correspondence with
the limits of available physiological analysis, and by then-
applicability in the actual control of life.

There is a library of books and pamphlets dealing with the
determination of sex, but a large number — redolent as they are
of good intentions — must be set aside at once because of obviously
fatal defects in their scientific procedure. Some lay stress on
what even the most tolerant must admit to be at least nnverifiable
factors, such as the desire of the parents or parent to have a
male child. Others allege the operation of factors which are
physiologically absurd. Others base a generalisation on an
outrageously small number of cases. The reason for the un-
usual copiousness of speculation in regard to this difficult biological
question is to be found rather in its practical than in its theoretical
interest.

The Problem Stated. — The general problem is : What deter-
mines whether a fertilised egg-cell will develop into a male or a
female organism ? But let us look at particular forms of the
problem. It is generally admitted that what are called " true
twins " in the human race arise from the division of a single ovum
into two independently developed ova, and they are said to be
always of the same sex, identical in this as in their other features.

THE DETERMINATION OF SEX 477

But ordinary twins, which arise from two distinct ova developing
simultaneously, are often of different sexes. Why is there this
difference ? The same question arises when we contrast the
" poly-embryony " (i.e. numerous embryos from one ovum)
which occurs in some insects with the ordinary simultaneous
production of many offspring from as many ova. In poly-
embryony the offspring are all of the same sex ; in ordinary
multiparity both sexes occur in varying proportions. As we shall
see, this particular case of the general problem is very suggestive.

In one household the family consists of boys and girls, in a
second of boys only, in a third of girls only — what determines
this ? A setting of hen's eggs gives rise to cocks and hens in
varying proportions — is the proportion practically modifiable ?
A guillemot usually lays a single egg in a season — what determines
the sex of the offspring ? It is well known that the unfertilised
eggs of a queen-bee develop into drones, while the unfertilised
eggs of aphides produced all through the summer months develop
into parthenogenetic females, until at the end of the season, in
autumn, males are produced. What does this mean ?

A great step would be gained if we could narrow the issue in
various cases by answering the question, When is the sex of the
offspring finally determined ? How long may a germ-cell
remain with the potentiality of either sex ? Is there sex-deter-
mination before fertilisation or during fertilisation, or not until
after fertilisation ? Are there cases where we must admit that
the embryo has the potentiality of either sex ? Is the deter-
mination early in some types, such as Mammals, and later in
other types, such as Amphibians ?

Prof. V. Haecker has proposed a useful terminology. Sex-
differentiation implies that one of the two sex-primordia in the
germ-cell is activated, while the other remains latent, (a) This
may occur before fertilisation — progamic sex-differentiation — as
in the large and small ova of Dinophilus, Rotifers, and Phylloxera.
(b) Or it may occur at the moment of fertilisation — syngamic

478 HEREDITY AND SEX

sex-differentiation — as in the case of the hive-bee, where the
fertilised ova become queens and workers and the unfertilised
ova drones, (c) Or it may (theoretically) occur after fertilisation
at some stage in development — epigamic sex-differentiation.
But the examples of this that used to be cited have given way
before criticism, and no convincing case is at present known.

Here we may refer to Prof. E. B. Wilson's proposal to draw
a distinction between sexual predetermination and sexual pre-
destination. " The definitive determination of maleness or
femaleness only occurs when all the factors necessary to their
production have been brought together. This may be effected
before fertilisation (' progamic determination ' of Haecker), but
may also first ensue upon union of the gametes (' syngamic deter-
mination '). Thus one may suppose that all the sexual eggs
of a queen-bee and of Maupas' Hydatina are predestined towards
maleness, but this is reversed by fertilisation when determination
occurs."

§ 3. Different Ways of Attacking the Problem

The problem of the determination of sex has been attacked
scientifically along three distinct lines, which are complementary,
not opposed. In some cases there has been a combination of two
methods.

Statistical. — Some conclusions as to the determination of the
sex of the offspring have been based on statistics, e.g. of the
relative numbers of male and female offspring in different
localities, at different times, with different ages of parents, and
so on. These statistics are valuable in proportion to the breadth
of their base, but it must be remarked that great care is necessary
in giving a physiological interpretation of statistical results.

Cytological.. — Some conclusions as to the determination of the
sex of the offspring have been based on observations of the germ-
cells in particular cases. Thus it has been shown that some
animals have two kinds of ova, the larger developing into females.

WAYS OF ATTACKING THE PROBLEM 479

Even in a type like the rabbit it is possible, according to Russo,
to distinguish two kinds of ova in the ovary. In quite a number

Fig. 45. — Decorative male and less adorned female of Spathura — a genus
of Humming-birds. (From Darwin, after Brehm.)

of animals there is dimorphism of spermatozoa, though it is
not known what significance attaches to this. In some cases,

480 HEREDITY AND SEX

among insects especially, one half of the spermatozoa have " an
accessory chromosome " absent in the other half, and there is
interesting indirect evidence that the ova fertilised by sper-
matozoa with the accessory chromosome develop into females,
while those fertilised by spermatozoa without the accessory
chromosome develop into males.

Experimental. — Some conclusions as to the determination of
the sex of the offspring have been based on experiment, e.g.
subjecting the eggs, or the embryos, or the parents to particular
conditions of nutrition, temperature, and the like, and observing
whether the relative numbers of the sexes in the offspring are in
any way different from those obtaining in ordinary conditions ;
or by contrasting the results of fertilising immature and over-
ripe ova ; or by trying particular breeding experiments in
reference to what are called sex-limited characters.

§ 4. Classification of the Theories

There are two main alternatives : (i) Are there two kinds
of germ-cells (male-producing and female-producing), which are,
in their occurrence and in their development, quite unaffected
by environmental influence ? or, (ii) Do environmental influences
give the germ-cell, either in its early stages or during its develop-
ment, a bias towards male-production or female-production ?

But a more detailed classification may be clearer and more
convenient for discussion. Five theories may be distinguished.

(a) That environmental influences, operating on the sexually
undetermined offspring (after fertilisation), may at least have
a share in determining the sex.

(b) That the sex is undetermined until the germ-cells unite
in fertilisation, when it is decided by their relative condition, or
by a balancing of the tendencies they bear, neither sperm nor
ovum being necessarily decisive.

(c) That the sex is fixed at a very early stage by the constitu-

CLASSIFICATION OF THE THEORIES 481

tion of the germ-cells as such, there being female-producing and
male-producing germ-cells, predetermined from the beginning
and arising independently of environmental influence.

(d) That maleness and femaleness are Mendelian characters.

(e) That environmental and functional influences, operating
through the parent's body, may alter the proportion of effective
female-producing and male-producing germ-cells.

It will be seen that these five theories are not in a strict
way mutually exclusive. Even if we conclude that there are.

Fig. 46. — Winged male and wingless female of Pneumora, a kind of
grasshopper. (From Darwin.)

for instance, two kinds of ova in the ovary, one set predestined
to develop into males and the other set predestined to develop
into females, it does not follow that the relative numbers of
these may not be changed as life goes on, e.g. by the diet of the
parent. And even if we conclude that there are two kinds of ova
predestined from the start, it does not follow that the predestina-
tion need be quite unalterable by the conditions of fertilisation
and of development.

Another preliminary caution must be noted. One must be
careful in arguing from one set of organisms to another. What
determines sex in frogs may not hold true for cattle ; whaf
determines sex in Rotifers may not apply to birds. Nature

31

482 HEREDITY AND SEX

is very manifold, and it may be that sex is determined by a variety
of factors operative in different cases and at different stages.

§ 5. First Theory: — That environmental influences, operating on
the sexually undetermined offspring (after fertilisation), may
at least have a share in determining the sex

In many 3'oung organisms it is for a time impossible to
distinguish the sexes, and the assumption is often made that
there is a prolonged indeterminateness as regards sex. The
first theory that we need discuss is, as stated above, that en-
vironmental influences give the bias towards maleness or female-
ness.

In support of this theory it has been customary to refer to
the interesting experiments on tadpoles made by Professor
Emile Yung, of Geneva, and although these are not so convincing
as some have thought, it is due to this zoologist to recognise
that he began experimental investigation of the subject at a
time when this mode of approach was little thought of.

Let us recall some of Yung's evidence. Tadpoles are said to
linger for some time in a state of sex-indifference or potential
hermaphroditism. In normal conditions there are about 57
females to 43 males in the hundred. But tadpoles fed with
beef, fish and frog-flesh, yielded respectively 78, 81 and 92
females in a hundred. This was, of course, a very interesting
result, but it has been pointed out that Yung did not pay sufficient
attention to differential mortality, that he had not sufficiently
large numbers, and that although some tadpoles are potentially
hermaphrodite (with testes around the ovaries), there are others
which are quite distinctly male or female even in young stages.
But the most important criticism is the first, which leads Beard,
for instance, to say that Yung's experiments are only of import-
ance in regard to the relative viability of the two sexes. It is
necessary to have renewed experiments on a large scale, and to

ENVIRONMENT AFFECTS OFFSPRING 483

have more precise data as to the time when the sex of the tadpole
is unmistakably distinguishable.

When a crowd of caterpillars are under-fed, there is an un-
usually large proportion of males (Landois, Treat, Gentry, and
others). But as it was shown long ago that the sex is determined
in the larva before it leaves the egg, the starving experiments
were irrelevant. They only show that there may be great
differences in the rate of juvenile mortality in the two sexes.
Thus Prof. Poulton points out in regard to the poplar hawk-
moth (Smerinthus populi), for instance, that the female cater-
pillars, being larger, require more food, and will therefore die first
when supplies are scarce.

Nor is there agreement among the results of experiment.
Kellogg and Bell found that the sex of the silkworm is not
appreciably affected by the nutrition of the parents or even
grandparents. Cuenot found that the proportion of the sexes
in blow-flies, where its visible determination is later than in
butterflies, was not affected by what the larvae ate, or by what
their parents ate.

What then is our conclusion in regard to the first theory ?
It must be admitted that there is no cogent evidence to show that
environmental influences operating on a developing organism may
decide what its sex is to be. Yet we should be slow to assert
that this is impossible. Consider, for instance, Nussbaum's
elaborate experiments on Hydra grisea, which he subjected to
varying nutritive conditions. In this species there are both
hermaphrodite and dioecious forms. Nussbaum found that the
optimum nutritive conditions resulted in predominance of female
polyps, and that groups wholly male could be produced by rela-
tive starving. From these experiments it seems that in Hydra
the nutrition of the body determines the production of ovary
or testis.

There are analogous experiments in regard to some plants.
Prantl found that spores of the Royal Fern (Osmunda) and of

484 HEREDITY AND SEX

Ceratopteris ihalictroides sown in soil without nitrogenous sup-
plies developed into male prothallia, that female organs were
formed when ammonium nitrate was supplied, and that wholly
male prothallia might become wholly female prothallia. Similar
results have been obtained for horsetails by Buchtien.

It is plain, of course, that in cases like fern-prothallia and
Hydra, which are normally hermaphrodite, what actually occurred
in the experiments was the inhibition or suppression of one set of
sexual organs in favour of another. None the less do the experi-
ments suggest that the first theory is not to be dismissed too
hurriedly.

Moreover, when we recall how a little nutritive attention
makes a worker-grub a queen-bee, or how Aphides produce
females parthenogenetically through months (or even years)
of high feeding and pleasant temperature, and how the advent
of autumn, with its cold and its scarcity of food, is followed by
a birth of males, and so on, we may not be able to share the dog-
matism of some who assert that the theory of the environmental
determination of sex is preposterous. We shall consider later
on the question of the influence of the environment on the
parents.

§ 6. Second Theory : — That the sex is undetermined until the
germ-cells unite in fertilisation, when it is decided by their
relative condition, or by a balancing of the tendencies they
bear, neither sperm nor ovum being necessarily decisive

It has been a favourite theory, especially in regard to man
and mammals, that the sex of the offspring depends upon the
relative condition of the germ-cells at fertilisation, the differences
in condition depending on the relative age of the parents and
other such circumstances. Let us consider various forms of
this second theory.

Hofacker (1828) and Sadler (1830) independently published

FERTILISATION IS DECISIVE 485

statistics in support of the theory that when the male parent
is the older the offspring are preponderatingly male, and that
when the female parent is the older the offspring are pre-
ponderatingly female. In short, the sex of the offspring depends
on the relative ages of the parents. Statistical evidence has been
found supporting and contradicting this theory. Schultze's
experiments on mice tell strongly against it.

Yet it seems fair to notice, that if the germ-cells remain for
some time undetermined in regard to the sex which they will
express' — if, in other words, they retain for some time the poten-
tiality of either — there is no a priori reason against the theory
that the absolute and relative ages of the parents may have
influence.

Or, again, even if there are two kinds of egg-cells and two kinds
of sperm-cells, which are from the first determined towards
female-production or towards male-production, the age of the
parent may favour the production of one kind rather than of
the other, or may favour the survival of one kind rather than
of the other.

It is hazardous for the inexpert to draw conclusions from
statistics, but there seems evidence in mankind of a correlation
between the age of the mother and the sex of the child. The
younger mothers tend to have more female children ; the older
mothers tend to have more male children. On this the self-
regulating balance of sex in a nation depends. When females
are scarce — for instance, in a colony — they mate early, and
supply the demand for girls. When men are scarce — for instance,
after war — there are more late marriages, and therefore more boys.

In connection with the general importance of age as a repro-
ductive factor, reference should be made to the remarkable work
of Dr. Matthews Duncan, Fecundity, Fertility, Sterility, and
Allied Topics (Edinburgh, 1866).

By many authors, e.g. Girou, and at various dates, the
theory has been propounded that the sex of the offspring tends

486 HEREDITY AND SEX

to be that of the more vigorous parent. This is a favourite
opinion among breeders and among the fathers of many boys,
but it lacks substantiation, and the concept of comparative
vigour is too vague to be useful.

So far as parental vigour may depend on what may be called
strained reproduction, or on deterioration supposed to result
from close in-breeding, Schultze's experiments on mice do not in
the least confirm the view that it has any effect on the proportions
of the sexes.

Starkweather was responsible for the theory that the sex of
the offspring tends to be the opposite of that of the " superior "
parent ; but " superiority " and " comparative vigour " are far
too vague to be scientifically discussable. Dr. Marshall notes
that Allison, an authority on the thoroughbred horse, accepts
Starkweather's theory. So far as we have been able to discover,
there are not any secure facts warranting the idea that a pre-
potent sire gives his offspring a bias either towards his own sex
or towards the opposite.

Van Lint maintains that the offspring has the sex of the sexually
weaker parent, i.e. the parent whose sex-cells are relatively the
weaker at the time of fertilisation. If a relatively feeble ovum
is fertilised by a relatively vigorous spermatozoon, the embryo
will be a female, but its body will follow the father. The author
explains under six heads what is meant by being sexually
weaker or stronger, but he naively points out that the sure
and certain sign of a man's being more sexually vigorous than
his wife is his having a daughter. " Le sexe de l'enfant tranchera
la question." The theory lacks scientific backing.

It has been repeatedly suggested that a determining factor
may be found in the relative maturity or freshness of the sex-
cells which unite in fertilisation. Thury and other breeders have
maintained that an ovum fertilised soon after ovulation is likely
to produce a female. That is to say, the fresher ovum, not
exhausted in any way, e.g. by continuing to live without feeding,

FERTILISATION IS DECISIVE 487

will tend to produce a female. An older egg tends to produce
a male. The bias of the ovum may be corroborated or con-
tradicted by the condition of the fertilising spermatozoon.

As the outcome of a very large series of experiments,
Prof. Richard Hertwig found that either over-ripeness or
under-ripeness of the eggs (due to artificially delaying or hasten-
ing fertilisation) led to a large excess of males. Elaborate experi-
ments by Sergius Kuschakewitsch have corroborated Hertwig's
results up to the hilt. The proportion of males is largely depen-
dent on the degree of over-ripeness in the ova, and cultures of
males only — with only 4-6 per cent, of deaths — were obtained.

In connection with fertilisation we may notice a theory
that has been suggested by Prof. H. E. Ziegler. He assumes
that the chromosomes derived from a grandmother tend to
produce a female, and those derived from a grandfather tend
to produce a male. He points out that the parental chromo-
somes include contributions from grandfather and grandmother,
and since the relative numbers of these depend on the chances of
the reduction division in maturation, it will be a " toss-up "
whether grandfatherly or grandmotherly chromosomes pre-
dominate. If the former, the child will be a boy ; if the latter,
a girl.

Suppose the potential offspring has 12 chromosomes from the
father and 12 from the mother, as in the human species. If
amongst the former there are 8 grandmother chromosomes and
amongst the latter 7 grandmother chromosomes, the child will be
a girl, for there are at least 15 of the 24 derived from the grand-
mother's side.

Probably, however, this speculation is inadmissible. We
must rid our minds of the view (held by many in the past) that
there is in ordinary cases any necessary intrinsic bias in the egg
to produce a female, any necessary intrinsic bias in the sper-
matozoon to incite the development of a male, and that there
is thus a combination of maleness and femaleness in the fertilised

488 HEREDITY AND SEX

egg. It is enough to recall the fact that the drone-bee has a
mother but no father, and the same is true of many Hymenoptera.
This is but a striking instance of the numerous facts which lead
one to conclude that every germ-cell — whether ovum or sper-
matozoon— has in it the potentiality of the distinctive characters
of both sexes. At some stage or other, as we are discussing,
something occurs, perhaps a fixing of the metabolism-rhythm,
perhaps some alteration of the ratio between nucleoplasm and
cytoplasm, perhaps the introduction of a specific qualitative
sex-determinant in fertilisation, which decides whether the
organism will become a male or a female and whether masculine
or feminine hereditary characters will find expression.

Our conclusion in regard to the second theory must be —
That there is little warrant for attaching much importance to
the relative condition of the germ-cells at the time of amphimixis.
The experiments of such a careful worker as Richard Hertwig
incline one to keep the question open, though O. Schultze's results
seem to close it in one case at least. He experimented with
enormous numbers of mice, which are very good subjects, being
ready to breed when seven weeks old, and littering, it may be,
ever)' three weeks, if not allowed to suckle. He found that
the proportions of the sexes were unaffected by the age of the
parents, by apparent vigour, by consanguineous unions, by
frequency of births, or by any kind of nutritive change.

§ 7. Third Theory '.—That the sex is fixed at a very early stage
by the constitution of the germ-cells as such, there being
female-producing and male-producing germ-cells, pre-deter-
mined from the beginning and arising independently of en-
vironmental influence.

On this view there are two kinds of germ-cells, constitu-
tionally pre-determined to be female-producers or male-pro-
ducers. This implies that the sex is determined before fertilisa-

TWO KINDS OF GERM-CELLS 489

tion, thus excluding the second theory. It also implies that the
influence of the environment is negligible after the germ-cells
have been established, and a fortiori after development has
begun, thus excluding the first theory.

Two Kinds of Ova— It may be that there are two kinds
of ova — one kind constitutionally predestined to developing
into males, the other kind constitutionally predestined to de-
veloping into females. This view is not inconsistent with the
assumption, which seems almost inevitable, that all ova carry
a complete hereditary equipment of both masculine and feminine
characters, though only one set usually finds expression. But
what evidence is there of two kinds of ova ?

Some animals normally produce two sizes of ova. Thus, in
Phylloxera among insects, and Hydatina senta among Rotifers,
there are large eggs which develop into females, and small ones
which develop into males. As both develop without fertilisation,
the problem is not complicated by the influence of the sperm.

In Dinophilus apatris, according to Von Malsen, and in a mite,
Pediculopsis, according to Reuter, where fertilisation occurs
as usual, there are large ova which develop into females and
small ova which develop into males. In Dinophilus the ovum
which becomes a male is only about one-tenth of the size of
that which becomes a female ; and the male himself is a degener-
ate pigmy !

Perhaps the occurrence of two sizes of ovum is much commoner
than we know. Thus Baltzer has recently described it in sea-
urchins. But we must not hastily assume that it is the size that
determines the sex, since it may be that the constitutional pre-
disposition to one sex or the other determines the size. On
our own theory, the ovum of relatively greater anabolic bias —
predestined to develop into a female — will tend to gather into
itself more reserve material than one predestined to develop
into a male. It seems probable that the size marks, but does
not make the difference,

490 HEREDITY AND SEX

In some of the higher Pteridophytes there are two kinds
of spores, micro- and macro-spores, which produce respectively
male and female prothallia. Prof. E. B. Wilson notes that
a similar predestination, not marked by visible differences, has
been proved by Blakeslee in both zygotes and spores of various
species of fungi, and that it has also been demonstrated in liver-
worts and mosses. He refers in particular to the recent studies
of the Marchals on dioecious mosses. " Isolation cultures prove
that the asexual spores, though similar in appearance, are in-
dividually predestined as male-producing and female-producing ;
and all efforts to alter this predestination by changes in the
conditions of nutrition, such as are known to be effective in the
case of fern prothallia, failed to produce the least effect."

The view that there are two kinds of ova, determined ab
initio as male-producers and female-producers, has a vigorous
supporter in Beard, who finds evidence in the skate. He main-
tains that the sex is determined when the primitive germ-cells
divide into oocytes. In his 1902 paper on *' The Determination
of Sex in Animal Development," Beard scouted the idea of en-
vironmental interference with the determination of sex. " Any
interference with, or alteration of, the determination of sex is
absolutely beyond human power. To hope ever to influence or
modify its manifestations would be not less futile and vain than
to imagine it possible for man to breathe the breath of life into
inanimate matter." To this, an experimenter like Russo would
answer that he has succeeded in effectively interfering with the
determination of sex. Although it may not be possible to alter
the bias of an egg which has become fixed as a male-producer
or a female-producer, it may be possible by altered nutrition to
change the proportions of these two kinds of eggs in the maternal
ovary, and it may be possible in other ways to change the normal
proportions of survival.

Of great interest in connection with the third theory are
the facts of poly-embryony— the production of multiple embryos

TWO KINDS OF GERM-CELLS 491

from one ovum. Like " identical twins," the " poly-embryonic "
offspring are always of the same sex. In one of the armadillos
(Praopus or Tatusia hybrida) von Jhering found on two occasions
eight embryos within one chorion — presumably, therefore, from
one ovum — and all were males. In some of the parasitic Hy-
menopterous insects, e.g. Encyrtus, investigated by Marchal and
Bugnion, Litomastix and Ageniaspis, investigated by Silvestri,
one segmented ovum forms a group of embryos, all of the same
sex — female if the egg be fertilised, male if it be not fertilised.
Now, it cannot be denied that these facts strongly confirm the
view that the sex of the offspring is already determined in the egg.

The theory has been more than once suggested that the
ova from one ovary develop into females and those from the
other ovary into males. Thus Dr. Rumley Dawson (The Causa-
tion of Sex, London, 1909) has maintained, for man, that the ova
produced by the right ovary develop into males, and that those
produced by the left ovary develop into females. This view
has been tested experimentally in the rat by Doncaster and
Marshall, who found that each rat, with one ovary completely
removed, produced young of both sexes. " This does not, of
course, prove that ' the right and left ovary hypothesis ' is
not true for man, but its definite disproof for another mammal
detracts from its probability." The theory has also been dis-
proved in Amphibians by H. D. King, and that it cannot apply
to birds is obvious, since they have only one ovary.

Two Kinds of Spermatozoa. — In about thirty different
kinds of animals, such as the freshwater snail, Paludina, and the
freshwater beetle, Dytiscus, there are two kinds of spermatozoa
which differ from one another in certain details of form. It has
been suggested that each kind is predestined towards the develop-
ment of one sex, but there is no definite evidence that the dimor-
phism has this significance.

The theory that in Vertebrates one testis yields male-pro-
ducing spermatozoa, the othsr female-producing spermatozoa.

492 HEREDITY AND SEX

has been disproved in rats by Copeman. Moreover, as Doncaster
and Marshall point out, it is known to stock-breeders that bulls
from which one testicle has been removed continue to give
calves of both sexes.

The Accessory Chromosome. — Of great interest are the
facts that have recently come to light regarding what is called
the accessory chromosome. In a number of insects, Myriopods
and Arachnids, the females have more chromosomes in their
cells than the males have. In the simplest cases (Anasa, Protenor)
the female has one more chromosome than the male, and the
egg has one more likewise. Now, half of the spermatozoa differ
from their neighbours in having the same number of chromosomes
as the egg, while the others have one fewer. This extra chromo-
some which half have and half have not is called the X-element
or accessory chromosome. There are facts which go to show
that fertilisation of the eggs by one class of spermatozoa results
in males, by the other in females. When two equal numbers
come together, the result is a female.

In the squash-bug, Anasa tristis, studied by Wilson, the eggs
have ii chromosomes and the sperms 10 or n. Egg n + sperm
ii produces a fertilised egg with 22 (2N) which develops into
a female. Egg 11+ sperm 10 produces a fertilised egg with 21
(2N — 1) which develops into a male.

The chromosomic dimorphism has been proved in about a
hundred species, but all are not equally convincing, and there
are many variations in detail. As the subject is difficult, especi-
ally without diagrams, and as the facts have been repeatedly
summed up in the last few years (e.g. by Wilson, who has con-
tributed more than any other to the investigation), we do not
propose to do more than refer to two or three important
points.

(a) In many cases, instead of there being an accessory chromo-
some in one half of the spermatozoa and no corresponding body
in the other half, there is a " large idiochromosome " or X-element

TWO KINDS OF GERM-CELLS 493

in one half and a " small idiochromosome " or Y-element in the
other half.

(b) The evidence that the one set of spermatozoa induce male-
development and the other set female-development is indirect ;
it is obtained by an examination of the state of the chromosomes
in the body-cells of the offspring. The Y-element, for instance,
is found only in the males, while the X-element is found in both
sexes, but doubled in the female, single in the male.

(c) Wilson gives the following formulae : —

(a) In the absence of a Y-element

Egg X + spermatozoon X = zygote XX (female).
Egg X + spermatozoon no X = zygote X (male).

(b) In the presence of a Y-element

Egg X + spermatozoon X = zygote XX (female).
Egg X + spermatozoon Y = zygote XY (male).

In the German cockroach (Blatta germanica) and in the " Red
Bug " [Pyrrhocovis apterus), half of the spermatozoa have the
same number of chromosomes as the ripe ova (N), including the
accessory chromosome ; the other half have one less (N — 1),
being without the accessory chromosome. An ovum with N
fertilised by a spermatozoon with N, results in a fertilised ovum
with 2N, and this develops into a female. An ovum with N
fertilised by a spermatozoon with N — 1 results in a fertilised
ovum with 2N — 1, and this develops into a male.

In the meal-worm (Tenebrio molitor) and in the house-fly
(Musca domestica) the number of chromosomes is the same
throughout, but in half of the spermatozoa one of the number
is small, and ova fertilised by these develop into males.

A fine corroboration of the importance of the chromosomes
has been recently afforded by the work of T. H. Morgan on
Phylloxera and of von Baehr on Aphis saliceii. In these forms
half of the spermatocytes degenerate (as Meves pointed out in the
bee), namely those without the accessory chromosome ; there-
fore all the spermatozoa are female-producers, and every one

494 HEREDITY AND SEX

knows that all the fertilised ova produce females. An interesting
accessory discovery is that in Phylloxera and Aphides the males
have in their bodies one chromosome fewer than the females have.
"The male-producing egg," Wilson notes, "must therefore
eliminate one chromosome, and this, we cannot doubt, is the
X-element."

These cytological studies are so very striking that one in-
quires anxiously as to the distribution of the phenomena in the
animal kingdom. There have been some noteworthy recent
extensions.

An accessory chromosome is reported by Boveri and Gulick in
Heterakis, a Nematode of the pheasant. The ovum has five
chromosomes ; the sperms are of two types, one with four, the
other with five — a condition similar to that described by Wilson
for Protenor, one of the Hemiptera. In the common Ascaris
megalocephala there is also evidence of an accessory chromosome,
but it seems at present somewhat discrepant and difficult. As
one would expect from the difficulty of the inquiry, there is still
considerable discrepancy of description in regard to many cases
in which an accessory chromosome has been affirmed. It is very
interesting to inquire whether there is any hint of an accessory
chromosome in Vertebrates. In a recent paper, Prof. M. F.
Guyer brings forward evidence to show that in man half of the
spermatids (or immature spermatozoa) have ten, and half twelve
chromosomes, which would correspond to one of Wilson's cases,
Syromastes, where half of the spermatids were found to possess
two more chromosomes than the others. Guyer has found
evidence, still unpublished, which leads him to think that, as
regards accessory chromosomes, conditions obtain among Verte-
brates (fowl, guinea-pig, rat, and man) similar to those found
in numerous Tracheates, and he ventures to express the expecta-
tion that the somatic cells of man will be found to contain
twenty-two chromosomes, and those of woman twenty-four
chromosomes.

TWO KINDS OF GERM-CELLS 495

The theory that the presence of one X-element in a fer-
tilised ovum means male offspring, and that the presence of two
means female offspring, is morphological, and our physiological
sense is left unsatisfied. Is the difference significant in itself,
or as an index of metabolic differences ? If the eggs with more
chromatin than their neighbours develop into females, and if
chromatin be an index of a relatively preponderant anabolism or
anabolic capacity, can the theory be brought into line with the
thesis of The Evolution of Sex, that the female is the outcome
and expression of relatively preponderant anabolism, and the
male of relatively preponderant katabolism ?

Baltzer has observed that about half of the eggs of the sea-
urchin are distinguished from the others by having one of the
eighteen chromosomes represented by a short " hook-chromo-
some " instead of a normal " rod-chromosome," and there is
indirect evidence that those ova with the short " hook-chromo-
some " become males. In discussing this case and comparing
it with the state of affairs in the various insects already referred
to, Boveri points out that in both cases the fertilised ovum from
which a female develops has more chromatin than that from which a
male develops, and that the amount of chromatin has a regulative
influence on the amount of cytoplasm. He recalls cases, such
as Issakowitsch's Daphnids, von Malsen's Dinophilus, and
Russo's rabbits, where it appears to him proved that nurtural
conditions influence sex-determination. The better-equipped
ova become females. He suggests, therefore, that in some cases
nurtural influence operates variably or unequally on sexually
indifferent germ-cells, giving them a bias to the one sex or the
other, and that in other cases the decision is due to an internal
factor such as the presence of stronger " assimilation-chromo-
somes " in some of the ova.

On the other hand, it may be that the additional chromatin
material is of qualitative importance. Thus, to give point to
his theory, Prof. E. B. Wilson suggests quite provisionally

496 HEREDITY AND SEX

that the X-element contains factors (enzymes or hormones ?)
that are necessary for the production of both the male and the
female characters ; that these are so adjusted that in the presence
of a single X-element the male character dominates, or is set
free ; while the association of two such elements leads to a
reaction which sets free the female character.

§ 8. Fourth Theory : — That Maleness and Femaleness are
Mendelian Characters

A Mendelian interpretation of sex, first suggested by
Strasburger, has been developed by Castle, Correns, Bateson,
and others. As Prof. Wilson points out, the interpretation
has taken " three forms, which exhaust the a 'priori possibilities.
These are, first, that both sexes are sex-hybrids, or heterozygotes
(Castle) ; second, that the male alone is a heterozygote, the
female being a homozygote recessive (Correns) ; third, that the
female is the heterozygote, the male being a homozygote re-
cessive (Bateson)."

As Prof. Wilson has shown, each of these forms of the theory
has its special difficulties, which seem to be most serious in the
case of the first.

Prof. Correns's theory was based on beautiful experiments
in crossing dioecious and monoecious forms of Bryony, which
showed that the monoecious condition behaves as a unit char-
acter, which is recessive to the dioecious.

The experiments made by Correns go to show that the pollen-
grains of the dioecious Bryony, though apparently all alike, must
be regarded as of two kinds in equal numbers — male-producing
and female-producing. What immediately arise, as a matter of
fact, are the rudimentary male prothallia, which produce the
reproductive gametes or pollen-nuclei, and the egg-cells fertilised
by half of these produce male plants, while the egg-cells fertilised
by the other half produce female plants.

MALENESS AND FEMALENESS 497

The third form of the Mendelian interpretation is supported by
a number of very striking facts, especially in regard to the
common currant-moth {Abraxas grossidariata) and the canary.
Let us re-state it very briefly. Assuming that there are sex-
determinants or ' factors ' of maleness and femaleness, the experi-
menters suggest (1) that these behave as Mendelian units, female-
ness being always dominant over maleness ; (2) that female
individuals are heterozygous as regards sex (having maleness
recessive) and that they give rise to equal contingents of male-
producing and female-producing ova ; (3) that male individuals
are homozygous as regards sex, being without the femaleness
factor, and give rise only to male-producing spermatozoa ; (4)
when a male-producing spermatozoon fertilises a male-producing
ovum the result is of course a male, when a male-producing sper-
matozoon fertilises a female-producing ovum the result is a
female, femaleness being by hypothesis dominant over maleness.

The study of sex-limited inheritance in the currant-moth,
in the barred Plymouth Rock (and according to some in the
canary), suggests the conclusion that the female is the heterozygous
sex. But Morgan's study of the inheritance of red eyes and
short wings in the pomace-fly (Drosophila ampelophila) suggests
that the male is the heterozygous sex. It may be, then, that
in some organisms it is the one way, and in some the other, as
regards maleness and fomaleness themselves.

Doncaster refers to the confirmation which the Mendelian
theory of sex receives from the results of castration. In Verte-
brates the castration of the male may prevent the expression of
masculine features, but it does not induce the expression of
feminine characters. This may mean that the male is homozy-
gous— that is, purely masculine, without any feminine characters
latent. We would, however, point out that in many cases there
is a lack of positiveness in the feminine characters ; it is mascu-
line characters which are positive and distinctive. In other
words, there might be a good deal of latent feminity in the

32

4g8 HEREDITY AND SEX

castrated male without there being much to show for it. It
would be extremely interesting to experiment with some case
like the Red-necked Phalarope, where the female bird is the more
masculine of the two.

When a Vertebrate female is castrated, or when the ovary
atrophies, there is often a development of masculine characters.
We must refer again to the case of the pullet. Guthrie has
shown that a castrated female chicken may acquire not only
the outward structural features of the opposite sex — cock's
comb, wattles, long hackle and tail feathers, spurs, etc. — but
the behaviour as well.

In Crustaceans the course of events is curiously the reverse of
what is true of Vertebrates. A female whose ovary has been
destroyed by a Rhizocephalous parasite has its secondary sex
characters reduced, but a castrated male assumes more or less
completely the characters of the female. It may be that in this
case the female characters are more positive, e.g. the broad
abdomen. " If the parasite dies and the host recovers, the ovary
of the female may again become functional ; but in the male under
such circumstances eggs may be produced in the testis. Geoffrey
Smith concludes from these observations and from others on
the Cirripedes, that the female is homozygous in sex and the
male heterozygous. There seems no a -priori reason," Mr.
Doncaster continues, " why this should not be true in the case
of Crustacea and flowering plants, while the converse is the case
in moths and vertebrates."

The fact that the proportions of the sexes are sometimes
very variable, as Heape points out in regard to canaries, does not
of itself tell against the view that the ova are determined at an
early stage to be male-producers or female-producers. There
may be a process of discriminate selection during the maturing
of the ova, and we know that in higher Vertebrates the possible
ova do not all come to maturity.

That the proportions of the sexes in different types are very

NURTURAL INFLUENCES 499

diverse seems at first sight to tell against the idea of an internal
automatic production of two kinds of gametes — " against the
existence of an intrinsic and uniform mechanism of sex-produc-
tion and against the specific assumption that sex is transmitted
as a Mendelian character." But Prof. E. B. Wilson suggests
that this difficulty may be overcome by supposing that there
is a disproportion in the number of one kind of spermatozoa
(like that which reaches a climax in Aphids, Daphnids, etc.,
where only the female-producing spermatozoa are left), or that
there be a certain proportion of impotent spermatozoa, as is
well known to be true of the pollen-grains of some flowering
plants, like Mirabilis.

§ 9. Fifth Theory : — That environmental and functional in-
fluences, operating through the parent's body, may alter the
proportion of effective female-producing and male-producing
germ- eel Is

This, like the first theory, admits the importance of nurture
(in the wide sense), but supposes it to be influential at an early
stage in determining the proportion of effective female-producing
and male-producing germ-cells. Supposing that the original
germ-cells are, as Mendelian theory would lead us to expect,
divided into two camps, male-producing and female-producing,
we can readily conceive that nurtural conditions may some-
times influence the relative rate of increase or the percentage of
survival in the two groups. Or supposing that the immature
germ-cells are constitutionally indifferent, as likely to develop
into males as into females, we can readily conceive that nurtural
conditions, such as a change in the nutrition of the parent, may
sometimes decide their destiny.

It seems fairly clear that there are many cases where this
theory of nurtural determination will not apply at all, e.g. when
numerous young are born at once and show an approximately

500 HEREDITY AND SEX

equal distribution of the sexes. Or how could it apply, for in-
stance, to such a clutch of eggs as Shufeldt reports in the case
of a sparrow-hawk ? The first became a male, the second a
female, the third a male, the fourth a female, and the fifth
a male, in regular alternation. Yet these were produced in a
short time from one ovary, and were probably fertilised by the
same set of spermatozoa.

On the other hand, there are cases where a mother produces
a long succession of offspring all of one sex, or produces one son
and a long succession of daughters, and so on. Such cases
suggest that the constitution of the parent may be of some
importance, and we know that the constitution is modifiable
by nutrition and the other factors in nurture.

When we pass from general considerations, such as the above,
and appeal to the facts, we find an interesting conflict of evidence.

From human statistics some have tried to prove that
abundant food favours the production of female offspring, and
vice versa ; but others have concluded, also from statistics, that
the parental nutrition is of no moment, unless in bringing about
a differential death-rate. The fact that 30 per cent, of human
twins are of different sexes seems enough to show that the dieting
of the parent is not of great importance. Schenk's notorious
theory (1898), that the sex of children could be adjusted by dieting
•the mothers, rested on entirely insufficient evidence — a very
small number of cases. Moreover, he supposed that the sex
was determined after conception.

In a statistical inquiry in London Mr. Punnett found that the
proportion of male to female infants is lowest in the poorest
quarter and highest in the wealthiest, yet the differences are
not great, and he concluded that they are due to differential
infantile mortality, birtn-rate, and probably marriage-rate. He
was inclined to believe that " in man, at any rate, the determina-
tion of sex is independent of parental nutrition. In any case its
influence can be but small."

NURTURAL INFLUENCES 501

Careful experiments have been made, e.g. by Cuenot and
Schultze, on the possible influence of the nutrition of the mam-
malian parent (e.g. mouse) on the sex of the offspring ; but the
results are all against the reality of this supposed influence, in
which, however, some breeders strongly believe. Schultze ex-
tended his experiments over three generations, but the high
feeding of grandparents as well as parents did not seem to have
any influence on the proportion of the sexes among the offspring.

Against these results, however, we have to balance the very
important work of Heape, who has brought forward evidence
for mammals and birds that peculiarities in nutrition and in
other environmental influences may exert a selective influence
on the germ-cells, affecting the proportion of male-producing
and female-producing gametes. "Through the medium of
nutrition supplied to the ovary, either by the quantity or the
quality of that nutrition, either by its direct effect upon the
ovarian ova or by its indirect effect, a variation in the proportion
of the sexes of the ova produced, and therefore of the young born,
is effected in all animals in which the ripening of the ovarian ova
is subject to selective action." ..." When no selective action
occurs in the ovary, the proportion of the sexes of ovarian ova
produced is governed by the laws of heredity."

Let us take one of Heape's interesting illustrations. Two
aviaries of canaries were kept under different conditions and the
proportions of the sexes were found to be notably different. In
one case, the aviary was kept at a regular temperature during
the breeding season ; it was comparatively well lighted and
sunned ; the birds did not receive specially rich food. In the
other case, the temperature of the aviary was allowed to vary
considerably during the breeding season ; it was in a room
facing north and east ; the birds had abundance of rich food.
" In the former of the two cases," to quote Marshall's summary,
" nesting, hatching, and moulting took place earlier, only about
half the percentage of loss was experienced, and from the nests

502 HEREDITY AND SEX

in which all the eggs were hatched, the percentage of males
produced was more than three times that which was obtained
from the other aviary, in which the environmental conditions
were less favourable. The results obtained in each case could
not be ascribed to the particular strains of canaries, since an
interchange of birds between the aviaries was not followed by
any material alteration in the proportion of the sexes in the two
environments. It is concluded, therefore, that the ova were
subject to a selective action on which depended the proportional
differences produced " (The Physiology of Reproduction, 1910,

P- 645)-

As the facts stand at present, they point to the conclusion that

if nutritive and other environmental influences are operative,

it is, in the main, by affecting the production and the survival of

sexually-predestined germ-cells.

Of great interest, and, as it seems to us, of importance are
Russo's experiments in treating rabbits with lecithin. They lend
support to the view that the germ-cells may be predisposed to
one sex or the other by the nutritive condition of the parent,
and to the view that the difference between the sexes is primarily
a question of the rhythm of metabolism. Russo attaches much
less importance to the chromosomes and much more importance
to the nature of the metabolism than do most biologists of to-day.
He says, in so many words, that he believes the sex of the off-
spring to depend on the specific metabolism of the germ-cells ;
and he thinks he has succeeded in artificially altering the meta-
bolism of the ovarian ova, and thus altering the normal propor-
tions of the sexes. In the normal ovary there are well-nourished
and ill-nourished ova, and the proportion of the former can be
increased by lecithin treatment.

Female rabbits treated by injections of Merits' lecithin
(solution of 15-20 per cent, in vaseline oil) developed large ovaries,
large Graaffian follicles, ova rich in nutritive material, and eventu-
ally an unusual number of female offspring. The sperm may, as

NURTURAL INFLUENCES 503

it were, corroborate the bias of the ovum, for the percentage of
female offspring is higher when both parents are fed with lecithin.
It is not possible to follow the ova and prove that a relatively
anabolic one always becomes a female, and never a male, and so
on, but the argument from altered proportions seems sound.
While the lecithin treatment is followed by an increase in the
number of ova of " an anabolic type, rich in lecithin globules,"
it often happens that the first litter after the beginning of the
treatment shows a marked preponderance of males. This Russo
regards as due to the fact that the injections stimulate the general
metabolism and inhibit the degeneration of the ova of the katabolic
type, capable of producing males. The increase in the number of
females occurs subsequently.

It has been objected to Russo's experiments that one of the
two kinds of ova which he distinguishes are ova in the course
of degenerative change ; that he worked with families of selected
rabbits (for it is admitted that some females produce more
females than others, though this is not known to be a hereditary
character) ; that the high nutrition should result rather in more
offspring than in female offspring ; and that the number of
experiments did not afford a sufficient basis for the conclusion.
The experiments have been repeated by Basile and by Punnett,
but with entirely negative results. It is desirable that they
should be extended to larger numbers and to a variety of types.

Several experimental investigations support the view that
changes in nutrition and other environmental conditions may
affect the mother so as to alter the ordinary proportions of the
sexes. Then Issakowitsch, working with the parthenogenetic
females of the Daphnid Simocephalits, von Malsen, working with
Dinophilus afialris, in which the ova are fertilised, found that
differences of temperature affected the proportion of the sexes,
apparently by affecting the nutrition of the mothers. Both
sets of experiments are the more satisfactory that they seem to
be free from any fallacy due to differential death-rate in the young

5o4 HEREDITY AND SEX

of the two sexes. It has been pointed out by Walker that pro-
duction of a preponderance of females when food is abundant and
a preponderance of males when food is scarce is an advantageous
automatic regulation which natural selection would tend to
perpetuate.

Many experiments have been made with the Rotifer Hydatina
senta, but the results are conflicting. There is a striking sex
dimorphism, the males being small and gutless. The females
are from birth either male-producers or female-producers ; and,
according to Maupas and Nussbaum, this is determined before
birth, while the female embryo is still within its mother's uterus,
by conditions of temperature and nutrition. Well-fed mothers
produce females which produce females only ; starved mothers
produce females which produce males only. According to
Punnett's researches, however, changes of temperature and
nutrition have no effect ; but some stocks give rise to many
male-producing females, others to few or none.

Against the theory of environmental influence are Stras-
burger's numerous experiments on dioecious Phanerogams, such
as Mercurialis perennis, spinach and hemp. He found that
changes in illumination, soil, crowding, and so on, had no effect
in altering the proportions of male and female offspring. He
is of opinion that in such cases the sex is fixed by the time the
seed is formed.

As regards the fifth theory, then, we find (a) that in certain
cases there is some evidence that the nurture of the parents may
influence the proportions of the male-producing and female-
producing germ-cells, affecting either the number formed or the
number that survive, and (b) that in other cases there is no
hint of any such influence, the facts pointing rather to the view
that the sex of the future offspring is not only predestined but
predetermined at a very early stage in the germ-cells.

With the facts as they are at present before us, it seems
impossible to give any one answer to the question under dis-

ANOTHER WAY OF LOOKING AT THE FACTS 505

cussion. As Prof. T. H. Morgan says : " Admitting that all
eggs and all sperms carry the material basis that can produce both
the male and female, the two conditions being mutually ex-
clusive when development occurs, the immediate problem of sex-
determination resolves itself into a study of the conditions that
in each species regulate the development of one or the other
sex. It seems not improbable that this regulation is different
in different species, and that, therefore, it is futile to search for
any principle of sex-determination that is universal for all species
with separate sexes ; for while the fundamental internal change
that stands for the male or the female condition may be the
same in all unisexual forms, the factor that determines which
of the alternative states is realised may be very different in
different species."

Looking back over the array of facts of which we have given
samples, we would say, with Dr. F. H. A. Marshall, that they
point to the conclusion that " the sex of the future organism is
determined in different cases by different factors and at different
stages of development — either in the unfertilised gamete, or at
the moment of fertilisation, or in the early embryo." We wish,
however, to look at the problem from another point of view.

§ 10. Another Way of Looking at the Facts

In a recent able article on sex-determination, Prof. H. E.
Jordan writes : ' The results of the newer investigations on
sex-determination seem, at least temporarily, to have brought
us back to the position of Geddes and Thomson, namely, that
femaleness is causally related to a dominating cell-anabolism,
and maleness to a relatively preponderant cell-katabolism.
This conclusion would seem to be the base from which future
investigations will start in the attempt to further elucidate the
fundamental mechanism of sex-differentiation."

To this physiological view of sex, first expounded in The
Evolution of Sex in 1889, a brief reference must now be made,

5o6 HEREDITY AND SEX

for we find ourselves unable to get away from the conviction
that there is no sex-determinant or factor at all, in the morpho-
logical or in the Mendelian sense, but that what settles the sex
is a metabolism-rhythm, or a relation of nucleoplasm and cyto-
plasm, or a relation between Anabolism and Katabolism.

All through the series of organisms — and of animals in par-
ticular— from the active Infusorians and the passive Sporozoa
to feverish birds and sluggish reptiles, we read alternatives or
antitheses between liberal expenditure of energy and a more
conservative habit of storing. This primarily depends on the
ratio between disruptive (katabolic) processes and constructive
(anabolic) processes, and we regard the sexes as expressions of
the same contrast within a given species.

According to this view, the deep constitutional difference
between the male and the female organism, which makes of the
one a sperm-producer and of the other an egg-producer, is due
to an initial difference in the balance of chemical changes.
" The female seems to be relatively the more constructive,
whence her greater capacity for sacrifices in maternity ; the male
relatively the more disruptive, whence his usually more vivid
life, his explosive energies in action." In short, the sexes
express a fundamental difference in the rhythm of metabolism.

As we have seen, many sets of facts lead to the conclusion
that each sex-cell has a complete equipment of masculine and
feminine characters, and it may be that the liberating stimulus
which calls the one set or the other into expression or develop-
ment, is afforded by the metabolism conditions that have been
set up in the field of operations, which lead also to the establish-
ment of ovary or spermary, as the case may be. As Dr. C. E.
Walker says in his interesting work Hereditary Characters (1910) :
" The evidence then seems to suggest that the secondary sexual
characters are dependent for their development upon the presence
of the sexual glands in the individual, and that the potentiality
of producing them is present in all individuals of both sexes."

ANOTHER WAY OF LOOKING AT THE FACTS 507

Let us consider the difference between the sexes in its
simplest expressions, such as we see, for instance, in Volvox,
that beautiful sphere of flagellate cells which well illustrates a
body in the making. From the ball of cells reproductive units
are sometimes set adrift, which divide to form other colonies
without more ado. But in other conditions, when nutrition
is checked, a less direct mode of reproduction occurs. Some
of the cells in the ball become large, well-fed elements — the
ova ; others, less anabolic, fade from green to yellow, divide
and re-divide into many minute units — the spermatozoa. The
large cells of one colony are fertilised by the small cells from
another. Here we see the formation of dimorphic reproductive
cells in different parts of the same organism. But we may also
find Volvox balls in which only ova are produced, and others in
which only sperms are produced. The former seem to be more
vegetative and nutritive than the latter ; we call them female
and male organisms respectively ; we are at the foundation of
the differences between the two sexes.

What we are suggesting is a physiological way of looking
at the problem, and the idea that the sex-contrast expresses a
physiological alternative. This is suggested in various ways.
For instance, there is the sometimes striking evidence that sex
is " a quality that pervades all the cells of the organism." Prof.
Wilson notes the extraordinary fact — surely of profound import-
ance— that " in the Mosses the Marchals demonstrate that all
the products of a single spore are likewise immutably determined,
since new plants formed by regeneration from fragments of the
protonema, or from any part of the gametophyte, are always
of the same sex."

It is very interesting also to consider cases where the sex
changes in the course of life ! Thus in the hag-fish (Myxine
glutinosa), according to Cunningham and Nansen, spermatozoa
are produced up to a certain size, after which the reproductive
organ is wholly ovarian. A case recently described by Prof.

508 HEREDITY AND SEX

F. Braem is very suggestive. He experimented with a simple
Annelid worm, Ophryotrocha puerilis. Taking a female which
had ripe eggs and showed no trace of hermaphroditism, he divided
it into two. The head portion, with thirteen segments, was
isolated. In three weeks it had regenerated seven segments with
parapodia. It was then killed and found to be male. The ova
had mostly disappeared from the reproductive organs, leaving
only a residue, and a functional testicular portion had developed,
which was producing spermatozoa. Braem suggests than in
consequence of the amputation the very young, indifferent
germ-cells had developed into male cells, which require less
subsistence than ova. What is certain is that the reproductive
organs had changed from producing eggs to producing sperms,
and such cases appear to us to favour the view that the sex-
difference is fundamentally physiological.

In this connection Dr. F. H. A. Marshall remarks : " When
once we admit the existence of latent {i.e. recessive) sexual char-
acters in individuals in which the characters of one sex are
dominant, and that under certain circumstances those of the
latent sex can develop at the expense of the dominant ones, in
response to appropriate physiological stimuli, we are compelled
to acknowledge also that the sex of the future individual is not
always predetermined in the gametes or even in the fertilised
ovum, but may be called into being at a later stage in life."
The prevalent view to-day, that sex is irrevocably determined in
the germ-cells before fertilisation or in the fertilised egg-cell,
seems to be true in certain cases, but it is in itself too simple.
It requires physiological re-statement, and it requires the addition
of a number of saving clauses.

It must be remembered that many at least of those who are
keenest on the scent of morphological criteria are also alive to the
importance of trying to get at the physiological realities behind
these. Thus we find Prof. Wilson saying, " Since the two classes
of spermatozoa differ in nuclear constitution, it is highly probable

CONCLUSION 509

that they differ in respect to their metabolic processes," or, again,
" Upon what conditions within the fertilised egg does the sexual
differentiation depend ? In some way, we may now be reason-
ably sure, upon the physiological reactions of nucleus and
protoplasm."

§ 11. Conclusion

In conclusion, our view is that the difference between an
ovum-producer and a sperm-producer is fundamentally a differ-
ence in the balance of chemical changes, i.e. in the ratio of ana-
bolic and katabolic processes, which may, of course, have its
structural expression in the relation of nucleoplasm and cyto-
plasm. Nor do we leave this difference in metabolism-rhythm
as a mere vague phrase, for we see its analogue in the contrast
between the ovum and the spermatozoon (though it is quite
unwarrantable to think of these as being in themselves respec-
tively female and male cells), between the macrogamete and the
microgamete, between the encysted and the flagellate cell,
between the plant and the animal, and in many a familiar con-
trast all through the series of Organisata. We adhere, in short,
to the thesis of The Evolution of Sex, that the sex-difference
is but one expression of a fundamental alternative in variation,
to be seen throughout the world of life.

CHAPTER XIV

SOCIAL ASPECTS OF BIOLOGICAL RESULTS

" Without heredity no amount of natural, sexual, or reproductive
selection would avail to progressively change, still less to differentiate,
living forms." — Karl Pearson.

" The causes refer to our ancestors, our teachers, and the surrounding
conditions of society, and with the causes must the responsibility be pushed
backwards. The unhealthy parents, and not the immoral children, are
responsible ; the unfitted teacher, and not the misbehaving pupil, should be
blamed ; society, and not the criminal, is guilty. To take it in its most
general meaning, the cosmical elements, with their general laws, and not
we single mortals, are the fools." — Munsterberg.

§ i. Relations of Biology and Sociology.

§ 2. The Chief Value of the Sociological Appeal to Biology.

§ 3. Originative Factors in Evolution.

§ 4. Social Aspects of Heredity.

§ 5. Directive Factors in Evolution.

As the general results of biological investigation must apply,
mutatis mutandis, to man as well as to other organisms, we
naturally look to Biology for some practical guidance in re-
lation to human affairs. Thus what we have said in regard to
the heritability of predispositions to disease may be of some
practical utility. Similarly, the long discussion regarding the
transmission of acquired characters has some practical corol-
laries. When all is said, however, we cannot but feel that the
application of biological results is only beginning, and beginning
with a tardiness which is a reproach to human foresight. There

510

BIOLOGY AND SOCIOLOGY 5"

can be no doubt that it would " pay " the British nation to put
aside a million a year for research on eugenics, or the improve-
ment of the human breed.

I may be permitted here to quote a notable passage from
the foremost British experimenter on heredity, Mr. William
Bateson (1905, p. 589) :

" There are others who look to the science of heredity with
a loftier aspiration ; who ask, Can any of this be used to help
those who come after to be better than we are — healthier, wiser,
or more worthy ? The answer depends on the meaning of the
question. On the one hand, it is certain that a competent
breeder, endowed with full powers, by the aid even of our present
knowledge, could in a few generations breed out several of the
morbid diatheses. As we have got rid of rabies and pleuro-
pneumonia, so we could exterminate the simpler vices. Vol-
taire's cry, ' Ecraser Vinfdme,' might well replace Archbishop
Parker's Table of Forbidden Degrees, which is all the instruction
Parliament has so far provided. Similarly, a race may con-
ceivably be bred true to some physical and intellectual char-
acters considered good. The positive side of the problem is
less hopeful, but the various species of mankind offer ample
material. In this sense science already suggests the way. No
one, however, proposes to take it ; and so long as, in our actual
laws of breeding, superstition remains the guide of nations,
rising ever fresh and unhurt from the assaults of knowledge,
there is nothing to hope or to fear from these sciences.

" But if, as is usual, the philanthropist is seeking for some
external application by which to ameliorate the course of de-
scent, knowledge of heredity cannot help him. The answer to
his question is No, almost without qualification. We have
no experience of any means by which transmission may be
made to deviate from its course ; nor from the moment of
fertilisation can teaching, or hygiene, or exhortation pick out
the particles of evil in that zygote, or put in one particle of good.

512 SOCIAL ASPECTS OR BIOLOGICAL RESULTS

From seeds in the same pod may come sweet peas climbing
five feet high, while their own brothers lie prone upon the ground.
The stick will not make the dwarf peas climb, though without
it the tall can never rise. Education, sanitation, and the rest
are but the giving or withholding of opportunity."

It seems to us that it may be useful to devote this chapter
to an elementary discussion of the relations of Biology and
Sociology, and especially to an inquiry into the bearings of
biological aetiology on sock.] problems.

Sociologists — that is to say, those who are engaged in the
scientific study of the origin, development, structure, and
functions of human societary forms — have admittedly a difficult
task, and it is not surprising that they should look about for
help on many sides. In recent years many writers on socio-
logical subjects have appealed to biology for assistance and
have used biological formulae in their interpretations. The
title of the admirable journal Archiv filr Rassen- und Gesellschafts-
Biologie is very significant. Let us try to illustrate at once the
value and the risks of the sociological appeal to biology. Our
point of view may seem very obvious to some, absurdly cautious
to others ; it seems to us consistent with scientific method.

§ i. Relations of Biology and Sociology

Every one admits that in biology — the scientific study of
the origin, development, structure, and functions of organisms
as such — it is useful to appeal to physics and chemistry.
Although it has not been possible, to our thinking, to translate
the biological description of any vital sequence into physical
and chemical terms, the methods of physical and chemical
analysis have been very valuable in biological study, deepening
it and broadening it, and enabling us to see more clearly what
is distinctively vital, the autonomy of the organism. The
utility of the analytic method has increased in proportion to

BIOLOGY AND SOCIOLOGY 513

the completeness with which it has been possible to discriminate
the numerous chemical and physical factors which contribute
to the result which we call vital activity.

By analogy, then, it seems on a priori grounds legitimate to
expect that biological analysis applied to the life and history
of societary forms will be fruitful ; and the few steady steps
already taken in this direction are full of promise. But the
analogy also suggests that the result of analysis in terms of
lower categories will in the long run be to bring the distinctively
social into stronger relief, and that certain progress in the utilisa-
tion of biological formulae will depend on the relative com-
pleteness with which the biological factors operative in social
activity can be discovered. A chemico-physical analysis of
organic processes which left out electrical factors would be
inept indeed ; a biological analysis of social processes which
left out, say, the " mutual aid " instinct would, we venture to
think, be equally fallacious.

From time to time in biology some success in physico-chemical
analysis has led to the fallacy which Comte called " a material-
ism " — the premature attempt to formulate the phenomena
of a higher order of facts in terms of the categories of a lower
order of facts, premature in that it attains an apparent success
only by ignoring the most essential features ; e.g. in this case,
those distinctive peculiarities of self-regulation, adaptive re-
sponse, and the like, which give organisms their peculiar apart-
ness from all inanimate systems. It is impossible to argue
the matter here, and it is impossible to tell what unification of
descriptive formulas may be in the lap of the future ; but we are,
we think, stating a matter of fact, not expressing a personal
opinion, when we say that it is at present an inaccurate " ma-
terialism " to pretend that we can formulate any distinctively
vital phenomenon in terms of mechanical (physico-chemical)
categories. In recognising and appreciating the operation of
the chemical and physical factors which contribute to the result

33

5*4 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

which we call the life of an organism, the biologist has so far
simply brought the distinctively vital into greater prominence.

Similarly, in regard to the biological analysis of social se-
quences, there seems to us in recent literature some warrant
for protesting against the " materialism " (in Comte's sense)
of pretending that sociology is merely a higher department of
biology, and a human societary group no more than a crowd of
mammals. We have little faith in a biology which does not
frankly admit that an organism is a new synthesis when com-
pared with inanimate systems, and we have equally little in a
sociology which does not consistently recognise that a human
societary unit, however simple, is also a new s}mthesis as com-
pared with the beasts of the field — a unity with a distinctive
mode of behaviour, with a whole that is more than the sum of
its parts ; in short, with a life and mind of its own.

The fallacy of regarding sociology as no more than a recondite
branch of biology is not merely verbal, implying differences
of opinion on the tedious question of the best definitions of
these two sciences ; it involves a misconception of what human
society is, a misconception which is discredited by the facts of
history and experience. No one doubts that the life of a social
group is made up of a complex of activities of individual per-
sons ; but these are integrated, harmonised, and regulated in
a manner as far beyond present biological analysis as the inte-
gration, harmonisation, and regulation of the chemical and
physical processes in the individual organism are at present
beyond mechanical analysis.

Nor is the "materialism" a theoretical fallacy merely; it
has its practical side. A cattle-breeder has been known to pro-
duce by careful selection a prize bull, almost perfect according
to the physical standard aimed at, but with the serious vital
defect of being sterile ; so preoccupation with a purely biological
ideal might, in relation to the human race, result in consequences
which were anything but advantageous socially. We venture

BIOLOGY AND SOCIOLOGY 515

to say this although there seems at present much more danger
of the converse practical fallacy of forgetting that the biological
ideal of a healthful, self-sustaining, evolving human breed is as
fundamental as the sociological ideal of a harmoniously integrated
society is supreme.

In any case, it is useful to recognise that the biological and
the sociological ideals are not synonymous. As a matter of
fact, though the former should contribute to the latter, which
should include it, the practical clashing of the two ideals is
familiar and interesting. Sociologically regarded, illegitimate
children do not appear to be very desirable ; biologically re-
garded, they are often very valuable assets. Sociologically
regarded, it seems quite consistent with progress that the trawling
industry should flourish ; but, what with pleasant food on the
one hand and pleasant dividends on the other, we run some
risk of forgetting what the biologist deplores, the elimination
of the splendid physical type of the line fisherman and the
threatened disappearance of one of the manliest of callings.
Scores of similar instances will occur to every one.

The danger of trying to press biological formulas into the
service of sociological interpretation is complicated by the
actual history of the sciences. It is well known that the socio-
logical inquiries of Malthus as to human population influenced
Darwin, Wallace, and Spencer, and that the concept of natural
selection in the struggle for existence came to biology from
above rather than from within its own sphere. The same is
true of the fruitful idea of division of labour, of the general
idea of evolution itself, and of others — they came to biology
from the human social realm.

To keep to the concept of selection for a moment : it was
applied to plants and animals, it was illustrated, justified if not
demonstrated, and formulated ; and now with the imprimatur
of biology it comes back to sociology as a great law of life. That
it is so we take for granted, but it is surely evident that in social

516 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

affairs, from which it emanated as a suggestion to biology, it
must be re-verified and precisely tested. Its biological form is
one thing, its sociological form may be another. Perhaps it
requires to be corrected by other laws of social life which have
meanwhile been recognised. Perhaps there may be other
hints from human social life as to the factors in evolution, whose
importance we shall not recognise until they have been projected
upon the world of plants and animals and verified there. In
any case, a formula borrowed from another science and applied
to a new order of facts — even to those in regard to which it first
arose as a suggestion — must be rigorously tested. Otherwise,
both organic and social sciences resolve themselves into socio-
morphic illusions.

§ 2. The Chief Value of the Sociological Appeal to Biology.

As it seems to us, the chief value of " the Appeal to Biology "
on the part of students of sociology is threefold :

(i) The analysis of biological factors operative in social
sequences may serve to bring into stronger relief what is dis-
tinctively social. Thus when we analyse out what is due to
natural inheritance, we see more clearly what social heredity
really is. When we analyse out the various forms of natural
selection operative in mankind, we see how much or how little
selection there is which cannot be expressed in that formula.

(2) The biological analysis may serve to show that certain
features of social life have what we may call organismal main-
springs, and become more intelligible when traced back to these.
Thus the relative lack of fertility in fine human stocks requires
biological as well as sociological interpretation. Again, no
one can do justice to the social significance of sex or of play
who does not know the biology of these. Or again, looking at
this value from another side, the relatively simpler biological
ideals, which must remain fundamental, e.g. of physical culture

ORIGINATIVE FACTORS IN EVOLUTION 5*7

and eugenics, may afford a useful touchstone for testing the
validity of the more complex sociological ideals.

(3) The parallelism of the two sciences is such that biological
conclusions and experiences may have great suggestive value
to sociology, aiding in the discovery of sociological laws and
indicating practicable possibilities of social evolution.

To illustrate this threefold value of the appeal to biology,
and at the same time the risk that biology, used unduly as a
support, may pierce the sociological hand, we propose in this
chapter to consider a few biological generalisations and to
inquire into their bearing on sociological problems.

§ 3. Originative Factors in Evolution

Variations. — Our biological knowledge of the nature and
origin of those changes or variations which form the raw material
of organic progress is still incipient ; yet the little we know
must be borne in mind in sociological discussions. There is
general agreement that inborn variations — which give every
organism its individuality — are the expression of changes in
the intricate architecture of the germ-plasm. It is suggested
that they are due (a) to the influences of the environing " body,"
with its variable nutritive stream, on the germ-cells ; (b) to
the intricate permutations and combinations preparatory to
and implied in fertilisation ; and (c) perhaps to what may be
called growth-changes in the germ-plasm as it is continued
from generation to generation. We are sure that these en-
dogenous or germinal changes, expressing themselves in develop-
ment, supply the raw material of evolution on which selection
operates, and we are not sure that there is any other source of
raw material.

Compared with most organisms, man is a slowly reproducing,
slightly varying, creature. In so far as deeply ingrained char-
acters are concerned, a bod'ly change in the race by natural

5i8 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

inheritance is likely to be slow. Thus we are led to look for
other than germinal origins of social variations ; thus we are
led to suspect that when a social evolutionary process — up or
down — is rapid, there must be super-organic factors at work.
The distinction between organismal and social variations is
obvious. The distinction between inborn variations and ac-
quired modifications (which may be very rapidly diffused) will
be alluded to later on.

While the facts seem to suggest that most of the organic
variations which occur in civilised communities are simply
slightly novel combinations and permutations in that complex
system of ancestral contributions which we call our natural
inheritance, the recent work of investigators like Bateson and
De Vries has led us to recognise that discontinuous or transilient
variations are of not infrequent occurrence in organisms. A
" new departure," a remarkable change of organic equilibrium
may suddenly appear, and may come to stay, especially if it be
favoured by inbreeding or some form of isolation. It seems
certain that a definite breed of cattle may arise in a single farm-
yard, may be inbred until it attains dominant prepotency, and
may after a while persist in its integrity in spite of occasional
inter-crossing. If this be so, we can better understand how a
particular human strain — such as " the Celtic type " — may be
so prepotent that it persists as an important social factor in spite
of much mingling of stocks. On the other hand, a genius is a
transilient variation who usually does not come to stay, except
as an immortal spirit embodied in literature or art.

The view that man has a range of psychical variability as
large as his range of physical variability is small, does not seem
to us supported by facts. The view that man's psychical varia-
tions are independent of natural inheritance is contradicted
by careful investigations, such as those of Karl Pearson (1903).
The useful fact to emphasise is that man, though slowly or
slightly variable, is rapidly and exceedingly modifiable, and that

MODIFICATIONS 5*9

social organisation provides a means — an external heritage —
whereby the results of modifications may be practically
though not organically entailed. To this elementary distinction
— necessary, however, for clear thinking — we must repeatedly
refer.

By a " social variation " we mean a change in the organisation
of a societary form, and it is not within the scope of this chapter
to discuss its nature and origin. That is part of the task of the
sociologist ; and its accomplishment lies far ahead. It may
not be presumptuous, however, to make this suggestion. A
variation expressing itself in an individual organism is marked
by changes in m|Pp individual units, and these changes have
to be described and measured. But the origin of the variation
was germinal, in the " immortal " germ-plasm which gives
continuity to the chain of transient generations. Thus we are
led to think that those social changes that really count must
have their basis in that which is to societary forms what the
germ-plasm is to generations of organisms, the esprit de corps
(in the unrealisable full meaning of the phrase !) which gives
unity to every societary form whether it be big or little.

Modifications. — Besides " variations " in the strict sense,
there are other organic changes, technically known as " modifi-
cations," or, more awkwardly, as " acquired characters." They
are definable as bodily structural changes acquired by the
individual organism as the direct result of changes in function
(use or disuse) or of changes in the environment, and so tran-
scending the limits of organic elasticity that they may persist
after the inducing conditions have ceased to operate. They
are exogenous, somatogenic changes, as contrasted with endo-
genous, blastogenic changes. They are the direct results of
peculiarities in " nurture," as contrasted with inborn changes
in the inherited " nature," to use the convenient words with
which Mr. Galton, following Shakespeare, has made us familiar.
That they are, after all, reactions of the inherited nature to

520 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

new conditions of stimulus, both positive and negative, is
obvious. Now, the important point is that we cannot with
any certainty count these "modifications" as part of the raw
material of evolution (progressive or retrogressive), for we have
no good evidence to show that they can be hereditarily entailed
as such, or even in any representative degree transmitted to
the offspring.

It is admitted that some deeply-saturating modifications
may, by affecting the nutritive stream, indirectly affect the
germ-plasm, but there is no proof of the transmission of any
modification as such. The evidence for this assertion will be
found, foi' instance, in preceding chapters.

It is admitted that the organism — notably the human or-
ganism— is often extraordinarily modifiable, and that similar
conditions may induce similar modifications on generation
after generation, so that an appearance of heritability results.

Moreover, as Professors Mark Baldwin, Lloyd Morgan, and
H. F. Osborn have pointed out, modifications that are effectively
advantageous — adaptive responses, in fact — may have an in-
direct evolutionary importance, for they may serve as sheltering,
life-preserving, or welfare-furthering screens until coincident
endogenous variations in the same direction have time and
opportunity to establish themselves. Thus a modificational
change may be gradually replaced by a strictly variational, and,
by hypothesis, heritable one. Then the screen or veneer may be
done without.

If the conclusion of the majority of biologists be correct,
that modifications are not as such transmitted, there are some
obvious sociological corollaries. We have, in the progress of
education, therapeutics, and hygiene, unceasingly striking
evidence that the human organism is very plastic ; but we
cannot delude ourselves with the belief that its precise gains
or losses are ever as such transmitted. Therefore, it has to be
our practical endeavour that advantageous modifications be

IMPORTANCE OF MODIFICATIONS 521

re-impressed on each successive generation, and that detrimental
modifications be avoided.

But the biological conclusion has to be in an important respect
corrected for the social realm, in view of the fact that man
has an external heritage of custom and tradition, institution
and legislation, literature and art, which is but slightly or not
at all represented in the animal world, which yet may be so
effective that its results come almost to the same thing as if
acquired characters were transmitted. They are re-impressed
on the bodies and minds of successive generations, though
never ingrained in the germ-plasm. It seems probable that
not a few of the biologically and socially unfit are only
modificationally veneered, or repressed, or arrested.

Moreover, while among plants and animals the organism is
often largely a creature of circumstances, very thoroughly in
the grip of its surroundings and mastered by them, it becomes
otherwise as we ascend the scale of being. Increasingly we find
the organism — be it bird or mammal or man — much more master
of its fate, able to select its own environment in some measure,
able to modify its surroundings as well as be modified by them.
As we take a bird's-eye view of the course of evolution, must
we not recognise the gradual emergence of the free agent — the
operation of what has been badly called " organic selection " ?

§ 4. Social Aspects of Heredity

We have defined heredity as the genetic relation between
successive generations, and inheritance as all that the organism
is or has to start with in virtue of its hereditary relation to
parents and ancestors. All sociological talk that appeals to a
" principle," " law," or " force " of heredity should be ruled out
of court.

The hereditary relation is sustained by the germinal material,
and the precise study of this physical basis has done much of

522 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

recent years to define the way in which generation is linked
to generation. The fundamental fact of the continuity of the
germinal material from generation to generation — the fact which
is in biology like the first law of motion in physics — secures that
persistence and continuity of organic kinship on which the
possibility of a society depends. The peculiar way in which
the germ-plasm accumulates within itself what we must regard
as multiple sets of hereditary contributions, and becomes like a
mosaic, or like capital growing at compound interest, is a funda-
mental fact for sociologist as well as for biologist. It is the
organic condition of the social instinct.

The great generalisation known as Galton's Law of Ancestral
Inheritance, according to which inheritances are on an average
made up of a half from the two parents, a quarter from the
four grandparents, an eighth from the great-grandparents, and
so on, may require some adjustment as regards the precise
fractions, and in relation to cases of inter-crossing, but the
general fact seems to have been well established, and it is eloquent.
Taking it along with Professor Karl Pearson's evidence that the
inheritance of psychical characters can be formulated like that
of physical characters, we are in a better position to understand
what is called " social solidarity " and " social inertia." We
are able to realise more vividly how the past' has a living hand
on and in the present, even to feel, perhaps, that there is a danger
of fallacy in insisting too much on either past or future when we
have to deal with the continuous stream of life. Mr. Galton's
generalisation makes reversions, survivals, recapitulations, and
the like more intelligible.

Very suggestive also is Mr. Galton's elucidation of Filial
Regression — that there is a continual and necessary tendency
to approximate to the mean of any stock. In proportion as
two parents are divergent from the mean of their stock, will be
the succession-tax levied upon their offspring, which will tend
to approximate, up or down, towards the general level. This

SOCIAL ASPECTS OF HEREDITY $23

is capable of statistical proof, and it follows from the broad fact
that each parental contribution is a mosaic of inheritance,
which, except in cases of very careful selection (for good or ill),
must eventually be traced to a crowd of ancestors representing
the average mediocrity of the stock.

Thus we have light thrown on the familiar facts that children
of exceptionally gifted pairs are often commonplace, and that
children of worse than commonplace parents are often very
fair samples of the breed. More generally, we see, as Mr. Galton
s,ays, that there is a general and inevitable levelling- up and
levelling- down, that a society biologically considered tends to
move like a great fraternity. Just as the " Hereditary Genius "
studies of Mr. Galton gave us a biological basis for pride of
race and a respect for true aristocracy, so his Filial Regression
formula is a message to democracy.

The facts of inheritance acquire profound sociological signifi-
cance when we inquire into the relative rates of fertility in
different sections of a population, and into the probabilities
of the production of highly endowed types in these different
sections. It seems to us that one of the most suggestive of
biological contributions to sociology is that famous " Huxley
Lecture " in which Mr. Galton indicated some of the probable
practical corollaries of his statistical laws.

Man is a slowly varying organism, and he is peculiarly liable
to have his inborn nature concealed by a veneer due to nurture,
but there is no ignoring the fact that there are great differences
in quality and quantity of hereditary endowment. As was long
ago expressed in immortal parable, there are those who have
ten talents, those who have five, and those who have only one.

Now, the differences in hereditary endowment — of strength
or intelligence, of stature or longevity, of fertility or social dis-
position, have a certain regularity of distribution, so far as we
can measure them at all. They conform to what is called the
Normal Law of Frequency, which is always illustrated when

524 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

variations are due to the combined action of many small and
different causes. Human variations, whether bodily or mental,
may be registered on a curve of frequency, just like the varia-
tions of poppies or jelly-fishes — on the same sort of curve as may
be illustrated by plotting out the marks round the bull's-eye
in target practice, or the numbers which come to the top
in so many thousand throws of the dice, or the marks in a
competitive examination with a large number of candidates.

Let us briefly recall Mr. Galton's argument. If we take a
precisely measurable quality like stature, we find that the
average height of a large number of adult Britons is 5 feet 8
inches ; above this line of mediocrity (R) there are taller men
who may be arranged in groups, the means of which are sepa-
rated from one another, by if inches; we may call these +S,
+ T, +U, +V, +W, and +X, till we end in giants of 6 feet
6 inches ; we may give to the distance between the groups
(if inches) the name " normal talent." Thus while the average
adult has 39 " normal talents " of stature (5 feet 8 inches), the six
groups above him, rapidly decreasing in numerical strength as
we ascend, have respectively 1 — 6 talents more than mediocrity.

On the other side of mediocrity, there are of course groups of
minus variations, groups which we may call — s, — t, — u,

— v, — w, and — x, with 1 — 6 talents fewer than the normal
equipment of 39 ; and the minus or left side of the curve exactly
reflects the plus or right side. A giant of 6 feet 6 inches would
belong to the small and very select sixth class above medio-
crity (+X), while a dwarf of 4 feet 10 inches would belong to
the sixth class below par ( — x) ; and there are apparently as
many of the one as of the other. Mr. Galton maintains that
the curve holds good for any particular measurable quality
taken separately, and that it also holds good when the qualities
are grouped. " It can be employed to give a general idea of the
distribution of civilisation, in so far as it is normally distributed

- • • and the same for any group of normal qualities."

SOCIAL ASPECTS OF HEREDITY 525

The next step in the argument is important and brings us
into closer touch with social problems. Mr. Charles Booth, in
his well-known demographic studies, has arranged the population
of East London into grades of " civic worth," beginning with
criminals, semi-criminals, and loafers, going on with increasing
numbers to casual workers, intermittent workers, and thence
to regular earners under 22s. a week, and so on. The results
show " a fair approximation to the normal law of frequency."
Again we have the groups, + S, +-T, -f U, etc., and the groups,
— s, — t, — u, etc., forming the two sides of an approximately
similar and symmetrical curve.

It is easy to say that one knows of this, that, and the
other one who rose into class +T by sheer luck; and of this,
that, and the other one who fell into class — t by the hand of
God — a fire, a wreck, an explosion, and what not ; but when
we are dealing with large numbers, it does not seem that these
exceptional exaltations and depressions of individuals are of
vital moment. It is also evident that the standard of civic
worth used by Booth is only one of many standards — that of
economic production under present conditions — but to begin
with we must measure by one standard at a time. We know
that it would be individually unjust to put, say, Arnold's " scholar
gipsy " on the minus side as a casual worker, but there are not
many scholar gipsies.

The next step in Mr. Galton's argument might be described
as a financial valuation of babies. Suppose we could import
at the present moment ten legions of boys of sound physique
and scouting intelligence, not crammed with intellectual fat
like Strasburg geese with the physical analogue, but alert in
understanding of methods and with unchecked inquisitiveness,
what great national gain it would mean ! It would be a good
investment, and it is within reach every year, since far more
than ten legions of this type of boy are being born annually in
our midst. That they do not effect all they might do, is partly

526 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

because of mis-education, but also because there is a simul-
taneous appearance of an enormously greater number of boys
who are emphatically not of this type.

Dr. Farr, the eminent statistician, tried to estimate the social
money-worth of the average baby born to an Essex labourer,
supposing him to live as long as and after the manner of his class.
Allowing for cost of maintenance during the two helpless periods
of infancy and senile infirmity, Dr. Farr came to the conclusion
that the national value of the baby was about £5. If £50 be
nearer the mark, it does not affect the argument.

" On a similar principle," Mr. Galton says, " the worth of a
+ X-class baby would be reckoned in thousands of pounds.
Some such ' talented ' folk fail, but most succeed, and may
succeed greatly. They found industries, establish vast under-
takings, increase the wealth of multitudes, and amass large
fortunes for themselves. Others," he continues, " whether they
be rich or poor, are the guides and lights of the nation, raising
its tone, enlightening its difficulties, and improving its ideals.
The great gain that England received through the immigration
of the Huguenots would be insignificant to what she would derive
from an annual addition of a few hundred children of the classes
+ W and +X."

Now, however, comes the crux of the whole argument. By
a method expounded in his " Natural Inheritance," Mr. Galton
has endeavoured to express in a standard table precisely how
each generation of a classified population is derived from its
predecessors. Keeping to the terminology that the groups
above mediocrity are +S, +T, +U, +V, + W, +X, let us
inquire with Galton into the origin of 35 male members of the
very excellent grade +V (fourth above mediocrity, 1 in 300).
(That these are not mainly due to marriages of + V-class
parents is probably suggested by our everyday experience,
and this observational conclusion is borne out by the statistics,
which, in regard to some qualities, such as stature, can be made

G ALTON'S HUXLEY LECTURE 527

very precise.) Mr. Galton's result is that of the 35 + V youths,
six come from + V (fourth) parentages ; ten from -f U (third) ;
ten from +T (second) ; five from + S (first) ; three from R,
and none from below R.

But along with this very suggestive result, we have to con-
sider the numerical strengths of the contributing parentages.
When this is done, " we see that the lower classes make their
scores owing to their quantity and not to their quality ; for
while 35 -f V-class parents suffice to produce six .sons of the
+ V-class, it takes 2,500 R-class fathers to produce three
of them." Thus from the point of view of eugenics, if we
wish to increase the number of + V-class offspring, the most
profitable source is to be found among the more prepotent
+ V-class parents ; they are three times more profitable than
those of the next class, + U, and 143 times more profitable
than those of class R !

Other Facts of Heredity. — One is tempted to linger over that
mode of inheritance which is called true reversion, where ancestral
characters that have lain latent for several generations suddenly
find opportunity to reassert themselves. It is true that " rever-
sion " has been a convenient " free toom " into which much
rubbish has been shot. It is true that reversion has been terribly
confused with arrests of development (usually of modificational
origin), with the not uncommon variations in those numerous
vestigial structures of which our body is a walking museum,
with independent variations that " happen to hit an old mark
in aiming at a new one " or simply suggest to the credulous a
harking-back to a more or less hypothetical ancestral type, and
even with the normal and everyday occurrence of filial regression.
Yet it is undeniable that ancestral traits may remain long latent,
apparently but never really lost, and that, in the intricate
shuffling of the cards which is associated with the maturation
and fertilisation of the germ-cells, they may suddenly find their
appropriate liberating stimulus, and assert themselves once more.

528 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

A shepherd's cottage garden was swallowed up in a deer-forest
and became a garden full of weeds ; generations passed and it
was once more delved ; the long dormant seeds were reawakened
and many old-fashioned flowers saw the light. So there may
be a reawakening of almost forgotten flowers and weeds in that
garden which we call our inheritance. Thus we interpret
biologically what we cannot ignore in the body politic, the
emergence of the old-fashioned type whom we— foxes without
tails — think to dispose of under the label " reactionary " ; of the
restless type " neither to haud nor bind," who may be a Moses
with reawakened nomad instincts capable of leading a people
through the desert to a new Promised Land ; or, as is often
the case, of the recrudescent vicious type, who, if he cannot be
pardoned when we know all, can at least be the better dealt with
the better he is understood.

Another aspect of heredity has an obvious sociological signifi-
cance, the dark and intricate business of hybridisation or cross-
breeding, in regard to which biologists are beginning to see
some daylight. If we call mankind a species, we must admit
that there are many sub-species or " elementary species," and
that within these again there are minor groups of more or less
well-marked stocks, and that there are also somewhat divergent
groups or varieties. As in the past, so still there is no small
amount of exogamy or cross-breeding, and it is much to be
desired that the whole matter should be carefully investigated.
How far is it true that cross-breeding provokes an " epidemic
of variations," that it tends to induce " reversions," that the
older stock is prepotent over the younger, and so on ? Ac-
cording to De Vries it is very generally true of plants, that a
retrogressive variety {i.e. one different from the parent species
in the marked absence of some character) will, if crossed by a
typical member of the species, produce offspring which return
to the original type. Is there any analogue of this " false
atavism or vicinism " in human kind ?

SOCIAL SIGNIFICANCE OF HEREDITY 529

One is tempted to speculate as to the possible sociological
interest of Mendel's Law, if it should be found to obtain in the
minglings of human races, but as yet we have not a sufficient
basis of fact. As we have seen, the inbreeding of hybrids of
peas, stocks, mice, etc., is followed by a splitting of the offspring
into true-breeding types like the two parents of the hybrids.
We may suggest that careful inquiry should be made as to the
results of inter-marriage among Eurasians, for if Mendel's Law
holds, there should be a sifting out of pure Asiatics and pure
Europeans, both probably more desirable than Eurasians, fine
mentally and physically as these often are.

There are still some who find satisfaction in pointing out
that as human evolution is par excellence a psychical evolution,
biological conclusions on the question of inheritance are irre-
levant, since they are based on the study of measurable physical
qualities. But those who would press this point must deal
with Professor Karl Pearson's " Huxley Lecture " for 1903,
" On the Inheritance of the Mental and Moral Characters in
Man, and its Comparison with the Inheritance of the Physical
Characters " (Joum. Anthropological Institute, xxxii. pp.
179-237). His method was to obtain for upwards of one thousand
families impartial data as to fraternal resemblance in physical
and psychical characters in school-children. His argument
was, " If fraternal resemblance for the moral and mental char-
acters be less than, equal to, or greater than fraternal resem-
blance for the physical characters, we may surely argue that
parental inheritance for the former set of characters is less
than, equal to, or greater than that for the latter set of char-
acters." His conclusion, after many years of investigation, was
that " the degree of resemblance of the physical and mental
characters of children is one and the same," or, more
concretely, " we inherit our parents' tempers, our parents'
conscientiousness, shyness, and ability as we inherit their
stature, forearm, and span." The psychical characters are

34

530

SOCIAL ASPECTS OF BIOLOGICAL RESULTS

inherited in the same way, and at the same rate as the
physical.

But one of the general points of this chapter may be illustrated
here. In proportion as we succeed in analysing out the biological
factors in our Natural Inheritance shall we see clearly what is
meant by " Social Heredity." What do we mean by it ? Not
merely that facts of family and stock inheritance may have
great social importance, whether they concern the history of
a dynasty or the physical deterioration of a proletariat ; not
merely that great biological generalisations, such as Filial Re-
gression, or the inverse ratio between rate of reproduction and
degree of individuation, have direct sociological relevancy ; not
merely that there are probably obscure laws of periodic re-
currence, such as " the law of generations "; we mean especially
that complex process by which much of what is most precious
to us appears to be sustained from generation to generation in
a social heritage, by tradition, conventions, institutions, laws,
and the whole framework of society itself. It is here that
the biologist leaves off, and the sociologist must come in.

§ 5. Directive Factors in Evolution

Selection. — Passing now to the directive factors in evolution
in contrast to those which are originative and conservative,
we find practical unanimity in recognising the importance of
selective processes. We use a plural phrase in protest against
the persistent fallacy of taking a narrow and crude view of what
occurs in many different modes, at many different levels, and
with very varied degrees of intensity.

Yariety of Modes, Levels, and Intensity in Selective Pro-
cesses.— As Darwin clearly indicated, the phrase " struggle
for existence " is to be taken in a wide and metaphorical sense.
In point of fact, it is in operation whenever and wherever the
degree of effectiveness of vital response is of critical moment,

PROCESSES OF SELECTION 53*

not merely in helping survival at the time, but in strengthening
foothold, increasing comfort, lengthening life, promoting re-
productive success, and so on.

It may be a miserable squabble around the platter of sub-
sistence, but it may be a gentle endeavour after well-being.
It may be prompted by " love " as well as by " hunger," using
both words in the widest sense ; it may be other-regarding as
well as self-preservative.

There may be struggle between foes of quite different natures,
e.g. birds of prey and vermin; competition between fellows of
the same kin, e.g. brown rat against black rat ; opposition
between the sexes (cf. courtship of spiders, in which the female
often devours the male, and human competition between male
and female doctors, clerks, etc.) ; self-assertion against the quite
indifferent, often merciless " weather " of the physical environ-
ment. The phases of "struggle " are as varied as life itself.

Interference with Natural Selection. — Not a few sociological
writers have echoed the warning of Herbert Spencer that modern
hygienic and therapeutic methods interfere with the natural
elimination of the weaklings whose survival consequently be-
comes a drag on the race, and there is doubtless some force in
the argument, especially if we could confine ourselves to an
entirely biological outlook. It appears to us, however, that
the practical corollary that we should cease from interfering
with natural selection, as the phrase goes, is as fallacious as it
is impossible, (i) It seems a little absurd to speak of, say,
the prevention of an artificially exaggerated infantile mortality
as if it were an interference with the order of nature. (2) Much
weakness which may readily become fatal is simply modificational,
due perhaps to lack of nutrition at a critical moment ; many
weakly children grow up thoroughly sound ; and even if we
do keep alive some whose constitutions are intrinsically bad,
we are at the same time saving and strengthening many whose
intrinsically good constitutions only require temporary shelter.

532 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

One enthusiast over microbic selection says : " The higher
the infantile death-rate which medicine so energetically combats,
the surer is the next generation of being purged of all weakly
and sickly organisms." But he omits to record the fact that
the infantile maladies also affect the intrinsically strong and
capable, and often weaken them, one might say, quite gratui-
tously. (3) Many of the microbic agents which thin our ranks
are very indiscriminate in their selection, and even if we believed
that in warring against microbes we are eliminating the elimi-
nators who have made our race what it is — as the enthusiastic
apologists for Bacteria declare — it is surely open to us to put
other modes of selection into operation. It were a sad confession
of incapacity if man could not select better than bacteria. (4)
Finally, since we cannot keep to the biological outlook, is it
ridiculously old-fashioned to plead that even when the physical
constitution is miserable, the weakling may be a national asset
worth saving, for its mental endowment, for instance, and for
other reasons ? That the weakling is to be allowed to breed more
weaklings if it can, is another matter. Every one agrees that the
reproduction of weaklings should be discouraged in every feasible
way — in every way compatible with rational social sentiment.

Multiplication of the Unfit. — We have to face a more difficult
problem when we consider the multiplication of the relatively
unfit. It is, we suppose, true that these have now a better chance
to survive and multiply than at any other epoch in the history
of our race. Especially perhaps in Britain do the weeds tend
to increase more rapidly than the flowers. It is impossible
to ignore the seriousness of the outlook. If, as Professor
Karl Pearson points out, 25 per cent, of the married couples
in Britain produce 50 per cent, of the next generation, how
much depends on the character of that 25 per cent. From the
most diverse regions we have reports of the alarming increase
of what not even the most optimistic can regard as other than
undesirables. In a fine climate and in a period of cheap food

MULTIPLICATION OF THE UNFIT 533

and high wages, the ratio of defectives — including deaf and
dumb, lunatics, epileptics, paralytics, crippled and deformed,
debilitated and infirm — is said to have increased from 5-4 per
1,000 above 15 years in 1874 to n*6 in 1896. Particular statis-
tics, such as these, may be open to criticisms, but there are scores
of similar statistics from almost every civilised country, and
there is no escape from the general result. As Emerson said,
we are breeding men with too much guano in their composition.

A Host of Practical Suggestions. — Needless to say, many
of the inquirers who have become impressed by the facts have
not been backward in making practical suggestions, which
might be arranged, if one had time, on an inclined plane. Some,
more trustful in natural selection than in any human device,
have taken up an extreme laissez-faire position, which, as human
society is constituted, is quite untenable. The other day we
passed by a rock village in Italy which was not so long ago in
the direst sense left to itself when cholera broke out within it,
sealed up, as it were, like a bee-hive diseased — but it is idle to
talk of leaving natural selection free play in any civilised com-
munity. Others, going to the opposite extreme, have advocated
what may be called surgical methods for both sexes to a degree
that is more than spartan. Between these extremes we find all
manner of suggestions. We need only refer to the marriage
examination and certificate system which is being increasingly
discussed — to much profit, it seems to us — in Germany ; the
segregation schemes which suggest that those obviously unfit
who have to fall back on the State {i.e. the relatively fit citizens)
for support should forfeit the right to reproduce, for which,
again, there is much to be said ; and the wise and gentle con-
structive eugenic proposals with which Mr. Galton has made
us all familiar.

Probably every one who is at all aware of the facts will admit
the desirability of giving attention to eugenics or the improve-
ment of the human breed, positively, if possible, in the way of

534 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

increasing the numbers of the effective, or negatively, in the
way of trying to reduce the multiplication of the unfit. Inquiry
into these subjects is comparatively new, discussion of them
is still rare, a superstitious attitude towards them is still very
common — we cannot tell what may come about when a con-
science relative to these things is developed, or in the wake of
great social changes.

Meanwhile, convinced as we are as to the hopefulness of various
forms of eugenic selection, we cannot but enter a protest against
the impetuous recommendations of some who suggest methods
of surgical elimination to an extent that is almost grotesque.

We would suggest the following cautions :

(i) We are far from being omniscient in regard to variations.
Some deteriorative changes are well known, and history has
given its verdict against them. There are surely few who
would encourage the marriage of those suffering from syphilis,
marked tuberculosis, senility, diabetes, deaf-mutism, chronic
nephritis, haemophilia, organic heart-disease, contracted pelvis,
and the like, Every one agrees that there should be no breeding
from epileptics, paralytics, lunatics, and so on, but many other
variations are unknown quantities. The unpromising bud may
burst into a fair flower. Virchow's thesis of the pathological
origin of some variations is not to be lightly brushed aside.
There is an optimism of pathology. No one would propose
to encourage the breeding of doubtful variants on the off-chance
of an occasional genius, but the race owes much to weaklings
none the less. A man belonging to a family which has been
manufacturing cystin for three generations should not have
children — he would not pass the German marriage examination
—but in himself he may be a very valuable national asset.
Some of the lists given by the social surgeons are quaint in
their unpracticality ; thus one includes " a criminal taint "—
as if that were a rarity, or as detectable as deaf-mutism— and
another includes "pauperism."

ELIMINATIVE MEASURES 535

(2) Is there not much to be said in support of the view that
many of the unfit are only modificalionally unfit — simply ill-
nourished plants in the crowded garden ? Are we not apt to
underrate the plasticity of human nature and the ready re-
pressibility of hereditary items ? Is there strictly speaking
such a thing as a transmissible disease, apart from pre-natal
infection ? Is not a predisposition to disease the most that
is transmitted ? Are not many criminals mere anachronisms ? —
people out of time or out of place, who require not incarceration
or worse, but only transplanting. Records of Jukes' families, or
of the woman whose 709 descendants cost the state a quarter of
a million are impressive, but one has to remember the modifi-
cational effect of social ostracism. One can hardly doubt that
the high rate of criminals among illegitimate children — said to
form one-tenth of the births in Germany — -is artificially created.
In passing we may note, as of interest, the formation of a League
in Germany to protect not merely illegitimates, but their
mothers.

(3) While it is undoubtedly true that strongly developed evil
characters may have a great power of persistence even beyond
the third and fourth generation, just as strongly developed good
characters may have, is there not a tendency to exaggerate
the consequent tainting of stock ? Dr. Archdall Reid has ex-
pounded the tendency of the uncontrolled alcoholic type to work
itself out, and the same is true of other types. If germinal
selection expresses a reality, we should expect taints to be
swamped, just as excellences often are.

(4) We do not know whether Mendelian phenomena of in-
heritance occur in man, but if they do, we should be slow to say
that it is not possible to bring a clean thing out of an unclean.
When an immune wheat plant and a non- immune are crossed,
the resulting hybrids are all susceptible to rust. When these
are self-fertilised, i.e. inbred, they produce seed from which
appear " rusty " plants and immune plants in the ratio of 3 : 1.

536 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

It may be that there are analogous phenomena awaiting dis-
covery in the case of man.

Our general position is that among civilised men the sentiments
of solidarity and sympathy are too precious and too strong to
admit of much social surgery, or of the more thoroughgoing
methods of reproductive elimination, which moreover assume
the possession of more science than is really available. On the
other hand, there seems much to be said for restricting the repro-
duction of undesirables who fall back on the State for support,
for some sort of marriage-tests, for developing a social prejudice
against reproduction among the victims of markedly bad in-
heritance, for a fuller and deeper recognition of woman's rights
both as to mating and maternity, for eugenic devices such as
Mr. Gaiton has suggested, and so on. But there is one other
suggestion we wish to try to express.

Militarism. — There is apt to be a vicious circle in our argu-
mentation over this difficult problem. To uphold our national
supremacy, it is said, we require, inter alia, a military organisa-
tion with alert scouting intelligence, not only among the officers
but in the rank and file. We are ceasing to breed this alert
scouting intelligence in sufficient numbers ; the nation is spawning
incapables. We cannot relax one spine of our bristling national
belligerence, for we have all our teeming millions to keep alive.
But the question rises whether it is not in great part our pre-
occupation with " Kriegspiel " that is responsible for that
relatively exaggerated multiplication of the repressed and non-
individuated, and for that relatively exaggerated infertility of
the fittest, or of what we think to be the fittest. If we indulged
in an era of " Friedenspiel," which may be even now approaching
like a long-delayed spring-time, might not the sociological
changes that ensued solve the problem which biologically seems
so hopeless ?

Statistics of what is often called " racial deterioration " are
only too plentiful, and though they require more critical analysis

MILITARISM 537

and more guarded treatment than they usually obtain, there
is no gainsaying that there are grim facts behind them ; and
without trying to make a scapegoat of militarism, it is difficult
to silence the thought that just as Napoleon reduced the physical
stature of the French nation, just as the wars of the Roman
Empire rooted out the best and left Rome to a mob who made
gods in their own image, so we are -now paying the biological
bill for past wars. Apart from the multiplication of " the
social precipitate " inter se, is there not a persistent deposit of
more precipitate from above, and may not the deterioration,
which the military examinations, for instance, reveal, be in great
part due to the crushing burden of militarism itself ? The
suggested surgical methods to eliminate the " precipitate "
from reproduction — if not from more — may be a little away from
the point if the persisting social conditions are meanwhile
securing a continuous deposit of more " precipitate."

If all the best heads in a deer-forest — such a dramatic illustra-
tion of reversed selection (" ob-selection ") in many ways — are
persistently shot down, the race of deer cannot keep up to the
desired standard ; if through militarism, and the spirit behind
it, a human breed is being left for the greater part of its con-
tinuance to the less fit, it will not be surprising if history repeats
itself, and " Vir " is replaced by a mere " Homo." When we
contemplate any national decadence — that of the Roman
Empire is at a convenient distance — we may interpret the facts
biologically, as an American zoologist, Professor D. S. Jordan,*
has recently done, in terms of the reversed selection which
spoiled the human harvest, or psychologically, in terms of the
changed ideas and ideals of the average man, or sociologically,
in terms of variations in the organisation of the societary form ;
but, fundamentally, these interpretations must be capable of
a unification, and this it is particularly the task of the sociologist

* See " The Human Harvest " (American Philosophical Society,
April 1906; also separately, Boston, 1907, pp. 122).

538 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

to work out. What more pressing problem has he than that of
discovering what factors are now threatening to bring about for
us results analogous to those which led to the Decline and Fall
of the Roman Empire ? Preoccupation with the biological
outlook — the breeder's point of view — will undoubtedly lead
to fallacy upon fallacy, to the " materialisms " to which we have
already referred ; on the other hand, an ignoring of the biological
point of view means a deliberate rejection of the order of facts
which we can most precisely measure and test. Moreover, the
commonplace is apt to be forgotten, that when changed ideas
and ideals find physical embodiment in flesh and blood,
they acquire, ipso facto, an inertia which no belated conversion
on the psychical plane can ever do away with. Even Pasteur
could not add " the cubit of stature " which Napoleon lopped
off Frenchmen.

Relative Infertility of more Individuated Stocks. — Let us
briefly refer to the other aspect of the fertility problem. The
biologist accustomed to interpret great results in terms of
selection and isolation acting on germinal variations, is not
likely to be lacking in faith in what may be accomplished by
attention to eugenics. But he finds it difficult to dispel the
shadow cast by the fact of the relatively great infertility of what
we believe to be types and stocks of high social efficiency. Over
and over again, in the history of mankind, elect castes — true
aristocracies — have arisen, only to disappear again in sterility,
or in the course of inter-societary struggle. Even if the latter
doom be averted by more evolved social organisation and racial
pacification, how are we to face the fact of the dwindling fertility
of what we believe to be the better stocks ? It may be that the
relatively recent diminution of the birth-rate among skilled
workmen and the like is partly modificational or artificial, an
adaptation to altered social conditions ; but what can we say
of the generally low fertility of the most individuated stocks ?

The factors which make towards this result are probably

RELATIVE INFERTILITY OF FITTEST

539

manifold. There are probably, as Spencer maintained, auto-
matically working physiological and psychical factors which
lessen reproductivity as individuation increases. It may be that
hyper-nutrition, sexual vice, the frequent absence of love
marriages, operate in the same direction ; it seems difficult to
doubt that selfish celibacy and selfish non-maternity are in
part to blame ; and there are all sorts of possible factors down

Fig. 47. Diagram illustrating the relation between Reproduc-
tion and Individuation (from " Evolution of Sex ")

Let the perpendiculars above the line A B denote the increasing degree
of total individuation of a series of forms 1, 2, 3, 4, 5, 6 (say Worm,
Fish, Frog, Bird, Man, Elephant). Similarly, let the perpendiculars
from the line C D represent the rate of multiplication of the same
forms. The curves joining the apices of the two sets of perpendiculars
indicate, by their inverted symmetry, the inverse ratio of individuation
and rate of multiplication.

to the marriage of heiresses who are often the sole survivors
of a dwindling family. Dr. Ireland points to the significant
fact that some of the high castes of India (Brahmins and Rajputs)
who are most exclusive in their marriages do not show the usual
dwindling tendency, which may be correlated with the circum-
stance that they are mostly poor and abstemious.

540 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

Is there any consolation in the thought that quality is always
safe against quantity, that eagles need never fear the frogs who
spawn, that an inheritance may persist socially even when a
lineage becomes extinct biologically ? Is there any warrant
for supposing that the race can continue producing from new
soil crop after crop of highly individuated types, each in its
turn destined to die out as a penalty for its own efficiency ?
Is there any truth in the inference that failure in reproductive
power is an expression of nature's verdict against dis-social
isolation of privileged classes, against every self-contradictory
denial of the solidarity of the social organism ? In any case, is
there not need for getting rid of a prudery of selfishness which
keeps some of the fitter types from recognising that they have
another contribution to make to the race besides their work ?

It should be borne in mind that precise thinking on the subject
of fertility is still very uncommon, that there is no general
awareness that the details of our dwindling birth-rate are sug-
gestive of disaster, and that very few have what may be called
an awakened conscience on the subject. The most common-
sense precautions are quite disregarded. Falling in love is out
of fashion, and almost non-mammalian types grow commoner.
In a sense, though it is a pity, it may be just as well that they
should die out. And who, for instance, ever thinks of the wise
Frenchman's saying, " My father was a farmer, I am a Professor,
my son must be a farmer again " ? But, apart from the slow
diffusion of an interest in eugenics, perhaps the most promiseful
line of activity is that of trying to promote social (including of
course ethical) variations which may bring about more whole-
some biological conditions.

Isolation. — The only other directive evolution- factor that
biologists are at all agreed about besides selection, is isolation — a
general term for all the varied ways in which the radius of
possible inter-crossing is narrowed. As expounded by Wagner,
Weismann, Romanes, Gulick, and others, isolation takes many

ISOLATION

54T

forms — spatial, structural, habitudinal, and psychical — and
it has various results.

It tends to the segregation of species into sub-species, it
makes it easier for new variations to establish themselves, it
promotes prepotency, or what the breeders call " transmitting
power," it fixes characters. One of the most successful breeds
of cattle (Polled Angus) seems to have had its source in one
farm-steading, its early history is one of close inbreeding, its
prepotency is remarkable, its success from our point of view
has been great. It is difficult to get secure data as to the results
of isolation in nature, but Gulick's recent volume on the subject
abounds in concrete illustrations, and we seem warranted in
believing that conditions of isolation have been and are of
frequent occurrence.

Reibmayr has collected from human history a wealth of
illustrations of various forms of isolation, and there seems
much to be said for his thesis that the establishment of a successful
race or stock requires the alternation of periods of inbreeding
(endogamy) in which characters are fixed, and periods of out-
breeding (exogamy) in which, by the introduction of fresh blood,
new variations are promoted. Perhaps the Jews may serve to
illustrate the influence of isolation in promoting stability of
type and prepotency ; perhaps the Americans may serve to
illustrate the variability which a mixture of different stocks
tends to bring about. In historical inquiry into the difficult
problem of the origin of distinct races, it seems legitimate to think
of periods of " mutation " — of discontinuous sporting — which led
to numerous offshoots from the main stock, of the migration of
these variants into new environments where in relative isola-
tion they became prepotent and stable.

Conclusion. — Our general position is that when we pass from
organisms to human societies, the whole venue changes so much
that we have to be very careful in our application of biological
formulae, (i) Thus, in regard to processes of selection, we have

542 SOCIAL ASPECTS OF BIOLOGICAL RESULTS

to recognise the intervention of rational selection as an ac-
celerant or as a brake on natural selection. (2) When a society
deliberately sets to work to select discriminately among the in-
dividualities which make up its own body politic, we have to do
with an infinitely subtler process than that observed when a
breeder selects in his stock, or when the physical environment
eliminates the ill-adapted members of a race. (3) There is in
human affairs a much more prominent occurrence of inter-
group, inter-societary, or inter-racial selection, which introduces
fresh complexities, e.g. that in the conflict of races the apparent
victors are sometimes, in some measure, conquered by the
vanquished.

In all selectionist proposals we have to face the difficulty
of agreeing what we are to select for. If selection processes
are to succeed, they must be consistent. As to the negative
ideal of trying to lessen the precipitate of undoubted incapables,
all will agree ; but the positive ideal of working towards evolu-
tion is necessarily vague, meaning different things to different
people. It will be generally admitted, however, that if we are
to avoid fallacious endeavour, our ideal must include " eutopias "
and " eutechnics " as well as " eugenics," and that it must be
not merely biological but distinctively sociological in its
outlook

BIBLIOGRAPHY

N.B. — This bibliography is simply representative, not in any way exhaustive. It may be
supplemented by those of Bateson (1909), Baur (1911), Delage (1903), Goldschmidt (1911),
Haecker (191 1).

The books which should be read first are indicated in the subject-indes under the title
" Best Book; to begin with."

1896-8. Ackermann, A. : Tierbastarde. Zusammenstellung der
bisherigen Beobachtungen. Abh. Vereins Naturkunde
Kassel, XL. pp. 103-121 ; xliii. pp. 1-79.
1 90 1. Adami, J. George : An Address on Theories of Inheritance,
with Special Reference to the Inheritance of Acquired
Conditions in Man. Brit. Med. Journal, No. 2109, June 1.
pp. 1317-1323, 2 figs. [Criticises Weismann's position
and proposes a modified theory.]
18x5. Adams : A Philosophical Treatise on Hereditary Peculiarities.
2nd ed. 1815.
Alcoholism : British Journal of Inebriety.
1904. Allen, G. M. : The Heredity of Coat-Colour in Mice. Proc.
Amer. Acad. xl. pp. 61-163, 7 figs.
y 1893. Ammon, Otto : Die natiirliche Auslese beim Menschen.
Fischer, Jena. pp. 326. [A valuable work.]

l%95- Die Vererbungerworbencr Eigenschaften. Nat. Wochen-

schr. Berlin, x. pp. 386.

1896. Der Abanderungsspielraum : ein Beitrag zur Theorie

der naturlichen Auslese. Nat. Wochenschr. xi. pp. 137-143,
149-155. Also separately, Dummler, Berlin, pp. 54.

/ 1900. Die Gesellschaftsordnung und ihre naturlichen Grund-

lagen. Fischer, Jena.
1899. Anthony, R. : Bull. Soc. Anthrop. Paris, x. 1899, pp. 303.
[Progeny of Manx cat.]
Archiv fur Entwickelungsmechanik. [Important Journal.]
Archiv fiir Rassen- und Gesellschaftsbiologie. [Important
Journal.]
1^-1892. Arlidge, J. T. : The Hygiene, Diseases and Mortality of
Occupations. Percival, London, pp. xx + 568.

543

544

BIBLIOGRAPHY

1890. Arreat, L. : Recents Travaux sur l'Heredite. Rev. Philos.

xxix. pp. 399-4 1 9-
1872. Askenasy, E. : Beitrage zur Kritik der Darwin'schen Lehre.

8vo, Leipsic.

1909. Baehr, W. B. von : Die Oogenese bei einigen viviparen
Aphiden und die Spermatogenese vora Aphis saliceti.
Arch. f. Zellenforschung, 111.

1894. Bailey, L. H. : Neo-Lamarckism and Neo-Darwinism. Amer.

Natural, xxviii. pp. 661-678.
1896. The Survival of the Unlike : a Collection of Evolution

Essays suggested by the Study of Domestic Plants. New

York, pp. 515, 3rd ed., 1899.
1904. Plant-Breeding : Five Lectures upon the Amelioration

of Domestic Plants. 3rd ed., MacMillan Co., New York.

PP- 334- [A- luminous study of plant-breeding with an

exhaustive bibliography.]
1893. Baillet, C. : Quelques Mots sur les Croisements dits au

Premier Sang, chez les Animaux Domestiques. Mem.

Acad. Toulouse, v. pp. 128-143.

1895. Du Croisement continu dans les Races d'Animaux

Domestiques. Mem. Acad. Toulouse, vn. pp. 141-160.

i8gg. De l'Atavisme et de l'Origine des Reproducteurs chez

les Principaux Especes d'Animaux Domestiques. Mem.

Acad. Toulouse, x. pp. 314-341.
1885. Balbiani, E. G. : Contribution a l'Etude de la Formation des

Organes Sexuels chez les Insectes. Rec. Zool. Suisse, 11.

PP- 525-588, 2 pis.
j 888. Les Theories Modernes de la Generation et de l'Heredite.

Revue Philos. No. 12, December.

1896. Baldwin, J. Mark. A New Factor in Evolution. Amer.

Natural, xxx. pp. 441-451, 536-553- Cf. Science, iv. p. 139.

j 896. Dictionary of Philosophy and Psychology.

!8g6. Heredity and Instinct. Science, m. pp. 438-441,

558-561. Cf. p. 669.
j 896. Physical and Social Heredity. Amer. Natural, xxx.

pp. 422-428.

j897. Organic Selection. Nature, lv. p. 558.

I9o2. Development and Evolution. Including Psychophysical

Evolution, Evolution by Orthoplasy, and the Theory of

Genetic Modes. MacMillan Co., New York, pp. 395.

BIBLIOGRAPHY 545

1907. Baldwin, J. Mark : Social and Ethical Interpretations of
Mental Development. 4th ed.

1890. Ball, W. Platt : Are the Effects of Use and Disuse In-
herited ? London, 156 pp.

1897. Ballantyne, J. W. : Teratogenesis : an Inquiry into the
Causes of Monstrosities. History of the Theories of the
Past. Oliver & Boyd, Edinburgh, pp. 62.

1904. Ballowitz, E. : Das Verhalten der Muskeln und Sehnen bei

Hyperdaktylie des Menschen im Hinblick auf die Aetiologie
dieser Missbildung. Verh. Anat. Ges. ; Anat. Anzeig.
xxv. Ergdnzungsheft, pp. 124-135, 3 figs. [Hyperdacty-
lism is due to a teratological splitting of the primordium,
and is not atavistic]
-1885. Bambeke, Ch. van : Pourquoi nous ressemblons a nos Parents.
Bruxelles. pp. 48. Also in Bull. Acad. Belg. x. pp.
901-944.

1 891. Barfurth, D. : Versuche zur funktionellen Anpassung.

Arch. mikr. Anat. xxxvu.

1894. Die experimentelle Regeneration uberschiissiger Glied-

massenteile (Polydactylie) bei den Amphibien. Arch.
Entwickelungsmech. 1.

1908. Experimentelle Untersuchung iiber die Vererbung der

Hyperdactylie bei Huhnern, 1. Der Einfluss der Mutter.
Arch. Entwickelungsmechanik, xxvi.

1909. 11. Der Einfluss des Vaters. Ibid. xxvn.

1905. Barrington, Amy ; Lee, Alice ; and Pearson, Karl :
On the Inheritance of Coat-Colour in the Greyhound.
Biometrika, in. p. 245.

1906. Barrington, A., and Pearson, K. : On the Inheritance of
Coat-Colour in Cattle. Part I. Shorthorn Crosses and
Pure Shorthorns. Biometrika, iv. pp. 427.

1900. Barthelet, M. : Experiences sur la Telegonie. Comptes
Rendus Acad. Sci. Paris, cxxxi. pp. 911-912.

1900. Bataillon, E. : Blastotomie Spontanee et Larves Jumelles
chez Petromyzon planeri. Comptes Rendus Acad. Sci.
Paris, cxxx. p. 1201. Cf. also Pression Osmotique de
l'CEuf et Polyembryonie Experimental . Ibid. pp. 1480-
1482.

1894. Bateson, W. : Materials for the Study of Variation. London,
598 pp., 209 figs.

1901. Heredity, Differentiation, and other Conceptions ol

Biology. Proc. Roy. Soc. London, lxix.

35

s

546 BIBLIOGRAPHY

1901. Bateson, W. : Introduction to Translation of Mendel's

Versuche iiber Pflanzenhybriden. Joum. R. Horticultural
Soc. xxvi. part 1.

1902. Mendel's Principles of Heredity, a Defence. With a

Translation of Mendel's Original Papers on Hybridisation.
University Press, Cambridge, pp. xii + 212. [Exposition
and corroboration of Mendel's conclusions.]

1902. and Collaborators, Reports to the Evolution Committee,

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Miss Saunders and R. C. Punnett, 53 pp.

1903. The Present State of Knowledge of Colour-Heredity in

Mice and Rats. Proc. Zool. Soc. London, pp. 71-99.

1904. Presidential Address, Section D, British Association, 1904,

Rep. Brit. Assoc. (Cambridge Meeting), pp. 574-589. [A
valuable introduction to the current study of variation
and heredity.] See also Nature, lxx., 1904, p. 406. (Cf.
pp. 538, 601, 627.)

1904. An Address on Mendelian Heredity and its Applica-
tion to Man. Brit. Med. Joum. 11., July 14, 1906,
pp. 60-67.

1906. Albinism in Sicily. Biometrika, iv. p. 231.

igo6. The Progress of Genetics since the Rediscovery of Men-
del's Papers. Progressus Rei Botanicce, 1.

1907. Facts limiting the Theory of Heredity. Science, xxvi.

pp. 649-60. Proc. Seventh Internal. Zool. Congress, 1907
(published 1912), pp. 306-319.

1908. The Methods and Scope of Genetics, pp. 49. [An in-
augural lecture.]

1909. Mendel's Principles of Heredity. [The most important

statement of what has been achieved by the experimental
study of genetics.]

1905. Bateson, W., and Gregory, R. P. : On the Inheritance

of Heterostylism in Primula. Proc. Roy. Soc. lxxvi.

p. 581.
1908. Bateson, W., and Punnett, R. C. The Heredity of Sex.

Science, xxvn.
1859. Baudement, E. : Art. Atavisme in I' Encyclopedic Pratique

de I'Agriculteur (de Moll et Gayot), n. 1859. [Cited by

Sanson as an early and good exposition.]
1907. Baur, E. : Untersuchungen iiber die Erblichkeitsverhalt-

/

f

BIBLIOGRAPHY 547

nisse einer nur in Bastardform lebensfahigen Sippe von
Antirrliinum ma jus. Ber. Deutsch. Bot. Ges. xxv.
pp. 4.| 2.

1910. Vererbungs- und Bastardierungsversuche mit Antirrhi-
num. Zeitschr. induktive Abstammangs- und Vererbungs-
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/

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1900. Some Aspects of Recent Biological Research. The

International Monthly, n. pp. 74-93.

Iq02. Mendel's Principles of Heredity and the Maturation of

the Germ-cells. Science, xvi. p. 991.

jgo^, The Chromosomes in relation to the Determination of

Sex in Insects. Science, xxn. Oct. 20.

1906. Mendclian Inheritance and the Purity of the Gametes.

Science, xxm. pp. 11 2-1 13.

BIBLIOGRAPHY 601

1906. Wilson, E. B. : Studies on Chromosomes : III. The Sexual
Differences of the Chromosome Groups in Hemiptera, with
some Considerations on the Determination and Inheritance
of Sex. Journ. Exper. Zool. in. pp. 1-39.

1906. A New Theory of Sex-Production. Science, xxm. pp.

1 89-1 91. [Re R. Hertwig's theory (1905).]

1907. Sex Determination in Relation to Fertilisation and

Parthenogenesis. Science, xxv. pp. 376-379.

1909. The Cell in Relation to Heredity and Evolution. In

" Fifty Years of Darwinism," pp. 92-113.

1909. Recent Researches on the Determination and Heredity

of Sex. Science, xxix. pp. 53-70.

1 910. The Chromosomes in Relation to the Determination of

Sex. Science Progress, v. pp. 570-592.

1896. Wilson, Gregg : Hereditary Polydactylism. Journ. Anat.
Physiol, xxx. pp. 437-449. [Theory of Polydactylism,
some interesting cases, bibliography.]

1908. Wilson, J. : Mendelian Characters among Short-horn Cattle.

Scientific Proc. R. Dublin Soc. xi.
The Origin of the Dexter- Kerry Breed of Cattle. Scientific

Proc. R. Dublin Soc. xn.
1910. The Inheritance of Coat-Colour in Horses. Scientific

Proc. R. Dublin Soc. xi.
1894. Wilson, W. P. : The Influence of External Conditions on

Plant Life. Wood's Holl, Biol. Lectures, n. pp. 163-

184.
1890. Windle, B. C. A. : Teratological Evidence as to the Heredity

of Acquired Conditions. Journ. Linn. Soc. xxm. See

also Nature, XL. p. 609.
1898. Wolff, E. : Beitrage zur Kritik der Darwin'schen Lehre.

Leipsic.

1908. Woltereck, R. : Ueber natiirliche und kunstliche Varietat-

bildung bei Daphniden. Verh. Deutsch. Zool. Ges.

1909. Weitere Experimented Untersuchungen uber Artverand-

erungen, speciell uber das Wesen quantitativer Artunter-
schiede bei Daphniden. Verh. Deutsch. Zool. Ges. pp.
1 10-172. [Important experiments on variation in pure
lines in Daphnids.]

1905. Wood, T. B. : Note on the Inheritance of Horns and Face
Colour in Sheep. Journ. Agric. Sci. 1. pp. 364-365, 1 pi.

1908. Wood, T. B., and Punnett, R. C. : Heredity in Plants and
Animals. Trans. Highland Agric. Soc. Scotland.

602 BIBLIOGRAPHY

1902-3. Woods, F. A. : Mental and Moral Heredity in Royalty.

Reprinted from Pop. Sci. Monthly, pp. 85. [A scholarly

series of studies.]
1903. Woods, F. A. : Mendel's Laws and some Records in Rabbit

Breeding. Biometrika, n. pp. 299-306.
1906. The Non-Inheritance of Sex in Man. Biometrika, v.

PP- 73-78.
1906. Mental and Moral Heredity in Royalty. A statistical

study in history and psychology. New York, pp. 312^

[A scholarly inquiry into the part heredity plays in

determining mental and moral character.]

1897. Yule, G. Udny : On the Theory of Correlation. Journ. Roy.
Statistical Soc. lx. pp. 1-44.

1902. Mendel's Laws and their Probable Relations to Intra-

racial Heredity. The New Phytologist, l. pp. 193-207,
222-238.

1912. Introduction to the Theory of Statistics. London, 1912.

/ 1878. Yung, E. : Contributions a l'Histoire de l'lnfluence des
Milieux Physiques sur les Etres Vivants. Arch. Zool.
Exper. vii. pp. 251-282, and Ibid. (1883) pp. 31-55.

/ 1885. De l'lnfluence des Variations du Milieu physico-chimique

sur le Developpement des Animaux. Arch. Sci. Phys.
Nat. xiv. pp. 502-522. See also Ibid. vol. vii. (1882),
p. 225 ; vol. 1. (1879), p. 209.

1908. Zederbauer, E. : Versuche liber Vererbung erworbener
Eigenschaften bei Capsella bursa pastoris. Oesterreich.
Bot. Zeitschrift.

1908. Zeitschrift der indukthe Abstammungs- und Vererbungslehre.
[Important Journal.]

1886. Ziegler, Ernst. : Konnen erworbene pathologische Eigen-
schaften vererbt werden, und wie entstehen erbliche
Krankheiten und Missbildungen ? 8vo, Jena. pp. 44.
From Beitrdge zur pathologischen Anat. und Physiol.
(Ziegler and Nauwerck), Bd. 1. [A valuable deliverance
by a renowned pathologist.]

1889. Die neuesten Arbeiten liber Vererbung und Abstam-

mungslehre und ihre Bedeutung fur die Pathologic Ibid.
vol. IV.

1894. Ziegler, H. E. : Die Naturwissenschaft und die Sozialdemo-
kratische Theorie. Stuttgart, pp. 252.

BIBLIOGRAPHY 603

1902. Ueber den derzeitigen Stand der Descendenzlehre in der

Zoologie. Fischer, Jena.

1904. Ziegler, H. E. : Der Begriff des Instinktes einst und jetzt.

Festschrift zu Weismann. Zool. Jahrb. Supplement vn.

1905. Die Vererbungslehre in der Biologic Fischer, Jena.

pp. 74, 2 pis. and 9 figs. [The best short introduction to
a study of the present position of theories of heredity and
inheritance.]

1906. Die Chromosomen-Theorie der Vererbung in ihrer

Anwendung auf den Menschen. Archiv. Rassen-Gesellsch.
Biologie, ill. pp. 797-812.

1910. Die Streitfrage der Vererbungslehre Naturwiss. Wochen-

schrift, ix.

SUBJECT-INDEX TO BIBLIOGRAPHY.

Abnormalities

Ballantyne (1897)

Ballowitz, Hyperdactylism
(1904)

Castle (1906)

Eugenics Laboratory Memoirs.

Farabee (1905)

Fere (1899)

Mehely (1905)

Rohde (1895)

Ryder (1893)

Sutton (1886)

Wilson (1896)
Accessory Chromosome

McClung (1902)

Stevens (1905)

Sutton (1902)

Wilson (1905)
Acquired Characters

Adami (1901)

Amnion (1895)

Bailey (1894)

Baldwin (1896)

Ball (1890)

Bemmelen (1890)

Bordage (1909)

Brock (1888)

Brooks (1896, 1899)

Brown-Sequard (1 869-1893)

Butler (1878, 1879)

Collins (1891)

Costantin (1901)

Cunningham (1892, 1895, 1896,
1908)

Darwin, F. (1908)

Delage (1903)

Acquired Characters (contd.)
Dendy (1903, 1912)
Detmer (1887)
Detto (1904)
Dingfelder (1887)
Du Bois-Reymond (188 1)
Eigenmann (1909)
Eimer (1888, 1890)
Elliot (1892)
Emery (1893)
Errera (1899)
Fischer (1901)
Gadow (1890)
Giard (1890, 1904)
Haeckel (1898)
Hartog (1889, 1893)
Henslow (1895)
His (1874)
Hoffmann (1888)
Hutton (1899)
Hyatt (1882, 1889, 1894)
Kammerer (1908)
Kidd (1892)
Kollmann (1887)
Kropotkin (191 2)
Lane (1887, 1888)
Lankester (1890)
Miles (1892)
Morgan, Lloyd (1896)
Morgan, T. H. (1907)
Ornstein (1889)
Orth (1887)

Osborn (1889, 1891, 1895)
Packard (1894)
Pauly (1905)
Poulton (1894, 1897)

605

6o6

SUBJECT-INDEX TO BIBLIOGRAPHY

Acquired Characters (contd.)
Reh (1894)

Reid, (1897, 1905, 1910)

Rignano (1907, 191 1)

Rohde (1895)

Romanes (1892-1897)

Rosenthal (1889)

Russell (1909)

Ryder (1889)

Semon (1905, 1907, 1910,
1911)

Spencer (1864, 1899)

Sumner (1910)

Thomson (1899, 1906)

Wallace (1889, 1893)

Weismann (1888, 1895, 1902)

Wettstein (1902, 1903)

Wilckens (1S93)

Windle (1890)

Ziegler, E. (1886)

Ziegler, H. E. (1905)
\lcoholism

British Journal of Inebriety

Hyslop (191 1 )

Mott (191 1)

Reid (1900)
- Ancestral Heredity, Law of

Darbishire (1909)

Galton (1897, 1898)

Pearson (1898, 1903A)

Ziegler, H. E. (1905)
Atavism. See Reversion

Baudement (1859)

Kohlbrugge (1897)

Mann (1893)

s

Best Books to begin with
Bailey (1904)
Bateson (1902, 1908)
Castle (191 1 )
Cuenot (191 1 )
Darbishire (191 1)
Dendy (1912)
Doncaster (1910)
Delage (1903)
Galton (1869, 1889)
Goldschmidt (191 1)

Best Books to begin with (contd.
Haecker (191 1)

Hertwig (1906)

Jordan (1898)

Lock (1906)

Lotsy (1906)

Martius (1905)

Morgan, C. I.. (1896, 1900)

Morgan, T. H. (1907)

Punnett (1905, 1907, 191 1)

Reid (1905)

Thomson (1909)

Vernon (1903)

Watson (19 1 2)

Weismann, (1891, 1892, 1893
1904)

Wilson (1900)

Ziegler, H. E. (1905)
Bibliography

Bailey (1904)

Bateson (1902, 1909)

Baur (191 1 )

Delage (1903)

Goldschmidt (191 1)

Haecker (191 1)

Osborn (1893)

Thomson (1889)
Blending

Castle (191 1)

East (1910)

Emerson (1910)
Breeding

Bateson (1909)

Biffen (1905, 1907, 1909)

Davenport (1907)

Fruwirth (1909)

Midler (1905)

Wilson (1908)

Wood and Punnett (1908)
Cattle

Wilson (1908)
Cats

Anthony (1899)

Doderlein (1887)

Doncaster (1904)

Torrey (1902)

SUBJECT-INDEX TO BIBLIOGRAPHY

607

Colour

Allen (1904)

Barrington, Lee, Pearson

(I9<>5)

Bateson (1903, 1909)

Castle (1905)

Castle and Allen (1903)

Crampe (1885)

Cuenot (1902, 1903, 1904)

Davenport (1904, iqo8)

Doncaster (1905)

Fischer (1874)

Haacke (1895)

Hurst (1906)

Jordan (191 1)

Noorduyn (1908*

Poulton (1892)

Riddle (1909, 1910)

Staples-Browne (1908)

Wheldale (1909, 19 10)

Wood (1905)
Consanguinity

Bos (1894)

Boudin

Bourgeois (1857)

Castle (191 1 )

Crampe (1877, 1883, 1884)

Darwin, C. (1876)

Darwin, G. H.

Debierre (1897)

Elderton, Eugenics Laboratory
Memoirs

Guaita (1898)

Mantegazza (1866)

Oesterlen

Voisin
Cultivation

Bailey (1904)

Fruwirth (1909)

Nilsson-Ehle (1909)

Tschermak (1908)

de Vries (1906)
Determination of Sex

Beard (1902)

Berner (1883)

Born (1881)

Determination of Sex (contd.)

Bugnion (1906, 1910)

Castle (1909, 191 1 )

Cohn (1898)

Correns (1907)

Cuenot (1899, 1896, 191 1 )

Doncaster (1909)

Diising {1883, 1884, 1885)

Gregory (1904)

Henneberg (1898)

Hertwig, R. (1905)

Issakowitsch (1905, 1906)

Joseph (1871)

Lenhossek (1903)

Lint (1902)

Loeb (1906)

McClung (1902)

Malsen (1906)

Marchal (1904)

Morgan (1905, 1907, 1909)

Nussbaum (1880)

Pfliiger (1881)

Pike (1907)

Punnett (1904, 1906)

Rauber (1900)

Raynor and Doncaster (1904)

Reed (1906)

Russo (1909)

Schenk (1902)

Seligson (1901)

Strasburger (19 10)

Walker (1910)

Wilson, (1905, 1906, 1910)
Development, Heredity and

Bergh (1895)

Bourne (1894)

Delage (1903)

Driesch(i889, 1901, 1904, 1905,
1906)

Herbst (1901)

Hertwig, O. (1896, 1898,
1906)

His (1874)

Kassowitz (1899)

Klebs (1903)

Minot (1892)

6o8

SUBJECT-INDEX TO BIBLIOGRAPHY

Development, Heredity and

( co ntd.)

Mitchell (1896)
Mivart (1894)
Morgan, T. H. (1900)
Pfeffer (1895)
Rabl (1906)

Roux (1881, 1893, 1895)
Vernon (1903)
Weismann (1893, 1904)
Whitman (1895)
Wilson (1893, 1900)
Woods (1906)
Yung (1878, 1885)
Disease

Binswanger (1896)

Bollinger (1882)

Brown-Sequard

Castle (1906)

Debierre (1897)

Dejerine (1886)

Deutschmann (1880)

Dollinger (1887)

Eugenics Laboratory Memoirs

Fay (1889)

Fere (1898)

Garrod (1908)

Hamilton (1900)

Hutchinson (1896)

Jung

Klebs (1887)

Leslie (1882)

Locher-Wild (1874)

Martius (1905)

Masoin (1879)

Mobius (1900)

Morel (1837)

Mott (191 1 )

Nettleship (1907, 1909)

Obersteincr (1875)

Ogilvie (1901)

Orschansky (1903)

Proc. Roy. Soc. Med. (1909)

Punnett (1908)

Raymond (1905)

Reibmayr (1899)

Disease (contd.)

Reid (1900, 1905, 1910)

Rentoul (1906)

Ribbert (1902)

Rohde (1895)

Sammelsohn (1880)

Schluter

Schuster (1906)

Senator and Kaminer (1904)

Sioli (1885)

Sollier (1889)

Sommer (1900)

Thomson (1901)

Virchow (1858, 1886)

Wallace, J. S. (1904)

Weldon (1905)

Ziegler, E. (1886)
Dogs

Barrington, Lee, Pearson

(1905)

Lang (1910)
Environment, Influence of

Cuenot (191 1 )

Eugenics Laboratory Memoirs

Kropotkin (1910)

Lefevre (1906)

Macdougal (1909)

Morgan (1907)

Paton (1903)

Pictet (1905, 1906)

Rabaud (191 1)

Schmankewitch (1875, 1877)

Semper (1881)

Standfuss (1896, 1902)

Thomson (1888)

Tower (1906, 19 10)

Vernon (1903)

Wallace, W. (1891)

Weismann (1894)

Wilson (1894)

Yung (1878, 1883, 1885)
Evolution Theory in general

Askenasy (1872)

Baldwin (1897, 1902)

Conn (1900)

Cope (1889, 1896)

SUBJECT-INDEX TO BIBLIOGRAPHY 609

Evolution Theory in general
(contd.)

Cuenot (1910)

Delage (1903)

Delage and Goldsmidt (1909)

Delbceuf (1877)

Geddes and Thomson (191 1 )

Haeckel (1866, 1898)

Headley (1900)

Jordan (1898)

Le Dantec (1903)

Lotsy (1906)

Morgan, Lloyd (1896, 1900)

— T. H. (1903)

Nageli (1884)

Osborn (1892, 1896)

Pearson (1900)

Poulton (1908)

Romanes (1892-1897)

Ryder (1894)

Schneider (1906, 191 1)

Seward (1909)

Spencer (1864, 1899)

Thomson (1901, 1903, 1906,

1909)
Wallace (1889)
Weismann (1904)
Wolff (1898)
Experimental Study of

Heredity
Bailey (1904)
Bateson (1902, 1904, 1905,

1906, 1909)
Baur (1910, 1911)
Biff en (1905)
Boveri (1889)
Castle (1900, 1905, 191 1)
Correns (1900, 1901, 1902,

1905)
Coutagne (1902)
Cuenot (1902, 1903)
Darbishire (1904, 1905)
Dareste (1891)
Darwin (1868)
Davenport (1909)
Ewart (1901)

39

Experimental Study of Here-
dity (contd.)

Fruwirth (1907)

Galton (1887)

Heape (1891)

Hurst (1902, 1903, 1904)

Johannsen (1909)

Kammerer (1908, 191 1)
Eugenics

Davenport (191 1)

Eugenics Laboratory Memoirs

Lang (1906)

Mendel (1865)

Morgan (1907)

Przibram (19 10)

Punnett (1905, 1907)

Saunders

Schuster (1905)

Seeliger (1894)

Standfuss (1902)

Staples-Browne (1904)

Sumner (1910)

Tschermak (1900, 1901, 1904)

de Varigny (1892)

de Vries (1900, 1901, 1905)
Experimental Embryology

Driesch (1901, 1905)

Herbst (1901)

Jenkinson (1909)

Maas (1903)

Wilson (1900)

Fertilisation

Boveri (1889, 1891, 1895,
1902, 1904)

Delage (1903)

Hatschek (1887)

Hertwig (1884)

Korschelt and Heider (1905)

Strasburger (1884, 1888, 190 1)

de Vries (1903)

Waldeyer (1898)
Fertility and Fecundity

Pearson (1899, 1900)

Rommel (1906)

Rommel and Philipp (1906)

6io SUBJECT-INDEX TO BIBLIOGRAPHY

Genealogy

Lorenz (1898)
Germ-cells

Beneden (1883)

Boveri(i88Q, 1891, 1895, 1904)

Haecker (1902)

Heider (1906)

Hertwig, O. (1898)

Kolliker (1885, 1886)

Korschelt and Heider (1905)

McClung (1905)

McFarland (1898)

Montgomery (1901, 1904)

Strasburger (1884, 1888, 1906,
1908, 1909)

Sutton (1902, 1903)

Weismann (1904)

Wilson (1900, 1906)

Ziegler (1905)
Germinal Continuity

Balbiani (1885)

Haecker (1902)

Jaeger (1876)

Nussbaum (1884)

Rauber (1886)

Richter (1887)

Riickert (1895)

Virchow (1858, 1900)

Weismann (1885, 1893, 1904)
Germinal Selection

Delage (1903)

Emery (1897)

Weismann (1895, 1896, 1904)
Guinea-pigs

Castle (1905, 1906)

Sollas (1909)
Heredity in General

Adams (181 5)

Bambeke (1885)

Biichner (1882)

Correns (1905)

Dall (1890)

Darwin (1868)

Debierre (1897)

Delbneuf (1887)

De Candolle (1885)

Heredity in General (contd.)

Delage (1903)

Emery (1893)

Galton (1889)

Goette (1898)

Goldschmidt (191 1)

Haacke (1893)

Hallez (1886)

Hensen (1885)

Herdman (1883)

Hertwig, O. (1905)

Jaeger (1897)

Jensen (1907)

Johannsen (1909)

Jordan (1898)

Le Dantec (1898, 1900, 1906)

Lock (1906)

Lotsy (1906)

Mann (1893)

Marshall (1888)

McKendrick (1888)

Merz (1903)

Mitchell (1903)

Montgomery (1906)

Nussbaum (1888)

Osborn (1892, 1893)

Pearson (1900)

Poulton (1889)

Reid (1905, 1910)

Saleeby (1906)

Schafer (1898)

Schneider (191 1)

Thomson (1889, 1898, 1902,
1906)

Turner (1889)

de Vries (1905)

Walker (19 10)

Weismann (1891, 1892, 1893)

Weldon (1906)

Wilson (1900)

Ziegler (1905)
Heredity, Laws of

Bateson (1909)

Buckman (1892)

Castle (1903, 191 1)

Cohen (1875)

SUBJECT-INDEX TO BIBLIOGRAPHY

611

Heredity, Laws of (coutd.)

Correns (1905, 1912)

Darbishire (1906)

Darwin (1868)

Davenport (1909)

Delage (1903)

Galton (1889, 1897)

Johannsen (1903, 1909)

Harris (1908)

Lang (1906)

Lucas (1847)

Mendel (1865)

Pearson (1896, 1898, 1900)

Reid (1910)

Tschermak (1900, 1901, 1904,
1905)

de Vries (1900, 1901, 1905)

Weldon (1905)

Ziegler (1905)
Heredity, Theory of

Beard (1904)

Brooks (1883)

Butler (1878, etc.)

Darwin (1868)

Delage (1903)

Dendy (1903)

Elsberg (1874)

Forel (1905)

Galton (1875)

Gautier (1886)

Geddes (1886)

Haeckel (1876)

Hatschek (1905)

Hering (1870)

Hertwig, O. (1884)

Jaeger (1876)

Lankester (1870, 1876, 1890)

Laycock (1875)

Lendl (1889)

Meyer (1906)

Orr (1893)

Petrunkewitsch (1904)

Rignano (191 1)

Ryder (1890, 1895)

Semon (1904)

de Vries (1889)

Heredity, Theory of (contd.)

Weigert (1887)

Weismann (1885, 1893, 1904)

Weldon (1905)

Ziegler (1905, 1906)
History of Investigations and
Speculations on Heredity

Arreat (1890)

Balbiani (1888)

Bemmelen (1890)

Delage (1903)

Giard (1905)

Merz (1903)

Nussbaum

Osborn (1894)

Overzier (1877)

Plarre (1881)

Poulton (1908)

Radl (1909)

Roth (1885)

Thomson (1889, 1899, 1902)

Thomson (1906, Spencer's
position)

Weigert (1887)

Ziegler (1902)
Horses

Blanchard (1903)

Davenport (1904)

Ewart (1899)

Hurst (1904, 1906)

Iwanoff (1905)

Lee (1903)

Pearson (1899)

Weldon (1904)

Wilson (1910)

Wood (1905)
Hybridisation in Animals

Ackermann (1896-8)

Baillet (1893)

Broca (1S58, 1859)

Coutagne (1902)

Crampe (1877, 1883, 1884)

Cuenot (1902, 1905, 1912)

Doncaster (1903)

Ewart (1899, 1 901)

Fischer (1874)

6i2 SUBJECT-INDEX TO BIBLIOGRAPHY

Hybridisation in Animals (contd.)

Guaita (1898, 1900)

Guyer (1900)

Haecker (1904)

Iwanoff (1905)

Lang (1906)

Pfliiger (1882)

Schuster (1905)

Standfuss (1896, 1912)

Stephan (1902, 1903)

Suchetet (1896)

Tutt (1898)

Vernon (1898, 1903)
Hybridisation in Plants

Bailey (1904)

Bateson (1901, 1902, 1905,1906)

Biffen (1907)

Blackman (1902)

Correns (1900, 1901, 1905)

Focke (188 1)

Gartner (1849)

Godron (1863, 1864, 1865, 1872)

Hurst (1901, 1902, 1903)

Kolreuter (1761 )

Lock (1904)

Macdougal (1905*)

Macfarlane (1891)

Mendel (1865, 1869)

Millardet (1894)

Nageli (1865, 1866)

Naudin (1865)

Nilsson-Ehle, H. (1909)

Poll (1907, 1910)

Rimpau (1891)

Spillman (1902)

Tschermak (1900, 1901, 1903,
1904)

de Vries (1900, 1901, 1903,
1905)

Webber (1900)

Wichura (1865)

Inbreeding. See Consanguinity
Bos (1894)

Castle, and others (1906)
Ewart (1899)

Inheritance of

Fertility and Fecundity, Pear-
son (1899)

Human Qualities, Eugenics La-
boratory Memoirs, Buchner
(1882), Lorenz (1898), Lucas
(1847) Woods (1906)

Longevity, Pearson (1900,
1901)

Mental and Moral Characters,
Lankester (1899), Pearsor
(1903c), Ribot (1902), Wilser
(1892) Baldwin (1907)

In Parthenogenesis, Warren
(1899)

Instincts, Baldwin (1896)
Loeb (1897), Lloyd Morgan,
(1896). Ziegler (1904)
Inheritance, Modes of

Alternative, Pearson (1903B),
Weldon (1902)

Bateson (1902)

de Vries (1905)

Walker (1910)

In Parthenogenesis, Warren
(1899, 1901)

In Asexual Reproduction, East
(1910)
Insects, Inheritance in

Coutagne (1902)

Kellogg and Bell (1904)

Pictet (1905)

Raynor and Doncaster (1904)

Schroeder (1904)

Standfuss (1896)

Tower (1906)

Tutt (1898)
Isolation

Gulick (1888, 1890, 1891, 1905)

Reibmayr (1897)

Romanes (1886)

Weismann (1904)

Journals

Archiv fur Rassen- und Gesell-

schaftsbiologie

SUBJECT-INDEX TO BIBLIOGRAPHY

613

Journals (contd.)

Archivfur Entwickelungsme-
chanik
Biometrika
Journal of Genetics
Zeitschriftfiiy induktive A bstam-
mungs- und V ererbungslehre

Man, Heredity in

Amraon (1893, 1900)

Broca (1865)

Biichner (1882)

Buckman (1892)

Castle (1903)

Castle and Farabee (1903)

Collins (1891)

Constable (1905)

Davenport, G. C. and C. B.

(1907, 1908, 1909, 1910)
Drinkwater (1908)
Ellis (1904)

Eugenics Laboratory Memoirs
Fay (1898)
Felkin (1887)
Fere (1898)
Galippe (1905)

Galton (1869, 1883, 1889, 1897)
Galton and Schuster (1906)
Goddard (1910)
Hurst (1908, 1912)
Huxley (1894)
Jordan (1898, 1906)
Kohlbrugge (1897)
Lane (1887, 1888)
Locher-Wild (1874)
Lorenz (1898)
Lucas (1847)
McKim (1900)
Metchnikoff (1903)
Mott (191 1 )
Nisbet (1890)
Odin (1895)

Pearson(i897, 1901, 1903, 1904)
Reibmayr (1897)
Reid (1896, 1897, 1905, 1910)
Rentoul (1906)

Man, Heredity in {contd.)

Ribot (1875, 1902)

Schimkewitsch (1906)

Sommer (1907)

Thomson, A. (1889)

Weinberg (1909)

Weldon (1904)

Wiedersheim (1887)

Woods (1902, 1903, 1906)

Ziegler (1906)
Material Basis of Inheritance

Bateson (1907)

Correns (1909)

Fick (1906)

Godlewski (1909)

Guyer (1907, 1909, 191 1)

Haecker (1907, 1910)

Hertwig, O. (1909)

Hickson (1907)

Meves (1908)

Ruzicka (1909)

Spiilman (1909)

Wilson (1909)
Mendelism

Bateson ^(1902, 1905, 1906,
1907, 1909)

Baur (1907, 1911)

Biff en (1905)

Blackman (1902)

Butler (1905)

Cannon (1902)

Castle (1903, 1907, 1909. 191 1 )

Correns (1900, 1901,1902,1905)

Coutagne (1902)

Cuenot (1908)

Darbishire (1904, 191 1)

Davenport (1901, 1904, 1909)

Doncaster (1903)

East (1910)

Galloway (191 1)

Gates (1910)

Giard (1903)

Gregoire (1907)

Guyer (1903)

Haacke (1906)

Hart (1909)

614 SUBJECT-INDEX TO BIBLIOGRAPHY

Mendelism (contd.)

Hurst (1901, 1902, 1903, 1904)

Kellogg (1908)

Kiister (1902)

Lang (1906, 1908)

Lock (1904, 1906)

Morgan (1905)

Mudge (1908)

Pearson (1904)

Punnett (1905, 1907, 1911)

Saunders (1904)

Shull (1908, 1909)

Spillman (1902, 1909)

Tower (1910)

Toyama (1906)

Tschermack (1900, 1901, 1902,
1903, 1905, 1906)

de Vries (1900, 1901, 1905)

Walker (1910)

Weldon (1902 1903, 1905)

Wilson (1902, 1906)

Yule (1902)

Ziegler (1903)
Mental and Moral Qualities

Eugenics Laboratory Memoirs

Galton (1869)

Pearson (1903c)

Ribot (1902)

Woods (1906)
Mice

Allen (1904)

Bateson (1903)

Cuenot (1902, 1903, 1904)

Darbishire (1902, 1903, 1904,

1905)
Davenport (1900, 1904)
Durham (1908)
Guaita (1898, 1900)
Plate (1910)
Schuster (1905)
Mnemic Theories of Inheritance
Butler (1878, 1910)
Darwin, F. (1908)
Hering (1870)
Semon (1904)
Rignano (191 1)

Modifications

Davenport (1896)

Fischer (1901)

Morgan (1896)

Pictet (1905)

Rabaud (191 1)

Standfuss (1896)

V( rnon (1903)
Mutation

Bumpus (1898)

Davenport (1905, 1909)

Lang (1906)

Macdougal (1905)

Mayer (1901)

Moll (1901)

Plate (1905)

Schroeder (1904)

Schroter (1906)

Scott (1894)

de Vries (1901, 1903, 1905, 191 1)

Wettstein (1908)

Ziegler (1905)

Philosophical

Bergson (191 1)

Brooks (1899, 1906)

Driesch (1908)

Jenkinson (1909)

Ribot (1975) '

Sandeman (1896)

Schneider (1908)

Schopenhauer (1873)
Plants, Heredity in

Bailey (1904)

Bateson (1902, 1909)

Baur (191 1 )

Biffen (1905, 1907)

Blackman (1902)

Castle (1900)

Correns (1900, 1901, 1902, 1905)

Focke (1881)

Fruwirth (1905)

Johannsen (1903, 1909)

Hurst (1902, 1903, 1904)

Laxton (1866, 1872, 1890)

Mendel (1865)

SUBJECT-INDEX TO BIBLIOGRAPHY

6i5

Plants, Heredity in (contd.)

Report, Horticultural Society

(1907)
Spillman (1902)
Tschermak (1900, 1901, 1903,

1904)
de Vries (1900, 1901, 1903,

1905, 1907)
Weismann (1888)
Weldon (1902)

POLYDACTYLISM

Barfurth (1908, 1909)

Castle (1906)

Wilson (1896)
Poultry

Davenport (1909)
Prepotency

Ewart (1899)

Galton (1898)
Protozoa, Heredity in

Jennings (1908)
Pure Lines

Jennings ( 1909 )

Johannsen (1909)

Woltereck (1909)

Rabbits

Castle (1903, 1905. I9°9)

Hurst (1905)

Lang (1910)

Loisel (19T0)

Raspail (1902)

Woods (1903)
Rats

Bateson (1903)

Crampe (1877, 1883, 1884)

Doncaster (1905)

Morgan (1909)

Mudge (1908)
Regeneration

Morgan (1900)

Rauber (1895, 1896)

Weismann (1899)
Regression

Pearson (1896)

de Vries (1905)

Reproduction, Physiology of

Marshall (19 10)
Reversion

Bateson (1909)

Castle (1907)

Davenport (19 10)

Ewart (1899, 1901)

Pearson (1900)

Punnett (191 1)

Weismann (1886)

Selection and Heredity

Ammon (1893)

Delage (1903)

Galton (1897)

Lapouge (1896)

Pearson (1896, 1899, 1901,
1902)

Plate (1903, 1908)

Rentoul (1906)

Weismann (1905)
Sex and Heredity

Bateson and Punnett (1908)

Bateson (1909)

Castle (1903)

Correns (1908)

Cuenot (191 1 )

Geddes and Thomson (1901)

Hart (1909)

Le Dantec (1903)

McClung (1902)

Meisenheimer (1909)

Morgan (1907, 1908, 1909,
1910)

Newcomb (1904)

Pearl and Surface (1909)

Punnett (1904)

Strasburger (1900)

Thomas (1907)

Walker (1910)

Wilson (1909)

Weininger (1903)

Ziegler (1905)
Social Problems

Ammon (1893, 1900)

Arlidge (1892)

6i6

SUBJECT-INDEX TO BIBLIOGRAPHY

Social Problems (contd.)
Baldwin (1896, 1907)
Beddoe (1896)
Butler (1901)
Castle (1903)
Chappie (1904)
Chatterton-Hill (1907)
Constable (1905)
Demoor, Massart, Vandervelde

(1897)
Duclaux (1902)
Durkheim (1897)
Eugenics Laboratory Memoirs
Galton (190 1, 1904)
Guyau (1889)
Huxley (1894)
Ireland (1900)
James (1896)
Jordan (1898, 1906, 1907)
Lankester (1905, 1907)
Lapouge (1888, 1896)
McDougall (1908)
McKim (1900)
Michaelis (1904)
Mott (191 1 )
Norton (1906)
Pearson (1901)
Ploetz (1895)
Reibmayr (1897, 1899)
Reid (1905)
Rentoul (1906)
Ruppin (1903)
Schallmayer (1903, 1905)
Senator and Kaminer (1907)
Sommer (1907)
Tarde (1896, 1905)
Tayler (1904)
Thomas (1907)
Thomson (1907, 1909)
Ziegler (1894)
Statistical Study of Heredity
Brooks (1899)
Darbishire (1905, 1906)
Davenport (1899, 190 1)
Eugenics Laboratory Memoirs
Galton (1889, 1901)

Statistical Study of Heredity
{contd.)

Johannsen (1909)

Lock (1906)

Ludwig (1901)

Merz (1903)

Pearson (1896, 1898, 1899,
1900, 1902, 1903, 1906)

Vernon (1903)

Weldon (1905, 1906)

Yule (1897, 1902, 1912)
Stock-breeding, Application to

Baillet (1889, 1895)

Barrington and Pearson (1906)

Castle (1905)

Cornevin (1891)

Darbishire (1906, 1907)

Davenport (1905, 1909)

Haecker (1904)

Iwanoff (1905)

Keller (1905)

Lapouge (1890)

Marshall (1905)

Punnett (191 1 )

Rommel (1906)

— and Phillipp (1906)

Sanson (1893)

Settegast (1888)

Shaw (1903)

Wilckens (1889)

Wood (1905)

Telegony

Barthelet (1900)

Darwin (1868)

Ewart (1899, 1 901)

Finn (1893)

Harvey (1851)

Morton (1821)

Romanes (1893)
Transplantation of Ova

Castle and Phillips (191 1)

Guthrie (1908)

Heape (1890, 1897)
Twins

Bataillon (1900)

SUBJECT-INDEX TO BIBLIOGRAPHY

617

Twins (contd.)

Brandes (1898)
Bugnion (1891)
Cuenot (1903)
Loeb (1893)
Marchal (1904)
Rosner (1901)

Unit Characters

See Mendelism, also
(1909)

Castle

Variation

Amnion (1872)

Bateson (1894, 1904, 1905)

Beard (1904)

Brandt (1895)

Brooks (1912)

Browne (1895)

Cuenot (191 1 )

Darwin (1868)

Davenport (1899, 1900, 1903)

Delage (1903)

Dendy (1912)

Ewart (1901)

Hurst (189 1)

Johannsen (1909)

Kammerer (1910)

Variation (contd.)

Kellogg and Bell (1904)
Klebs (1903)
Lang (1904, 1906)
Ludwig (1901)
Mauck (1901)
Mayer (1901)
Mcintosh (1903)
Rosa (1905)
Sedgwick (1899)
Tower (1906)
Vernon (1903)
de Vries (1905)
Walker (1910)
Wallace (1889)
Warren (1901)
Ziegler (1905)

Weismannism

Adami (1901)
Delage (1903)
Hartog (1891, 1893)
Kolliker (1886)
Morgan (1907)
Romanes (1896)
Spitzer (1886)
Vines (1889)
Ziegler (1905)

INDEX

Abnormal conditions, acquired and
innate, 256

Abnormalities, 287 ; expressed only
in one sex, 290

Accessory chromosome, 492

Acquired characters, transmission
of, 164; defined, 173, 211, 212

Adami, Prof., 186, 278

Adjustments, temporary and in-
dividual, 72

Age of parents as a factor in sex-
determination, 484

Albuminuria, 261

Alcoholism, 189, 219, 273

Allbutt, T. Clifford, on disease, 251

Allelomorphs, simple and com-
pound, 352

Alpine plants, modification in, 1 10

Alternative inheritance, 116

Amnion on supernumerary mam-
mae, 129

Amphidasys, variation in, 87

Amphimixis, 49, 54 ; a cause of
variation, 103 ; maturation and,

435

Amputations, 224

Anabolism, 71

Ancestors, reduction of, 322

Ancestral plasms, 427

— inheritance, Mendelism in rela-
tion to, 371 ; Galton's law of,

323. 522
Andalusian fowls, 352
Antenatal modifications, 271
Anticipation of disease, 302
Architecture of inheritance, 392

Argyll, Duke of, on inheritance of

acquired characters, 196
Aristotle's Historia Animalium, 167
Atavism, 132; "systematic," 123,

137
Amelia aurita, discontinuous varia-
tion in, 87

Bailey, 14

Balbiani, 410

Baldwin, Prof. Mark, 243

Ballantyne, Dr. J. W., 161, 163
223, 229

Barclay, R. W., 388

Baron, 154

Basset-hounds, coat colour in, 323

Bateson, 65, 75, 83, 85, 120; on
Mendel's theory, 354, 383 ; on
congenital cataract, 371 ; on
practical applications of biology,

Bats, fertilisation in, 152

Beard, Dr. John, 490

Bedart, 289

Bees, queen, fertilisation of, 152

Behring, 271

Bell, Dr. Joseph, 126

Beneden, Van, 46, 437

Bergson, Prof., 235

Bernard, Claude, 153

Bert, Paul, experiments on Daph-

niae, 189
Besler, W., 228
Biometrika, 79, 80
Bleeding, 272
Blended inheritance, 382

619

620

INDEX

Blumenbach, 167

Bollinger, 210

Bond, Dr., experiments, 149

Bonnet, Charles, 396

Booth, Mr. Charles, 525

Bouchut, 226

Boveri, 48, 62, 450

Breeder's evidence in favour of

modification-inheritance, 216
Brewer, Prof.,W. H., 208, 216, 218,

229
Brine-shrimps, experiments on, 213
Brock, 167
Bronn, 228
Brooks, Prof., W. K., 68, 166, 322,

405, 408
Browne on variation in jelly-fish,

87

Browne, Sir Thomas, 248

Brown-Sequard, 153; on eye
defects, 227 ; experiments on
guinea-pigs, 230, 231, 264

Brucke, 399

Buchner, Prof. L., 22, 228

Buckle, 23, 24

Buffon, 113, 399

Bunge, Prof. G. \on, 277

Butler, Samuel, 454

Butterflies, experiments on, 214

Candolle, De, 20

Carnations, wheat-ear, 141

Carncri, 148

Castle, W. E., 366, 388, 496

Cats, alleged telegony in, 147 ; Manx
and Japanese, 225 ; colour in, 365

Cattle, alleged telegony in, 148

Changes, cyclic, 71 ; involved in
functioning, 72

Chapuis, Dr., on supposed telegony
in birds, 149

Charrin, 234

Chelidonium majus, mutation in, 97

Chromosomes, number of, 46 ; com-
binations of, 103 ; in man, Ziegler
on, 300 ; accessory. 492

Clermont-Tonnerre, Tillet de, 160

Climatic changes, 210

Clouston, T. S , on alcoholism, 276 ;

on nervous diseases, 281
Co-adaptations, 237
Colour-blindness, 272
Consanguinity, 386
Conservative types of organisation,

69
Constitutional units, Spencer's

theory of, 199
■ — vulnerability, 286
Continuity, theory of genetic or

germinal, 407
Cope, 216, 322
Cornevin, 145, 146, 153, 154
Correns, 159, S37, 357. 358. 379,

496
" Courtier," pedigree of, 390
Coutagne, experiments in hybridi-
sation, 359
Cuenot, 189, 362, 483, 501
Currant, white-flowering, reversion

in, 140

Dachauer women, 210

Dahlias, reversion in, 141

Dalton, 425

Daltonism, or colour-blindness,
291

Darbishire, A. D., 354, 362, 375 ;
on statistical laws, 329-30

Darwin, 22 ; on discontinuous
variations, 83 ; on reversion,
119; on intercrossing, 120,
on telegony, 144, 146, 148, 154,
155 ; on inheritance of wounds,
226 ; on inbreeding, 387

— G. H., 386

Davenport, 116, 120, 354, 360

Dawson, Dr. Rumley, 491

Deafness, statistics as to, 256

Debierre, 237, 282, 283

Defects, 287

Deformations, 227

Degeneration, alcoholic, 277

INDEX

621

Degrees of transmissibility, 191

Dejerine, 291

Delage, Prof. Y., 27, 54-5, 64, 145,
160, 224, 225, 420, 446, 454

Delamare, 234

Democritus, 399

Dendy, 371

Determinants and determinates,
434. 435 I breaking up of deter-
minants, 442 ; objections to theory

of, 445

Determination of sex, 475 ; classi-
fication of theories of, 480

Development, theory of, 392,440;
arrests of, 125 ; heredity and,
412

Differential division, 422, 441

Dingfelder, 224

Directive factors in evolution, 526

Discontinuity in variation, 82, 370

Disease, heredity and, 250 ; what is,
251

Diseases due to innate predisposi-
tions and acquired modifications,
252

Dogs, telegony in, 146

Domestic animals, size in, 216

Dominance, 338, 353

Dominant characters, 357

Doncaster, 321, 497

Driesch, Dr. Hans, 62, 413, 417,
421, 424

Dupuy, 234

Duration of life, 334

Ehrenberg, 87

Eisen on complexity of the nucleus,
28

" Elementary Species," 94

Entelechy, 413

Environment, relation between or-
ganism and, 174

Epigenesis, 416

Errera, Prof. I.., experiments with
moulds, 187

Evening primrose, 91

Evolutio, 416

Evolution, general theory of, 12 ;

new view of, 367
Ewart, Prof. Cossar, 103, 138 ; on

telegony, 146, 147, 149, 152, 157
Exclusive inheritance, 385
Experimental school, 425
External heritage, man's, 246, 249

Famine, Irish, 162

Farabee, 287

Farr, Dr., 526

Fay, E. A., 256

Felkin, R. W., 77

Fere, 274

Fertilisation, 60, 62, 440

Filial regression, 142, 314, 522

Finn, Mr. Frank, on telegony, 149,

Fischer, experiments on butter-
flies, 214

Fluctuations, cumulative effect of,
80 ; and mutations, 90, 95

Focke on xenia, 1 59

Food as a factor in sex-determina-
tion, 482

Foster, Sir Michael, 43

Galton, Fr., 14, 15, 43, 70, 82, 168,
314, 315, 404, 408

Galtonian v. Mendelian theories,
330-32, 37i

Galton's law of ancestral inherit-
ance, 323, 327, 522

Gametes, segregation of, 344

Gautier, 273

Gemmules, Darwin's theory of, 450

Germ-cells, nature and origin of,
7,7 ; maturation of, 45, 48 ;
completely equipped potential
organisms, 56 ; apartness of,
197 ; uniqueness of, 399

Germinal continuity, theory of, 42,
407

— selection, 454, 459

— variations, 169

'

622

INDEX

Germ-nuclei as bearers of heredi-
tary qualities, 58
Germ-plasm, persistence of, 449
Girou, 22
Goethe, 113
Golden Rod, 180

Gout, transmissibllity of, 182, 261
Grandidier, 20
Guaita, Von, 115, 387
Gulick, 540, 541
Guyer, 65, 494

Haacke, Dr. W., 196

Habits and instincts, modifications
of, 220

Haeckel, 32, 224, 407, 454 ; on
transmission of acquired char-
acters, 196, 228

Haecker, observations of water-
fleas, 51

Haemophilia, 20, 265, 291

Haller, Albrecht von, 397

Hamilton, Prof. D. J., 255, 296;
on gout, 131, 182 ; on nervous
diseases, 263 ; on nerve-cells,
279, 284

Hands, large and small, 209

Hannot, 291

Hare -lip, 288

Hartmann, R.. 227

Hartog, Prof. M., 222

Harvey, Dr. Alexander, on cross-
breeding sheep, 147, 154, 155

— William, 397, 416

Hatschek, 103

Heape, 501

Heine, 249

Herdman, Prof. W. A., 406

Heredity, definitions of, 15 ; re-
statement of central problem of,
37 ; and variation, 66 ; and
evolution, 85 ; and disease, 250 ;
experiments on, 374 ; and in-
heritance, 391 ; old theories of,

394
Hering, E., 454

Hertvvig, Prof. O., 62, 63, 186, 416,
420, 422, 436

Hertwig, Prof. R., 487

Hickson, 65

Hilaire, Etienne Geoffroy St., 83

Hill, Dr. Leonard, 232

His, 168, 416

Histonal selection, 456

Hofacker, 484

Homochronous heredity, 19

Horse and zebra, hybridisation of,
138

Horses, reversion in, 1 30 ; improve-
ment in trotting, 207

Hurst, C. C, 354, 363

Hutchins, D. E., 224

Hutchinson, 261, 264, 288, 292

Huxley, 398, 431, 438 ; on La-
marckian hypothesis, 172

Hybridisation in general, 380 ; of
distinct species, 380 ; resultsof, 383

Hypothesis of development, 440

Idioplasm, 429

Idiosyncrasies, physiological, 268

Ids and idants, 430

Immunity, 218, 296

Individual contribution, law of

diminishing, 324
Individuated stocks, relative in-
fertility of, 538
" Infection " hypothesis, 153
Infections, 298
Inheritance, the physical basis of,

26; dual nature of, 49, 51 ; in

cases of parthenogenesis, 57 ;

unilateral, 112 ; " crossed," 113 ;

particulate, 114; Mendelian, 116;

statistical study of, 309 ; law

of ancestral, 323
Innate and acquired diseases, 258
Integral division, 419, 441
Intercrossing, swamping effects of,

370
Intracellular pangenesis, De Vries'

theory of, 453

INDEX

623

Intra-organismal selection, 455
Intra-uterine contagion, 219
Ireland, Dr., 539
Isolation, 540
Issakowitch, 495

Jaeger's theory, 404, 407
Jelly-fish, symmetry of common,

177
Jenkin, Prof. Fleeming, 370
Johannsen, Prof., experiments on

" pure lines," uj
Jordan, Prof. D. S., 537
Jordan, H. E., 505

Kammerer, 215

Kant, 167

Kanthack, A. A., 283, 297

Karma, 431

Katabolism, 70

Keller, Prof., 99

Kidd, Dr. W., 196

Kiener, 146

Klebs, 272

Lamarck on intercrossing, 103

Lamarckism, 172

Lamarck's laws, 170

Lane, Dr. Arbuthnot, 208

— C. H., on dogs, 147

Lang, experiments with snails, 360

Lankester, E. Ray, 16, 253, 402,403 ;
on Lamarckian position, 172, 206

Lanugo, 130

Leprosy, 260

Lina Lapponica, hereditary rela-
tions in, 359

Lint, Van, 486

Lock, R. II., 16, 310, 497

Loeb, Prof., experiments on fertili-
sation, 55, 64

Lucas, Prosper, 9, 67, 108, 254, 289

Lundstrom, 229

McClung, 496, 498
McCracken, Miss, 359

Macfarlane, Prof. J. M.; on plant-
hybrids, 1 1 1
Mackenzie, Dr. Leslie, 262, 304
Mairet, 274
Maize, Mendelian phenomena in,

356-8
Malformations of parts, 289
Malsen, von, 495
Mammae, supernumerary, 129
Man, alleged telegony in, 145 ;
proportion of male and female
births in, 500
Mantegazza, 224
Marchal, 28, 491, 507
Marchant, 85

Mare, the case of Lord Morton's, 144
Marshall, Prof. Milnes, 322
Martius, 257, 265, 271, 277, 303
Material basis of inheritance, 426
Maternal impressions, 154, 161
Maturation of germ-cells, 45-9, 59,

— and amphimixis, 435

Mayer, A. G., on Pseudoclytia, 87-9
Medical arguments as to inheritance

of acquired characteristics, 219
Medusoids, variation in, 140
Mendel, Gregor Johann, 336 ; Men-
del's law, 337, 343 ; his experi-
ments, 337 ; theoretical interpre-
tation, 343 ; theory summarised,
^348 ; corroborations of, 351 ;
discovery in relation to other
conclusions, 365 ; practical ap-
plication, 373
Mendelian inheritance, 84 ; phe-
nomena, 1 34

— interpretation of reversion, 133

— and Galtonian theories com-
pared, 371

Merogony, experiments on, 417

Metabolism, 70

Metaphysical theories of heredity,

395
Mice, experiments on, 224 ; hy-
bridisation of, 342 ; Mendelian
phenomena in, 361

624

INDEX

Microbic diseases, 283 ; selection,

532

Miles, Manly, 218

Militarism, 536

Minot, 436

Misunderstandings, current, as to
acquired characters, 179-91

Mitchell, Chalmers, 422

Modifications, 73, what are. 175,
519, and variations, 11, 176;
acquired, resembling ancestral
characters, 127 ; secondary re-
sults of, 242 ; indirect import-
ance of, 242, 519

Montgomery, T. H., 15

Moral character, inheritance of, 247

Morgan, Prof. Lloyd, on mechanism
of transmission, 200 ; on habit and
instinct, 220 ; on importance of
modifications, 243

— T. H., 94, 234, 416, 465, 493,
505 ; experiments on mice, 234-5 ;

Mosaic evolutio theory, 416

Mott, F. W., 276, 292, 303

Moussu, 234

Multicellular organisms, the heredi-
tary relation in the asexual mul-
tiplication of, 34

Multiplication, table of modes of, 29

Multiplicities, 288

Mutation, the oldest known, 96

" Mutation theory " of de Vries, 90

Mutilations, 168 ; transmissibility
of, 221

Nageli, Alpine plants, no, 184;
his " idioplasm," 417 ; on the
material basis of inheritance, 426

Natural selection, interference with,

S3i
" Nature," 246
Nature and nurture, 6
Nawaschin and Guignard, 160
Neo-Darwinian position, 240
Neo-Lamarckian position, 240
Nervous diseases, 220, 263, 278

Nettleship, 273, 302, 364
Non-Mosaic theories, 420
Nurture, importance of, 242, 245
Nussbaum, 409, 483

Obersteiner, 232

CEnothera (evening primrose,) 91

Ogilvie, Dr., on transmission of in-
fectious diseases, 185, 201

Oogenesis, 47

Organic change, different kinds of,
70

Organism and environment, rela-
tion between, 174 ; conception of,

365
Originative factors in evolution, 517
Osborn, H. F., 243
Ovum, the typical, 39
Owen, 407

Packard, 189

Pangen theory of de Vries, 199

Pangenessis, Darwin's theory of, 199,
402

Panmixia, 238, 463

Pavamacium, division in, 31

Parents, influence of, in determining
sex, 484

Paris, siege of, 162

" Parsimony, law of," 241

Parthenogenesis, inheritance in re-
lation to, 57 ; reversion in, 140

Parthenogenetic development, ar-
tificial, 53— S

Particulate inheritance, 385

Pathology, optimism of, 251

Pearson, Prof. Karl, on reversion,
122 ; on stature, 128 ; telegony,
I51. J55 I statement of Galton's
law, 324 ; law of ancestral in-
heritance, 326 ; statistical re-
sults, 334 ; inheritance of mental
and moral characters, 529

Peas, differentiating characters of,
338 ; hybridisation of, 341

Peculiarities, persistent, 70

INDEX

62'

Penycuik experiments, the, 149
Peron, 235

Persistence of characters, 67
Personal selection, 455

Pfliiger, 168

Physiological units, Spencer's
theory of, 399, 451

Pieri, experiments of, 64

Pigeons, experiments on crossing,
138. 36i

Pigs, alleged case of telegony in, 148

Plants, modifications in, due to en-
vironment ,211

Plasticity of organisms, J2

Plastids, 432

Plutarch, 120

Poisoning, 188

Polarity, 400

Polar nuclei, 160

Polydactylism, 129, 287

Poulton, Prof. E. B., 167, 171, 483

Poultry, breeding experiments with,
360

" Practical men," opinion of, as to
transmissibility of acquired char-
acters, 193

Prantl, 483

Prediction, 319

Predisposition to disease, 255, 266

" Preformationist " theories, 396

Pre-natal infection, 189, 255 ; in-
fluences, 289

Presence and Absence Theory, 347

Pressure, results of on sole of foot,
210

Prichard, James Cowles, 167

Primrose, species of, 137

Protenor belfmgi, accessory chro-
mosome in, 492

Protophyta, differentiation of, 33

Protozoa, differentiation of, 33

Pseudoclytia, variability of, 87, 88

Psychosis, abnormal, 183

Punjabis, skeletal peculiarities of,
208

Punnett, R. C, on Mendelism, 343,

40

349, 352- 36o; on pioportion of
male and female births, 500
" Pure Lines," 377-9

Quatrefages, De, 114, 120
Quetelet, statistical methods, 312

Rabbits, Mendelian phenomena in,

363
Rath, Dr. vom, on telegony, 158, 155
Rauber, 407

Reappearance of modifications, 184
— not equivalent to inheritance,

254
Recapitulation theory, 125
Recessive characters, 339, 357
Regeneration, 444
Regression, 320 ; filial, 128, 314
Reibmayr, 541
Reid, Dr. Archdall on Alcoholism,

276
Re-infection, 184
Representative particles, 450
Reproduction, diverse modes of,

29 ; asexual, 35
Resemblance, complete hereditary,

10S
Retrogressive varieties, 94 ; rever-
sion of, 134
Reversion, 119; illustrations of,
1 20 ; definitions of, 1 2 1 ; in crosses,
133 ; interpretations in terms
of, 137 ; in parthenogenesis, 140
Rohde, 281
Riedel, 224

Ritzema-Bos on inbreeding, 387
Rodents, experiments on, in regard

to telegony, 148
Romanes, 191,230 ; on experiments

on guinea-pigs, 233
Rommel and Philipps, 333
Rosenthal, 224
Ross, James, 278
Roux, 419 ; struggle of parts, 455
Russell, Dr. William, 297

626

INDEX

Russo, 502

Rust in wheat, 358

Sachs, 421

Sadler, 484

Saint-Hilaire, Etienne Geoffjoy, 367

Salisbury, Lord, on natural selec-
tion, 334

Sandeman, George, on congenital
and acquired characters, 177

Sanson on telegony, 145

Schliiter, Dr. R., on congenital
tuberculosis, 284

Schmankewitsch, experiments, 213

Schultze, 485

Secondary effects of disease, 269

Seed-reversion, 136

Segregation, law of, 339

Selection, 530 ; and stimulus, 243 ;
Mendelism, 369

Settegast on telegony, 145, 194, 227

Sex, heredity, and 472 ; determina-
tion of, 475 ; what it means,

473

Shoemaker, anatomical peculiari-
ties of, 208

Short-sightedness, 272

Silkworms, experiments with 359

Snails, experiments with, 360

Social aspects of heredity, 510

Sociology, relations of biology and,
512

Sociomorphism, 366

Somatic cells and germ-cells, differ-
ence between, 10 1 ; modifica-
tions, 172

Sommer, Max, experiments, 235 ;
results, 236

Species, elementary, 94 ; hybrids,
382

Spencer, Herbert, on acquired cha-
racters, 166, 181, 183, 194, 205,
237; on telegony, 145, 146; on
plant-modifications, 212 ; his
theory of physiological units, 399,
454

Spermatogenesis, 48

Spermatozoon, the typical, 39, 40

Spore-formation, 418

" Sports," jy

Sprenger, 97

Standfuss, hybridisation of butter-
flies, 214

Staples-Brown, R., 361

Starkweather, 486

Star primrose, origin of, 86

Statistical methods, 309, 425 ; re-
sults, 332

Stature, inheritance of, 112

Stephens, 113

Strasburger, Prof., on fertilisation
in higher plants, 63 ; material
basis of inheritance, 426

Sutton on combinations of chro-
mosomes 103, 104

Syphilis, " hereditary " or " con-
genital," 185, 286

Telegony, 143 ; representative al-
leged cases of, 145

Thaer, 229

Theological theories of heredity, 394

Tietz, 224

Toe, dwindling of little, 210

Tours, Moreau de, 282

Tower, W. L., 105, 376, 377

Toxins, 219

Toyama, 354, 359

Transient adjustments, 174

Transmission of acquired charac-
ters, 164 ; hypotheses as to
mechanism of, 199 ; Spencer's
theory, 199

Trophoplasm, 430

Tschermak, 337, 354

Tuberculosis, 283

Tulase, 271

Turner, Sir William, on telegony,
154; on transmission of acquired
characters, 195 ; on haemophilia,
272 ; on abnormalities, 2S7

INDEX

627

Uncertainties in inheritance, 295
Unfit, multiplication of, 532
Unicellular organisms, multiplica-
tion in, 29 ; hereditary relation in
31 ; transmission in, 185
Unit characters, 366
Units, theoretical conception of ele-
mentary, 90

Variability, 100, 268

Variation, study of, 11, 75 ; discon-
tinuous, 82 ; causes of, 100, 103,
104 ; and modifications, 176 ; in-
dependent, resembling reversions,
128 ; inherited and independent,
269 ; theory of, 517

Varieties, " ever-sporting," 95

Varigny, H. de, on telegony, 147

Vernon, Dr. H. M., experiments on
hybridisation of sea-urchins, 117;
variation, 310

Vestigial structures, 127

Vicinism, 133

Villar, S., 147

Virchow, Prof., 210, 257, 534

Voigt, experiments on Planarian
worm, 35

Voisin, 235

Vries, H. de, on reversion, 133,
136, 141 ; mutations, 83, 90 ;
Pangen theory, 199, 421, 453

Wallace, A. R., on Lamarckism, 172

Waltzing mice, 362

Wart-hog, African, habits of, 180

Weismann, on reversion, 122, 129,
140; on telegony, 148, 152 ; on
transmission of acquired charac-
ters, 168 ; on climatic influences,

211 ; on Brown-Sequard's results,
233 ; theory of continuity of the
germ-plasm, 410, 419 ; summary,
434 ; germinal selection, 454

Weismannism, Mendelism and, 366

Weldon, Prof., on the law of ances-
tral inheritance, 320, 321, 328

Westphal, 232

Whitman, Prof. C. O., 398

Wilder, H. H., 211

Wilson, Dr. George, 271

Wilson, Prof. E. B., on number of
chromosomes, 45, 46, 48, 50 ; on
amphimixis, 50 ; nuclear division,

439, 49o, 494
Winkler, experiments of, 64
Wolff, C. F.,415
Wollaston, 23
" Wonder horses," 86
Wounds, 225

Xenia, 159, 160

Young birds, experiments with, 21
Yule, G. Udny, 15 ; on regression,
321 ; on Mendel's and Galton's
laws, 327
Yung, experiments on sex-deter-
mination, 482

Zebra and horse, hybrids, 138
Ziegler, Prof. Ernst, on acquired
characters, 182, 259 ; trans-
missibility of nervous disorders,
282
— Prof, H. E., on chromosomes.
103, 300 ; dwindling of parts,
determination of sex, 487

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