YALE UNIVERSITY
MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES

PROBLEMS OF GENETICS

SILLIMAN MEMORIAL LECTURES
PUBLISHED BY YALE UNIVERSITY PRESS

ELECTRICITY AND MATTER. By JOSEPH JOHN THOMSON,
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PROBLEMS OF GENETICS

BY

WILLIAM BATESON, M.A., F.R.S.

DIRECTOR OF THE JOHN INNES HO"firiCULTURAL INSTITUTION, HON. FELLOW OF ST. JOHN'S
COLLEGE, CAMBRIDGE, AND FORMERLY PROFESSOR OF BIOLOGY IN THE UNIVERSITY

WITH ILLUSTRATIONS

NEW HAVEN: YALE UNIVERSITY PRESS

LONDON: HUMPHREY MILFORD

OXFORD UNIVERSITY PRESS

MCMXIII

Copyright, 1913
By YALE UNIVERSITY

First printed August, 1913, 1000 copies

THE SILLIMAN FOUNDATION

In the year 1883 a legacy of about eighty-five thousand
dollars was left to the President and Fellows of Yale College
in the city of New Haven, to be held in trust, as a gift from her
children, in memory of their beloved and honored mother, Mrs.
Hepsa Ely Silliman.

On this foundation Yale College was requested and directed
to establish an annual course of lectures designed to illustrate
the presence and providence, the wisdom and goodness of God,
as manifested in the natural and moral world. These were to be
designated as the Mrs. Hepsa Ely Silliman Memorial Lectures.
It was the belief of the testator that any orderly presentation
of the facts of nature or history contributed to the end of this
foundation more effectively than any attempt to emphasize the
elements of doctrine or of creed; and he therefore provided that
lectures on dogmatic or polemical theology should be excluded
from the scope of this foundation, and that the subjects should be
selected rather from the domains of natural science and history,
giving special prominence to astronomy, chemistry, geology, and
anatomy.

It was further directed that each annual course should be made
the basis of a volume to form part of a series constituting a
memorial to Mrs. Silliman. The memorial fund came into the
possession of the Corporation of Yale University in the year 1901 ;
and the present volume constitutes the fifth of the series of
memorial lectures.

PREFACE

This book gives the substance of a series of lectures delivered
in Yale University, where I had the privilege of holding the office
of Silliman Lecturer in 1907.

The delay in publication was brought about by a variety of
causes.

Inasmuch as the purpose of the lectures is to discuss some of
the wider problems of biology in the light of knowledge acquired
by Mendelian methods of analysis, it was essential that a fairly
full account of the conclusions established by them should first
be undertaken and I therefore postponed the present work till
a book on Mendel's Principles had been completed.

On attempting a more general discussion of the bearing of
the phenomena on the theory of Evolution, I found myself
continually hindered by the consciousness that such treatment
is premature, and by doubt whether it were not better that the
debate should for the present stand indefinitely adjourned.
That species have come into existence by an evolutionary
process no one seriously doubts; but few who are familiar with
the facts that genetic research has revealed are now inclined to
speculate as to the manner by which the process has been ac-
complished. Our knowledge of the nature and properties of
living things is far too meagre to justify any such attempts.
Suggestions of course can be made: though, however, these
ideas may have a stimulating value in the lecture room, they
look weak and thin when set out in print. The work which may
one day give them a body has yet to be done.

The development of negations is always an ungrateful task
apt to be postponed for the positive business of experiment.
Such work is happily now going forward in most of the centers
of scientific life. Of many of the subjects here treated we already
know more than we did in 1907. The delay in production has
made it possible to incorporate these new contributions.

The book makes no pretence at being a treatise and the

vii

Vlll

PREFACE

number of illustrative cases has been kept within a moderate
compass. A good many of the examples have been chosen from
American natural history, as being appropriate to a book in-
tended primarily for American readers. The facts are largely
given on the authority of others, and I wish to express my
gratitude for the abundant assistance received from American
colleagues, especially from the staffs of the American Museum
in New York, and of the Boston Museum of Natural History.
In connexion with the particular subjects personal acknowledg-
ments are made.

Dr. F. M. Chapman was so good as to supervise the prepara-
tion of the coloured Plate of Colaptes, and to authorize the loan
of the Plate representing the various forms of Helminthophila,
which is taken from his North American Warblers.

I am under obligation to Messrs. Macmillan & Co., for per-
mission to reproduce several figures from Materials for the Study
of Variation, illustrating subjects which I wished to treat in
new associations, and to M. Leduc for leave to use Fig. 9.

In conclusion I thank my friends in Yale for the high honour
they did me by their invitation to contribute to the series of
Silliman Lectures, and for much kindness received during a
delightful sojourn in that genial home of learning.

TABLES OF CONTENTS.

CHAPTER PAGE

I. INTRODUCTORY. THE PROBLEM OF SPECIES AND

VARIETY i

II. MERISTIC PHENOMENA 31

III. SEGMENTATION, ORGANIC AND MECHANICAL 60

IV. THE CLASSIFICATION OF VARIATION AND THE NATURE

OF SUBSTANTIVE VARIATION 83

NOTE TO CHAPTER IV 94

V. THE MUTATION THEORY 97

NOTE TO CHAPTER V 116

VI. VARIATION AND LOCALITY 118

VII. LOCAL DIFFERENTIATION — continued. OVERLAPPING

FORMS 146

VIII. LOCALLY DIFFERENTIATED FORMS — continued. CLI-
MATIC VARIETIES 164

IX. THE EFFECTS OF CHANGED CONDITIONS 187

X. THE EFFECTS OF CHANGED CONDITIONS — continued.

THE CAUSES OF GENETIC VARIATION 212

XI. THE STERILITY OF HYBRIDS. CONCLUDING RE-
MARKS 233

APPENDIX TO CHAPTER X 250

INDEX 251

PROBLEMS OF GENETICS

PROBLEMS OF GENETICS

CHAPTER I

INTRODUCTORY

THE purpose of these lectures is to discuss some of the
familiar phenomena of biology in the light of modern discoveries.
In the last decade of the nineteenth century many of us per-
ceived that if any serious advance was to be made with the group
of problems generally spoken of as the Theory of Evolution,
methods of investigation must be devised and applied of a kind
more direct and more penetrating than those which after the
general acceptance of the Darwinian views had been deemed
adequate. Such methods obviously were to be found in a
critical and exhaustive study of the facts of variation and heredity,
upon which all conceptions of evolution are based. To construct
a true synthetic theory of Evolution it was necessary that vari-
ation and heredity instead of being merely postulated as axioms
should be minutely examined as phenomena. Such a study
Darwin himself had indeed tentatively begun, but work of a
more thorough and comprehensive quality was required. In
the conventional view which the orthodoxy of the day prescribed,
the terms variation and heredity stood for processes so vague
and indefinite that no analytical investigation of them could be
contemplated. So soon, however, as systematic inquiry into
the natural facts was begun it was at once found that the ac-
cepted ideas of variation were unfounded. Variation was seen
very frequently to be a definite and specific phenomenon, af-
fecting different forms of life in different ways, but in all its
diversity showing manifold and often obvious indications of
regularity. This observation was not in its essence novel.
Several examples of definite variation had been well known to

2 PROBLEMS OF GENETICS

Darwin and others, but many, especially Darwin himself in his
later years, had nevertheless been disposed to depreciate the
significance of such facts. They consequently then lapsed into
general disparagement. Upon more careful inquiry the abun-
dance of such phenomena proved to be far greater than was
currently supposed, and a discussion of their nature brought
into prominence a consideration of greater weight, namely
that the differences by which these definite or discontinuous
variations are constituted again and again approximate to and
are comparable with the class of differences by which species
are distinguished from each other.

The interest of such observations could no longer be denied.
The more they were examined the more apparent it became that
by means of the facts of variation a new light was obtained on
the physiological composition and capabilities of living things.
Genetics thus cease to be merely a method of investigating
theories of evolution or of the origin of species but provide a
novel and hitherto untried instrument by which the nature of
the living organism may be explored. Just as in the study of
non-living matter science began by regarding the external
properties of weight, opacity, colour, hardness, mode of occur-
rence, etc., noting only such evidences of chemical attributes
and powers as chance spontaneously revealed; and much later
proceeded to the discovery that these casual manifestations of
chemical properties, rightly interpreted, afford a key to the
intrinsic nature of the diversity of matter, so in biology, having
examined those features of living things which ordinary obser-
vations can perceive, we come at last to realize that when studied
for their own sake the properties of living organisms in respect
of heredity and variation are indications of their inner nature
and provide evidences of that nature which can be obtained from
no other source.

While such ideas were gradually forming in our minds, came
the rediscovery of Mendel's work. Investigations which before
had only been imagined as desirable now became easy to pursue,
and questions as to the genetic inter-relations and compositions
of varieties can now be definitely answered. Without prejudice

INTRODUCTORY 3

to what the future may disclose whether by way of limitation
or extension of Mendelian method, it can be declared with
confidence and certainty that we have now the means of be-
ginning an analysis of living organisms, and distinguishing many
of the units or factors which essentially determine and cause the
development of their several attributes.

Briefly put, the essence of Mendelism lies in the discovery
of the existence of unit characters or factors. For an account
of the Mendelian method, how it is applied and what it has
already accomplished, reference must be made to other works.1
With this part of the subject I shall assume a sufficient ac-
quaintance. In these lectures I have rather set myself the task
of considering how certain problems appear when viewed from
the standpoint to which the application of these methods has
led us. It is indeed somewhat premature to discuss such ques-
tions. The work of Mendelian analysis is progressing with
great rapidity and anything I can say may very soon be super-
seded as out of date. Nevertheless a discussion of this kind
may be of at least temporary service in directing inquiry to the
points of special interest.

THE PROBLEM OF SPECIES AND VARIETY

Nowhere does our new knowledge of heredity and variation
apply more directly than to the problem what is a species and
what is a variety? I cannot assert that we are already in a
position to answer this important question, but as will presently
appear, our mode of attack and the answers we expect to re-
ceive are not those that were contemplated by our predecessors.
If we glance at the history of the scientific conception of Species
we find many signs that it was not till comparatively recent
times that the definiteness of species became a strict canon of
the scientific faith and that attempts were made to give precise
limits to that conception. When the diversity of living things
began to be accurately studied in the sixteenth and seventeenth

1 In Mendel's Principles of Heredity (Cambridge University Press, 1909)
I have dealt with this subject, giving an account of the principal facts discovered
up to the beginning of 1909.

4 PROBLEMS OF GENETICS

centuries names were applied in the loosest fashion, and in giving
a name to an animal or a plant the naturalists of those times had
no ulterior intention. Names were bestowed on those creatures
about which the writer proposed to speak. When Gesner or Aldro-
vandi refer to all the kinds of horses, unicorns, dogs, mermaids,
etc., which they had seen or read of, giving to each a descriptive
name, they do not mean to "elevate" each named kind to "spe-
cific rank"; and if anyone had asked them what they meant by
a species, it is practically certain that they would have had not
the slightest idea what the question might imply, or any suspicion
that it raised a fundamental problem of nature.

Spontaneous generation being a matter of daily observation,
then unquestioned, and supernatural events of all kinds being
commonly reported by many witnesses, transmutation of species
had no inherent improbability. Matthioli,2 for instance, did not
expect to be charged with heresy when he declared Stirpium
mutatio to be of ordinary occurrence. After giving instances
of induced modifications he wrote, "Tan turn enim in plantis
naturae germanitas potest, ut non solum saepe praedictos
praestet effectus, sed etiam ut alteram in alteram stirpem facile
vertat, ut cassiam in cinnamomum, sisymbrium in mentham,
triticum in lolium, hordeum in avenam, et ocymum in serpyllum."

1 do not know who first emphasized the need for a clear
understanding of the sense in which the term species is to be
applied. In the second half of the seventeenth century Ray
shows some degree of concern on this matter. In the intro-
duction to the Historia Plantarum, 1686, he discusses some
of the difficulties and lays down the principle that varieties
which can be produced from the seed of the same plant are to
be regarded as belonging to one species, being, I believe, the
first to suggest this definition. That new species can come into
existence he denies as inconsistent with Genesis 2, in which it is
declared that God finished the work of Creation in six days.
Nevertheless he does not wholly discredit the possibility of a
"transmutation" of species, such that one species may as an
exceptional occurrence give rise by seed to another and nearly

2 Matthioli Opera, Ed. 1598, p. 8, originally published 1565.

INTRODUCTORY 5

allied species. Of such a phenomenon he gives illustrations the
authenticity of which he says he is, against his will, compelled to
admit. He adds that some might doubt whether in the cases
quoted the two forms concerned are really distinct species, but
the passage is none the less of value for it shews that the con-
ception of species as being distinct unchangeable entities was not
to Ray the dogma sacrosanct and unquestionable which it
afterwards became.3

In the beginning of the eighteenth century Marchant,4 having
observed the sudden appearance of a lacinated variety of Mercu-
rialis, makes the suggestion that species in general may have
arisen by similar mutations. Indeed from various passages it is
manifest that to the authors of the seventeenth and early eigh-
teenth centuries species appeared simply as groups more or less
definite, the boundaries of which it was unnecessary to determine
with great exactitude. Such views were in accord with the
general scientific conception of the time. The mutability of

3 Ray's instances relate to Kales, and in most of these examples we can
see that there was no question of mutation or transmutation at all, but that the
occurrence was due either to mistake or to cross-fertilisation. Sharrock, to whom
Ray refers, was inclined to discredit stories of transmutation, but he has also this
passage (History of the Propagation and Improvement of Vegetables by the Concurrence
of Art and Nature, Oxford, 1660, p. 29):

" It is indeed growen to be a great question, whether the transmutation of a
species be possible either in the vegetable, Animal, or Minerall Kingdome. For the
possibility of it in the vegetable; I have heard Mr. Bobart and his Son often report it,
and proffer to make oath that the Crocus and Gladiolus, as likewise the Leucoium,
and Hyacinths by a long standing without replanting have in his garden changed
from one kind to the other: and for satisfaction about the curiosity in the presence
of Mr. Boyle I tooke up some bulbs of the very numericall roots whereof the re-
lation was made, though the alteration was perfected before, where we saw the
diverse bulbs growing as it were on the same stoole, close together, but no bulb
hah" of the one kind, and the other half of the other: But the changetime being past
it was reason we should believe the report of good artists in matters of their own
faculty."

Robert Sharrock was a fellow of New College, Oxford. Both the Bobarts were
professional botanists, the father was author of a Catalogue of the plants in the
Hortus Medicus at Oxford, and the son was afterwards Curator of the Oxford
Garden.

*Mem. Ac. roy. des Sci. for 1719 (1721), p. 59-

6 PROBLEMS OF GENETICS

species is for example sometimes likened (see for instance Shar-
rock, loc. cit.) to the metamorphoses of insects, and it is to be
remembered that the search for the Philosopher's Stone by which
the transmutation of metals was to be effected had only recently
fallen into discredit as a pursuit.

The notion indeed of a peculiar, fixed meaning to be attached
to species as distinct from variety is I think but rarely to be
found categorically expressed in prae-Linnaean writings.

But with the appearance of the Systema Naturae a great
change supervened. Linnaeus was before all a man of order.
Foreseeing the immense practical gain to science that must come
from a codification of nomenclature, he invented such a system.

It is not in question that Linnaeus did great things for us and
made Natural History a manageable and accessible collection of
facts instead of a disorderly heap ; but orderliness of mind has
another side, and inventors and interpreters of systems soon attri-
bute to them a force and a precision which in fact they have not.

The systematist is primarily a giver of names, as Ray with
his broader views perceived. Linnaeus too in the exordium to
the Systema Naturae naively remarks, that he is setting out to
continue the work which Adam began in the Golden Age, to give
names to the living creatures. Naming however involves very
delicate processes of mind and of logic. Carried out by the light
of meagre and imperfect knowledge it entails all the mischievous
consequences of premature definition, and promotes facile
illusions of finality. So was it with the Linnaean system. An
interesting piece of biological history might be written respecting
the growth and gradual hardening of the conception of Species.
To readers of Linnaeus's own writings it is well known that his
views cannot be summarized in a few words. Expressed as they
were at various times during a long life and in various connexions,
they present those divers inconsistencies which commonly
reflect a mind retaining the power of development. Nothing
certainly could be clearer than the often quoted declaration of the
Philosophia Botanica, "Species tot numeramus quot diversae
formae in principio sunt creatae," with the associated passage
"Varietates sunt plantae ejusdem speciei mutatae a caussa

INTRODUCTORY 7

quacunque occasional!." Those sayings however do not stand
alone. In several places, notably in the famous dissertation
on the peloric Linaria he explicitly contemplates the possibility
that new species may arise by crossing, declaring nevertheless
that he thinks such an event to be improbable. In that essay
he refers to Marchant's observation on a laciniate Mercurialis,
but though he states clearly that that plant should only be
regarded as a variety of the normal, he does not express any
opinion that the contemporary genesis of new species must be an
impossibility. In the later dissertation on Hybrid Plants he
returns to the same topic. Again though he states the belief
that species cannot be generated by cross-breedings, he treats
the subject not as heretical absurdity but as one deserving
respectful consideration.

The significance of the aphorisms that precede the lectures
on the Natural Orders is not easy to apprehend. These are
expressed with the utmost formality, and we cannot doubt that
in them we have Linnaeus's own words, though for the record
we are dependent on the transcripts of his pupils.

The text of the first five is as follows:

1. Creator T. O. in primordio vestiit Vegetabile Medullare
principiis constitutivis diversi Corticalis unde tot difformia
individua, quot Ordines Naturales prognata.

2. Classicas has (i) plantas Omnipotens miscuit inter se,
unde tot Genera ordinum, quot inde plantae.

3. Genericas has (2) miscuit Natura, unde tot Species con-
generes quot hodie existunt.

4. Species has miscuit Casus, unde totidem quot passim
occurrunt, Varietates.

5. Suadent haec (1-4) Creatoris leges a simplicibus ad
Composita.

Naturae leges generationis in hybridis.

Hominis leges ex observatis a posteriori.

I am not clear as to the parts assigned in the first sentence
respectively to the "Medulla" and the "Cortex" beyond that
Linnaeus conceived that multiformity was first brought about
by diversity in the "Cortex" The passage is rendered still

8 PROBLEMS OF GENETICS

more obscure if read in connection with the essay on "Generatio
Ambigena" where be expresses the conviction that the Medulla
is contributed by the mother, and the Cortex by the father, both
in plants and animals.5

But however that may be, he regards this original diversity
as resulting in the constitution of the Natural Orders, each rep-
resented by one individual.

In the second aphorism the Omnipotent is represented as
creating the genera by intermixing the individual plantae classicae,
or prototypes of the Natural Orders.

The third statement is the most remarkable, for in it he
declares that Species were formed by the act of Nature, who by
inter-mixing the genera produced Species congener es, namely
species inside each genus, to the number which now exist.
Lastly, Chance or Accident, intermixing the species, produced
as many varieties as there are about us.

Linnaeus thus evidently regarded the intermixing of an
originally limited number of types as the sufficient cause of
all subsequent diversity, and it is clear that he draws an an-
tithesis between Creator, Natura, and Casus, assigning to each
a special part in the operations. The acts resulting in the
formation of genera are obviously regarded as completed within
the days of the Creation, but the words do not definitely show
that the parts played by Nature and Chance were so limited.

Recently also E. L. Greene6 has called attention to some
curious utterances buried in the Species Plantarum, in which
Linnaeus refers to intermediate and transitional species, using
language that even suggests evolutionary proclivities of a
modern kind, and it is not easy to interpret them otherwise.

Whatever Linnaeus himself believed to be the truth, the
effect of his writings was to induce a conviction that the species

B Amoen. Acad., 1789, vol. 6. I do not know whether attention has been called
to the curious mistake which Linnaeus makes in the course of this argument. He
cites the differences between the Mule and the Hinny in illustration of his thesis,
pointing out that the Mule is externally more like a horse and the Hinny more like
an ass. This, he says, is because the Mule has the horse for a father, and the
Hinny the ass, thus inverting the actual facts!

*Proc. Washington Ac. Sci., 1909, XI, pp. 17-26.

INTRODUCTORY 9

of animals and plants were immutably fixed. Linnaeus had
reduced the whole mass of names to order and the old fantastical
transformations with the growth of knowledge had lapsed into
discredit; the fixity of species was taken for granted, but not
till the overt proclamation of evolutionary doctrine by Lamarck
do we find the strenuous and passionate assertions of immutability
characteristic of the first half of the nineteenth century.

It is not to be supposed that the champions of fixity were
unacquainted with varietal differences and with the problem
thus created, but in their view these difficulties were apparent
merely, and by sufficiently careful observation they supposed
that the critical and permanent distinctions of the true species
could be discovered, and the impermanent variations detected
and set aside.

This at all events was the opinion formed by the great body
of naturalists at the end of the eighteenth and beginning of the
nineteenth centuries, and to all intents and purposes in spite
of the growth of evolutionary ideas, it remains the guiding
principle of systematists to the present day. There are 'good
species' and 'bad species' and the systematists of Europe and
America spend most of their time in making and debating them.

In some of its aspects the problem of course confronted earlier
naturalists. Parkinson for instance (1640) in introducing his
treatment of Hieracium wrote, "To set forth the whole family of
the Hawkew^eedes in due forme and order is such a world of
worke that I am in much doubt of mine own abilitie, it having
lyen heavie on his shoudiers that hath already waded through
them ... for such a multitude of varieties in forme pertaining
to one herbe is not to be found againe in rerum natura as I
thinke," and the same idea, that the difficulty lay rather in
man's imperfect powers of discrimination than in the nature of
the materials to be discriminated, is reflected in many treatises
early and late.

It was however with the great ouburst of scientific activity
which followed Linnaeus that the difficulty became acute.
Simultaneously vast masses of new material were being collected
from all parts of the world into the museums, and the products

io PROBLEMS OF GENETICS

of the older countries were re-examined with a fresh zeal and on
a scale of quantity previously unattempted. But the problem
how to name the forms and where to draw lines, how much
should be included under one name and where a new name was
required, all this was felt, rather as a cataloguer's difficulty
than as a physiological problem. And so we still hear on the
one hand of the confusion caused by excessive "splitting" and
subdivisions, and on the other of the uncritical "lumpers" who
associate together under one name forms which another collector
or observer would like to see distinguished.

In spite of Darwin's hopes, the acceptance of his views has
led to no real improvement — scarcely indeed to any change at
all in either the practice or aims of systematists. In a famous
passage in the Origin he confidently declares that when his
interpretation is generally adopted "Systematists will be able
to pursue their labours as at present; but they will not be in-
cessantly haunted by the shadowy doubt whether this or that
form be a true species. This, I feel sure, and I speak after
experience, will be no slight relief. The endless disputes whether
or not some fifty species of British brambles are good species
will cease." Those disputes nevertheless proceed almost ex-
actly as before. It is true that biologists in general do not,
as formerly, participate in these discussions because they have
abandoned systematics altogether; but those who are engaged
in the actual work of naming and cataloguing animals and
plants usually debate the old questions in the old way. There
is still the same divergence of opinion and of practice, some in-
clining to make much of small differences, others to neglect
them.

Not only does the work of the sytematists as a whole proceed
as if Darwin had never written but their attitude towards these
problems is but little changed. In support of this statement I may
refer to several British Museum Catalogues, much of the Biologia
Centrali- Americana, Ridgway's Birds of North America, the
Fauna Hawaiensis, indeed to almost any of the most important
systematic publications of England, America, or any other
country. These works are compiled by the most proficient

INTRODUCTORY n

systematists of all countries in the several groups, but with
rare exceptions they show little misgiving as to the fundamental
reality of specific differences. That the systematists consider
the species-unit as of primary importance is shown by the
fact that the whole business of collection and distribution of
specimens is arranged with regard to it.

Almost always the collections are arranged in such a way that
the phenomena of variation are masked. Forms intermediate
between two species are, if possible, sorted into separate boxes
under a third specific name. If a species is liable to be constantly
associated with a mutational form, the mutants are picked out,
regardless of the circumstances of their origin, from the samples
among which they were captured, and put apart under a special
name. Only by a minute study of the original labels of the
specimens and by redistributing them according to locality and
dates, can their natural relations be traced. The published
accounts of these collections often take no notice of variations,
others make them the subject of casual reference. Very few
indeed treat them as of much importance. From such indi-
cations it is surely evident that the systematists attach to the
conception of species a significance altogether different from that
which Darwin contemplated.

I am well aware that some very eminent systematists regard
the whole problem as solved. They hold as Darwin did that
specific diversity has no physiological foundation or causation
apart from fitness, and that species are impermanent groups,
the delimitations of which are ultimately determined by en-
vironmental exigency or "fitness." The specific diversity of
living things is thus regarded as being something quite different
in nature from the specific diversity of inorganic substances.
In practice those who share these opinions are, as might be an-
ticipated, to be found among the Mumpers' rather than among
the 'splitters/ In their work, certainly, the Darwinian theory
is actually followed as a guiding principle; unanalysed inter-
gradations of all kinds are accepted as impugning the integrity
of species; the underlying physiological problem is forgotten,
and while the product is amost valueless as a contribution to

12 PROBLEMS OF GENETICS

biological research, I can scarcely suppose that it aids greatly
in the advances of other branches of our science.

But why is it that, with these exceptions, the consequences
of the admittedly general acceptance of a theory of evolution
are so little reflected in the systematic treatment of living things?
Surely the reason is that though the systematist may be con-
vinced of the general truth of the evolution theory at large, he
is still of opinion that species are really distinct things. For
him there are still 'good' species and 'bad' species and his ex-
perience tells him that the distinction between the two is not
simply a question of degree or a matter of opinion.

To some it may seem that this is mere perversity, a refusal
to see obvious truth, a manifestation of the spirit of the collector
rather than of the naturalist. But while recognising that from
a magnification of the conception of species the systematists
are occasionally led into absurdity I do not think the grounds
for their belief have in recent times been examined with the
consideration they deserve. The phenomenon of specific
diversity is manifested to a similar degree by living things be-
longing to all the great groups, from the highest to the lowest,
Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae,
and Bacteria, all present diversities of such a kind that among
them the existence of specific differences can on the whole be
recognised with a similar degree of success and with very similar
limitations. In all these groups there are many species quite
definite and unmistakable, and others practically indefinite.
The universal presence of specificity, as we may call it, simi-
larly limited and characterised, is one of its most remarkable
features. Not only is this specificity thus universally present
among the different forms of life, but it manifests itself in respect
of the most diverse characteristics which living things display.
Species may thus be distinguished by peculiarities of form, of
number, of geometrical arrangement, of chemical constitution
and properties, of sexual differentiation, of development, and of
many other properties. In any one or in several of these features
together, species may be found distinguished from other species.
It is also to be observed that the definiteness of these distinctions

INTRODUCTORY 13

has no essential dependence on the nature of the characteristic
which manifests them. It is for example sometimes said that
colour-distinctions are of small systematic importance, but every
systematist is familiar with examples (like that of the wild species
of Callus) in which colours though complex, show very little
variation. On the other hand features of structure, sexual dif-
ferentiation, and other attributes which by our standards are
estimated as essential, may be declared to show much variation
or little, not according to any principle which can be detected,
but simply as the attention happens to be applied to one species
or group of species, or to another. In many groups of animals and
plants observers have hit upon characters which were for a time
thought to be finally diagnostic of species. The Lepidoptera and
Diptera for instance, have been re-classified according to their
neura tion . Through a considerable range of forms determinations
may be easily made on these characters, but as is now well
known, neura tion is no more immune from variation than any
other feature of organisation, and in some species great varia-
bility is the rule. Again it was once believed by some that the
genitalia of the Lepidoptera provided a basis of final determination
— with a similar sequel. In some groups, for example the Lycae-
nidae, or the Hesperidae, there are forms almost or quite in-
distinguishable on external examination, but a glance at the
genitalia suffices to distinguish numerous species, while on the
contrary among Pieridae a great range of species show scarcely
any difference in these respects: and again in occasional species
the genitalia show very considerable variations.

The proposition that animals and plants are on the whole
divisible into definite and recognisable species is an approxi-
mation to the truth. Such a statement is readily defensible,
whereas to assert the contrary would be palpably absurd. For
example, a very competent authority lately wrote: "In the
whole Lepidopterous fauna of England there is no species of
really uncertain limits." 7 Others may be disposed to make
certain reservations, but such exceptions would be so few as
scarcely to impair the validity of the general statement. The

7 J. W. Tutt, in Ent. Rec., 1909, XXI, p. 185.

H PROBLEMS OF GENETICS

declaration might be extended to other orders and other
lands.

We know, of course, that the phenomenon of specific diversity
is complicated by local differentiation: that, in general, forms
which cannot disperse themselves freely exhibit a multitude of
local races, and that of these some are obviously adaptative,
and that a few even owe their peculiarity to direct envitonmental
effects. Every systematist also is perfectly aware that in dealing
with collections from little explored countries the occurrence
of polymorphism or even of sporadic variation may make the
practical business of distinguishing the species difficult and
perhaps for the time impossible; still, conceding that a great
part of the diversity is due to geographical differentiation, and
that some is sporadic variation, our experience of our own floras
and faunas encourages the belief that if we were thoroughly
familiar with these exotic productions it would usually be
possible to assign their specific limitations with an approach
to certainty.

For apart from any question of the justice of these wider
inferences, if we examine the phenomenon of specificity as it
appears in those examples which are nearest to hand, surely we
find signs in plenty that specific distinction is no mere consequence
of Natural Selection. The strength of this proposition has
lain mainly in the appeal to ignorance. Steadily with the growth
of knowledge has its cogency diminished, and such a belief
could only have been formulated at a time when the facts of
variation were unknown.

In Darwin's time no serious attempt had been made to ex-
amine the manifestations of variability. A vast assemblage of
miscellaneous facts could formerly be adduced as seemingly
comparable illustrations of the phenomenon "Variation."
Time has shown this mass of evidence to be capable of analysis.
When first promulgated it produced the impression that varia-
bility was a phenomenon generally distributed amongst living
things in such a way that the specific divisions must be arbitrary.
When this variability is sorted out, and is seen to be in part a
result of hybridisation, in part a consequence of the persistence

INTRODUCTORY 15

of hybrids by parthenogenetic reproduction, a polymorphism
due to the continued presence of individuals representing various
combinations of Mendelian allelomorphs, partly also the tran-
sient effect of alteration in external circumstances, we see how
cautious we must be in drawing inferences as to the indefiniteness
of specific limits from a bare knowledge that intermediates exist.
Conversely, from the accident of collocation or from a mislead-
ing resemblance in features we deem essential, forms genetically
distinct are often confounded together, and thus the divergence
of such forms in their other features, which we declare to be
non-essential, passes as an example of variation. Lastly, and
this is perhaps the most fertile of all the sources of confusion,
the impression of the indefiniteness of species is created by the
existence of numerous local forms, isolated geographically from
each other, forms whose differences may be referable to any one
of the categories I have enumerated.

The advance has been from many sides. Something has
come from the work of systematists, something from cultural
experiments, something from the direct study of variation as it
appears in nature, but progress is especially due to experimental
investigation of heredity. From all these lines of inquiry we
get the same answer; that what the naturalists of fifty years
ago regarded as variation is not one phenomenon but many,
and that what they would have adduced as evidence against
the definiteness of species may not in fact be capable of this
construction at all.

If we may once more introduce a physical analogy, the dis-
tinctions with which the systematic naturalist is concerned in
the study of living things are as multifarious as those by which
chemists were confronted in the early days of their science.
Diversities due to mechanical mixtures, to allotropy, to differences
of temperature and pressure, or to degree of hydration, had all
to be severally distinguished before the essential diversity due
to variety of chemical constitution stood out clearly, and I
surmise that not till a stricter analysis of the diversities of animals
and plants has been made on a comprehensive scale, shall we
be in a position to declare with any confidence whether there is

16 PROBLEMS OF GENETICS

or is not a natural and physiological distinction between species
and variety.

As I have said above, it is in the cases nearest to hand that
the problem may be most effectively studied. Comparison
between forms from dissimilar situations contributes something;
but it is by a close examination of the behaviour, especially the
genetic behaviour, of familiar species when living in the presence
of their nearest allies that the most direct light on the problem
is to be obtained. I cannot understand the attitude of those who,
contemplating such facts as this examination elicits, can com-
placently declare that specific difference is a mere question of
degree. With the spread of evolutionary ideas to speak much
of the fixity of species has become unfashionable, and yet how
striking and inscrutable are the manifestations of that fixity!

Consider the group of species composing the agrestis section
of the genus Veronica, namely Tournefortii, agrestis, and
polita.

These three grow side by side in my garden, as they do in
suitable situations over a vast area of the temperate regions.
I have for years noticed them with some care and become familiar
with their distinctions and resemblances. Never is there any
real doubt as to the identity of any plant. The species show
some variability, but I have never seen one which assumed any
of the distinguishing features of the others. A glance at the
fruits decides at once to which species a plant belongs. I find
it impossible to believe that the fixity of these distinctions is
directly dependent on their value as aids in the struggle for
existence. The mode of existence of the three forms in so far
as we can tell is closely similar. By whatever standard we reckon
systematic affinity I suppose we shall agree that these species
come very near indeed to each other. Bentham even takes
the view that polita is a mere variety of agrestis.

Now in such cases as this it has been argued that the specific
features of the several types have been separately developed
in as many distinct localities, and that their present association
is due to subsequent redistribution. Of these Veronicas indeed
we know that one, Tournefortii (=Buxbaumii) is as a matter of fact

INTRODUCTORY 17

a recent introduction from the east.8 But this course of argument
leads to still further difficulties. For if it is true that the peculiari-
ties of the several species have been perfected and preserved on
account of their survival-value to their possessors, it follows
that there must be many ways of attaining the same result.
But since sufficient adaptation may be ensured in so many ways,
the disappearance of the common parent of these forms is dif-
ficult to understand. Obviously it must have been a plant
very similar in general construction to its modern representatives.
Like them it must have been an annual weed, with an organisation
conformable to that mode of life. Why then, after having been
duly perfected for that existence should it have been entirely
superseded in favour of a number of other distinct contrivances
for doing the same thing, and — if a gradual transition be predi-
cated— not only by them, but by each intermediate stage
between them and the original progenitor? Surely the obvious
inference from such facts is that the burden cast upon the theory
of gradual selection is far greater than it can bear; that adapta-
tion is not in practice a very close fit, and that the distinctions
between these several species of Veronica have not arisen on
account of their survival-value but rather because none of their
diversities was so damaging as to lead to the extermination of
its possessor. When we see these various Veronicas each rigidly
reproducing its parental type, all comfortably surviving in
competition with each other, are we not forced to the conclusion
that tolerance has as much to do with the diversity of species
as the stringency of Selection? Certainly these species owe their
continued existence to the fact that they are each good enough
to live, but how shall we refer the distinctions between them di-
rectly or indirectly to the determination of Natural Selection?

SE. Lehmann (Bull. I'Herb. Boissier, Sen 2, VIII, 1908, p. 229) has published
an admirable paper on the interrelationships of these species and has instituted
cultural experiments which will probably much elucidate the nature of their specific
distinctness. As regards the existence of intermediate forms he comes to the conclu-
sion that two only can be so regarded. The first was described by Kuntze from
specimens found on a flower-pot on board a Caspian steamer, from which Leh-
mann proposes the new specific name Siaretensis. This comes between polita and
filifarmis, a close ally of Tournefortii. The other, which combines some of the
features of both polita and Tournefortii, was found in the province of Asterabad.
3

i8 PROBLEMS OF GENETICS

The control of Selection is loose while the conformity to
specific distinction is often very strict and precise, and no less
so even when several closely related species co-exist in the same
area and in the same circumstances.

The theory of Selection fails at exactly the point where it
was devised to help: Specific distinction.

Let us examine a somewhat different set of facts in the case
of another pair of nearly allied species Lychnis diurna and ves-
pertina. The two plants have much in common. Both are
dioecious perennials, with somewhat similar flowers, the one
crimson, the other white. Each however has its peculiarities
which are discernible in almost any part of its structure, whether
flower, leaf, fruit or seed, distinctions which would enable a
person thoroughly familiar with the plants to determine at once
from which species even a small piece had been taken. There
is so much resemblance however as readily to support the surmise
that the two were mere varieties of one species. Bentham,
following Linnaeus, in fact actually makes this suggestion,
with what propriety we will afterwards consider. Now this case
is typical of many. The two forms have a wide distribution,
occurring sometimes separately, sometimes in juxtaposition.
L. diurna is a plant of hedgerows and sheltered situations. L.
vesper tina is common in fields and open spaces, where diurna
is hardly ever found; but not rarely vespertina occurs in associa-
tion with diurna in the places which that plant frequents. In
this case I do not doubt that we have to do with organisms of
somewhat different aptitudes. That L. vespertina has powers
which diurna has not is shown very clearly by the fact that
diurna is sometimes entirely absent from areas where vespertina
can abound.9 But in order to understand the true genetic
relations of the two plants to each other it is necessary to observe
their behaviour when they meet as they not unfrequently do.

g In Cambridgeshire for example vespertina is common but diurna is absent.
Whether this absence is connected with the general presence of chalk I cannot say.
When introduced artificially diurna establishes itself, for a time at least, without any
apparent difficulty and occasionally escapes from the garden on to the neighbouring
roadside.

INTRODUCTORY 19

If the Lychnis population of such a locality be examined it will
be found to consist of many undoubted and unmodified diurna,
a number — sometimes few, sometimes many — of similarly
unmodified vespertina, and an uncertain but usually rather small
proportion of plants obviously hybrids between the two. How
is it possible to reconcile these facts with the view that specific
distinction has no natural basis apart from environmental
exigency?

Darwinian orthodoxy suggests that by a gradual process of
Natural Selection either one of these two types was evolved from
the other, or both from a third type. I cannot imagine that
anyone familiar with the facts would propose the first hypothesis
in the case of Lychnis, nor can I conceive of any process, whether
gradual or sudden, by which diurna could have come out of
vespertina, or vespertina out of diurna. Both however may no
doubt have been derived from some original third type. It is
conceivable that Lychnis macrocarpa of Boissier, a native of
Southern Spain and Morocco, may be this original form. This
species is said to combine a white flower (like that of L. ves-
pertina), with capsule- teeth rolled back (like those of diurna).10
But whatever the common progenitor may have been, if we are
to believe that these two species have been evolved from it by
a gradual process of Natural Selection based on adaptation,
enormous assumptions must be made regarding the special fitness
of these two forms and the special unfitness of the common
parent, and these assumptions must be specially invoked and
repeated for each several feature of structure or habits distin-
guishing the three forms.

Why, if the common parent was strong enough to live to give
rise to these two species, is it either altogether lost now, or at
least absent from the whole of Northern Europe? Its two
putative descendants, though so distinct from each other, are,
as we have seen, able often to occupy the same ground. If
they were gradually derived from a common progenitor —
necessarily very like themselves — can we believe that this original

10 Conceivably however it may be a segregated combination. For an account
of this plant see Boissier, Voy. Bot. Midi de I'Espagne, 1839, II, 722.

20 PROBLEMS OF GENETICS

form should always, in all the diversities of soil and situation
which they inhabit, be unable to exist? Some one may fancy
that the hybrids which are found in the situations occupied by
both forms are this original parental species. But nothing can
be more certain than that these plants are simply heterozygous
combinations made by the union of gametes bearing the characters
of diurna and vespertina. n For they may be reproduced exactly
in FI or in later generations of that cross when it is artificially
made ; when bred from their families exhibit palpable phenomena
of segregation more or less complex; and usually, if perhaps not
always, they are partially sterile.12 In a locality on the Norfolk
coast that I know well, there is a strip of rough ground chiefly
sand-bank, which runs along the shore. This ground is full of
vespertina. Not a hundred yards inland is a lane containing
diurna, and among the vespertina on the sand-bank are always
some of the hybrid form, doubtless the result of fertilisation
from the heighbouring diurna population. Seed saved from these
hybrids gave vespertina and hybrids again, having obviously been
fertilised by other vespertina or by other hybrids, and I have
no doubt that such hybrid plants if fertilised by diurna would
have shown some diurna offspring. The absence of diurna
in such localities may fairly be construed as an indication that
diurna is there at a real disadvantage in the competition for
life.

But if, admitting this, we proceed to consider how the special
aptitude of vespertina is constituted, or what it is that puts
diurna at a disadvantage, we find ourselves quite unable to
show the slightest connexion between the success of one or the

11 A discussion of this subject with references to literature is given by Rolf e,
in an excellent paper on " Hybridisation viewed from the standpoint of Systematic
Botany" (Jour. R. Hort. Soc., XXIV, 1900, p. 197). He concludes: "The simple
fact is that the two plants (L. diurna and vespertina) are thoroughly distinct in
numerous particulars, and affect such different habitats that in some localities
one or the other of them is completely wanting. But when their stations are
adjacent they hybridise together very readily, and it is here that these intermediate
forms occur which have puzzled botanists so much." The same paper contains
valuable information concerning several cognate illustrations.

12 In only two cases have I seen such plants (both females) completely sterile.

INTRODUCTORY 21

failure of the other on the one hand, and the specific character*
istics which distinguish the two forms on the other. The or-
thodox Selectionist would, as usual, appeal to ignorance. We
ask what can vespertina gain by its white flowers, its more lan-
ceolate leaves, its grey seeds, its almost erect capsule- teeth,
its longer fruits, which diurna loses by reason of its red flowers,
more ovate leaves, dark seeds, capsule-teeth rolled back, and
shorter fruits? We are told that each of these things may
affect the viability of their possessors. We cannot assert that
this is untrue, but we should like to have evidence that it is true.
The same problem confronts us in thousands upon thousands
of examples, and as time goes on we begin to feel that speculative
appeals to ignorance, though dialectically admissible, provide
an insufficient basis for a proposition which, if granted, is to
become the foundation of a vast scheme of positive construction.

One thing must be abundantly clear to all, that to treat two
forms so profoundly different as one, because intermediates of
unknown nature can be shown to exist between them, is a mere
shirking of the difficulties, and this course indeed creates artificial
obstacles in the way of those who are seeking to discover the
origin of organic diversity.

In the enthusiasm with which evolutionary ideas were re-
ceived the specificity of living things was almost forgotten.
The exactitude with which the members of a species so often
conform in the diagnostic, specific features passed out of account;
and the scientific world by dwelling with a constant emphasis
on the fact of variability, persuaded itself readily that spe-
cies had after all been a mere figment of the human mind.
Without presuming to declare what future research only can
reveal, I anticipate that, when variation has been properly
examined and the several kinds of variability have been suc-
cessfully distinguished according to their respective natures,
the result will render the natural definiteness of species increas-
inlgy apparent. Formerly in such a case as that of the two
Lychnis species, the series of "intermediates" was taken to be a
palpable proof that vespertina "graded" to diurna. It is this
fact, doubtless, upon which Bentham would have relied in sug-

22 PROBLEMS OF GENETICS

gesting that both may be one species.13 Genetic tests, though as
yet imperfectly applied, make it almost certain that these inter-
grading forms are not in any true sense variations from either
species in the direction of the other, but combinations of elements
derived from both.

The points in which very closely allied species are distin-
guished from each other may be found in the most diverse
features of their organisation. Sometimes specific difference
is to be seen in a character which we can believe to be important
in the struggle, but at least as often it is some little detail that
we cannot but regard as trivial which suffices to differentiate
the two species. Even when the diagnostic point is of such a
nature that we can imagine it to make a serious difference in the
economy we are absolutely at a loss to suggest why this feature
should be a necessity to species A and unnecessary to species B
its nearest ally. The house sparrow (Passer domesticus) is in
general structure very like the tree sparrow (P. montanus).
They differ in small points of colour. For instance montanus
has a black patch on the cheek which is absent in domesticus.
The presence in the one species and the absence in the other
are equally definite, and in both cases we are equally unable to
suggest any consideration of utility in relation to these features.
The two species are distinguished also by a characteristic that
may well be supposed to be of great significance. In domesticus
the two sexes are strongly differentiated, the cock being more
ornate than the hen. On the other hand the two sexes in mon-
tanus are alike, and, if we take a standard from domesticus, we
may fairly say that in montanus the hen has the colouration of
the male. It is not unreasonable to suppose that such a dis-
tinction may betoken some great difference in physiological
economy, but the economical significance of this perhaps im-
portant distinction is just as unaccountable as that of the seem-
ingly trivial but equally diagnostic colour-point.

I have spoken of the fixed characteristics of the two species.

13 As is well known, in an even more notorious example, he proposed to unite
Primula vulgaris, P. elatior, and P. acaulis, similarly relying on the existence of
"intermediates," which we now well know to be mongrels between the species.

INTRODUCTORY 23

If we turn to a very different feature, their respective liability
to albinistic variation, we find ourselves in precisely similar
difficulty. Passer domesticus is a species in which individuals
more or less pied occur with especial frequency, but in P. mon-
tanus such variation is extremely rare if it occurs at all. The
writer of the section on Birds in the Royal Natural History
(III., 1894-5, P- 393) calls attention to this fact and remarks
that in that species he knows no such instance.

The two species therefore, apart from any differences that we
can suppose to be related to their respective habits, are charac-
terised by small fixed distinctions in colour-markings, by a
striking difference in secondary sexual characters, and by a
difference in variability. In all these respects we can form
no surmise as to any economic reason why the one species
should be differentiated in the one way and the other in the other
way, and I believe it is mere self-deception which suggests the
hope that with fuller knowledge reasons of this nature would be
discovered.

The two common British wasps, Vespa vulgaris and Vespa
germanica, are another pair of species closely allied although
sharply distinguished, which suggest similar reflexions. Both
usually make subterranean nests but of somewhat different
materials. F. vulgaris uses rotten wood from which the nest
derives a characteristic yellow colour, while F. germanica scrapes,
off the weathered surfaces of palings and other exposed timber,
material which is converted into the grey walls of the nest. The
stalk by which the nest is suspended (usually to a root) in the
case of germanica passes freely through a hole in the external
envelope, but vulgaris unites this external wall solidly to the
stalk. In bodily appearance and structure the two species are
so much alike that they have often been confounded even by
naturalists, and to the untrained observer they are quite indis-
tinguishable. There are nevertheless small points of difference
which almost though not quite always suffice to distinguish the
two forms. For example the yellow part of the sinus of the eyes
is emarginate in vulgaris but not emarginate in germanica. V.
vulgaris often has black spots on the tibiae while in germanica the

24 PROBLEMS OF GENETICS

tibiae are usually plain yellow. In both species there is a hori-
zontal yellow stripe on the thorax, but whereas in vulgaris this is
a plain narrow stripe, it is in germanica enlarged downwards in
the middle. These and other apparently trivial details of colour-
ation, though not absolutely constant, are yet so nearly constant
that irregularities in these respects are quite exceptional. Lastly
the genitalia of the males, though not very different, present
small structural points of distinction which are enough to distin-
guish the two species at a glance.14

In considering the meaning of the distinctions between these
two wasps we meet the old problem illustrated by the Sparrows.
The two species have somewhat different habits of life and we
should readily expect to find differences of bodily organisation
corresponding with the differences of habits. But is that what
we do find? Surely not. To suppose that there is a corre-
spondence between the little points of colour and structure which
we see and the respective modes of life of the two species is
perfectly gratuitous. We have no inkling of the nature of such
a correspondence, how it can be constituted, or in what it may
consist.

Is it not time to abandon these fanciful expectations which
are never realised? Everywhere both among animals and plants
does the problem of specific difference reiterate itself in the same
form. In view of such facts as I have related and might indefi-
nitely multiply, the fixity of specific characters cannot readily
be held to be a measure of their economic importance to their
possessors. The incidence of specific fixity is arbitrary and
capricious, sometimes lighting on a feature or a property which
can be supposed to matter much, but as often is it attached to the
most trifling of superficial peculiarities.

The incidence of variability is no less paradoxical, and without
investigation of the particular case no one can say what will be

"For an account of the distinctions between Vespa vulgaris and germanica
see Ch. Janet, Etudes sur les Fourmis, les GuZpes el les Abeilles, ne, Note. Sur
Vespa germanica et V. vulgaris. Limoges (Ducourtieux), 1895; and R. du Buysson,
Monographic des Gudpes, Ann. Soc. Ent. France, 1903, Vol. LXXII, p. 603, PI.
VIII.

INTRODUCTORY 25

found to show much or little variability. The very charac-
teristic which in one species may exhibit extreme variability
may in an allied species show extreme constancy. Illustrations
will occur to any naturalist, but nowhere is this truth more
strikingly presented than in the British Noctuid Moths. Many
are so variable that, in the common phrase, "scarcely two can
be found alike," while others show comparatively slight variation.
It need scarcely be remarked that, in the instances I have in
mind, the evidence of great variability is in no way due to the
abundance with which the particular species occurs, for common
species may show constancy, and less abundant species may show
great variability. The polymorphism seems to be now at least
a general property of the variable species, as the fixity is a
property of the fixed species. In illustration I may refer to the
following examples.

Dianthoecia capsincola is a common and widely distributed
moth which feeds on Lychnis. It shows little variation. Dian-
thoecia carpophaga is another species which feeds chiefly on
Silene. Its habits are very similar to those of capsincola. Like
that species it has a wide geographical range and is abundant
in its localities, but in contrast to the fixity of capsincola, car-
pophaga exhibits a complex series of varieties. A gratis suffusa
(= ypsilon) is a moth widely spread through the southern half
of England. It is very constant in colour and markings. Ag-
rotis segetum and tritici are excessively variable both in ground
colour and markings, being found in an immense profusion of
dissimilar forms throughout their distribution. Of these and
several other species of A gratis there are many named varieties,
some of which have by various writers been regarded as speci-
fically distinct. Of the genus Nocttia many species (e. g. f estiva)
show a similar polymorphism, but N. triangulum, though showing
some variation in certain respects, is usually very constant to
its type, and the same is true of N. umbrosa.

In several species of Taeniocampa, especially instabilisf the
multiplicity of forms is extreme, while cruda (= pulverulenta)
is a comparatively constant species. The genus Plusia contains
a number of constant species, but in Plusia interrogationis we

26 PROBLEMS OF GENETICS

meet the fact that the central silvery mark undergoes endless
variation. "Truly no two are alike," says Mr. Tutt, "and to
look down a long series of interrogationis is something like looking
at a series of Chinese characters." In contrast to this we have
the fact that in Plusia gamma the very similar silvery mark is
by no means variable.

I have taken this series of cases from the Noctuid moths,
but it would be as easy to illustrate the same proposition from
the Geometridae or the Micro-Lepidoptera.15 I have a long
series of Peronea cristana,ior example, which was given tome by
Mr. W. H . B . Fletcher, of Bognor . All were beaten out of the same
hedge, and their polymorphism is such that no one unaccustomed
to such examples could suppose that they belonged to a single
species. Another common form, P. schalleriana, which lives in
similar circumstances, exhibits comparatively slight variability.

It should be expressly noted that the variation of which I am
speaking is a genuine polymorphism. Several of the species
enumerated exhibit also geographical variation, possessing defi-
nite and often strikingly distinct races peculiar to certain
localities; but apart from the existence of such local differen-
tiation, stands out the fact upon which I would lay stress, that
some species are excessively variable while others are by com-
parison constant, in circumstances that we may fairly regard
as comparable.

This fact is difficult to reconcile with the conventional view
that specific type is directly determined by Natural Selection

15 The statements made above are for the most part taken from Barrett, C. G.,
Lepidoptera of the British Islands, and from Tutt, J. W., The British Noctuae and
their Varieties. The reader who is unfamilar with the amazing polymorphism
exhibited by some of these moths should if possible take an opportunity of looking
over a long series in a collection, or, if that be impossible, refer to the admirable
coloured plates published by Barrett. It may not be superfluous to observe that
plenty of similar examples are known in other countries. For instance Plotheia
frontalis, a Noctuid which often abounds in Ceylon, shows an equally bewildering
wealth of forms. If a dozen specimens of such a species were to be brought home
from some little known country, each individual would almost certainly be described
as the type of a distinct species. (See the coloured plate published by Sir G. Hamp-
son, Cat. Brit. Mus., Heterocera, Vol. IX.)

INTRODUCTORY 27

and that the precision with which a species conforms to its
pattern is an indication of the closeness of that control. Anyone
familiar with the characteristics of Moths will agree that the
Noctuids, Geometrids and Tortricids are creatures whose existence
depends in some degree on the success with which they can escape
detection by their enemies in the imaginal state. We are there-
fore not surprised to find that some species of these orders
exhibit definite geographical variation in conformity with the
character of the ground, which may reasonably be supposed to
aid in their protection. If this were all, there would be nothing to
cause surprise. We might even be disposed to allow that varia-
bility might contribute to the perpetuation of animals so situ-
ated, on the principle that among a variety of surroundings
some would probably be in harmony with the objects on which
they rest. But we cannot admit the plausibility of an argument
which demands on the one hand that the extreme precision with
which species A adheres in the minutest details of its colour and
pattern to a certain type shall be ascribed to the protective fitness
of those details, and on the other hand that the abundant varia-
bility of species B shall be ascribed to the same determination.
If it is absolutely necessary for A to conform to one type how
comes it that B may range through some twenty distinct forms,
any two of which differ more from each other than the regular
species of many other genera? The only reply I can conceive
is a suggestion that there may be some circumstance which
differentiates the various classes of cases, that the exigencies
of the fixed species may be different from those of the variable.
Those who make such appeals to ignorance do not always perhaps
realise whither this course of reasoning may lead. If admissible
here the same argument would lead us to suggest that because
albino moles have for an indefinite period occurred on a certain
land near Bath there may be something in the soil or in the
conditions of life near Bath which requires a proportion of albinos
in its mole population. Or again, because the butterfly Thais
rumina in one locality, Digne in the south of France, has a per-
centage of individuals of the variety Honoratii (with certain
normally yellow spots on the hind wing coloured bright red)

28 PROBLEMS OF GENETICS

and nowhere else throughout its distribution, that therefore we
may suggest that there is some difference in the condition of
life at Digne which makes the continuance of Honoratii there
possible and beneficial.

A polymorphism offering a parallel to that of the variable
moths is afforded by the breeding plumage of the Ruff, the
male of Machetes pugnax. The variety of plumage which these
cocks exhibit is such that the statement that no two can be
found alike is only a venial exaggeration. Newton remarks16
"that all this wonderful 'show* is the consequence of the poly-
gamous habit of the Ruff can scarcely be doubtful"; but even if
it be conceded that the great external differentiation of the
cocks may be a result of sexual selection, the problem of their
polymorphism remains unsolved, for, as we are well aware,
polygamy is not usually associated with polymorphism of the
male. The Black Cock (Tetrao tetrix), for example, is as polyga-
mous as the Ruff, but in that and countless other cases, both
sexes are constant to one type of plumage.

When we thus compare the polymorphism of one species with
the fixity of another, and attempt to determine the causes which
have led to these extraordinary contrasts, two distinct lines of
argument are open to us. We may ascribe the difference either
to causes external to the organisms, primarily, that is to say,
to a difference in the exigencies of Adaptation under Natural
Selection; or on the other hand we may conceive the difference
as due to innate distinctions in the chemical and physiological
constitutions of the fixed and the variable respectively. There
is truth undoubtedly in both conceptions. If the mole were
physiologically incapable of producing an albino that variety
would not have come into being, and if the albino were totally
incapable of getting its living it would not be able to hold its

18 Diet, of Birds, p. 800. It would be interesting and profitable to attempt in
a long series of Ruffs to determine the Mendelian factors which by their combinations
give rise to this complex assemblage of varietal forms. A few such factors both of
colour and pattern can be at once distinguished, and it is noticeable that some of
the resulting types of barring, spangling and penciling show a perceptible corre-
spondence with some of the types of colouration found in the breeds of domestic
fowls.

INTRODUCTORY 29

own. Were Plotheia frontalis constructed on a chemical plan
which admitted of no variation, the countless varieties would
not have been produced ; and if one of its varieties had an over-
whelming success out of all proportion to that of the rest, then
the species would soon become monomorphic again. We
cannot declare that Natural Selection has no part in the deter-
mination of fixity or variability; nevertheless looking at the whole
mass of fact which a study of the incidence of variation provides,
I incline to the view that the variability of polymorphic forms
should be regarded rather as a thing tolerated than as an element
contributing directly to their chances of life; and on the other
hand that the fixity of the monomorphic forms should be looked
upon not so much as a proof that Natural Selection controls
them with a greater stringency, but rather as evidence of a natural
and intrinsic stability of chemical constitution.

Compare the condition of a variable form like the male
Ruff (or in a less degree the Red Grouse in both its sexes) with
that of the common Pheasant which is comparatively constant.
In the Pheasant no doubt variations do occur as in other wild
birds, but apart from the effects of mongrelisation the species
is unquestionably uniform. Could it seriously be proposed
that we should regard the constancy of the pheasant's plumage
in this country as depending on the special fitness of that type
of colouration? Even if the pheasant be not an alien in Western
Europe, it has certainly been protected for centuries, and for a
considerable period has existed in a state of semi-domestication.
Such conditions should give good opportunity for polymorphism
to be produced. In some coverts various aberrations do of
course occur and persist, yet there is nothing indicative of a
general relaxation of the fixity of the specific type, and the pheas-
ant remains substantially a fixed species.1 The common pheasant
(Phasianus colchicus) even shows little of that disposition to

1 Howard Saunders (Illust. Manual of British Birds, 1899, p. 499) states that
there is evidence that the pheasant had become naturalized in the south of England
before the Norman invasion. He adds, "little, if any, deviation from the typical
P. colchicus took place up to the end of last century, when the introduction of
the Chinese Ring-necked P. torquatus commenced, which has left almost indelible
marks, especially with regard to the characteristic white collar."

3o PROBLEMS OF GENETICS

form local races which appears in the species of Further India.
Are we not then on safer ground in regarding the fixity of our
species as a property inherent in its own nature and consti-
tution? Just as in ages of domestication no rose has ever given
off a blue variety so has the pheasant never broken out into the
polymorphism of the Ruff.

As soon as it is realised how largely the phenomena of vari-
ation and stability must be an index of the internal constitution
of organisms, and not mere consequences of their relations to
the outer world, such phenomena acquire a new and more
profound significance.

CHAPTER II

MERISTIC PHENOMENA

Twenty years ago in describing the facts of Variation, argu-
ment was necessary to show that these phenomena had a special
value in the sciences of Zoology and Botany. This value is
now universally understood and appreciated. In spite however
of the general attention devoted to the study of Variation, and
the accumulation of material bearing on the problem, no satis-
factory or searching classification of the phenomena is possible.
The reason for this failure is that a real classification must pre-
suppose knowledge of the chemistry and physics of living things
which at present is quite beyond our reach.

It is however becoming probable that if more knowledge of
the chemical and physical structure of organisms is to be at-
tained, the clue will be found through Genetics, and thus that
even in the uncoordinated accumulation of facts of Variation
we are providing the means of analysis applicable not only to
them, but to the problems of normality also.

The only classification that we can yet institute with any
confidence among th phenomena of Variation is that which
distinguishes on the one hand variations in the processes of
division from variations in the nature of the substances divided.

Variations in the processes of division are most often made
apparent by a change in the number of the parts, and are therefore
called M eristic Variations, while the changes in actual composition
of material are spoken of as Substantive Variations. The Me-
ristic Variations form on the whole a natural and fairly well
defined group, but the Substantive Variations are obviously
a heterogeneous assemblage.

Though this distinction does not go very far, it is useful,
and in all probability fundamental. It is of value inasmuch as
it brings into prominence the distinct and peculiar part which

31

32 PROBLEMS OF GENETICS

the process of division, or, more generally, repetition of parts,
plays in the constitution of the forms of living things.

That there may be a real independence between the M eristic
and the Substantive phenomena is evident from the fact both
that M eristic changes may occur without Substantive Variation,
and that the substances composing an organism may change
without any perceptible alteration in its meristic structure.
When the distinction between these two classes of phenomena
is perceived it will be realised that the study of genetics has on
the one hand a physical, or perhaps more strictly a mechanical
aspect, which relates to the manner in which material is divided
and distributed; and also a chemical aspect, which relates to
the constitution of the materials themselves. Somewhat as
the philosophers of the seventeenth and eighteenth centuries
were awaiting both a chemical and a mechanical discovery which
should serve as a key to the problems of unorganised matter,
so have biologists been awaiting two several clues. In Mendelian
analysis we have now, it is true, something comparable with the
clue of chemistry, but there is still little prospect of penetrating
the obscurity which envelops the mechanical aspect of our phe-
nomena. To make clear the application of the terms chemical
and mechanical to the problem of Genetics the nature of that
problem must be more fully described. In its most concrete
form this problem is expressed in the question, how does a cell
divide? If the organism is unicellular, and the single cell is
the whole body, then the process of heredity is accomplished
in the single operation of cell-division. Similarly in animals and
plants whose bodies are made up of many cells, the whole process
of heredity is accomplished in the cell-divisions by which the
germ-cells are formed. When therefore we see a cell dividing,
we are witnessing the process by which the form and the proper-
ties of the daughter-cells are determined.

Now this process has the two aspects which I have called
mechanical and chemical. The term " Entwicklungsmechanik "
has familiarised us with the application of the word mechanics
to these processes, but on reflexion it will be seen that this com-
prehensive term includes two sorts of events which are sometimes

MERISTIC PHENOMENA 33

readily distinguishable. There is the event by which the cell
divides, and the event by which the two halves or their descend-
ants are or may be differentiated. It is common knowledge that
in some cell-divisions two similar halves, indistinguishable in
appearance, properties, and subsequent fate, may be produced,
while in other divisions daughter-cells with distinct properties
and powers are formed. We cannot imagine but that in the
first case, when the resulting cells are idenfical, the division
is a mechanical process by which the mother-cell is simply
cut in two; while in order that two differentiated halves may be
produced, some event must have taken place by which a chemical
distinction between the two halves is effected.1 In any ordinary
Mendelian case we have a clear proof that such a chemical dif-
ference may be established between germ-cells. The facts of
colour-inheritance for instance prove that germ-cells, otherwise
identical, may be formed possessing the chromogen-f actor which
is necessary to the formation of colour in the flowers, or destitute
of that factor. Similarly the germ-cells may possess the ferment
which, by its action on the chromogenic substance, produces
the colour, or they may be without that ferment. The same
line of argument applied to a great range of cases. Nevertheless,
though differences in chemical properties are often thus consti-
tuted by cell-divisions, and though we are thus able to make a
quasi-chemical analysis of the individual by determining and
enumerating these properties, yet it is evident that the dis-
tribution of these factors is not itself a chemical process. This
is proved by the fact that similar divisions may be effected be-
tween halves which are exactly alike, and also by the fact that
the numbers in which the various types of germ-cells are formed
negative any suggestion of valency between them. The recog-
nition of the unit-factors may lead — indeed must lead — to great
advances in chemical physiology which without that clue would
have been impossible, but in causation the chemical phenomena
of heredity must be regarded as secondary to the physical or

1 In saying this we make no assumption as to the particular cell-division at
which differentiation occurs. This may be one of the maturation-divisions, or it
may perhaps be much earlier.

34 PROBLEMS OF GENETICS

mechanical phenomena by which the cells and their constituents
are divided and separated. When therefore we speak of the
essential phenomena of heredity we mean the mechanics of divi-
sion, especially, though not, as we shall see, exclusively, of cell-
division ; and in the relation between the two halves of the divid-
ing cell we have the problem presented in what seems to be its
simplest form.

In attempting to form some conception of the processes by
which bodily characteristics are transmitted, or — to avoid that
confusing metaphor of "transmission" — how it comes about
that the offspring can grow to resemble its parent, continuity of
the germ-substance which in some animals is a visible phenom-
enon,2 gives at least apparent help. An egg for example on be-
coming adult develops in certain parts a particular pigment.
The eggs of that adult when they reach the appropriate age de-
velop the same pigment. We have no clear picture of the
mechanism by which this process is effected, but when we realise
that the pigment results from the interaction of certain sub-
stances, and that since all the eggs are in reality pieces of the same
material, it seems, unless we inquire closely, not unnatural that
the several pieces of the material should exhibit the same colours
at the same periods of their development. The continuity of
the material of the germs suggests that there is a continuity
of the materials from which the pigment is formed, and that
thus an actual bit of those substances passess into each egg
ready at the appropriate moment to generate the pigment.
The argument thus outlined applies to all substantive character-
istics. In each case we can imagine, if we will, the appearance
of that characteristic as due to the contribution of its rudiment
from the germ tissues.

When we consider more critically it becomes evident that
the aid given by this mental picture is of very doubtful reality,
for even if it were true that any predestined particle actually
corresponding with the pigment-forming materials is definitely

2 From the recent discoveries of Envin Baur we are led to surmise that in the
flowering plants the sub-epidermal layer, or some of its elements, may legitimately
be regarded as a similar germ-substance, continuous in Weismann's sense.

MERISTIC PHENOMENA 35

passed on from germ to germ, yet the power of increase which
must be attributed to it remains so incomprehensible that the
mystery is hardly at all illuminated.

When however we pass from the substantive to the meristic
characters, the conception that the character depends on the
possession by the germ of a particle of a specific material becomes
even less plausible. Hardly by any effort of imagination can we
see any way by which the division of the vertebral column into x
segments or into y segments, or of a Medusa into 4 segments or
into 6, can be determined by the possession or by the want of a
material particle. The distinction must surely be of a different
order. If we are to look for a physical analogy at all we should
rather be led to suppose that these differences in segmental
numbers corresponded with changes in the amplitude or number
of dividing waves than with any change in the substance or
material divided.

PHENOMENA OF DIVISION

I have said that in the division of a cell we seem to see the
problem in its simplest form, but it is important to observe that
the problem of division may be presented by the bodies of animals
and plants in forms which are independent of the divisions be-
tween cells. The existence of pattern implies a repetition of
parts, and repetition of parts when developed in a material
originally homogeneous can only be created by division. Cell-
division is probably only a special case of a process similar to
that by which the pattern of the skeleton is laid down in a uni-
cellular body such as that of a Radiolarian or Foraminiferan.
Attempts have lately been made to apply mathematical treat-
ment to problems of biology. It has sometimes seemed to
me that it is in the geometrical phenomena of life that the most
hopeful field for the introduction of mathematics will be found.
If anyone will compare one of our animal patterns, say hat of a
zebra's hide, with patterns known to be of purely mechanical
production, he will need no argument to convince him that there
must be an essential similarity between the processes by which
the two kinds of patterns were made and that parts at least of

36 PROBLEMS OF GENETICS

the analysis applicable to the mechanical patterns are applicable
to the zebra stripes also. Patterns mechanically produced are
of many and very diverse kinds. One of the most familiar
examples, and one presenting some especially striking analogies
to organic patterns, is that provided by the ripples of a mackerel
sky, or those made in a flat sandy beach by the wind or the ebbing
tide. With a little search we can find among the ripple-marks, and
in other patterns produced by simple physical means, the closest
parallels to all the phenomena of striping as we see them in our
animals. The forking of the stripes, the differentiation of two
"faces," the deflections round the limbs and so forth, which in the
body we know to be phenomena of division, are common both to
the mechanical and the animal patterns. We cannot tell what in
the zebra corresponds to the wind or the flow of the current, but
we can perceive that in the distribution of the pigments, that is
to say, of the chromogen-substances or of the ferments which
act upon them, a rhythmical disturbance has been set up which
has produced the pattern we see; and I think we are entitled to
the inference that in the formation of patterns in animals and
plants mechanical forces are operating which ought to be, and
will prove to be, capable of mathematical analysis. The com-
parison between the striping of a living organism and the sand-
ripples will serve us yet a little farther, for a pattern may either
be formed by actual cell-divisions, and the distribution of dif-
ferentiation coincidently determined, or — as visibly in the pig-
mentation of many animal and plant tissues — the pattern may
be laid down and the pigment (for example) distributed through
a tissue across or independently of the cell-divisions of the tissue.
Our tissues therefore are like a beach composed of sands of
different kinds, and different kinds of sands may show distinct
and interpenetrating ripples. When the essential analogy be-
tween these various classes of phenomena is perceived, no one
will be astonished at, or reluctant to admit, the reality of dis-
continuity in Variation, and if we are as far as ever from knowing
the actual causation of pattern we ought not to feel surprised that
it may arise suddenly or be suddenly modified in descent. Biol-
ogists have felt it easier to conceive the evolution of a striped

38 PROBLEMS OF GENETICS

animal like a zebra from a self-coloured type like a horse (or
of the self-coloured from the striped) as a process involving many
intergradational steps; but so far as the pattern is concerned, the
change may have been decided by a single event, just as the
multitudinous and ordered rippling of a beach may be created
or obilterated at one tide.

This point is well illustrated by the tusk of an Indian elephant
which I lately found in a London sale-room. This tusk is by
some unknown cause, presumably a chronic inflammation,
thrown up into thirteen well-marked ridges which closely simulate
a series of segments (Fig. i). Whatever the cause the condition
shows how easily a normally unsegmented structure may be
converted into a series of repeated parts.

The spread of segmentation through tissues normally unseg-
mented is very clearly exemplified in the skates' jaws shown in
in Fig. 2. The right side of the upper figure shows the normal
arrangement in the species Rhinoptera jussieui, but the structure
on the left side is very different. The probable relations of the
several rows of teeth to the normal rows is indicated by the let-
tering, but it is evident that by the appearance of new planes
of division constituting separate centers of growth, the series has
been recast. The pattern of the left side is so definite that had
the variation affected the right side also, no systematist would
have hesitated to give the specimen a new specific name. The
other two drawings show similar variations of a less extensive
kind, the nature of which is explained by the lettering of the
rows of teeth.

This power to divide is a fundamental attribute of life, and
of that power cell-division is a special example. In regard to
almost all the chief vital phenomena we can say with truth that
science has made some progress. If I mention respiration, meta-
bolism, digestion, each of these words calls to mind something
more than a bare statement that such acts are performed by an
animal or a plant. Each stands for volumes of successful ex-
periment and research, But the expression cell-division, the
fundamental act which typifies the rest, and on which they all
depend, remains a bare name. We can see with the microscope

MERISTIC PHENOMENA

39

the outward symptoms of division, but we have no surmise as
to the nature of the process by which the division is begun or
accomplished. I know nothing which to a man well f rained in
scientific knowledge and method brings so vivid a realisation

E

FIG. 2. Jaws of Skates (Rhinoptera) showing meristic variation. (For a detailed
discussion see Materials for the Sttidy of Variation, p. 259.)

of our ignorance of the nature of life as the mystery of cell-
division. What is a living thing? The best answer in few words
that I know is one which my old teacher, Michael Foster, used
to give in his lectures introductory to biology. "A living thing

40 PROBLEMS OF GENETICS

is a vortex of chemical and molecular change." This description
gives much, if not all, that is of the essence of life. The living
thing is unlike ordinary matter in the fact that, through it, matter
is always passing. Matter is essential to it; but, provided
that the flow in and out is unimpeded, the life-process can go
on so far as we know indefinitely. Yet the living "vortex"
differs from all others in the fact that it can divide and throw
off other "vortices," through which again matter continually
swirls.

We may perhaps take the parallel a stage further. A simple
vortex, like a smoke-ring, if projected in a suitable way will twist
and form two rings. If each loop as it is formed could grow and
then twist again to form more loops, we should have a model
representing several of the essential features of living things.

It is this power of spontaneous division which most sharply
distinguishes the living from the non-living. In the excellent
book dealing with the problems of development, lately published
by Mr. Jenkinson a special emphasis is very properly laid on the
distinction between the processes of division, and those of dif-
ferentiation. Too often in discussions of the developmental
processes the distinction is obscured. He regards differentiation
as the "central difficulty." "Growth and division of the nucleus
and the cells," he tells us, are side-issues. This view is quite
defensible, but I suspect that the division is the central difficulty,
and that if we could get a rationale of what is happening in
cell-division we should not be long before we had a clue to the
nature of differentiation. It may be self-deception, but I do
not feel it impossible to form some hypothesis as to the mode of
differentiation, but in no mood of freest speculation are we ever
able to form a guess as to the nature of the division. We see
differentiations occurring in the course of chemical action, in
some phenomena of vibration and so forth: but where do we
see anything like the spontaneous division of the living cell?
Excite a gold-leaf electroscope, and the leaves separate, but we
know that is because they were double before. In electrolysis
various substances separate out at the positive and negative
poles respectively. Now if in cell-division the two daughter-

MERISTIC PHENOMENA 41

cells were always dissimilar — that is to say, if differentiation
always occurred — we could conceive some rough comparison
with such dissociations. But we know the dissimilarity between
daughter-cells is not essential. In the reproduction of unicel-
lular organisms and many other cases, the products formed at the
two poles are, so far as we can tell, identical. Any assumption
to the contrary, if we were disposed to make it, would involve
us in difficulties still more serious. At any rate, therefore, if
differentiation be really the central difficulty in development,
it is division which is the essential problem of heredity.

Sir George Darwin and Professor Jeans tell us that "gravita-
tional instability" consequent on the condensation of gases is
"the primary agent at work in the actual evolution of the uni-
verse," which has led to the division of the heavenly bodies. The
greatest advance I can conceive in biology would be the dis-
covery of the nature of the instability which leads to the c ntinual
division of the cell. When I look at a dividing cell I feel as an
astronomer might do if he beheld the formation of a double
star: that an original act of creation is taking place before me.
Enigmatical as the phenomenon seems, I am not without hope
that, if it were studied for its own sake, dissociated from the
complications which obscure it when regarded as a mere incident
in development, some hint as to the nature of division could be
found. It is I fear a problem rather for the physicist than for
the biologist. The sentiment may not be a popular one to utter
before an assembly of biologists, but looking at the truth imper-
sonally I suspect that when at length minds of first rate analytical
power are attracted to biological problems, some advance will
be made of the kind which we are awaiting.

The study of the phenomena of bodily symmetry offers
perhaps the most hopeful point of attack. The essential fact
in reproduction is cell-division, and the essential basis of heredi-
tary resemblance is the symmetry of cell-division. The phenom-
ena of twinning provide a convincing demonstration that this
is so. By twinning we mean the production of equivalent struc-
tures by division. The process is one which may affect the whole
body of an animal or plant, or certain of its parts. The term

42 PROBLEMS OF GENETICS

twin as ordinarily used refers to the simultaneous birth of two
individuals. Those who are naturalists know that such twins
are of two kinds, (i) twins that are not more alike than any other
two members of the same family, and (2) twins that are so much
alike that even intimate friends mistake them. These latter
twins, except in imaginative literature, are always of the same
sex.

It is scarcely necessary for me to repeat the evidence from
which it has been concluded that without doubt such twins arise
by division of the same fertilised ovum. There is a perfect
series of gradations connecting them with the various forms of
double monsters united by homologous parts. They have been
shown several times to be enclosed in the same chorion, and the
proofs of experimental embryology show that in several animals
by the separation of the two first hemispheres of a dividing egg
twins can be produced. Lastly we have recently had the ex-
traordinarily interesting demonstration of Loeb, to which I may
specially refer. Herbst some years ago found that in sea water,
from which all lime salts had been removed, the segments of the
living egg fall apart as they are formed. Using this method
Loeb has shown that a temporary immersion in lime-free sea
water may result in the production of 90 per cent, of twins.
We are therefore safe in regarding the homologous or "identical"
twins as resulting fro the divisions of one fertilised egg, while
the non-identical or "fraternal" twins, as they are called, arise
by the fertilisation of two separate ova.3

In the resemblance of identical twins we have an extreme case

3 These fraternal twins, which show no special resemblance to each other,
are like the multiple births of other animals, and there is no disposition for them to
be of the same sex. In the sheep, for example, statistics show that the frequency
of pairs of twins, male and female, is approximately double that of the frequency
of pairs, both male or both female, as it should be if the sex-distribution were for-
tuitous. For instance Bernadin (La Bergerie de Rambouillet, 1890, p. 100) gives
the following figures for twin- lambs in Merinos: both male, 87; both female, 83;
sexes mixed, 187. The p-banded Armadillo (Dasypus novemcinctus) , in which
the young born in one litter are said to be always of one sex, is the only known
exception in Vertebrates, and is presumably a genuine case of normal polyembryony
(see especially, Rosner, Bull. Ac. Soc. Cracovie, 1901, p. 443, and Newman and
Patterson, Biol. Bull., XVII, 1909, p. 181, and an important paper lately published
by H. H. Newman and J. T. Patterson, Jour. Morph., 1911, XXII, p. 855.

MERISTIC PHENOMENA 43

of hereditary likeness4 and a proof, if any were needed, that the
cause of individual variation is to be sought in the differentiation
of germ-cells. The resemblance of identical twins depends on
two circumstances, First, since only two germ-cells take part
in their production, difference between the germ cells of the
same individual cannot affect them. Secondly the division of
the fertilised ovum, the process by which they became two in-
stead of one, must have been a symmetrical division. The
structure of twins raises however one extremely significant
difficulty, which as yet we cannot in any way explain. The
resemblance between twins is a phenomenon of symmetry,
like the resemblance between the two sides of a bilaterally sym-
metrical body. Not only is the general resemblance readily
so interpreted, but we know also that in double monsters, namely
unseparated twins, various anatomical abnormalities shown by
the one half-body are frequently shown by the other half-
also. 5 The two belong to one system of symmetry How then
does it happen that the body of one of a pair of twins does not
show a transposition of viscera? We know that the relation of
right and left implies that the one should be the mirror-image of
the other. Such a relation of images may be maintained even
in minute details. For example if the same pattern of finger-
print is given by the fingers of the two hands, one is the reverse
of the other. In double monsters, namely unseparated twins,
there is evidence that an inversion of viscera does occur with
some frequency. Evidence from such cases is not so clear and
simple as might be expected, because as a matter of fact, the
heart and stomach, upon which the asymmetry of the viscera
chiefly depend, are usually common to the two bodies. Du-
plicity generally affects either the anterior end alone, or the pos-
terior end alone. The division is generally from the heart forwards ,
giving two heads and two pairs of anterior limbs on a common
trunk, or from the heart backwards, giving two pairs of posterior
limbs with the anterior body common. In either case, though

* A good collection of evidence as to disease in homologous twins was lately
published by E. A. Cockayne, Brit. Jour. Child. Diseases, Nov., 1911.
6 Cp. Windle, B. C. A., Jour. Anat. Phys., XXVI, p. 295.

44 PROBLEMS OF GENETICS

the bodies may be grouped in a common system of symmetry,
neither can be proved to show definite reversal of the parts. To
see that reversal recourse must be had to more extreme duplica-
tions, such as the famous Siamese Twins. They, as a matter of
fact, were an excellent instance of the proposition that twins
are related as mirror-images, for both of them had eleven pairs
of ribs instead of the normal twelve, and one of them had a
partial reversal of viscera.6 (Kiichenmeister, Verlagerung, etc.,
p. 204.)

If anyone could show how it is that neither of a pair of
twins has transposition of viscera the whole mystery of division
would, I expect, be greatly illuminated.7 At present we have
simply to accept the fact that twins, by virtue of their detach-
ment from each other, have the power of resuming the polarity
which is proper to any normal individual. It was nevertheless
with great interest that I read Wilder's recent observation 8 that
occasionally in identical twins the finger-print of one or both
the index-fingers may be reversed, showing that there is after
all some truth in the notion that reversal should occur in them.

There is another phenomenon by twinning which, if we could
understand it, might help. I refer to the free-martin, the subject
of one of John Hunter's masterpieces of anatomical description.
In horned cattle twin births are rare, and when twins of opposite
sexes are born, the male is perfect and normal, but the repro-

6 Mr. E. Nettleship tells me that in the course of collecting pedigrees of families
containing colour-blind members he has discovered two cases (shortly to be pub-
lished) of pairs of twins, which on account of their very close resemblances must
be deemed homologous, one of each pair being colour-blind and the other normal.
Such a distinction between closely similar twins is most curious and unexpected.

7 Another paradoxical phenomenon of the same nature occurs in the Narwhal
The males normally have the left tusk alone developed, the corresponding right
tusk remaining as an undeveloped rudiment in its socket. The left tusk is a
left-handed screw. Occasionally the right tusk is also developed and grows to
the same length as that of the left side, but in such specimens the right tusk is
also a left-hand screw like the tusk of the other side, instead of being reversed
as we should certainly have expected. It need scarcely be remarked that in the
case of the horns of antelopes, and in other examples of spiral organs arranged in
pairs, that of one side of the body is the mirror image of that on the other side.
The Narwhal's tusks in being both twisted in the same direction are thus highly
anomalous, and are comparable with pairs of twins.

8 Wilder, H. H., Amer. Jour. Anat., 1904, III, p. 452.

MERISTIC PHENOMENA 45

ductive organs of the female are deformed and sterile, being
known as a free-martin. The same thing occasionally happens
in sheep, suggesting that in sheep also twins may be formed by
the division of one ovum; for it is impossible to suppose that
mere development in juxtaposition can produce a change of this
character. I mention the free-martin because it raises a question
of absorbing interest. It is conceivable that we should interpret
it by reference to the phenomenon of gynandromorphism, seen
occasionally in insects, and also in birds as a great rarity. In
the gynandromorph one side of the body is male, the other female.
A bullfinch for instance has been described with a sharp line of
division down the breast between the red feathers of the cock
on one side and the brown feathers of the hen on the other.
(Poll,H., SB. Ges. Nat. Fr.t Berlin, 1909, p. 338.) In such cases
neither side is sexually perfect. If the halves of such a gynan-
dromorph came apart, perhaps one would be a free-martin.

The behaviour of homologous twinning in heredity has been
little studied. It does not exist as a normal feature in any animal
which is amenable to experiment, and we cannot positively
assert that a comparable phenomenon exists in plants; for in
them — the Orange, for example — polyembryony may evidently
be produced by a parthenogenetic development of nucellar tissue.
It is possible that in Man twinning is due to a peculiarity of the
mother, not of the father. It may and not rarely does descend
from mother to daughter, but whether it can be passed on
through a male generation to a daughter again, there is not
sufficient evidence to show. The facts as far as they go are
consistent with the inference which may be drawn from Loeb's
experiment, that the twinning of a fertilized ovum may be de-
termined not by the germ-cells which united to form it, but by
the environment in which it begins to develop. The opinion that
twinning may descend through the male directly has been lately
expressed by Dr. J. Oliver in the Eugenics Review (1912), on the
evidence of cases in which twins had occurred among the rela-
tions of fathers of twins, but I do not know of any comprehen-
sive collection of evidence bearing on the subject.

Besides twinning of the whole body a comparable duplicity

46 PROBLEMS OF GENETICS

of various parts of the same body may occur. Such divisions
affect especially those organs which have an axis of bilateral
symmetry, such as the thumb, a cotyledon, a median petal,
the frond of a fern or the anal fin of a fish. From the little
yet known it is clear that the genetic analysis of these conditions
must be very difficult, but evidence of any kind regarding them
will be valuable. We want especially to know whether these
divisions are due to the addition of some factor or power which
enables the part to divide, or whether the division results from
the absence of something which in the normal body prevents
the part from dividing. Breeding experiments, so far as they
go, suggest that the less divided state is usually dominant to
the more divided.9 The two-celled Tomato fruit is dominant to
the many-celled type. The Manx Cat's tail, with its suppression
of caudal segmentation is a partial dominant over the normal
tail. The tail of the Fowl in what is called the "Rumpless"
condition is at least superficially comparable with that of the
Manx Cat, and though the evidence is not wholly consistent,
Davenport obtained facts indicating that this suppressed con-
dition of the caudal vertebrae is an imperfect dominant.10

Some evidence may also be derived from other examples of
differences which at first sight appear to be substantive though
they are more probably meristic in ultimate nature. The
distinction between the normal and the "Angora" hair of the
Rabbit is a case in point. We can scarcely doubt that one of
the essential differences between these two types is that in the
Angora coat the hair-follicles are more finely divided than they
are in the normal coat, and we know that the normal, or less-
divided condition, is dominant to the Angora, or more finely
divided.

In the case of the solid-hoofed or "mule-footed" swine, the

9 Polydactylism which is often a dominant and the web-foot of Pigeons which
is recessive should be remembered as possible exceptions (see p. 49).

10 Davenport inclined at first to regard rumplessness as a recessive, but in his
latest publication on the subject he definitely concludes that it is an imperfect
dominant. This conclusion accords well with evidence quoted by Darwin (An.
and Pits., II, ed. 2, p. 4) that rumpless fowls may throw tailed offspring. (Amer.

.. 1910. XLIV, p. 134-)

MERISTIC PHENOMENA

47

evidence shows, as Spillman has lately pointed out,u that the
condition behaves as a dominant. The essential feature of
this abnormality is that the digits III and IV are partially
united. The union is greatest peripherally. Sometimes the

n

IV

FIG. 3. /, //, ///, various degrees of syndactyly affecting the medius and
annularis in the hand; IV, syndactyly affecting the index and medius in the foot.
(After Annandale.)

third phalanges only are joined to form one bone, but the second
and even the first phalanges may also be compounded together.
Here the variation is obviously meristic and consists in a failure

"Spillman, W. J., Amer. Breeders Mag., 1910, I, p. 178.

48 PROBLEMS OF GENETICS

to divide, the normal separation of the median digits of the foot
being suppressed.

Webbing between the digits, in at least some of its mani-
festations, is a variation of similar nature. The family recorded
by Newsholme12 very clearly shows the dominance of this con-
dition. The case is morphologically of great interest and must
undoubtedly have a bearing on the problems of the mechanics
of Division. In discussing the phenomena of syndactylism
I pointed out some years ago that the digits most frequently
united in the human hand are III and IV, while in the foot,

a

_ m+in

'v

FIG. 4. Case of complete syndactyly in the foot. 77 and 777, digit apparently
representing the index and medius. c2 + cs, bone apparently representing the
middle and external cuneiform; cb, cuboid; cl, internal cuneiform. (After Gruber.)

union most frequently takes place between II and III.13 In
Newsholme's family the union was always between II and III
of the foot, except in the case of one male who had the digits
III and IV of the right hand alone webbed together. There
can be little doubt that the geometrical system on which the
foot is planned has an axis of symmetry passing between the
digits II and III, while the corresponding axis in the hand passes
between III and IV. Union between such digits may therefore
be regarded as comparable with any non-division or " coalescence "
of lateral structures in a middle line, and when as in these ex-

12 Newsholme, Lancet, December 10, 1910, p. 1690.
18 Materials for the Study of Variation, 1894, p. 358.

MERISTIC PHENOMENA 49

amples such a condition is shown to be a dominant we cannot
avoid the inference that some concrete factor has the power of
suppressing or inhibiting this division. Figs. 3 and 4 illus-
trate degrees of union between digits in the human hand and
foot.

It is not in question that various other forms of irregular
webbing and coalescence of digits exist, and respecting the genetic
behaviour of these practically nothing is as yet known, Such
a case is described by Walker,14 in which the first and second
metacarpals of both feet were fused in mother and daughter,
and several more are found in literature. Contrasted with these
phenomena we have the curious fact that in the Pigeon, Staples-
Browne found webbing of the toes a recessive character. The
question thus arises whether this webbing is of the same nature
as that shown to be a dominant in Man, and indeed whether the
phenomenon in pigeons is really meristic at all. There is some
difference perceptible between the two conditions; for in Man
there is not so much a development of a special web-like skin
uniting the digits as a w^ant of proper division between the digits
themselves, and in extreme cases two digits may be represented
by a single one. In the Pigeon I am not aware that a real
union of this kind has ever been observed, and though the web-like
skin may extend the whole length of the digits and be so narrow
as to prevent the spread of the toes, it may, I think, be main-
tained that the unity of the digits is unimpaired. For the
present the nature of this variation in the pigeon's feet must be
regarded as doubtful, and we should note that if it is actually
an example of a more perfect division being dominant to a less
perfect division, the case is a marked exception to the general
rule that non-division is dominant to division.

Reference must also be made to the phenomenon of fasciation
in the stems of plants. As Mendel showed in the case of Pisum
this condition is often a recessive. The appearances suggest
that the difference between a normal and a fasciated plant
consists in the inability of the fasciated plant to separate its
lateral branches. The nature of the condition is however very

14 Walker, G., Johns Hopkins Hospital Bulletin, XII, 1901, p. 129.

5

So PROBLEMS OF GENETICS

obscure and it is equally likely that some multiplication of the
growing point is the essential phenomenon.16

Stockard's interesting experiments16 illustrate this question.
He showed that by treating the embryos of a fish (Fundulus
heteroclitus) with a dilute solution of magnesium salts, various
cyclopian monstrosities were frequently produced. These have
been called cases of fusion of the optic vesicles. I would prefer
to regard them as cases of a division suppressed or restricted by
the control of the environment. Conversely, the splendid dis-
covery of Loeb, that an unfertilised egg will divide and develop
parthenogenetically without fertilisation, as a consequence of
exposure to various media, may be interpreted as suggesting that
the action of those media releases the strains already present
in the ovum, though I admit that an interpretation based on the
converse hypothesis, that the medium acts as a stimulus, is as
yet by no means excluded.

In these cases we come nearest to the direct causation or
the direct inhibition of a division, but the meaning of the
evidence is still ambiguous. I incline to compare Loeb's par-
thenogenesis with the development (and of course accompany-
ing cell-division) of dormant buds on stems which have been
cut back.

It is interesting to note that sometimes as an abnormality,
the faculty of division gets out of hand and runs a course ap-
parently uncontrolled. A remarkable instance of this condition
is seen in Begonia " phyllomaniaca" which breaks out into buds
at any point on the stem, petioles, or leaves, each bud having,
like other buds, the power of becoming a new plant if removed.
We would give much to know the genetic properties of B. phyl-
lomaniaca, and in conjunction with Mr. W. O. Backhouse I have
for some time been experimenting with this plant. It proved
totally sterile. Its own anthers produce no pollen, and all at-
tempts to fertilise it with other species failed though the pollen
of a great number of forms was tried.

Recently however we have succeeded in making plants which

15 Cp. R. H. Compton, New Phytologist, 1911, p. 249.

16 Arch, f, Entwickelungsmech., 1907, XXIII, p. 249.

MERISTIC PHENOMENA 51

are in every respect Begonia phyllomaniaca, so far as the char-
acters of stems and leaves are concerned. These plants, of
which we have sixteen, were made by fertilising B. heracleifolia
with B. polyantha. They are all beginning to break out in
"phyllomania." As yet they have not flowered, but as they
agree in all details with phyllomaniaca there can be little doubt
that the original plant bearing that name was a hybrid similarly
produced. The production of "phyllomania" on a hybrid
Begonia has also been previously recorded by Duchartre.17
In this case the cross was made between B. incarnata and lucida.
The synonymy of the last species is unfortunately obscure, and
I have not succeeded in repeating the experiment.

FIG. 5. Piece of petiole of Begonia phyllomaniaca. The proximal end is to the

right of the figure.

From these facts it seems practically certain that the condition
is one which is due to the meeting of complementary factors.
At first sight we may incline to think that the phyllomania is in
some way due to the sterility. This however cannot be seriously
maintained; for not only is sterility in plants not usually associ-
ated with such manifestations, but we know a Begonia called
"Wilhelma" which is exactly phyllomaniaca and equally sterile,
though it has no trace of phyllomania. This plant arose in the
nurseries of MM. P. Bruant of Poitiers, and has generally been
described as a seedling of phyllomaniaca, but from the total
sterility of that form this account of its origin must be set aside.

The phenomenon in this case can hardly be regarded as

17 Bull. Soc. Bot. de France, xxxiv, 1887, p. 182.

PROBLEMS OF GENETICS

due to the excitation of dormant buds, for it is apparent on
examination that the new growths are not placed in any fixed
geometrical relation to the original plant. They arise on the

tic

a»

c*
s»

c*

d*

d*.

'<t2

FIG. 6. Two right hind feet of polydactyle cats. // shows the lowest de-
velopment of the condition yet recorded. The digit, di, which stands as hallux is
fully formed and has three phalanges. Both it and the digit marked fa are formed
as left digits. In the normal hind foot of the cat the hallux is represented by a
rudiment only.

I shows a further development of the condition. In this foot there are six
digits, di has two phalanges, but both it and d2 and d* are shaped as left digits.
Thus d3, which in the normal foot would be shaped as a right digit, is transformed
so as to look like a left digit.

petiole, for example, as small green outgrowths each of which
gradually becomes a tiny leaf. The attitude of these leaves is
quite indeterminate, and they may point in any direction,

MERISTIC PHENOMENA 53

some having their apices turned peripherally, some centrally, and
others in various oblique or transverse positions (Fig. 5). These
little leaves are thus comparable with seedlings, in that their
polarity is not related to, or consequent upon that of the parent
plant. They have in fact that "individuality," which we asso-
ciate with germinal reproduction.

There are many curious phenomena seen in the behaviour of
parts normally repeated in bilateral symmetry which may some
day guide us towards an understanding of the mechanics of
division. A part like a hand, which needs the other hand to
complete its symmetry, cannot twin by mere division, yet by
proliferation and special modifications on the radial side of the
same limb, even a hand may be twinned. In the well known poly-
dactyle cats a change of this kind is very common and indeed
almost the rule. When extra digits appear at the inner (tibial)
side of the limb, they are shaped as digits of the other side, and
even the normal digit II (index) is usually converted into the
mirror-image of its normal self. The limb then develops a
new symmetry in itself. Nevertheless it is not easy to interpret
these facts as meaning that there has been some interruption
in the control which one side of the body exercises over the
other. The heredity of polydactylism is complex but there is
little doubt that the condition familiar in the Cat is a dominant.
In some human cases also the descent is that of a dominant, but
irregularities are so frequent that no general rule can yet be
perceived. The dominance of such a condition is an exception
to the principle that the less-divided is usually dominant to the
more-divided, a fact which probably should be interpreted as
meaning that divisions are of more than one kind.

Among ordinary somatic divisions, whether of organs, cells,
or patterns of differentiation, the control of symmetry is usually
manifested. There is however one class of somatic differenti-
ations which are exceptionally interesting from the fact that they
may show a complete independence of such geometrical control.
The most familiar examples of these geometrically uncontrolled
Variations are to be seen in bud-sports. The normal differ-
entiation of the organs of a plant is arranged on a definite geo-

54 PROBLEMS OF GENETICS

metrical system, which to those who have never given special
attention to such things before, will often seem surprisingly
precise. The arrangement of the leaves on uninjured, free-
growing shoots can generally be seen to follow a very definite
order, just as do the flowers or the parts of the flowers. If
however bud sports occur, then though the parts included in
the sports show all the geometrical peculiarities proper to the
sport- variety, yet the sporting-buds themselves are not related
to each other according to any geometrical plan.

A very familiar illustration is provided by the distribution
of colour in those Carnations that are not self-coloured. The
pigment may, as in Picotees, be distributed peripherally with
great regularity to the edges of the petals; or, as in Bizarres and
Flakes, it may be scattered in radial sectors which show no
geometrical regularity. Now in this case the pigments are the
same in both types of flower, and the chemical factors concerned
in their production must surely be the same. The difference
must lie in the mechanical processes of distribution of the pig-
ment. In the Picotee we see the orderly differentiation which we
associate with normality; in the Bizarre we see the disorderly
differentiation characteristic of bud-sports. The distribution of
colour in this case lies outside the scheme of symmetry of the
plant.

Such a distribution is characteristic of bud-sports, and of
certain other differentiations in both plants and animals, which
I cannot on this occasion discuss. Now reflexion will show that
these facts have an intimate bearing on the mechanical problems
of heredity. For first in the bud-sports we are witnessing the
distribution of factors which distinguish genetic varieties. We
do not know the physical nature of those factors, but if we must
give them a name, I suppose we should call them "ferments"
exactly as Boyle did in 1666. He is discussing how it comes about
that a bud, budded on a stock, becomes a branch bearing the
fruit of its special kind. He notes that though the bud inserted
be "not so big oftentimes as a Pea," yet "whether by the help
of some peculiar kind of Strainer or by the Operation of some
powerful Ferment lodged in it, or by both these, or some other

MERISTIC PHENOMENA 55

cause," the sap is "so far changed as to constitute a Fruit quite
otherwise qualify 'd." 18 We can add nothing to his speculation,
and we believe still that by a differential distribution of "fer-
ments" the sports are produced. All the factors are together
present in the normal parts; some are left out in the sport.
In an analogous case however, that of a variegated Pelargonium
which has green and also albino shoots, Baur proved that the
shoots pure in colour are also pure in their posterity. There
can be no doubt that the sports of Carnations, Azaleas, Chrysan-
themums, etc., would behave in the same way.

The well-known Azaleas Perle de Ledeburg, President
Kerchove, and Vervaeana are familiar illustrations. Perle de
Ledeburg is predominantly white, but it has red streaks in some
of its flowers. It not very rarely gives off a self-red sport. This
is evidently due to the development of a bud in a red-bearing
area of the stem. The red in this plant is not under "geometrical
control." Many plants have white flowers with no markings,
but if the red markings are geometrically ordered differentiations,
no self-coloured sports are formed. The case of Vervaeana is a
good illustration of this proposition. It has white flowers with
red markings arranged in an orderly manner on the lower parts
of the petals, especially on the dorsal petals. This is one of the
Azaleas most liable to have red sports, and at first sight it might
seem that the sport represented the red of the central marks.
Examination however of a good many flowers shows that irregular
red streaks like those of Perle de Ledeburg occur, about as com-
monly as in that variety. Vervaeana in fact is Perle de Ledeburg
with definite red markings added, and its red sports obviously are
those branches the germs of which came in a patch of the stem
bearing these red elements. That this is the true account is
rendered quite obvious by the fact that the red of the sport is a
colour somewhat different from that of the definite marks, and
that these marks are still present on the red ground of the sporting
flowers.

It will be understood that these remarks apply to those cases
in which the production of sports is habitual or frequent, and

18 R. Boyle, The Origine of Formes and Qualities, Oxford, 1666.

56 PROBLEMS OF GENETICS

I imagine in all such examples it will be found that there are
indications of irregularity in the distribution of the differ-
entiations such as to justify the view that they are not under that
geometrical control which governs the normal differentiation
of the parts. The question next arises whether these consider-
ations apply also to the production of a bud-sport as a rare
exception, but by the nature of the case it is not possible to
say positively whether the appearance of an exceptional sport
is due to the unsuspected presence of a pre-existing fragment of
material having a special constitution, or to the origin, de novo,
of such a material. For instance one of the garden forms of
Pelargonium known as altum is liable perhaps once in some
hundreds of flowers to have one or two magenta petals. The
normal colour is a brilliant red ; and as we may be fairly sure that
this red is recessive to magenta the interpretation would be
quite different according as the appearance of the magenta is
regarded as due to the presence of small areas endowed with
magentaness, or to the spontaneous generation of the factor
for that pigment. Either interpretation is possible on the facts,
but the view that the whole plant has in it scarce mosaic particles
of magenta seems on the whole more consistent with present
knowledge.

In Pelargonium altum the enzyme causing the magenta colours
must be distributed in very small areas, but a case in which the
magenta is similarly arranged in a much coarser patchwork
may be seen in the Pelargonium " Don Juan," which often bears
whole trusses or branches of red flowers upon plants having the
normal dominant magenta trusses. In most cases there is little
doubt that though the magenta flowered parts can " sport" to
red, the red parts could not produce the magenta flowers.

The asymmetrical, or to speak more precisely, the disorderly,
mingling of the colours in the somatic parts is thus an indication
of a similarly disorderly mixing of the factors for those colours
in the germ-tissues, so that some of the gametes bear enough of
the colour-factors to make a self-coloured plant, while others
bear so little that the plant to which they give rise is a patch-
work. If this view is correct we may extend it so far as to con-

MERISTIC PHENOMENA 57

sider whether the fineness or coarseness of the mixture visible
in the flowers or leaves may not give an indication of the degree
to which the factors are subdivided among the germ-cells. We
know very little about the genetic properties of striped varieties.
In both Antirrhinum and Mirabilis it has been found that the
striped may occasionally and irregularly throw self-coloured
plants, and therefore the striping cannot be regarded simply as
a recessive character. On the other hand in Primula Sinensis
there are well-known flaked varieties which ordinarily at least
breed true. Whether these ever throw selfs I do not know,
but if they do it must be quite exceptionally. The power of
these flaked plants to breed true is, I suspect, connected with
the fact that in their flowers the coloured and white parts are
intimately mixed, this intimate mixture thus being an indication
of a similarly intim te mixture in the germ-cells. It would be
important to ascertain whether self-fertilised seed from the oc-
casional flowers in which the colour has run together to join a
large patch gives more self-coloured plants than the intimately
flaked flowers do.

The next fact may eventually prove of great importance.
We have seen that in bud-sports the differentiation is of the same
nature as that between pure types, and also that in the sporting
plant this differentiation is distributed without any reference
to the plant's axis, or any other consideration of symmetry.
Now among the germ-cells of a Mendelian hybrid exactly such
characters are being distributed allelomorphically, and there
again we have strong evidence for believing that the distribution
obeys no pattern. For example, we can in the case of seeds still
in situ perceive how the characters were distributed among the
germ-cells, and there is certainly no obvious pattern connecting
them, nor can we suppose that there is an actual pattern obscured.

Of this one illustration is especially curious. Individual
plants of the same species are, as regards the decussations of
their leaves and in other respects, either rights or lefts. The fact
is not emphasized in modern botany and is in some danger of
being forgotten. When, as in the flowers of Arum, some Gladioli,
Exacum, St. Paulia, or the fruits of Loasa, rights and lefts occur

58 PROBLEMS OF GENETICS

on the same stem, they come off alternately. But if, as in the
seedlings of Barley the twist of the first leaf be examined, it
will be seen to be either a right- or left-handed screw. An
ear of barley, say a two-row barley, is a definitely symmetrical
structure. The seeds stand in their envelopes back to back in
definite positions. Each has its organs placed in perfectly
definite places. // these seeds were buds their differentiations
would be grouped into a common plan. One might expect that
the differentiations of these embryos would still fall into the
pattern; but they do not, and so far as I have tested them, any
one may be a right or a left, just as each may carry any of the
Mendelian allelomorphs possessed by the parent plant, without
reference to the differentiation of any other seed. The fertil-
isation may be responsible, but our experience of the allelo-
morphic characters suggest that the irregularity is in the egg-
cells themselves.19

Germ cells thus differ from somatic cells in the fact that their
differentiations are outside the geometrical order which governs
the differentiation of the somatic cells. I can think of possible
exceptions, but I have confidence that the rule is true and I
regard it as of great significance.

The old riddle, what is an individual, finds at least a partial
solution in the reply that an individual is a group of parts dif-
ferentiated in a geometrically interdependent order. With the
germ-cell a new geometrical order, with independent polarity is
almost if not quite always, begun, and with this geometrical inde-
pendence the power of rejuvenescence may possibly be associated.

The problems thus raised are unsolved, but they do not look
insoluble. The solution may be nearer than we have thought.
In a study of the geometry of differentiation, germinal and
somatic, there is a way of watching and perhaps analyzing what
may be distinguished as the mechanical phenomena of heredity.
If any one could in the cases of the Picotee and the Bizarre Carna-
tion, respectively, detect the real distinction between the two

19 Remarkable experiments on this question have lately been carried out by
R. H. Compton (Cantb. Phil. Soc., XV, 1910, p. 495), showing that in a certain
Barley, "Plumage Corn," the average ratio of left to right is about 1.5. A fuller
paper has since been published by Compton, Jour. Genetics, 1912, II, i, p. 53.

MERISTIC PHENOMENA 59

types of distribution, he would make a most notable advance.
Any one acquainted with mechanical devices can construct a
model which will reproduce some of these distinctions more or less
faithfully. The point I would not lose sight of is that the analogy
with such models must for a long way be a true and valuable
guide. I trust that some one with the right intellectual equip-
ment will endeavor to follow this guide ; and I am sanguine enough
to think that a comprehensive study of the geometrical phenomena
of differentiation will suggest to a penetrative mind that critical
experiment which may one day reveal the meaning of spontaneous
division, the mystery through which lies the road, perhaps the
most hopeful, to a knowledge of the nature of life.

CHAPTER III

SEGMENTATION, ORGANIC AND MECHANICAL

Models may be and often have been devised imitating some
of the phenomena of division, but none of them have reproduced
the peculiarity which characterises divisions of living tissues,
that the position of chemical differentiation is determined by those
divisions. For example, models of segmentation, whether
radial or linear, may be made by the vibration of plates as in
the familiar Chladni figures of the physical laboratory, or by
the bowing of a tube dusted on the inside with lycopodium
powder, and in various other ways. The sand or the powder
will be heaped up in the nodes or regions of least movement, and
the patterns thus formed reproduce many of the geometrical
features of segmentation. But in the segmentations of living
things the nodes and internodes, once determined by the dividing
forces, would each become the seat of appropriate and distinct
chemical processes leading to the differentiation of the parts,
and the deposition of the bones, petals, spines, hairs, and other
organs in relation to the meristic ground-plan. The "ripples" of
meristic division not merely divide but differentiate, and when
a "ripple" forks the result is not merely a division but a re-
duplication of the organ through which the fork runs. An
example illustrating such a consequence is that of the half-ver-
tebrae of the Python. On the left side the vertebra is single
(Fig. 7) and bears a single rib, but on the right side a division
has occurred with the result that two half-vertebrae, each
bearing a rib, are formed, one standing in succession to the
other. We cannot, indeed, imagine any operation of physio-
logical division carried out in such an organ as a vertebra, passing
through a plane at right angles to the long axis of the body, which
does not necessarily involve the further process of reduplication.

As the meristic system of distribution spreads through the
body, chemical differentiations follow in its track, with seg-

60

SEGMENTATION

61

mentation and pattern as the visible result. Could we analyse
these simultaneous phenomena and show how it is that the places
of chemical differentiation are determined by the system of
division, progress would then be rapid. It is here that all
speculation fails.

167

FIGS. 7 and 8. Two examples of imperfect division in the vertebrae of a python.
I, the vertebrae 147-150 from the right side, showing imperfect division between
the 1 48th and I49th. The condition on the left side of this vertebra was the same.
//, the dorsal surface of vertebrae 165-167. On the right side the i66th is double
and bears two ribs, but on the left side it is normal and has one rib only.

Many attempts have been made to interpret the processes of
division and repetition, in terms of mechanics, or at least to
refer them to their nearest mechanical analogies, so far with

62 PROBLEMS OF GENETICS

little success. The problem is beset with difficulties as yet
insurmountable and of these one must be especially noticed. In
the living thing the process by which repetition and patterns
come into being consists partly in division but partly also in
growth. We have no means of studying the phenomena of
pattern-formation except in association with that of growth.
Growth soon ceases unless division takes place, and if growth is
impossible division soon ceases also. In consequence of this
fact that the final pattern is partly a product of growth, it can
never be used as unimpeachable evidence of the primary geo-
metrical relations of the members as laid down in the divisions.

In the last chapter in referring to the problem of repetition
I introduced an analogy, comparing the patterns of the organic
world with those produced in unorganised materials by wave-
motion. In the preliminary stage of ignorance, having no more
trustworthy clue, I do not think it wholly unprofitable to consider
the applicability of this analogy somewhat more fully. It
possesses, as I hope to show, at least so much validity as to
encourage the belief that morphology may safely discard one
source of long-standing error and confusion.

Those who have studied the structure of parts repeated in
series will have encountered the old morphological problem of
"Serial Homology," which has absorbed so much of the attention
of naturalists and especially of zoologists at various periods.
This problem includes two separate questions. The first of
these is the origin in evolution of the resemblance between two
organs occurring in a repeated series, of which the fore and hind
limbs of Vertebrates are the prerogative instance. From the
fact that these resemblances can be traced very far, often into
minute details of structure, many anatomists have inclined to
the opinion that the resemblance must originally have been still
more complete, and that the two limbs, for instance, must have
acquired their present forms by the differentiation of two iden-
tical groups of parts.

Similar questions arise whenever parts are repeated in series,
whether the series be linear or radial, and, though less obviously,
even when the repetition is bilateral only. In each such example

SEGMENTATION 63

the question arises, is the resemblance between the parts the
remains of a still closer resemblance, or is differentiation original?
Sometimes the view that these parts have arisen by the dif-
ferentiation of a series of identical parts is plausible enough,
as for example when the peculiarities of various appendages of a
Decapod Crustacean are referred to modifications of the Phyl-
lopod series. In application to other cases however we soon meet
with difficulty, and the suggestion that the segments of a verte-
brate were originally all alike is seen at once to be absurd, for
the reason that a creature so constituted could not exist, and that,
differentiation of at least one anterior and one posterior segment,
is an essential condition of a viable organism consisting of parts
repeated in a linear series. Between these two terminal segments
it is possible to imagine the addition of one segment, or of a
series of approximately similar segments; but when once it is
realised that the terminals must have been differentiated from
the beginning, it will be seen that the problem of the origin of
the resemblance between segments is not rendered more com-
prehensible by the suggestion that even the intervening members
were originally alike. Seeing indeed that some differentiation
must have existed primordially it is as easy to imagine that the
original body was composed of a series grading from the condition
of the anterior segment to that of the posterior, as any other
arrangement. The existence of a linear or successive series in
fact postulates a polarity of the whole, and in such a system the
conception of an ideal segment containing all the parts represented
in the others has manifestly no place. The introduction of that
conception though sanctioned by the great masters of com-
parative anatomy, has, as I think, really delayed the progress of
a rational study of the phenomena of division. The same notion
has been applied to every class of repetition both in animals and
plants, generally with the same unhappy results. In the cruder
forms in which this doctrine was taught thirty years ago it is
now seldom expressed, but modified presentations of it still
survive and confuse our judgments.

The process of repetition of parts in the bodies of organisms
is however a periodic phenomenon. This much, provided we

PROBLEMS OF GENETICS

remain free from prejudice as to the nature and causation of the
period or rhythm, we may safely declare, and a comparison may
thus be instituted between the consequences of meristic repetition

FIG. 9. Osmotic growths simulating segmentation. (After Leduc.)

in the bodies of living things and those repetitions which in
the inorganic world are due to rhythmical processes. Of such
processes there is a practically unlimited diversity and we have
nothing to indicate with which of them our repetitions should
rather be compared.

SEGMENTATION 65

In some respects perhaps the best models of living organisms
yet made are the "osmotic growths" produced by Leduc.1
These curious structures were formed by placing a fragment of
a salt, for instance calcium chloride, in a solution of some col-
loidal substance. As the solid takes up water from the solution
a permeable pellicle or membrane is formed around it. The ves-
icle thus enclosed grows by further absorption of water, often
extending in a linear direction, and in many examples this growth
occurs by a series of rhythmically interrupted extensions. Some
of the growths thus formed are remarkably like organic structures,
and might pass for a series of antennary segments or many other
organs consisting of a linear series of repeated parts. In admitting
the essential resemblance between these " osmotic growths"
and living bodies or their organs I lay less stress on the general con-
formation of the growths, which often as Leduc points out, recall
the forms of fungi or hydroids, but rather on the fact that the
interruptions in the development of these systems are so closely
analogous to the segmentations or repetitions of parts character-
istic of living things (Fig. 9). In the same way I am less im-
pressed by Leduc's models of Karyokinesis, wonderful as they
nevertheless are, for the division is here imitated by putting
separate drops on the gelatine film. What we most want to know
is how in the living creature one drop becomes two. The models
of linear segmentation have the remarkable merit that they do in
some measure imitate the process of actual division or repetition.
So in a somewhat modified method Leduc, by causing the diffusion
of a solution in a gelatine film, produced rhythmical or periodic
precipitations strikingly reminiscent of various organic tissues,
for here also the process of periodic repetition is imitated with
success.

It is a feature common to these and to all other rhythmical
repetitions produced by purely mechanical forces that there
is resemblance between the members of the series, and that this
similarity of conformation may be maintained in most complex
detail. When however in the mechanical series some of the
members differ from the rest we have no difficulty in recognising

1 Stephane Leduc, Theorie Physico-Chymiqite de la Vie, Paris, 1910.
6

66 PROBLEMS OF GENETICS

that these differences — which correspond with the differenti-
ations of the organic series — are due to special heterogeneity in
the conditions or in the materials, and it never occurs to us to
suppose that all the members must have been primordially alike.
For example, in the case of ripple-marks on the sand, which I
choose as one of the most familiar and obvious illustrations
of a repeated series due to mechanical agencies, if we notice
one ripple different in form from those adjacent to it, we do
not suppose that this variation must have been brought about by
deformation of a ripple which was at first formed like the others,
but we ascribe it to a difference in the sand at that point, or to a
difference in the way in which the wind or the tide dealt with it.
We may press the analogy further by observing that in as much
as such a series of waves has a beginning and an end, it possesses
polarity like that of the various linear series of parts in organisms,
and even the formation of each member must influence the
shape of its successor. Since in an organism the beginning and
end of the series are always included, some differentiation among
the repetitions must be inevitable. If therefore it be conceded,
as I think it must, that segmentation and pattern are the con-
sequence of a periodic process we realize that it is at least as
easy to imagine the formation of such a series of parts having
family likeness combined with differentiation as it would be to
conceive of their arising primordially as a series of identical repe-
titions. The suggestion that the likenesses which we now per-
ceive are the remains of a still more complete resemblance
is a substitution of a more complex conception for a simpler one.
The other question raised by the problem of Serial Homology
is how far there is a correspondence between individual members
of series when the series differ from each other either in the
number of parts, or in the mode of distribution of differentiation
among them. Students, for example, of vertebrate morphology
debate whether the nth vertebra which carries the pelvic girdle
in Lizard A is individually homologous with the n+xth vertebra
which fulfils this function in Lizard B,or whether it is not more
truly homologous with the vertebra standing in the nth ordinal
position, though that vertebra in Lizard B is free.

SEGMENTATION 67

In various and more complex aspects the same question is
debated in regard to the cranial and spinal nerves, the branches
of the aorta, the appendages of Arthropoda, and indeed in re-
gard to all such series of differentiated parts in linear or suc-
cessive repetition. Persons exercised with these problems
should before making up their minds consider how similar
questions would be answered in the case of any series of rhyth-
mical repetitions formed by mechanical agencies. In the case
of our illustration of the ripples in the sand, given the same forces
acting on the same materials in the same area, the number of
ripples produced will be the same, and the nth ripple counting
from the end of the series will stand in the same place whenever
the series is evoked. If any of the conditions be changed, the
number and shapes can be changed too, and a fresh "distribution
of differentiation" created. Stated in this form it is evident
that the considerations which would guide the judgment in the
case of the sand ripples are not essentially different from those
which govern the problem of individual homology in its applica-
tion to vertebrae, nerves, or digits.

The fact that the unit of repetition is also the unit of growth
is the source of the obscurity which veils the process. When we
compare the skeleton of a long-tailed monkey with that of a
short-tailed or tailless ape we see at once how readily the addi-
tional series of caudal segments may be described as a conse-
quence of the propagation of the "waves" of segmentation
beyond the point where they die out in the shorter column, and
we see that with an extension of the series of repetitions there is
growth and extension of material.

The considerations which apply to this example will be found
operating in many cases of the variation of terminal members of
linear series. Some of these series, like the teeth of the dog,
end in a terminal member of a size greatly reduced below that of
the next to it. Even when there is thus a definite specialisation
of the last member of the series it not infrequently happens that
the addition, by variation, of a member beyond the normal
terminal, is accompanied by a very palpable increase in size of
the member which stands numerically in the place of the normal

68 PROBLEMS OF GENETICS

terminal.2 So also with variation in the number of ribs, when a
lumbar vertebra varies homoeotically into the likeness of the
last dorsal and bears a rib, the rib placed next in front of this,
which in the normal trunk is the last, shows a definite increase
in development.

The consequences of such homoeoses are sometimes very
extensive, involving readjustments of differentiation affecting
a long series of members, as may easily be seen by comparing
the vertebral columns of several individual Sloths3 (whether
Brady pus or Choloepus) to take a specially striking example.

It may be urged that no feature as yet enables us to perceive
wherein lies the primary distinction which determines such
variation, whether it is due to a difference in the dividing forces
or in the material to be divided. If for instance we were to
imitate such a series of segments by pressing hanging drops of
a viscous fluid out of a paint-tube by successive squeezes, the
number of times the tube is contracted before it is empty will give
the number of the segments, but their size may depend either
on the force of the contractions or on the capacity of the tube,
or on various other factors. Nevertheless in the case of the
variation of terminal members, whatever be the nature of the
rhythmical impulse which produces the series of organs, the ele-
vation of the normally terminal member in correspondence with
the addition of another is what we should expect.

If the organism acquired its full size first and the delimitation
of the parts took place afterwards, there might be some hope that
the resemblance between living patterns and those mechanically
caused by wave-motion might be shown to be a consequence of
some real similarity of causation, but in view of the part played
by growth, appeal to these mechanical phenomena cannot be
declared to have more than illustrative value. Similarly in as
much as living patterns appear, and almost certainly do in reality
come into existence by a rhythmical process, comparisons of
these patterns with those developed in crystalline structures, and
in the various fields of force are, as it seems to me, inadmissible,
or at least inappropriate.

2 Materials for the Study of Variation, No. 249, p. 217; and p. 272.

3 Materials, p. 118.

SEGMENTATION 69

However their intermittence be determined, the rhythms of
division must be looked upon as the immediate source of those
geometrically ordered repetitions universally characteristic of
organic life. In the same category we may thus group the seg-
mentation of the Vertebrates and of the Arthropods, the concen-
tric growth of the Lamellibranch shells or of Fishes' scales, the
ripples on the horns of a goat, or the skeletons of the Foraminifera
or of the Heliozoa. In the case of plant-structures Church 4 has
admirably shown, with an abundance of detail, how on analysis
the definiteness of phyllotaxis is an expression of such rhythm in
the division of the apical tissues, and how the spirals and "ortho-
stichies" displayed in the grown plant are its ultimate conse-
quences. The problem thus narrows itself down to the question
of the mode whereby these rhythms are determined.

It is natural that we should incline to refer them to a chemical
source. If we think of the illustration just given, of the seg-
mentation of a viscous fluid into drops by successive contractions
of a soft-walled tube we can, I think, conceive of such rhythmic
contractions as due to summations of chemical stimuli, somewhat
as are the beats of the heart. But when we recognize the vast
diversity of materials the distribution of which is determined by
an ostensibly similar rhythmic process it seems hopeless to look
forward to a directly chemical solution. That the chemical
degradation of protoplasm or of materials which it contains is
the source of the energy used in the divisions cannot be in dispute,
but that these divisions can be themselves the manifestations
of chemical action seems in the highest degree improbable.

We may therefore insist with some confidence on the dis-
tinction between the Meristic and the substantive constitution
of organisms, between, that is to say, the system according to
which the materials are divided and the essential composition
of the materials, conscious of the fact that the energy of division
is supplied from the materials, and that in the ontogeny the
manner in which the divisions are effected must depend secon-
darily on the nature of the substances to be divided. The me-

4 Church, A. H.f On the Relation of Phyllotaxis to Mechanical Laws, London,
1904.

70 PROBLEMS OF GENETICS

chanical processes of division remain a distinguishable group of
phenomena, and variations in the substances to be distributed in
division may be independent of variations in the system by which
the distribution is effected.

Modern genetic analysis supplies many remarkable examples
of this distinction. When formerly we compared the leaves of a
normal palmatifid Chinese Primula with the pinnatifid leaves6 of
its fern-leaved variety we were quite unable to say whether the
difference between the two types of leaf was due to a difference
in the material cut up in the process of division or to a difference
in that process itself. Knowledge that the distinction is deter-
mined by a single segregable factor tends to prove that the
critical difference is one of substance. So also in the Silky fowl
we know that the condition of its feathers is due to the absence
of some one factor present in the normal form. We may con-
ceive such differences as due to change of form in the successive
"waves" of division, but we cannot yet imagine segregation
otherwise than as acting by the removal or retention of a material
element. Future observation by some novel method may suggest
some other possibility, but such cases bring before us very clearly
the difficulties by which the problem is beset.

In another region of observation phenomena occur which as
it seems to me put it beyond question that the meristic forces are
essentially independent of the materials upon which they act,
save, in the remoter sense, in so far as these materials are the
sources of energy. The physiology of those regenerations and
repetitions which follow upon mutilation supplies a group of
facts which both stimulate and limit speculation. No satis-
factory interpretations of these extraordinary occurrences has
ever been found, but we already know enough to feel sure that
in them we are witnessing indications which should lead to the
discovery of the true mechanics of repetition and pattern.
The consequences of mutilation in causing new growth or perhaps
more strictly in enabling new growth to take place, are such that
they cannot be interpreted as responses to chemical stimuli in

5 It is a question whether the dominance of the palmatifid leaf over the pin-
natifid is not really an example of the dominance of a lower number of segmentations

SEGMENTATION 71

any sense which the word chemical at present connotes. Powers
are released by mutilation of which in the normal conditions of
life no sign can be detected. All who have tried to analyse the
phenomena of regeneration are compelled to have recourse to the
metaphor of equilibrium, speaking of the normal body as in a state
of strain or tension (Morgan) which when disturbed by muti-
lation results in new division and growth. The forces of division
are inacessible to ordinary means of stimulation. Applications,
for example, of heat or of electricty excite no responses of a
positive kind unless the stimuli are so violent as to bring about
actual destruction.8 These agents do not, to use a loose expres-
sion, come into touch with the meristic forces. Changes in the
chemical environment of cells may, as in the experiments of
Loeb and of Stockard produce definite effects, but the facts
suggest that these effects are due rather to alterations in the
living material than to influence exerted directly on the forces
of division themselves.

By destruction of tissue however the forces both of growth
and of division also may often be called into action with a re-
sulting regeneration. Interruption of the solid connexion
between the parts may produce the same effects, as for example
when the new heads or tails grow on the divided edges of Pla-
narians (Morgan), or when from each half embryo partially sepa-
rated from its normally corresponding half, a new half is formed
with a twin monster as the result.

Often classed with regenerations but in reality quite distinct
from them are those special and most interesting examples
where the growth of a paired structure is excited by a simple

over a higher. From the uncertainty whether two given leaves of two separate
plants are actually comparable one cannot institute quite satisfactory numerical
comparisons, but I think the view that the "Fern" leaf has more lobes than an
otherwise similar "Palm" leaf may be fairly maintained. If this be admitted,
the "Palm" leaf represents the dominant low number and its round shape is a
consequence of the greater powers of growth which are so often possessed by the
members of a shorter series.

6 It is perhaps of importance to remember that in certain species of bacteria
(e. g. Bacillus Anthracis) division may cease where the organism is cultivated under
certain artificial conditions though growth continues. In this way very long un-
segmented threads are produced.

72 PROBLEMS OF GENETICS

wound. Some of the best known of these instances are presented
by the paired extra appendages of Insects and Crustacea. Some
years ago I made an examination of all the examples of such
monstrosities to which access was to be obtained, and it was with
no ordinary feeling of excitement that I found that these super-
numerary structures were commonly disposed on a recognizable
geometrical plan, having definite spatial relations both to each
other and to the normal limb from which they grew. The more
recent researches of Tornier7 and especially his experiments on
the Frog have shown that a cut into the posterior limb-bud
induces the outgrowth of such a pair of limbs at the wounded
place. Few observations can compare with this in novelty or
significance; and though we cannot yet interpret these phenomena
or place them in their proper relations with normal occurrences,
we feel convinced that here is an observation which is no mere
isolated curiosity but a discovery destined to throw a new light
on biological mechanics. The supernumerary legs of the Frog
are evidently grouped in a system of symmetry similar to that
which those of the Arthropods exhibit, and though in Arthropods
paired repetitions have not been actually produced by injury
under experimental conditions we need now have no hesitation
in referring them to these causes as Przibram has done.

At this point some of the special features of the super-
numerary appendages become important. First they may arise
at any point on the normal limb, being found in all situations
from the base to the apex. Nor are they limited as to the surface
from which they spring, arising sometimes from the dorsal,
anterior, ventral, or posterior surfaces, or at points intermediate
between these principal surfaces.

With rare and dubious exceptions, the parts which are con-
tained in these extra appendages are only those which lie periph-
eral to their point of origin. Thus when the point of origin is
in the apical joint of the tarsus, the extra growth if completely
developed consists of a double tarsal apex bearing two pairs
of claws. If they arise from the tibia, two complete tarsi are

* Arch. /. Entwm., XX, 1905, p. 76; Sitzungsb. d. Ges. Naturf., Berlin, 1907,
p. 41, etc.

SEGMENTATION

73

added. If they spring from the actual base of the appendage
then two complete appendages may be developed in addition to
the normal one. We must therefore conclude that in any point
on a normal appendage the power exists which, if released, may
produce a bud containing in it a paired set of the parts peripheral
to this point.

Next the geometrical relations of the halves of the super-
numerary pair are determined by the position in which they stand

DA

DAA

FIG. n. Diagrams of the geometrical relations which are generally exhibited
by extra pairs of appendages in Arthropoda. The sections are supposed to be
those of the apex of a tibia in a beetle. A, anterior, P, posterior, D, dorsal, V,
ventral, if1, MJ are the imaginary planes of reflexion. The shaded figure is in
each case a limb formed like that of the other side of the body, and the outer
unshaded figures are shaped like the normal for the side on which the appendages
are. On the several radii are shown the extra pairs in then: several possible re-
lations to the normal from which they arise. The normal is drawn in thick lines
in the center.

74 PROBLEMS OF GENETICS

in regard to the original appendage. These relations are best
explained by the diagram (Fig. n), from which it will be seen
that the two supernumerary appendages stand as images of each
other; and, of them, that which is adjacent to the normal ap-
pendage forms an image of it. Thus if the supernumerary pair
arise from a point on the dorsal surface of the normal appendage,
the two ventral surfaces of the extra pair will face each other.
If they arise on the anterior surface of the normal appendage,
their morphologically posterior surfaces will be adjacent, and so on.

These facts give us a view of the relations of the two halves
of a dividing bud very different from that which is to be derived
from the exclusive study of normal structures. Ordinary mor-
phological conceptions no longer apply. The distribution of the
parts shows that the bud or rudiment which becomes the super-
numerary pair may break or open out in various ways according
to its relations to the normal limb. Its planes of division are
decided by its geometrical relations to the normal body.

Especially curious are some of the cases in which the extra
pair are imperfectly formed. The appearance produced is then
that of two limbs in various stages of coalescence, though in
reality of course they are stages of imperfect separation. The
plane of "coalescence" may fall anywhere, and the two appen-
dages may thus be compounded with each other much as an
object partially immersed in mercury "compounds" with its
optical image reflected from the surface.

Supernumerary paired structures are not usually, if ever,
formed when an appendage is simply amputated. Cases oc-
casionally are seen which nevertheless seem to be of this nature.
Borradaile,8 for example, described a crab (Cancer pagurus)
having in place of the right chela three small chelae arising from
a common base, where the appearances suggested that the three
reduced limbs replaced a single normal limb. From the details re-
ported however it seems still possible that one of the chelae
(that lettered F. I in Borradaile's figure) may be the normal
one, and the other two an extra pair. The chela which I suspect
to be the normal is in several respects deformed as well as being

8 Borradaile, L. A., Jour. Marine ZooL, 1897, No. 8.

SEGMENTATION 75

reduced in size, and this deformity may perhaps have ensued as
a consequence of the same wound which excited the growth of the
extra pair. Its reduced size may be due to the same injury,
which may quite well have checked its growth to full proportions.

Admitting doubt in these ambiguous cases it seems to be a
general rule that for the production of the extra pair the normal
limb should persist in connexion with the body. Moreover it is
practically certain that in no case can a single, viz. an unpaired,
duplicate of the normal appendage grow from it. Many examples
have been described as of this nature, but all of them may be with
confidence regarded as instances of a supernumerary pair in
which only the two morphologically anterior or the two mor-
phologically posterior surfaces are developed. We have thus
the paradox that a limb of one side of the body, say the right,
has in it the power to form a pair of limbs, right and left, as an
outgrowth of itself, but cannot form a second left limb alone.

A very interesting question arises whether it is strictly
correct to describe the extra pair as a right and a left, or whether
they are not rather two lefts or two rights of which one is reversed.
This question did not occur to me when in former years I studied
these subjects. It was suggested to me by Dr. Przibram.
The answer might have an important bearing on biological
mechanics, but I know no evidence from which the point can
be determined with certainty. In order to decide this question
it would be necessary to have cases in which the paired repetition
affected a limb markedly differentiated on the two sides of the
body, and of course the development of the extra parts in order
to be decisive must be fairly complete. One example only is
known to me which at all satisfies these requirements, that of the
lobster's chela figured (after Van Beneden) in Materials for the
Study of Variation, p. 531, Fig. 184, III.

Here the drawing distinctly suggests that one of the extra
dactylopodites, namely that lettered R, is differentiated as a
left and not merely a reversed right. For the teeth on this
dactylopodite are those of a cutting claw, not of a crushing claw,
whereas the dactylopodites R' and L' bear crushing teeth. The
figure makes it fairly certain also that the limb affected was a

76 PROBLEMS OF GENETICS

crushing claw. Accepting this interpretation, we reach the
remarkable conclusion that the bud of new growth consisted of
halves differentiated into cutter and crusher as the normal claws
are, and that the extra crusher is geometrically a left but physi-
ologically a right. Though shaped as a left in respect of the
direction in which it points, the extra crusher is really an op-
tically reversed right, while the dactylopodite R, which is
placed pointing like a right, is really a reversed left (Fig. 12).

If these indications are reliable 9 and are established by further
observation we shall be led to the conclusion that the bud which

FIG. 12. Right claw of lobster bearing a pair of extra dactylopodites (after
van Beneden). The fine toothing on R suggests that this is part of a cutting
claw, though the limb bearing it is a crusher.

becomes an extra pair of limbs does not merely contain the parts
proper to the side on which it grows, but is comparable with
the original zygotic cell, and consists not simply of two halves,
but of two halves differentiated as a right and a left like the two
halves of the normal body.

Phenomena of this kind, evoked by mutilation or injury,
together with the cognate observations on regeneration throw

9 Dr. Przibram, I should mention, concludes that on the whole the facts are
against this interpretation, but as more evidence is certainly required, I call at-
tention to the possibility.

SEGMENTATION 77

very curious lights on the nature of living things. To an under-
standing of the nature of the mechanics of living matter and its
relation to matter at large they offer the most hopeful line of
approach. I allude especially to the examples in which it has
been established that the part which is produced after mutilation
is a structure different from that which was removed. The
term "regeneration" was introduced before such phenomena
were discovered, and though every one recognizes its inapplica-
bility to these remarkable cases, the word still misleads us by
presenting a wrong picture to the mind. The expression " hetero-
morphosis" (Loeb) has been appropriately applied to various
phenomena of this kind, and Morgan has given the name "mor-
phallaxis" to another group of cases in which the renewal occurs
by the transformation of a previously existing part.10 But we
must continually remember that all these occurrences which we
know only as abnormalities and curiosities must in reality be
exemplifications of the normal mechanics of division and growth.
The conditions needed to call them forth are abnormal, but the
responses which the system makes are evidences of its normal
constitution. When therefore, for example, the posterior end
of a worm produces a reversed tail from its cut end we have a
proof that there must be in the normal body forces ready to
cause this outgrowth. The new structure is not an ill-shaped
head-end, for, as Morgan shows, the nephridial ducts have their
funnels perforating the segments in a reversed direction. The
"tension" of growth is actually reversed.11 So also when in a
Planarian amputation of the body immediately behind the head
leads to the formation of a new reversed head at the back of
the normal head, while amputation further back leads to the
regeneration of a new tail, these responses give indications
of forces normally present in the body of the Planarian. Such
facts open up a great field of speculation and research. Espe-
cially important it would be to determine where the critical
region may be at which the one response is replaced by the

10 Morgan, T. H., Regeneration, 1901.

11 It would be interesting to know whether growth continues at the original
posterior end after the new "posterior" end has been formed in front.

78 PROBLEMS OF GENETICS

\

other. I suppose it is even possible that there is some neutral
zone in which neither kind of response is made.

Physical parallels to the phenomena of regeneration are
not easy to find and we still cannot penetrate beyond the empirical
facts. Przibram has laid stress on the general resemblance
between the new growth of an amputated part in an animal and
the way in which a broken crystal repairs itself when placed in
the mother-solution. That the two processes have interesting
points of likeness cannot be denied. It must however never be
forgotten that there is one feature strongly distinguishing the
two; for I believe it is universally recognized by physicists that
all the phenomena of geometrical regularity which crystals
display are ultimately dependent on the forms of the particles
of the crystalline body. This cannot in any sense be supposed
to hold in regard to protoplasm or its constituents. The de-
finiteness of crystals is also an unlikely guide for the reason that
it is absolute and perfect, or in other words because this kind of
regularity cannot be disturbed at all without a change so great
that the substance itself is altered; whereas we know that the
forms of living things are capable of such changes, great and small,
that we must regard perfection of form, whether manifested in
symmetry or in number, as an ideal which will only be produced
in the absence of disturbance. The symmetry of the living
things is like the symmetry of the concentric waves in a pool
caused by a splash. Perfect circles are made only in the imagi-
nary case of mathematical uniformity, but the system maintains
an approximate symmetry though liable to manifold deformation.

Since the geometrical order of the living body cannot be a
direct function of the materials it must be referred to some more
proximate control. In renewing a part the body must possess
the power of seizing particles of many dissimilar kinds, and whirl
them into their several and proper places. The action in re-
newal, like that of original growth, may be compared — very
crudely — with the action of a separator which simultaneously
distributes a variety of heterogeneous materials in an orderly
fashion ; but in the living body the thing distributed must rather
be the appetency for special materials, not the materials them-
selves.

SEGMENTATION 79

If the analogy of crystals be set aside and we seek for other
parallels to regeneration there are none very obvious. I have
sometimes wondered whether it might not be possible to institute
a fruitful comparison between the renewal of parts and the refor-
mation of waves of certain classes after obliteration. In several
respects, as I have already said, some curious resemblances with
the repetitions formed by wave-motion are to be traced in our
organic phenomena, and though admitting that I cannot develop
these comparisons, I think nevertheless they may be worth
bearing in mind. When, after obliteration, an eddy in a stream,
or a ripple-mark (a more complex case of eddy-formation) in
blown sand is re-formed, we have an example in which pattern is
reconstituted and growth takes place not by virtue of the com-
position of the materials — in this case the water or the sand —
but by the way in which they are acted upon by extraneous
forces.

A feature in the actual mode by which ripple-marks are
reconstituted may not be without interest in connexion with
our phenomena of regeneration. When, for example, the wind
is blowing steadily over a surface of fine, dry sand, the familiar
ripple-marks are formed by a heaping of the sand in lines trans-
verse to the direction of the wind. The heaping is due to the
formation of eddies corresponding with positions of instability.
When the wind is steady and the sand homogeneous, the dis-
tances between the ripples, or wave-lengths, are sensibly equal.
If while the wind continues to blow, the ripples are obliterated
with a soft brush they will quickly be re-formed over the whole
area, but I have noticed that at first their wave-length is ap-
proximately half that of the ripples in the undisturbed parts of
the system.12 The normal wave-length is restored by the gradual
accentuation of alternate ripples. Of course the sand-ripples are
in reality slowly travelling forward in the direction towards
which the wind is. blowing, and for this our living segmentations
afford no obvious parallel, but the appearances in the area of

12 In the actual case observed, the ripples unsmoothed had a wave-length of
about 2>6 inches; and when the new ones were first formed, there were about 30
ridges in the length originally traversed by 15 or 16.

8o PROBLEMS OF GENETICS

reformation, and especially the forking of the old ridges where
they join the new ones, are curiously reminiscent of the irregu-
larities of segmentation seen in regenerated structures. The
value of the considerations adduced in the chapter is, I admit,
very small. The utmost that can be claimed for them is that
mechanical segmentations, like those seen in ripple-mark, or
in Leduc's osmotic growths, show how by the action of a contin-
uous force in one direction, repeated and serially homologous
divisions can be produced having features of similarity common
to those repetitions by which organic forms and patterns are
characterised. The analogy supplies a vicarious picture of the
phenomena which in default of one more true may in a slight
degree assist our thoughts. It suggests that the rhythms of
segmentation may be the consequence of a single force definite
in direction and continuously acting during the time of growth.
The polarity of the organism would thus be the expression of
the fact that this meristic force is definitely directed after it has
once been excited, and the reversal seen in some products of re-
generation suggest further that it is capable of being reflected.
This polarity cannot be a property of the material, as such,
but is determined by a force acting on that material, just as the
polarity of a magnet is not determined by the arrangement of its
particles, but by the direction in which the current flows.

To some it may appear that even to embark on such discussions
as this is to enter into a perilous flirtation with vitalistic theories.
How, they may ask, can any force competent to produce chem-
ical and geometrical differentiation in the body be distinguished
from the "Entelechy" of Driesch? Let me admit that in this
reflexion there is one element of truth. If those who proclaim
a vitalistic faith intend thereby to affirm that in the processes
by which growth and division are effected in the body, a part is
played by an orderly force which we cannot now translate into
terms of any known mechanics, what observant man is not a
vitalist? Driesch's first volume, putting as it does into intel-
ligible language that positive deduction from the facts — es-
pecially of regeneration — should carry a vivid realisation of this
truth to any mind. If after their existence is realised, it is

SEGMENTATION 81

desired that these unknown forces of order should have a name,
and the word entelechy is proposed, the only objection I have to
make is that the adoption of a term from Aristotelian philosophy
carries a plain hint that we propose to relegate the future study
of the problem to metaphysic.

From this implication the vitalist does not shrink. But
I cannot find in the facts yet known to us any justification of so
hopeless a course. It was but yesterday that the study of
Entwicklungsmechanik was begun, and if in our slight survey
we have not yet seen how the living machine is to be expressed
in terms of natural knowledge that is poor cause for despair.
Driesch sums up his argument thus:13

"It seems to me that there is only one conclusion possible.
If we are going to explain what happens in our harmonious-
equipotential sytems by the aid of causality based upon the
constellation of single chemical factors and events, there must
be some such thing as a machine. Now the assumption of the
existence of a machine proves to be absolutely absurd in the
light of the experimental facts. Therefore there can be neither
any sort of a machine nor any sort of causality based upon con-
stellation underlying the differentiation of harmonious-equipotential
systems.

"For a machine, typical with regard to the three chief dimen-
sions of space, cannot remain itself if you remove parts of it
or if you rearrange its parts at will."

To the last clause a note is added as follows:

"The pressure experiments and the dislocation experiments
come into account here; for the sake of simplicity they have
not been alluded to in the main line of our argument."

I doubt whether any man has sufficient knowledge of all
possible machines to give reality to this statement. In spite also
of the astonishing results of experiments in dislocation, doubt
may further be expressed as to whether they have been tried in
such variety or on such a scale as to justify the suggestion that
the living organism remains itself if its parts are rearranged at

13 The Science and Philosophy of the Organism; Gifford Lectures, 1907. London,
1908, p. 141.

82 PROBLEMS OF GENETICS

will. All we know is that it can "remain itself" when much is
removed, and when much rearrangement has been affected,
which is a different thing altogether.

I scarcely like to venture into a region of which my ignorance
is so profound, but remembering the powers of eddies to re-form
after partial obliteration or disturbance, I almost wonder whether
they are not essentially machines which remain themselves
when parts of them are removed.

Real progress in this most obscure province is not likely to be
made till it attracts the attention of physicists; and though they
for long may have to forego the application of exact quantitative
methods, I confidently anticipate that careful comparison
between the phenomena of repetition formed in living organisms
and the various kinds of segmentation produced by mechanical
agencies would be productive of illuminating discoveries.

CHAPTER IV

THE CLASSIFICATION OF VARIATION AND THE NATURE OF
SUBSTANTIVE FACTORS

We have now seen that among the normal physiological
processes the phenomena of division form a recognisable, and
in all likelihood a naturally distinct group. Variations in these
respects may thus be regarded as constituting a special class
among variations in general.

The substantive variations have only one property in com-
mon— the negative one that they are not Meristic. The work
of classifying them and distinguishing them according to their
several types demands a knowledge of the chemistry of life far
higher than that to which science has yet attained. In reference
to some of the simplest variations Garrod has introduced the
appropriate term "Chemical sports." The condition in man
known as Alkaptonuria in which the urine is red is due especially
to the absence of the enzyme which decomposes the excretory
substance, alkapton. The " chemical sport" here consists in the
inability to break up the benzene ring. The chemical feature
which distinguishes and is the proximate cause of several colour-
varieties can now in a few cases be declared. The work of Miss
Wheldale has shown that colour-varieties may be produced by
the absence of the chromogen compound the oxidation of which
gives rise to sap-colours, by differences in the completeness of
this process of oxidation, and by a process of reduction super-
vening on or perhaps suppressing the oxidation. Some of these
processes moreover may be brought about by the combined action
of two bodies, the one an enzyme, for example an oxygenase, and
the other a substance regarded as a peroxide, contributing the
oxygen necessary for the oxidation to take place. Variation in
colour may thus be brought about by the addition or omission
of any one of the bodies concerned in the action.

Similar variations, or rather similar series of variations will

83

84 PROBLEMS OF GENETICS

undoubtedly hereafter be identified in reference to all the various
kinds of chemical processes upon which the structure and func-
tions of living things depend. The identification of these proc-
esses and of the bodies concerned in them will lead to a real
classification of Substantive Variations.

To forecast the lines on which such classification will proceed
is to look too far ahead. We may nevertheless anticipate with
some confidence that future analysis will recognise among the
contributing elements, some which are intrinsic and inalienable,
and others which are extrinsic and superadded.

We already know that there may be such interdependence
among the substantive characters that to disentangle them will
be a work of extreme difficulty. The mere fact that in our
estimation characters belong to distinct physiological systems is
no proof of their actual independence. In illustration may be
mentioned the sap-colour in Stocks and the development of
hoariness on the leaves and stems, which Miss Saunders's experi-
ments have shown to be intimately connected, so that in certain
varieties no hoariness is produced unless the elements for sap-
colour are already present in the individual plant.

The first step in the classification of substantive variations
is therefore to determine which are due to the addition of new
elements or factors, and which are produced by the omission of
old ones. A priori there is no valid criterion by which this can
be known, and actual experiments in analytical breeding can
alone provide the knowledge required. Some very curious results
have by this method been obtained, which throw an altogether
unexpected light on these problems. For example, in order that
the remarkable development of mesoblastic black pigment char-
acteristic of the Silky Fowl should be developed, it is practically
certain that two distinct variations from such a type as Callus
bankiva must have occurred. I assume, as is reasonable, that
G. bankiva has genetic properties similar to those of the Brown
Leghorn breed which has been used in the experiments which Mr.
Punnett and I have conducted. Callus bankiva was not available
but the Brown Leghorn agrees with it very closely in colouration,
and probably in the general physiology of its pigmentation.

CLASSIFICATION OF VARIATIONS 85

Setting aside the various structural differences between the two
breeds, the Silky is immediately distinguished from the Leghorn
by the fact that the skin of the whole body including that of the
face and comb appears to be of a deep purplish colour. The
face and comb of the Leghorn are red and the skin of the body
is whitish yellow. On examination it is found that the purple
colour of the Silky is in reality due to the distribution of a deep
black pigment in the mesoblastic membranes throughout the
body. The somatopleura, the pleura, pia mater, the dermis, and
in most organs the connective tissue and the sheaths of the blood-
vessels, are thus impregnated with black. No such pigmentation
exists in the Leghorn. As the result of an elaborate series of
experimental matings we have proved that the distinction be-
tween the Leghorn and the Silky consists primarily in the fact
that the Silky possesses a pigment-producing factor, P, which is
not present in the Leghorn.

This variation must undoubtedly have been one of addition.
But besides this there is another difference of an altogether dis-
similar nature; for the Brown Leghorn possesses a factor which
has the power of partially or completely restricting the operation
of the pigment-producing factor, P. Moreover in respect of
this pigment-restricting factor which we may call D, the sexes
of the Brown Leghorn differ, for the male is homozygous or DD,
but the female is heterozygous, Dd. Thus in order that the
black-skinned breed could be evolved from such a type as a
Brown Leghorn it must be necessary both that P should be added
and that D should drop out. We have not the faintest concep-
tion of the process by which either of these events have come to
pass, but there is no reasonable doubt that in the evolution of
the Silky fowl they did actually happen.

We may anticipate that numerous interdependences of this
kind will be discovered.

Before any indisputable progress can be made with the prob-
lem of evolution it is necessary that we should acquire some real
knowledge of the genesis of that class of phenomena which
formed the subject of the last chapter. So long as the process
of division remains entirely mysterious we can form no conception

86 PROBLEMS OF GENETICS

even of the haziest sort as to the nature of living organisms, or
of the proximate causes which determine their forms, still less
can we attempt any answer to those remoter questions of origin
and destiny which form the subject of the philosopher's con-
templation. It is in no spirit of dogmatism that I have ventured
to indicate the direction in which I look for a solution, though
I have none to offer. It may well be that before any solution
is attained, our knowledge of the nature of unorganised matter
must first be increased. For a long time yet we may have to
halt, but we none the less do well to prepare ourselves to utilise
any means of advance that may be offered, by carefully recon-
noitering the ground we have to traverse. The real difficulty
which blocks our progress is ignorance of the nature of division,
or to use the more general term, of repetition.

Let us turn to the more familiar problem of the causes of
variation. Now since variation consists as much in meristic
change as in alteration in substance or material, there is one
great range of problems of causation from which we are as yet
entirely cut off. We know nothing of the causation of division,
and we have scarcely an observation, experiment or surmise
touching the causes by which the meristic processes may be
altered.

Of the way in which variations in the substantive composition
of organisms are caused we have almost as little real evidence,
but we are beginning to know in what such variations must con-
sist. These changes must occur either by the addition or loss
of factors.

We must not lose sight of the fact that though the factors
operate by the production of enzymes, of bodies on which these
enzymes can act, and of intermediary substances necessary to
complete the enzyme-action, yet these bodies themselves can
scarcely be themselves genetic factors, but consequences of their
existence. What then are the factors themselves? Whence do
they come? How do they become integral parts of the organism?
Whence, for example, came the power which is present in a White
Leghorn of destroying — probably reducing — the pigment in its
feathers? That power is now a definite possession of the breed,

CLASSIFICATION OF VARIATIONS 87

present in all its germ-cells, male and female, taking part in their
symmetrical divisions, and passed on equally to all as much as
is the protoplasm or any other attribute of the breed. From the
body of the bird the critical and efficient substance could in all
likelihood be isolated by suitable means, just as the glycogen
of the liver can be. But even when this extraction has been
accomplished and the reducing body isolated, we shall know no
more than we did before respecting the mode by which the power
to produce it was conferred on the fowl, any more than we know
how the walls of its blood-vessels acquired the power to form a
fibrin-ferment.

It is when the scope of such considerations as this are fully
grasped that we realise the fatuousness of the conventional treat-
ment which the problem of the causes of variation commonly
receives. Environmental change, chemical injury, differences in
food supply, in temperature, in moisture, or the like have been
proposed as "causes." Admitting as we must do, that changes
may be produced — usually inhibitions of development — by sub-
jecting living things to changes in these respects, how can we
suppose it in the smallest degree likely that very precise, new,
and adaptative powers can be conferred on the germs by such
treatment? Reports of positive genetic consequences observed
comparable with those I have mentioned, become from time to
time current. We should I think regard them with the gravest
doubt. Few, so far as I am aware, have ever been confirmed,
though clear and repeated confirmation should be demanded
before we suffer ourselves at all to build upon such evidence.
In a subsequent chapter some of these cases will be considered in
detail.

In no class of cases would the transmission of an acquired
character superficially appear so probable as in those where power
of resisting the attack of a pathogenic organism is acquired in
the lifetime of the zygote. The possession of such a power is
moreover a distinction comparable with those which differentiate
varieties and species. It is due to the development in the blood
of specific substances which pervade the whole fluid. This
development is exactly one of those "appropriate responses to

88 PROBLEMS OF GENETICS

stimuli" which naturalists who incline to regard adaptation as
a direct consequence of an environmental influence might most
readily invoke as an illustration of their views. And yet all
evidence is definitely unfavourable to the suggestion of an
inheritance of the acquired power of resistance. Such change as
can be perceived in the virulence of the attacks on successive
generations may be most easily regarded as due to the exter-
mination of the more susceptible strains, and perhaps in some
measure to variation in the invading organisms themselves, an
"acquired character" of quite different import.

The specific "anti-body" may have been produced in re-
sponse to the stimulus of disease, but the power to produce it
without this special stimulus is not included in the germ-cells
any more than a pigment. All that they bear is the power to
produce the anti-bodies when the stimulus is applied.

If we could conceive of an organism like one of those to
which disease may be due becoming actually incorporated with
the system of its host, so as to form a constituent of its germ-cells
and to take part in the symmetry of their divisions, we should
have something analogous to the case of a species which acquires
a new factor and emits a dominant variety. When we see the
phenomenon in this light we realise the obscurity of the problem.
The appearance of recessive varieties is comparatively easy to
understand. All that is implied is the omission of a constituent.
How precisely the omission is effected we cannot suggest, but
it is not very difficult to suppose that by some mechanical fault
of cell-division a power may be lost. Such variation by unpack-
ing, or analysis of a previously existing complex, though unac-
countable, is not inconceivable. But whence come the new
dominants? Whether we imagine that they are created by some
rearrangement or other change internal to the organism, or
whether we try to conceive them as due to the assumption of
something from without we are confronted by equally hopeless
difficulty.

The mystery of the origin of a dominant increases when it
is realised that there is scarcely any recent and authentic account
of such an event occurring under critical observation, which can

CLASSIFICATION OF VARIATIONS 89

be taken as a basis for discussion. The literature of horticulture
for example abounds in cases alleged, but I do not think anyone
can produce an illustration quite free from doubt. Such evidence
is usually open to the suspicion that the plant was either intro-
duced by some accident, or that it arose from a cross with a pre-
existing dominant, or that it owed its origin to the meeting of
complementary factors. In medical literature almost alone how-
ever, there are numerous records of the spontaneous origin of
various abnormal conditions in man which habitually behave
as dominants, and of the authenticity of some of these there
can be no doubt.

When we know that such conditions as hereditary cataract
or various deformities of the fingers behave as dominants, we
recognize that those conditions must be due to the addition of
some element to the constitution of the normal man. In the
collections of pedigrees relating to such pathological dominants
there are usually to be found alleged instances of the origin of the
condition de novo. Not only do these records occur with such
frequency that they cannot be readily set aside as errors, but from
general considerations it must be obvious that as these mal-
formations are not common to normal humanity they must at
some moment of time have been introduced. The lay reader
may not be so much impressed with the difficulty as we are. He
is accustomed to regard the origin of any new character as equally
mysterious, but when once dominants are distinguished from
recessives the problem wears a new aspect. Thus the appearance
of high artistic gifts, whether as an attribute of a race or as a
sporadic event among the children of parents destitute of such
faculties, is not very surprising, for we feel fairly sure that the
faculty is a recessive, due to the loss of a controlling or inhibiting
factor; but the de novo origin of brachydactylous fingers in a
child of normal parents is of quite a different nature, and must
indicate the action of some new specific cause.

Whether such evidence is applicable to the general problem
of evolution may with some plausibility be questioned; but
there is an obvious significance in the fact that it is among these
pathological occurrences that we meet with phenomena most

90 PROBLEMS OF GENETICS

nearly resembling the spontaneous origin of dominant factors,
and I cannot see such pedigrees as these without recalling Vir-
chow's aphorism that every variation owes its origin to some
pathological accident. In the evolution of domestic poultry,
if Callus bankiva be indeed the parent form of all our breeds,
at least some half dozen new factors must have been added
during the process. In bankiva there is, for example, no factor
for rose comb, pea comb, barring on the feathers, or for the
various dominant types of dark plumage. Whence came all
these? It is, I think, by no means impossible that some other
wild species now extinct did take part in the constitution of
domestic poultry. It seems indeed to me improbable that the
heavy breeds descend from bankiva. Both in regard to domestic
races of fowls, pigeons, and some other forms, the belief in origin
within the period of human civilization from one simple primi-
tive wild type seems on a balance of probabilities insecurely
founded, but allowing something for multiplicity of origin we
still fall far short of the requisite total of factors. Elements
exist in our domesticated breeds which we may feel with con-
fidence have come in since their captivity began. Such ele-
ments in fowls are dominant whiteness, extra toe, feathered
leg, frizzling, etc., so that even hypothetical extension of the
range of origin is only a slight alleviation of the difficulty.

Somehow or other, therefore, we must recognize that dominant
factors do arise. Whether they are created by internal change,
or whether, as seems to me not wholly beyond possibility, they
obtain entrance from without, there is no evidence to show.
If they were proved to enter from without, like pathogenic
organisms, we should have to account for the extraordinary
fact that they are distributed with fair constancy to half the
gametes of the heterozygote.

In proportion as the nature of dominants grows more clear
so does it become increasingly difficult to make any plausible
suggestion as to their possible derivation. On the other hand
the origin of a recessive variety by the loss of a factor is a process
so readily imagined that our wonder is rather that the phenom-
enon is not observed far more often. Some slip in the accurate

CLASSIFICATION OF VARIATIONS 91

working of the mechanical process of division, and a factor
gets left out, the loss being attested by the appearance of a
recessive variety in some subsequent generation.

Consistently with this presentation of the facts we find that,
as in our domesticated animals and plants, a diversity of recessives
may appear within a moderately short period, and that when
variations come they often do not come alone. Witness the
cultural history of the Sweet Pea, Primula Sinensis, Primula
obconica, Nemesia strumosa and many such examples in which
variation when it did come was abundant. The fact cannot
be too often emphasized that in the vast proportion of these
examples of substantive variation under domestication, as well
as of substantive variation in the natural state, the change
has come about by omission, not by addition. To take, for
example, the case of the Potato, in which so many spontaneous
bud-variations have been recorded, East after a careful study
of the evidence has lately declared his belief that all are of this
nature, and the opinion might be extended to many other groups
of cases whether of bud or seminal variation. Morgan draws
the same conclusion in reference to the many varieties he has
studied in Drosophila.

In the Sweet Pea, a form which is beyond suspicion of having
been crossed with anything else, and has certainly produced
all the multitude of types which we now possess by variations
from one wild species, there is only one character of the modern
types which could, with any plausibility, be referred to a factor
not originally forming part of the constituents of the wild species.
This is the waved edge, so characteristic of the "Spencer'*
varieties; for the cross between a smooth-edged and a waved
type gives an intermediate not unfrequently. Nevertheless
there is practically no doubt that this is merely an imperfection
in the dominance of the smooth edge, and we may feel sure
that any plant homozygous for smooth edge would show no wave
at all. Hence it is quite possible that even the appearance of
the original waved type, Countess Spencer, was due to the loss
of one of the factors for smooth edge at some time in the history
of the Sweet Pea.

92 PROBLEMS OF GENETICS

In the case of the Chinese Primrose (Primula Sinensis) one
dominant factor has been introduced in modern times, probably
within the last six years at most. This is the factor which
causes suppression of the yellow eye, giving rise to the curious
type known as " Queen Alexandra." Mr. R. P. Gregory's
experiments proved that this was a very definite dominant, and
the element responsible for this development is undoubtedly an
addition to the original ingredient-properties, with which the
species wras endowed. Unfortunately, as happens in almost every
case of the kind, the origin of this important novelty appears
to be lost. Its behaviour, however, when crossed with various
other types is that of a simple dominant giving an ordinary 3 : I
ratio. There is therefore no real doubt that it came into exist-
ence by the definite addition of a new factor, for if it was simply
a case of the appearance of a new character made by combination
of two previously existing complementary factors we should
expect that when Queen Alexandra was self-fertilised a 9 : 7
ratio would be a fairly common result, which is not in practice
found.

In Oenothera Gates l has observed the appearance, in a large
sowing of about 1 ,000 Oenothera rubrinervis, of a single individual
having considerably more red pigment in the calyx than is usual
in rubrinervis. The whole of the hypanthium in the flowers of
this plant was red instead of green as in rubrinervis, and the
whole of the sepals were red in the bud-stage, except for small
green areas at the base. This type behaved as a dominant over
rubrinervis, but so far a pure-breeding individual was not found.
Admittedly the variation of this plant from the type of rubrinervis
can be represented as one of degree, though there is a very
sensible gap in the series between the new form which Gates
names "rubricalyx" and the reddest rubrinervis seen in his cul-
tures. It must certainly be recognised as a new dominant.
Gates, rightly as I consider, regards the distinction between
rubrinervis and rubricalyx as a quantitative one, and the same
remark applies to certain other types differing in the amount of
anthocyanin which they produce. I do not understand the argu-

i Gates, R. R., Zts. f. Abstammungslehre, 1911, IV, pp. 341 and 361.

CLASSIFICATION OF VARIATIONS 93

ment which Gates introduces to the effect that the difference
between such quantitative types cannot be represented in terms
of presence and absence. We are quite accustomed to the fact
that in the rabbit self-colour segregates from the Dutch-marked
type. These two types differ in a manner which we may reason-
ably regard as quantitative. It is no doubt possible that the
self-coloured type contains an ingredient which enables the colour
to spread over the whole body, but it is, I think, perhaps more
easy to regard the Dutch type as a form from which a part of
the colour is absent. It may be spoken of in terms I have used,
as a subtraction-stage in colour. Following a similar method we
may regard rubricalyx as an addition-stage in colour- variation.
The fact that crosses between rubrinervis, or rubricalyx and
Lamarckiana give a mixture of types in Fi, does not I think show,
as Gates declares, that there is any system here at work to
which a factorial or Mendelian analysis does not apply; but that
question may be more fitly discussed in connexion with the other
problems raised by the behaviour of Oenothera species in their
crosses.

I do, however, feel that, interesting as this case must be
admitted to be, we cannot quite satisfactorily discuss it as an
illustration of the de novo origin of a dominant factor. The
difference between the novelty and the type is quantitative, and
it is not unreasonable to think of such a difference being brought
about by some "pathological accident" in a cell-division.

Recognition of the distinction between dominant and reces-
sive characters has, it must be conceded, created a very serious
obstacle in the way of any rational and concrete theory of evolu-
tion. While variations of all kinds could be regarded as mani-
festations of some mysterious instability of organisms this diffi-
culty did not occur to the mind of evolutionists. To most of
those who have taken part in genetic analysis it has become a
permanent and continual obsession. With regard to the origin
of recessive variations, there is, as we have seen, no special
difficulty. They are negative and are due to absences, but as
soon as it is understood that dominants are caused by an addition
we are completely at a loss to account for their origin, for we

94 PROBLEMS OF GENETICS

cannot surmise any source from which they may have been
derived. Just as when typhoid fever breaks out in his district
the medical officer of health knows for certain that the bacillus of
typhoid fever has by some means been brought into that district
so do we know that when first dominant white fowls arose in the
evolution of the domestic breeds, by some means the factor for
dominant whiteness got into a bird, or into at least one of its
germ-cells. Whence it came we cannot surmise.

Whether we look to the outer world or to some rearrangement
within the organism itself, the prospect of finding a source of
such new elements is equally hopeless.

Leaving this fundamental question aside as one which it is
as yet quite unprofitable to discuss, we are on safe ground in
foreseeing that the future classification of substantive variations,
which genetic research must before long make possible, will be
based on a reference to the modes of action of the several factors.
Some will be seen to produce their effects by oxidation, some by
reduction, some by generating substances of various types,
sugars, enzymes, activators, and so forth. It may thus be
anticipated that the relation of varieties to each other and to
types from which they are derived will be expressible in terms
of definite synthetical formulae. Clearly it will not for an in-
definite time be possible to do this in practice for more than a
few species and for characters especially amenable to experi-
mental tests, but as soon as the applicability of such treatment
is generally understood the influence on systematics must be
immediate and profound, for the nature of the problem will at
length be clear and, though the ideal may be unattainable, its
significance cannot be gainsaid.

Note. — With hesitation I allow this chapter to appear in
the form in which it was printed a year ago, but in passing it for
the press after that interval I feel it necessary to call attention
to a possible line of argument not hitherto introduced.

In all our discussions we have felt justified in declaring that
the dominance of any character indicates that some factor is

CLASSIFICATION OF VARIATIONS 95

present which is responsible for the production of that character.
Where there is no definite dominance and the heterozygote is
of an intermediate nature we should be unable to declare on
which side the factor concerned was present and from which side
it was absent. The degree of dominance becomes thus the
deciding criterion by which we distinguish the existence of factors.
But it should be clearly realized that in any given case the argu-
ment can with perfect logic be inverted. We already recognize
cases in which by the presence of an inhibiting factor a character
may be suppressed and purely as a matter of symbolical expres-
sion we might apply the same conception of inhibition to any
example of factorial influence whatever. For instance we say
that in as much as two normal persons do not have brachydacty-
lous children, there must be some factor in these abnormal persons
which causes the modification. Our conclusion is based on the
observed fact that the modification is a dominant. But it may
be that normal persons are homozygous in respect of some factor
N, which prevents the appearance of brachydactyly, and that in
any one heterozygous, Nn, for this inhibiting factor, brachy-
dactyly can appear. Similarly the round pea we say contains
R, a factor which confers this property of roundness, without
which its seeds would be wrinkled. But here we know that the
wrinkled seed is in reality one having compound starch-grains,
and that the heterozygote, though outwardly round enough, is
intermediate in that starch-character. If we chose to say that
the compoundness of the grains is due to a factor C and that two
doses of it are needed to make the seed wrinkled, I know no
evidence by which such a thesis could be actually refuted. That
such reasoning is seemingly perverse must be conceded; but
when we consider the extraordinary difficulties which beset
any attempt to conceive the mode of origin of a new dominant
factor, we are bound to remember that there is this other line of
argument which avoids that difficulty altogether. In the case of
the " Alexandra "-eye in Primula, or the red calyx in Gates's
Oenothera, inverting the reasoning adopted in the text, we may
see that only the Primula homozygous for the yellow eye can
develop it and that two doses of the factor for the rubrinervis

96 PROBLEMS OF GENETICS

calyx are required to prevent that part of the plant from being
red.

We may proceed further and extend this mode of reasoning to
all cases of genetic variation, and thus conceive of all alike as
due to loss of factors present in the original complex. Until we
can recognize factors by means more direct than are provided by
a perception of their effects, this doubt cannot be positively
removed. For all practical purposes of symbolic expression we
may still continue to use in our analyses the modes of representa-
tion hitherto adopted, but we must not, merely on the ground of
its apparent perversity, refuse to admit that the line of argument
here indicated may some day prove sound.

CHAPTER V

THE MUTATION THEORY

When with the thoughts suggested in the last chapter we
contemplate the problem of Evolution at large the hope at the
present time of constructing even a mental picture of that process
grows weak almost to the point of vanishing. We are left
wondering that so lately men in general, whether scientific or
lay, were so easily satisfied. Our satisfaction, as we now see,
was chiefly founded on ignorance.

Every specific evolutionary change must represent a definite
event in the construction of the living complex. That event
may be a disturbance in the meristic system, showing itself in
a change in the frequency of the repetitions or in the distribution
of differentiation among them, or again it may be a chemical
change, adding or removing some factor from the sum total.

If an attempt be made to apply these conceptions to an actual
series of allied species the complexity of the problem is such that
the mind is appalled. Ideas which in the abstract are appre-
hended and accepted with facility fade away before the concrete
case. It is easy to imagine how Man was evolved from an
Amoeba, but we cannot form a plausible guess as to how Veronica
agrestis and Veronica polita were evolved, either one from the
other, or both from a common form. We have not even an
inkling of the steps by which a Silver Wyandotte fowl descended
from Callus Bankiva, and we can scarcely even believe that it
did. The Wyandotte has its enormous size, its rose comb, its
silver lacing, its tame spirit, and its high egg production. The
tameness and the high egg production are probably enough both
recessives, and though we cannot guess how the corresponding
dominant factors have got lost, it is not very difficult to imagine
that they were lost somehow. But the rose comb and the silver
colour are dominants. The heavy weight also appears in the
crosses with Leghorns, but we need not at once conclude that it

97

98 PROBLEMS OF GENETICS

depends on a simple dominant factor, because the big size of
the crosses may be a consequence of the cross and may depend on
other elements.

Now no wild fowl known to us has these qualities. May we
suppose that some extinct wild species had them? If so, may
we again make the same supposition in all similar cases? To do
so is little gain, for we are left with the further problem, whence
did those lost wild species acquire those dominants? Supposi-
tions of this kind help no more than did the once famous
conjecture as to the origin of living things — that perhaps they
came to earth on a meteorite. The unpacking of an original
complex, the loss of various elements, and the recombination of
pre-existing materials may all be invoked as sources of specific
diversity. Undoubtedly the range of possibilities thus opened
up is large. It will even cover an immense number of actual
examples which in practice pass as illustrations of specific dis-
tinction. The Indian Rock pigeon which has a blue rump
may quite reasonably be regarded as a geographically separated
recessive form of our own Columba lima, for as Staples-Browne
has shown the white rump of livid is due to a dominant factor.
The various degrees to which the leaves of Indian Cottons are
incised have, as Leake says, been freely used as a means of classi-
fication. The diversities thus caused are very remarkable,
and when taken together with diversities in habit, whether
sympodial or monopodial, the various combinations of points
of difference are sufficiently distinctive to justify any botanist
in making a considerable number of species by reference to them
alone. Nevertheless Leake's work goes far to prove that all of
these forms represent the re-combinations of a very small number
of factors. The classical example of Primula Sinensis and its
multiform races is in fact for a long way a true guide as to the
actual interrelations of the species which systematists have
made. That they did make them was due to no mistake in
judgment or in principle, but simply to the want of that ex-
tended knowledge of the physiological nature of the specific
cases which we now know to be a prime necessity.

But will such analysis cover all or even most of the ordinary

THE MUTATION THEORY 99

cases of specific diversity between near allies? Postponing the
problem of the interrelations of the larger divisions as altogether
beyond present comprehension, can we suppose, that in general,
closely allied species and varieties represent the various con-
sequences of the presence or absence of allelomorphic factors
in their several combinations? The difficulty in making a
positive answer lies in the fact that in most of the examples in
which it has been possible to institute breeding experiments with
a view to testing the question, a greater or less sterility is en-
countered. Where, however, no such sterility is met with, as
for instance in the crosses made by E. Baur among the species
of Antirrhinum there is every reason to think that the whole
mass of differences can and will eventually be expressed in terms
of ordinary Mendelian factors. Baur has for example crossed
species so unlike as Antirrhinum majus and molle, forms differing
from each other in almost every feature of organisation.1 The
Fz generation from this cross presents an amazingly motley
array of types which might easily if met with in nature be de-
scribed as many distinct species. Yet all are fertile and there
is not the slightest difficulty in believing that they can all be
reduced to terms of factorial a'nalysis.

If allowance be made for the complicating effects of sterility,
is there anything which prevents us from supposing that such
good species as those of Veronica or of any other genus comprising
well-defined forms may not be similarly related? I do not know
any reason which can be pointed to as finally excluding such a
possibility. Nevertheless it has been urged with some plausi-
bility that good species are distinguished by groups of differ-
entiating characters, whereas if they were really related as the
terms of a Mendelian F2 family are, we should expect to find
not groups of characters in association, but rather series of forms
corresponding to the presence and absence of the integral factors
composing the groups of characters. I am not well enough
versed in systematic work to be able to decide with confidence
how much weight should be attached to this consideration. Some

1 See Lotsy and Baur, Rep. Genetics Conf., Paris, 1911, pp. 416-426. Com-
pare Lecoq on Mirabilis jalapa X longiflora, Fecondation des Vegetaux, 1862, p. 311.

ioo PROBLEMS OF GENETICS

weight it certainly has, but I cannot yet regard it as forming a
fatal objection to the application of factorial conceptions on
the grand scale. It may be recalled that we are no longer under
any difficulty in supposing that differences of all classes may be
caused by the presence or absence of factors. It seemed at first
for example that such characters as those of leaf shape might
be too subtle and complex to be reducible to a limited number of
factors. But first the work of Gregory on Primula Sinensis
showed that several very distinct types of leaves were related
to each other in the simplest way. In that particular example,
intermediates are so rare as to be negligible, but subsequently
Shull dealing with such a complicated example as Capsella, and
Leake in regard to Cottons, both forms in which intergrades occur
in abundance, have shown that a simple factorial scheme is
applicable. We need not therefore, to take an extreme case,
doubt that if it were possible to examine the various forms of
fruit seen in the Squashes by really comprehensive breeding
tests, even this excessive polymorphism in respect of structural
features would be similarly reducible to factorial order.

It must always be remembered also that in a vast number
of cases, nearly allied forms which are distinct, occupy distinct
ground. Moreover, by whatever of the many available mechan-
isms that end be attained, it is clear that nature very often does
succeed in preventing intercrossing between distinct forms so
far that the occurrence of that phenomenon is a rarity under
natural conditions. The facts may, I think, fairly be summarized
in the statement that species are on the whole distinct and not
intergrading, and that the distinctions between them are usually
such as might be caused by the presence, absence, or inter-com-
bination of groups of Mendelian factors; but that they are so
caused the evidence is not yet sufficient to prove in more than a
very few instances.

The alternative, be it explicitly stated, is not to return to the
view formerly so widely held, that the distinctions between
species have arisen by the accumulation of minute or insensible
differences. The further we proceed with our analyses the more
inadequate and untenable does that conception of evolutionary

THE MUTATION THEORY 101

change become. If the differences between species have not
come about by the addition or loss of factors one at a time, then
we must suppose that the changes have been effected by even
larger steps, and variations including groups of characters, must
be invoked.

That changes of this latter order are really those by which
species arise, is the view with which de Vries has now made us
familiar by his writings on the Mutation Theory. In so far as
mutations may consist in meristic changes of many kinds and
in the loss of factors it is unnecessary to repeat that we have
abundant evidence of their frequent occurrence. That they may
also more rarely occur by the addition of a factor we are, I think,
compelled to believe, though as yet the evidence is almost en-
tirely circumstantial rather than direct. The evidence for the
occurrence of those mutations of higher order, by which new
species characterized by several distinct features are created,
is far less strong, and after the best study of the records which
I have been able to make, I find myself unconvinced. The facts
alleged appear capable of other interpretations.

The most famous and best studied examples are of course the
forms of Oenothera raised by de Vries from Oenothera Lamarckiana
in circumstances well known to all readers of genetic literature.
Whatever be the true significance of these extraordinary "mu-
tations" there can be no question about the great interest which
attaches to them, and the historical importance which they will
long preserve. Apart also from these considerations it is be-
coming more and more evident that in their peculiarities they
provide illustrations of physiological phenomena of the highest
consequence in the study of genetics at large.

De Vries found, as is well known, that Oenothera Lamarckiana
gives off plants unlike itself. These mutational forms are of
several distinct and recognizable types which recur, and several
of them breed true from their first appearance. The obvious
difficulty, which in my judgment should make us unwilling at
present to accept these occurrences as proof of the genesis of new
species by mutation, is that we have as yet no certainty that the
appearance of the new forms is not an effect of the recombination

102 PROBLEMS OF GENETICS

of factors, such as is to be seen in so many generations of plants
derived from a cross involving many genetic elements. The
first question is what is Oenothera Lamarckiana? Is it itself a
plant of hybrid origin? To this fundamental question no satis-
factory answer has yet been given. All attempts to find it as a
wild plant in America have failed. It existed in Europe in the
latter half of the eighteenth century. Whence it came is still
uncertain, but the view that it came into existence in Europe and
perhaps in Paris, seems on the whole the most probable. The
question has been debated by Macdougal, Gates, and Davis.
From historical sources there is little expectation of further
light. Those who favour the notion of a hybrid origin look on
Oenothera biennis as one of the putative parents. It has been
conjectured that a species called grandifiora lately re-discovered
on the Alabama river was the other parent. Experiments have
been instituted by Davis to discover whether Lamarckiana can
be made artificially by crossing these two species. The results
so far have shown that while plants approximating in various
respects to Lamarckiana have thus been produced, none agree
exactly with that form. Davis, to whom reference should be
made for a full account of the present state of the enquiry,
points out that there are many strains of biennis in existence
and that it is by no means impossible that by using others of
these strains a still closer approximation can be made. None
of Davis's artificial productions as yet breed at all true, as
Lamarckiana on the whole does. In such a case, however, where
several characters are involved, this is perhaps hardly to be
expected.

One feature of the Oenotheras is very curious. Not only
Lamarckiana, but all the allied species so far as I am aware,
have a considerable proportion of bad and shrivelled pollen
grains. This is undoubtedly true of species living in the wild
state as well as of those in cultivation. I have had opportunities
of verifying this for myself in the United States. No one looking
at the pollen of an Oenothera would doubt that it was taken from
some hybrid plant exhibiting partial sterility. On the other
hand, it is difficult to suppose that numbers, perhaps all, of the

THE MUTATION THEORY 103

" species" of the genus are really hybrids, and many of them breed
substantially true. I regard this constant presence of bad
pollen grains as an indication that the genetic physiology of
Oenothera is in some way abnormal, and as we shall presently
see, there are several other signs which point in the same direction.

Discussion of the whole series of phenomena is rendered
exceedingly difficult first, by reason of the actual nature of the
material. The characteristics of many of the types which de Vries
has named are evasive. A few of these types, for instance, gigas,
nanella, albida, brevistylis, and perhaps a few more are evidently
clear enough, but we have as yet no figures and descriptions
precise enough to enable a reader to appreciate exactly the pe-
culiarities of the vast number of forms which have now to be
considered in any attempt to gain a comprehensive view of the
whole mass of facts. It is also not in dispute that the forms are
susceptible of great variations due simply to soil and cultural
influences.

The fact that no Mendelian analysis has yet been found
applicable to this group of Oenotheras as a whole is perhaps largely
due to the fact that until recently such analysis has not been
seriously attempted. Following the system which he had
adopted before the rediscovery of Mendelism, or at all events,
before the development of that method of analysis, de Vries has
freely applied names to special combinations of characters and
has scarcely ever instituted a factorial analysis. Before we can
get much further this must be attempted. It may fail, but we
must know exactly where and how this failure comes about.
There are several indications that such a recognition of factorial
characters, could be carried some way. For example, the height,
the size of the flowers, the crinkling of the leaves, the brittleness
of the stems, perhaps even the red stripes on stems and fruits,
and many more, are all characters which may or may not depend
on distinct factors, but if such characters are really transmitted
in unresolved groups, the limitations of those groups should be
carefully determined. The free use of names for the several
forms, rather than for the characters, has greatly contributed
to deepen the obscurity which veils the whole subject.

104 PROBLEMS OF GENETICS

1 do not mean to suggest that these Oenotheras follow a simple
Mendelian system. All that we know of them goes to show that
there are curious complications involved. One of these, prob-
ably the most important of all, has lately been recognized by
de Vries himself, namely, that in certain types the characters
borne by the female and the male germ-cells of the same plant
are demonstrably different. There can be little doubt that
further research will reveal cognate phenomena in many unsus-
pected places. The first example in which such a state of things
was proved to exist is that of the Stocks investigated by Miss
Saunders.2 By a long course of analysis she succeeded in estab-
lishing in 1908 the fact that if a plant of Matthiola is of that
eversporting kind which gives a large proportion of double-
flowered plants among its offspring (produced by self-fertili-
sation), then the egg-cells of such a plant are mixed in type, but
the pollen of the same plant is homogeneous. Some of the egg-
cells have in them the two factors for singleness, but some of
them are short of one or both of these factors. The pollen-
grains, however, are all recessives, containing neither of these
factors. The egg-cells, in other words, are mixed, "singles" and
"doubles," while the pollen-grains are all "doubles." The same
is true of the factor differentiating "white," or colourless plastids
from cream-coloured plastids in Matthiola, the egg-cells being
mixed "whites" and "creams," while the pollen-grains are all
"creams," viz: recessives. Later in the same year (1908)
de Vries3 announced a remarkable case which will be discussed
in detail subsequently. It relates to certain Oenotheras hetero-
zygous for dwarf ness, in which (p. 113) the ovules were mixed,
tails and dwarfs, while the pollen is all dwarf.

Again in Petunia Miss Saunders's4 work has shown that
a somewhat similar state of things exists, but with this remark-
able difference, that though the egg-cells are mixed, singles and
doubles, the pollen-grains are all singles, viz: dominants. All
the Petunias yet examined have been in this condition, including

2 Rep. Evol. Ctee. R. S., IV, 1908, p. 38.

3 Ber. Deut. Bot. Ges., 1908, XXVI, a, p. 672.

4 Jour. Genetics, i, 1910, p. 57.

THE MUTATION THEORY 105

some which in botanic gardens pass for original species. Whether
actual wild plants from their native habitats are in the same
state, is not yet known, but it is by no means improbable. The
case may be compared with that of the moth Abraxas grossu-
lariata studied by Doncaster and Raynor, in which the females
are all heterozygous, or we may almost say "hybrids" of grossu-
lariata and the variety lacticolor. Similarly we may say that at
least garden Petunias are heterozygous in respect of singleness.
The proof of this is of course that when fertilised with the pollen
of doubles they throw a mixture of doubles and singles. The
statements which de Vries has published regarding the behaviour
of several of the Oenotheras go far to show that they must have
a somewhat similar organisation. On the present evidence it is
still quite impossible to construct a coherent scheme which will
represent all the phenomena in their interrelations, and among
the facts are several which, as will appear, seem mutually incom-
patible. The first indication that the Oenotheras may have
either mixed ovules or mixed pollen appears in the fact that
Lamarckiana and several of its "mutants" used as males, with
several other forms as females, give a mixed offspring. For
example, de Vries (1907) found that

biennis 9 X Lamarckiana c?

biennis cruciata 9 X Lamarckiana cf

muricata 9 X Lamarckiana cf

biennis 9 X rubrinervis d71

biennis cruciata 9 X rubrinervis c?

all give a mixture of two distinct types which he names laeta
and velutina, consisting of about equal numbers of each. On
account of the fact that the two forms are produced in association
de Vries has called these forms "twin hybrids," a designation
which is not fortunate, seeing that it is impossible to imagine
that any kind of twinning is concerned in their production. The
distinction between these two seems to be considerable, laeta
having leaves broader, bright green in colour, and flat, with
pollen scanty, while vehitina has leaves narrower, grayish green,
more hairy, and furrow-shaped, with pollen abundant.

We next meet the remarkable fact that these two forms,

io6 PROBLEMS OF GENETICS

laeta and velutina breed true to their respective types, and do not
reproduce the parent-types among their offspring resulting from
self-fertilisation. This statement must be qualified in two
respects. When muricata cf is fertilised by brevistylis the forms
laeta and velutina are produced, but each of them subsequently
throws the short-styled form as a recessive (de Vries, 1907,
p. 406). It may be remembered that de Vries's previous publi-
cations had already shown that the short style of brevistylis,
one of the Lamarckiana "mutants," behaves as a recessive
habitually (Mutationstheorie, II, p. 178, etc.).

Also when nanella, the dwarf "mutant" of Lamarckiana is
used as male on muricata as female, laeta and velutina are pro-
duced, but one only of these, namely, velutina, subsequently
throws dwarfs on self -fertilisation. The dwarfs thus thrown are
said to form about 50 per cent, of the families in which they
occur (de Vries, 1908, p. 668). The fact that the two forms,
laeta and velutina, are produced by many matings in which
Lamarckiana and its mutant rubrinervis are used as males is
confirmed abundantly by Honing, who has carried out extensive
researches on the subject. After carefully reading his paper,
I have failed to understand the main purport of the argument
respecting the "double nature" of Lamarckiana which he founds
on these results, but I gather that in some way laeta is shown to
partake especially of the nature of Lamarckiana, while velutina
is a form of rubrinervis. The paper contains many records which
will be of value in subsequent analysis of these forms.

Before considering the possible meaning of these facts we
must have in our minds the next and most novel of the recent
extensions of knowledge as to the genetic properties of the
Oenotheras. In the previous statement we have been concerned
with the results of using either Lamarckiana itself or one of its
"mutants" rubrinervis, brevistylis, or nanella as male, on one of
the species biennis or muricata. The new experiments relate
to crosses between the two species biennis and muricata
themselves.

De Vries found :

I. That the reciprocal hybrids from these two species differed,

THE MUTATION THEORY 107

biennisXmuricata producing one type of Fi and muricataX
biennis producing another. Each Fi resembled the father more
than the mother.

2. That each of the hybrids so produced breeds true on self-
fertilisation.

3. That if we speak of the hybrid from biennisXmuricata
as BM and of the reciprocal as MB, then

BMXMB

gives exclusively offspring of biennis type but that

MB XBM

gives exclusively offspring of muricata type. Evidently, apart
from all controversy as to the significance of the "mutants"
of Lamarckiana, we have here a series of observations of the first
importance.

The fact that reciprocal crossings give constantly distinct
results must be taken to indicate that the male and female sides
of one, if not of both, of the parents are different in respect of
characters which they bear. This is de Vries's view, and he
concludes rightly, I think, that the evidence from all the experi-
ments shows that both biennis and muricata are in this condition,
having one set of characters represented in their pollen-grains
and another in their ovules. The plants breed true, but their
somatic structures are compounded of the two sets of elements
which pass into them from their maternal and paternal sides
respectively. This possibility that species may exist of which
the males really belong to one form and the females to another,
is one which it was evident from the first announcement of the
discovery of Mendelian segregation might be found realised in
nature.6

Oe. biennis and muricata were crossed reciprocally with each
other and with a number of other species, and the behaviour of
each, when used as mother, was consistently different from its
behaviour when used as father. De Vries is evidently justified

5 In Rep. i to Evol. Committee, 1902, p. 132, attention was called to this pos-
sibility, though of course at that date it was in sexual animals alone that it was
supposed to exist. It had not occurred to me that even a hermaphrodite plant
might be hi this condition.

io8 PROBLEMS OF GENETICS

by the results of this series of experiments in stating that the
"Bild," as he terms it, or composition of the male and female
sides of these two species, biennis and muricata, are distinct.
On the evidence before us it is not, however, possible to form a
perfectly clear idea of each, and until details are published, a
reader without personal knowledge of the material cannot do
more than follow the general course of the argument. For fuller
comprehension a proper analysis of the characters with a clear
statement of how they are distributed among the several types
and crosses is absolutely necessary. According to de Vries the
female of biennis possesses a group of characters which he defines
as "conica" in allusion to the shape of the flower-buds. Besides
the conical buds, this group of features includes imperfect
development of wood, rendering the plant very liable to attacks
of Botrytis, and comparatively narrow leaves.

The female of muricata carries a group of features which he
calls "frigida" and, though this is not quite explicitly stated in a
definition of that type, it is to be inferred6 that its characteristics
are regarded as greater height, strong development of wood with
comparative resistance to Botrytis, and broad leaves.

The characters borne by the male parts of the two species
are in general those by which they are outwardly distinguished.
For example, the leaves of Oe. biennis are comparatively broad
and are bright green, while those of muricata are much narrower
and of a glaucous green, and I understand that de Vries regards
these properties as contributed by the male side in each case and
to be carried by the male cells of each species. The suggestion
as regards biennis and muricata comes near the conception often
expressed by naturalists in former times (e. g., Linnaeus) and
not rarely entertained by breeders at the present day, that the
internal structure is contributed by the mother and the external
by the father.

On the other hand, the offspring of each species when used
as mother is regarded as possessing in the main the features of
the maternal "Bild," but the matter is naturally complicated
by the introduction of features from the father's side, and it is

6 From the description of the offspring of muricata used as mother.

THE MUTATION THEORY 109

here especially that the account provided is at present unsatis-
factory and inconclusive. There seems, however, to be no serious
doubt that biennis and muricata each in their outward appearance
exhibit on the whole the features which their pollens respectively
carry, and that the features borne by their ovules are in many
respects distinct.

The types are thus "hybrids" which breed true. The results
of intercrossing them each way are again " hybrids" which breed
true. It will be remembered that on former occasions de Vries
has formulated a general rule that species-hybrids breed true,
but that the cross-breds raised by interbreeding varieties do not.
One of these very cases was quoted7 as an illustration of this
principle, viz: muricata Xbiennis. The grounds for this general
statement have always appeared to me insufficient, and with the
further knowledge which the new evidence provides we are
encouraged to hope that when a proper factorial analysis of the
types is instituted we shall find that the phenomenon of a con-
stant hybrid will be readily brought into line with the systems
of descent already worked out for such cases as that of the Stocks,
and others already mentioned.

In further discussion of these facts de Vries makes a suggestion
which seems to me improbable. Since the egg-cells of muricata,
for instance, bear a certain group of features which are missing
on the male side, and conversely the pollen bears features absent
from the female side, he is inclined to regard the bad pollen grains
as the bearers of the missing elements of the male side and to
infer that there must similarly be defective ovules representing
the missing elements of the female side. No consideration is
adduced in support of this view beyond the simple fact that the
characters borne by male and female are dissimilar, whereas
it would be more in accord with preconception if the same sets
of combinations were represented in each — as in a normal
Mendelian case. There is as yet no instance in which the absence
of any particular class of gametes has been shown with any
plausibility to be due to defective viability, though there are, of
course, cases in which certain classes of zygotes do not survive

7 de Vries, Species and Varieties, 1905, p. 259.

no

PROBLEMS OF GENETICS

owing to defective constitution (e. g., the albinos of Antirrhinum
studied by Baur, and the homozygous yellow mice). I am
rather inclined to suppose that in these examples of hybrids
breeding true we shall find a state of things comparable with
that to which we formerly applied the terms " coupling" and
"repulsion." In these cases certain of the possible combinations
of factors occur in the gametic series with special frequency,
being in excess, while the gametes representing other combina-
tions are comparatively few. In a recent paper on these cases
Professor Punnett and I have shown that these curious results
vary according to the manner in which the factors are grouped
in the parents. If A and B are two factors which exhibit these
phenomena we find that the gametic series of the double heterozy-
gote differs according as the combination is made by crossing
ABXab, or by crossing AbXaB. In a normal Mendelian case
the FI form, AaBb, produces gametes AB, Ab, aB, ab, in equal
numbers; but in these peculiar cases those gametes which contain

Gametic series

AB Ab

aB ab

u gaiiicica

in series

^yguic
forme(

Partial repulsion
from zygote .
of form

3i
IS

(n-i) I
31 I
IS I

2«
64
32

4tt2
4096
IO24

AbXaB

7

7 I

16

256

3

3 i

8

64

i

i

4

16

!3 I

3

8

64

Partial coupling

7 I

7

16

256

from zygote ^

IS i

IS

32

1024

of form

3i

64

4096

ABXab

63 I

63

128

16384

(n-i) i

i (w-i)

2n

4n2

Nature of zygotic series

AB

Ab

aB

ab

Partial repulsion
from zygote J
of form

2W-|-I
2049
513

nf — i
1023
2SS

W— I

1023
255

AbXaB

129

63

63

(^

33

IS

IS

9

3

3

41

7

7

9

177

IS

IS

49

737

225

3009

63

63

961

12161

127

127

3969

— i 2n—i n2 — 2w —

THE MUTATION THEORY in

the parental combinations are in excess. This excess almost
certainly follows the system indicated by the accompanying
table. In the general expressions n is half the number of gametes
required to express the whole system. Now if we imagine that
sex-factors are involved with the others concerned in such a re-
lationship as this we have a system of distribution approximating
to that found in biennis and muricata. The difference in re-
ciprocals is represented in a not improbable way. It cannot yet
be said that the rarer terms in the series are formed at all, and
perhaps they are not. As we pointed out in our discussion of
these phenomena, the peculiar distribution of factors in these
cases must be taken to mean that the planes of division at some
critical stage in the segregation are determined with reference
to the parental groups of factors, or in other words, that the
whole system has a polarity, and that the distribution of factors
with reference to this polarity differs according to the grouping
of factors in the gametes which united in fertilization to produce
the plant. Subsequent proliferation of cells representing certain
combinations would then lead to excess of the gametes bearing
them. It is on similar lines that I anticipate we shall hereafter
find the interpretation of the curious facts discovered by de Vries,
though it is evident that a long course of experiment and analysis
must be carried through before any certainty is reached. The
work must be begun by a careful study of the descent of some
single factor, for example, that causing the broader leaf of
biennis, and we may hope that the study of Oenothera by proper
analytical methods will no longer be deferred.

We have now to return to the relations of laeta and velutina.
These two forms, it will be remembered are frequently produced
when Lamarckiana or one of its derivatives is used as male,
and the most unexpected feature in their behaviour is that both
breed true as regards their essential characteristics, on self-fertili-
sation. If one only bred true the case might, in view of the
approximate numerical equality of the two types, be difficult
to interpret on ordinary lines, but as both breed true it must be
clear that some quite special system of segregation is at work.
What this may be cannot be detected on the evidence, but with

H2 PROBLEMS OF GENETICS

the results from the biennis-muricata experiments before us,
it is natural to suspect that we may here again have to recognise
a process of allocation of different factors to the male and female
sides in laeta and velutina. That some such system is in operation
becomes the more probable from the new fact which de Vries
states in describing the group of characters which he calls conica,
namely that this type is the same as that of velutina.

There are many collateral observations recorded both by
de Vries and others which have a bearing on the problems, but
they do not yet fall into a coherent scheme. For example, we
cannot yet represent the formation of laeta and velutina from the
various species fertilised by Lamarckiana cf . That this is not
due to any special property associated with the pollen of La-
marckiana is shown by the fact that a species called Hookeri
gives laeta and velutina in both its reciprocal crosses with La-
marckiana (de Vries, 1909, p. 3), and also by the similar fact that
Lamarckiana 9 fertilised by the pollen of a peculiar race of
biennis named biennis Chicago throws the same types. Before
these very complicated phenomena can be usefully discussed
particulars must be provided as to the individuality of the various
plants used. This criticism applies to much of the work which
de Vries has lately published, for, as we now know familiarly,
plants to which the same name applies can be quite different in
genetic composition.

Attention should also be called to one curiously paradoxical
series of results. When the dwarf "mutant" of Lamarckiana
which de Vries names "nanella" is used as father on muricatay
Fi consists of laeta and velutina in approximately equal numbers.
Both forms breed true to their special characteristics, but
velutina throws dwarfs of its own type, while laeta does not
throw dwarfs. Subsequent investigation of the properties of
these types has led to some remarkable conclusions, and it was
in a study of these plants that de Vries first came upon the phe-
nomen of dissimilarity between the factors borne by the male
and female cells of the same plant, a condition which had been
recently detected in the Stocks as a result of Miss Saunders's
investigations. The details are very remarkable. We have

THE MUTATION THEORY 113

first the fact that muricata 9 X dwarf nanella c? gives about
50 per cent, laeta and about 50 per cent, of velutina.
As regards Velutina it was shown that:

Tails, Dwarfs,

per cent. per cent.

I. Velutina selfed gave 38 62

Velutina 9 X dwarf nanella c? gave 39 61

do. X do. gave 49 51

do. X dwarf c? derived from velutina gave 43 57
3. Dwarfs X velutina cf gave — all dwarfs

The three experiments taken together prove, as de Vries says,
that the ovules of velutina are mixed, tails and dwarfs, and that
the pollen is all dwarf. The condition is almost the same as
that of the Stocks. It may be noted also that in the Stocks the
egg-cells of the "double" type are in excess, being approximately
9 to 7 of the "single" type, but de Vries regards the two types
in velutina as probably equal in number. The figures (169 : 231)
rather suggest some excess of the recessives, perhaps 9:7, and
the point would be worth a further investigation.

As regards laeta, by self -fertilisation no dwarfs were produced,
but in all other respects it behaved almost exactly like velutina.
The ovules are evidently mixed tails and dwarfs, and whether
fertilised by dwarfs or by the pollen of velutina, which is already
proved to be all dwarf, the result was a steady 50 per cent, of
tails and 50 per cent, of dwarfs. The pollen of laeta used on
dwarfs gives nothing but dwarfs, and in three series of such ex-
periments 226 dwarfs were produced.

We are thus faced with this difficulty. Since the egg-cells of
laeta are evidently mixed, tails and dwarfs, and the pollen used
on dwarfs gives all dwarfs, why does not self-fertilisation give
a mixed result, tails and dwarfs, instead of all tails? De Vries
regards the result of self-fertilisation as showing the real nature
of the pollen, and declares it to be all tails, while he represents
the behaviour of the same pollen used on dwarfs by stating that
in these combinations the dwarf character dominates. This
does not seem to me a natural interpretation. I should regard
the pollen of laeta as identical with that of velutina, namely dwarf,
and I suspect the difficulty is really created by the behaviour of
laeta on self-fertilisation. Until a proper analysis is made in

114 PROBLEMS OF GENETICS

which the identity of the different individuals used is recorded,
no further discussion is possible.8

Other results of a complicated kind involving production of
laeta and velutina together with a third form have been published
by de Vries in his paper on "Triple Hybrids." To these also
the same criticism applies. Some of the observations seem cap-
able of simple factorial representation and others are conflicting.

Taking the work on Oenothera as a whole we see in it con-
tinually glimpses of order which further on are still blocked by
difficulties and apparent inconsistencies. Through such a stage
all the successful researches in complicated factorial analysis
have passed and I see no reason for supposing that with the
application of more stringent methods this more difficult set of
problems will be found incapable of similar solutions. To
return to the original question whether in Oenothera we can claim
to see a special contemporaneous output of new species in actual
process of creation, it will be obvious that while the interrelation
of the several types is still so little understood, such a claim has
no adequate support. It is true that many of the "mutants"
of Lamarckiana can well pass for species, but this is equally true
of many new combinations of pre-existing factors as we have
seen in Primula Sinensis and other cases. Still less can it be
admitted that these facts of uncertain import supply a justi-
fication for the conception which has played a prominent part
in the scheme of the Mutationstheorie, namely that there are
special periods of Mutation, when the parent-species has peculiar
genetic properties. To conclude: The impression which the
evidence leaves most definitely on the mind is that further dis-
cussion of the bearing which the Oenotheras may have on the
problem of evolution should be postponed until we have before
us the results of a searching analysis applied to a limited part of
the field. In such an analysis it is to be especially remembered
that we have now a new clue in the well-ascertained fact that the
genetic composition of the male and female germ-cells of the

8 Zeijlstra in a recent paper announces that many nanella plants are the subject
of a bacterial disease to which he attributes their dwarfness. I gather that this
does not apply to all nanella plants and that some are dwarfs apart from disease.
The matter may no doubt be further complicated from this cause.

THE MUTATION THEORY 115

same individual may be quite different. When with this pos-
sibility in view the behaviour of the types is re-examined I
anticipate that many of the difficulties will be removed.

Outside the evidence from Oenothera, which, as we have seen,
is still ambiguous, I know no considerable body of facts favour-
able to that special view of Mutation which cfe Vries has pro-
mulgated. Of variation, or if we will, Mutation, in respect of
some one character, or resulting from recombination, there is
proof in abundance ; but of that simultaneous variation in several
independent respects to which de Vries especially attributes the
origin of new specific types I know only casual records which
have yet to undergo the process of criticism.

Besides de Vries's " Mutationstheorie" and "Species and Vari-
eties" the chief publications relating to the subject of the be-
haviour of Oenothera are the following: (Many other papers
relating especially to the cytology of the forms have appeared.)

Davis, B. M. Genetical Studies on Oenothera, I. Amer. Nat., XLIV, 1910, p. 108.

Genetical Studies on Oenothera, II. Ibid., XLV, 1911, p. 193.
Gates, R. R. An Analytical Key to some of the Segregates of Oenothera. Twen-
tieth Annual Report of the Missouri Botanical Garden, 1909.
Studies on the Variability and Heritability of Pigmentation in Oenothera.

Ztsch.f. Abstammungslehre, 1911, IV, p. 337.

Honing, J. A. Die Doppelnatur der Oenothera Lamarckiana. Ztsch. f. Abstam-
mungslehre, 1911, IV, p. 227.

Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K. Small). Mutants and
Hybrids of the Oenotheras. Carnegie Institution's Publication, No. 24,
1905.

Macdougal, D. T., Vail, A. M., Shull, J. H. Mutations, Variations and Relation-
ships of the Oenotheras. Carnegie Institution's Publication, No. 81, 1907.
de Vries, H. On Atavistic Variation in Oenothera cruciata. Bull. Torrey Club,

1903. Vol. 30, p. 75-

On Twin Hybrids, Bot. Gaz., Vol. 44, 1907, p. 401.
Ueber die Zwillingsbastarde von Oenothera nanella. Ber. Deut. Bot. Ges.,

1908, XXVI, a, p. 667.

Bastarde von Oenothera gigas. Ibid., p. 754.
On Triple Hybrids. Bot. Gaz., 1909, Vol. 47, p. I.
Ueb. doppeltreziproke Bastarde von Oenothera biennis L. und Oenothera

muricata L. Biol. Cbltt., 1911, XXXI, p. 97.

Zeijlstra, H. H. Oenothera nanella de Vries, eine krankhafte Pflanzenart. Biol.
Cbltt., 1911, XXXI, p. 129.

Ii6 PROBLEMS OF GENETICS

NOTE.

Since this chapter was written two contributions of special
importance have been made to the study of the Oenothera prob-
lems. The first is that of Heribert-Nilsson.9 The author begins
by giving a critical account of the evidence for de Vries's inter-
pretation of the nature of the mutants. In general this criticism
pursues lines similar to those sketched in the foregoing chapter,
concluding, as I have done, that the chief reason why factorial
analysis has been declared to be inapplicable to the Oenothera
mutants is because no one has hitherto set about this analysis
in the right way. He has also himself made a valuable beginning
of such an analysis and gives good evidential reasons for the belief
that at least the red veining depends on a definite factor which
also influences the size of certain parts of the plant. He argues
further that many of the distinctions between the mutants are
quantitative in nature. With great plausibility he suggests that
the system of cumulative factors which Nilsson-Ehle discovered
in the case of wheat (subsequently traced by East in regard to
maize) may be operating also in these Oenotheras. According
to this system several factors having similar powers may coexist
in the same individual, and together produce a cumulative effect.
Scope would thus be given for the production of the curious and
seemingly irregular numbers so often recorded in the "mutating"
families.

Another remarkable observation relating to the crosses of
muricata and biennis has been published by Goldschmidt.10
He finds that in the formation of this cross the female pronucleus
takes no part in the development of the zygotic cell, but that
when the male pronucleus enters, the female pronucleus is
pushed aside and degenerates. As de Vries observed, the recip-
rocal hybrids are in each case very like the father ("stark
patroklin"), a consequence which finds a natural explanation in
the phenomenon witnessed by Goldschmidt. The results of
the subsequent matings can also be readily interpreted on the
same lines. Indications of maternal characters are nevertheless

9 Zts. f. Abstamm., 1912, VIII.

10 Arch. f. Zellforschung, 1912, IX, p. 331.

THE MUTATION THEORY 117

mentioned by de Vries, and if Goldschmidt's account of the
cytology is confirmed, these must presumably be referred to the
influence of the maternal cytoplasm. Clearly this new work
opens up lines of exceptional interest. The interpretation I
have offered above must probably be reconsidered. The dis-
tinction between the male and female cells of the types may no
doubt be ultimately factorial, but it is difficult to regard such a
distinction as created by a differential distribution of the ordinary
factors.

CHAPTER VI

VARIATION AND LOCALITY

In all discussions of the modes of Evolution the phenomena of
Geographical Distribution have been admitted to be of para-
mount importance. First came the broad question, were the
facts of distribution consistent with the Doctrine of Descent?
I suppose all naturalists are now agreed that they are thus
consistent, and that though some very curious and as yet in-
explicable cases remain to be accounted for, the distribution of
animal and plant life on the face of the earth is much what we
might expect as a result of a process of descent with modification.
Passing from this general admission to the more particular ques-
tion whether the facts of distribution favour one special con-
ception of the mode of progress of evolution rather than another,
no agreement has yet been reached. One outstanding feature
is hardly in dispute, namely that prolonged isolation is generally
followed by greater or less change in the population isolated.
Groups of individuals which from various causes are debarred
from free intermixture with other groups almost always exhibit
peculiarities, but on the other hand, cosmopolitan types which
range over wide areas are on the whole uniform, or nearly so
throughout their distribution. Examples of these two categories
will be familiar to all naturalists. The barriers to intercourse
may be seas, deserts, prairies, mountain-chains, or circumstances
of a much less obvious character which isolate quite as effectually.
The local unit is not necessarily an island, a district, or an area
of special geological formation, but may, as every collector knows,
be a valley, a pond, a creek, a "bank" in the sea, a clump of
trees, a group of rocks in a bay, or a particular patch of ground
on a mountain side. All the great groups provide examples of
such specially isolated forms. The botanist knows them well;
the conchologist, the entomologist, the ornithologist and the
student of marine life are all equally aware that special varieties

118

VARIATION AND LOCALITY 119

or special species come from special places and from nowhere
else. In one remarkable case the season of appearance plainly
acts as the isolating barrier. Tephrosia bistortata is a small
Geometrid moth which has two broods, appearing in March
and July respectively. It is closely allied to T. crepuscularia
which emerges in May and June. From the fact that occasional
specimens cannot be quite certainly referred to one or other of
the two, many have held that the two are one species. Never-
theless, in general they present distinctions which are plain
enough. Some localities have one form only, but in several
woods they co-exist. Experiment has shown that the two can
be crossed, and that the cross-breds can breed inter se and with
at least one of the parent stocks.1 Some diminution in fertility
was observed, but perhaps not more than is commonly encountered
when wild forms are bred in captivity. In such a case it can
scarcely be doubted that the distinctness of the two forms in
the places where they co-exist is maintained by the seasonal
isolation.

Just as the consequences of isolation are to be seen in the
most different forms of life so may they also affect the most di-
verse features of organisation, such as size, colour, sculpture,
shape, or number of parts. In the Sloth (Choloepus) the geo-
graphical races differ in the number of cervical vertebrae — or
in other words, in the distribution of vertebral differentiation.
The geographical races of Cistudo differ in the number of claws
and phalanges.2

In Shetland, the males of Hepialus humuli (the Ghost Moth)
are not sharply differentiated in colour from the females, as they
are elsewhere, but in varying degrees resemble them.8 No such
males are found in other localities, and even in the other Scottish
islands they are normal. In the island of Waigiu the converse
phenomenon has been observed in Phalanger maculatus. Gen-

1 For the evidence see Tutt, J. W., Trans. Ent. Soc., 1898, p. 17. Compare
the remarkable case given by Gulick (Evolution Racial and Habitudinal, p. 123)
of the two races of Cicada, which are separated by reason of their life-cycles, one
having a period of 13, the other 17 years.

2 For references see Materials, p. 396, and also G. Baur, Amer. Nat., 1893,
July, p. 677.

1 Jenner Weir, Entomologist, 1880, XIII, p. 251.

120 PROBLEMS OF GENETICS

erally the male is spotted with white, and the female is without
spots, but in Waigiu the females are spotted like the males.4

The following striking illustration was pointed out to me by
Dr. W. D. Miller. Euphonia elegantissima as it occurs in Mexico
and Central America has the two sexes very distinct from each
other. The male has the lower parts orange and the upper
parts a dark indigo blue, with a bright turquoise-blue head and
neck. The female, except for the head, is of a bright olive green.
A form in which the sexes are similarly differentiated exists in
Porto Rico and is known as E. Sclateri. But in many of the
other West Indian islands the representative "species" (E.
flavifrons) has the two sexes closely resembling the female of
E. elegantissima. This form is found in Antigua, Barbados,
St. Vincent, and Guadeloupe, from which localities the British
Museum has specimens. All three so-called species are very
much alike otherwise.

In the genus Pyrrhulagra (Loxigilla) to which Mr. Outram
Bangs called my attention, several distinct and alternative pos-
sibilities occur. The genus has many local species occurring
on the various West Indian islands. These species are char-
acterized by differences in size, colour, and the shape of the bill.
The colours have a narrow range, being black or greyish, with
or without chestnut marks about the head and throat. In
most of the islands the males are in general colour a full black,
and the females are distinctly grey. They are thus found in
San Domingo, Jamaica, Bahama, and most of the Lesser Antilles.
In Porto Rico we meet the peculiarity that the hens are almost
as black as the males (Ridgway describes the black of the hens
as slightly less intense). This form is called portoricensis.
A larger type, known as grandis, similarly coloured, inhabits
St. Kitt's. Then, on the contrary, in Barbados, both sexes are
a dull blackish grey, like the hens of the Lesser Antilles in general.

The local species of Agelaius show similarly capricious dis-
tinctions. A. phoeniceus is a widely spread species, found over
a great part of North America. The male is black with red-orange

4 Jentink, Notes Leyden Mus., 1885, VII, p. in. Specimens illustrating this
peculiarity are in the British Museum.

VARIATION AND LOCALITY 121

bars on the wings, but the female is somewhat thrush-like in
colour. In the island of Porto Rico there is a form called xan-
thomus, in which both sexes are like the males of the mainland.
A similar species called humeralis, also with both sexes male-like,
lives in Cuba. The island of Cuba, curiously enough, has also a
distinct species named assimilis, in which the female is a dull
black all over, though the male is like the mainland type.

So also may local races differ in respect of variability. Ar-
gynnis paphia, the Silver Washed Fritillary, through a great
part of its distribution has only one female form. In the English
New Forest a second female form, valesina, co-exists with the
ordinary paphia female. But in the southern valleys of the Alps
the valesina female is much the commoner of the two, and indeed
in some localities where the species is abundant, I have seen no
paphia females in many days collecting.

The beetle Gonioctena variabilis furnishes an illustration of a
comparable phenomenon affecting the male sex. In 1894 an(i
1895 I studied the curious colour variations of this species es-
pecially in the neighbourhood of Granada, and Mr. Doncaster
ten years later repeated the observations on the same ground,
and also collected the insect in other places in the south of Spain.
The distinctions are not easy to give in words and the reader is
referred to the colour plate accompanying my paper.5 The
essential fact is that the males commonly have the elytra red
with black spots and the females for the most part have greenish
grey elytra with black stripes. In some localities a large minority
of males closely resemble the female type, being identical in
colour and then only distinguishable by structural differences.
In two Granada localities I found the proportion of such males
quite different. In the Darro valley about 38 per cent, (in

5 Proc. ZooL Soc., 1895, p. 850. Plate. Many points beyond that mentioned
above are involved in this remarkable case. For example, not only are there males
like females, but a small proportion of females resemble the ordinary male type.
The stripes are not merely the spots produced, for they occupy different anatomical
positions. The spots almost always go with a black ventral surface, but the striped
forms nearly always have that region testaceous. Spartium retama, the food-plant,
will not grow in England, but if it could be naturalised in America the whole problem
might be investigated there and results of exceptional interest would almost cer-
tainly be attained.

122 PROBLEMS OF GENETICS

718) were of this feminine type, but on the hills some 300 feet
above only 19 per cent, (in 3,230) were like the females. At
Castillejo, not far from Toledo I found no such male in 75 speci-
mens.

Mr. Doncaster collected from several localities, especially
from two areas near Malaga, about 5 miles apart. In one of
these the female-like males were, as usual, in a minority, but
in the other these were actually in great excess, amounting to
about 8 1 per cent, in the 173 taken. Doncaster found a doubtful
indication that the composition of the population varies with
the season, which is quite possible, but it is most interesting
to note that in my chief locality after the lapse of ten years he
found the proportions very much the same as I had done at the
same season, for where I had 19 per cent, of the female-like males
his collecting gave 16 per cent. In other respects also, his sta-
tistics corresponded very closely with mine.6

The various forms of Heliconius erato are well known to en-
tomologists. They are strikingly distinguished by the colours
of the strong comb-like marking on the hind wing, which may be
red, yellow, green or blue. In various parts of the distribution
in South America sometimes two and sometimes three of these
distinct types co-exist.7

The distribution of the varieties of Noctua castanea typifies
a large range of cases. The form which is reckoned the normal
of the species has red fore-wings. It is practically restricted to
Great Britain and Germany, according to Tutt. The other
common form, neglecta, has grey fore-wings, and in this pattern
it ranges through West Central Europe from North Italy to
Germany. In the British Isles it extends up to Orkney. In
Britain this grey form is by far the commoner, occurring where-

6 Doncaster, L., Proc. Zool. Soc., 1905, II, p. 528.

7 1 am not aware that the details of this striking case have ever been worked
out. It should be noted that the green and blue forms are not due to simple modi-
fication of the red pigment; for these colours, due to interference, fork over the
area occupied by the red lines. The distinctions between these forms cannot
therefore be simply chemical, as we may suppose them to be, for instance, in the
case of many red and yellow forms, and the genetic relationships of the Hehconid
varieties would raise many novel problems and be well worth studying experi-
mentally.

VARIATION AND LOCALITY 123

ever the species is found. The red form is much scarcer in
England, and does not occur at all in many localities where the
grey form is common. Mr. Woodforde, from whom this account
is taken,8 states that in August, 1899, he saw considerably over a
hundred of the grey in the New Forest at sugar, but only two
red ones. In Staffordshire however the red is proportionately
more numerous and he estimates them as 40 per cent, of the
population. Lastly a form has been taken in Staffordshire as a
rarity in which the red is replaced by yellow, and this has hitherto
been seen nowhere else. It is beyond our immediate purposes
to discuss the genetic relationships of such forms, but the details
of this case are interesting as making fairly clear the fact that
the distinctions between castanea and neglecta are due to com-
binations of the presence of and absence of two pairs of factors,
of which one produces a red pigment in the ground colour of the
forewing and the other irrorates the same region with black
scales. Mr. Woodforde states that all intermediates exist,
and that in Staffordshire the greys always have a pinkish tinge.
The yellow is doubtless another recessive to the red.

Species which are uniform in some localities may be poly-
morphic in others. Such a phenomenon is well exemplified by
the orchid Acer as hircina. Of this species distinct varieties had
previously been known in Germany, but Galle 9 has lately given a
detailed account of a number of most diverse forms found growing
in a district of Eastern France. Without reference to his plates
it is impossible to give any adequate conception of the profusion
of types which the flowers of the species there assume. In some
the lip is elongated to many times its usual length, twisting
and dividing in a fashion suggesting some of the strangest of the
Tropical Orchids. In others the labellum and the lateral petals
are all comparatively short and wide (Fig. 13). Intermediates,
combining these qualities in various degrees, were abundant, and
the condition of the species, which was the only representative of
the genus in the locality, recalls the extreme polymorphism of
many of the Noctuid Moths.

8 Woodeforde, F. C., Trans. North Staffordshire Field Club, XXXV, 1901, Plate.

9 E. Galle, Compte Rendus du Congres Internal, de Bot. a I'Expos. Univ., 1900,
p. 112.

124

PROBLEMS OF GENETICS

FIG. 13. Various forms of Acer as hircina. (After Galle.) This figure only shows
a few of the more striking forms illustrated in Galle's plates.

VARIATION AND LOCALITY 125

Somewhat comparable variability has been seen in another
Orchid genus Ophrys. In Great Britain the species apifera,
aranifera and muscifera though variable are fairly distinct, but
Moggridge has published two series of plates10 showing a very
different state of things as regards the Ophrys population of the
Riviera. Here the outward diversity is such that the ordinary
specific names cannot be applied with any confidence and the
limits of the species are quite uncertain. It may well be supposed
that these Riviera plants are interbreeding, and indeed we may
safely assume that they are. It is, however, to be remembered
that Darwin showed apifera in this country to be habitually self-
fertilised, so that the different behaviour on the Riviera may
itself constitute a local peculiarity. Moreover it is to be gathered
from Moggridge's account that in the districts which he examined
the condition was not to be described by the statement that our
three types were there co-existing and hybridising, but rather
we should say that the population was polymorphic, containing
these three types amongst others. Conchologists are aware
that on the Dogger Bank Modiola attains a size unparalleled
elsewhere. The same is true of the sponges Grantia compressa
and Grantia ciliata in the estuary of the Orwell.11 Conversely,
as we know so well in the case of Man, dwarf races occur in
several special localities. Such examples may be multiplied
indefinitely.

The relation of local forms to species has often been dis-
cussed from many points of view, but I know no treatment of
the subject clearer or more comprehensive than an excellent
account of some of the various manifestations of local dif-
ferentiation as they appear in Helicidae published by Coutagne12
and a reader interested in the problem which they raise would

10 Flora of Mentone, 1864-8, Nova Ada Acad. Goes., XXXV, 1869.

11 I owe these facts to Canon A. M. Norman, who showed me illustrative
specimens. They were originally described by Bower bank (Monogr. Brit. Spongi-
adae, vol. II, pp. 18 and XX; vol. Ill, Pis. I and III). A specimen of G. compressa
measured 5 inches, with a greatest width of 3 M in. G. ciliata was found measuring
3 in. long and M in. wide. These dimensions are many times those of normal
specimens.

12 Coutagne, G., Recherches sur le Polymorphisme des Mollusques de France
Annales Soc. d'Agric. Set. et Industr. Lyon, 1895.

126 PROBLEMS OF GENETICS

do well to make himself acquainted with the original from
which the following notes are taken. He speaks for example
of Helix lapicida. This is on the whole a constant form ranging
up to the altitude of 1,300 m., common all over France except
at great heights and in the Olive regions where it is restricted
to moist places. Though subjected to such diverse conditions
it shows only trivial variations in colour and other respects
throughout its distribution, excepting that on both sides of the
Pyrenees it has a very distinct sporadic variety called Andorrica
or microporus. This variety occurs here and there, together
with the type-form sometimes in colonies (pp. 26-30 and 86).
Bulimus detritus though more restricted in geographical range
is a much more variable form. It exhibits great variations in
colour, form, and size, and as Coutagne well insists, these are
independent of each other. Foreshadowing the methods of
factorial analysis he suggests that distinctions in each respect,
the "modes" as he calls them, should be denoted by a letter,
or if desired, by a name, and the several combinations of differ-
ences might thus be most logically and usefully expressed. Of
such combinations he says there are at least 18, all of which can
be found. The whole possible series does not necessarily occur
in the same place, and various localities are characterised by
the presence or absence of certain of the combinations as Cou-
tagne calls them, and by the relative frequency with which they
occur. The ideas thus enunciated are much in advance of the
ordinary practice of systematists, who give names to forms which
are nothing but accidental combinations of factors, just as the
horticulturists for practical reasons give names to similar com-
binations, which as we now know are merely specially noticeable
terms in a long series of possibilities. In each case it is rather
the factors which should be named than the forms which are
constituted by their casual collocation. In this special example
of Bulimus detritus the 18 forms are made by the combinations
of three pairs of independent factors. Besides these combina-
tions which may occur anywhere or almost anywhere in the dis-
tribution there are two more distinct local forms, each of which
is regarded by Coutagne as probably constituting a fresh " mode,"
perhaps compatible with the others.

VARIATION AND LOCALITY 127

Helix striata (Draparnauld)13 is truly polymorphic; and its
various forms have been described under various specific names.
It abounds in the calcareous hills of Provence and Languedoc,
disappearing in the alluvial lowlands and equally in the upper
levels at about 800-1,000 m. From this district it extends
through regions of similar altitude over a great part of France
(details given).

Locard in his monograph of this group, which he calls col-
lectively the group of Helix Heripensis, tabulates 27 distinct
named forms. The characteristics in which these forms differ
have been reckoned as 17, and as several of these vary in degree
of development, the number of modes may be increased to 109.
For practical purposes however Coutagne considers that the
various developments of 7 characteristics in their several com-
binations are enough to express the various forms, and he gives
examples of this method of definition. As he observes, though
names may be required to define the modes, no one need be
alarmed at that, for the same names of modes will be applicable
to a great range of distinct species, and the formulae expressing
their combinations will replace the varietal names.

This particular example of polymorphism is but little limited
by locality. Occasional colonies present some special physiog-
nomy which may in a given place seem almost invariable, though
in this very respect the colonies found elsewhere may be highly
variable, but such limitations are exceptional for H. striata.

Some distinct and obvious susceptibilities to the influence
of soil and climate are however noticeable. For example on
siliceous ground the shells are thinner, while on calcareous soils
they are thicker ; similarly those from the Northern districts
attain a larger size than those from further South. Moreover
those subjected to curtailed development, whether from drought,
heat or cold often show a shortening of the spire. In contrast
with this case Coutagne describes the varieties of Helix caespitum,
which he says are for the most part localised, quoting many il-
lustrative cases.

Another remarkable case in which locality plays a curious
part is provided by the two species Helix trochoides and pyra-

18 As to the synonymy and references see Coutagne, p. 45.

128 PROBLEMS OF GENETICS

midata. In France generally they are distinct enough from
each other, trochoides being smaller and having a characteristic
keel. Coutagne says that after having collected these species
from more than a score of localities he came upon a colony of
trochoides on the island of Pomegues in which the shells were
relatively enormous, most of them having only a slight keel,
and a few none at all. On the other hand he received a con-
signment of pyramidata from four localities in Sicily, all small,
and one of them exactly like the trochoides from Pomegues.
Judging by the samples received from Sicily, trochoides is there
not more variable than it is in Provence, while the Sicilian
pyramidata is protean.

The relations of the two species Helix nemoralis and hortensis
provide an illustration of another kind of manifestation of local
peculiarity. H. hortensis and nemoralis as usually met with,
are two very distinct forms. H. hortensis is smaller and duller,
and its peristome is white. H. nemoralis is larger and more shiny,
and its peristome is brown. In several anatomical points,
moreover, especially in the shape of the dart, there are great
differences. For a full account of these peculiarities of the two
forms and a discussion of their inter-relations the reader is re-
ferred to the elaborate work of A. Lang14 who has studied them
extensively and has also succeeded in experimentally raising
hybrids between them. These hybrids were in a slight degree
fertile with both the parent species, but up to the time of pub-
lication no young had been reared from hybrids inter se.

Coutagne describes the result of collections made in 62
French localities. Some had exclusively hortensis, some ex-
clusively nemoralis, and in some the two were found in associ-
ation. He gives details of five of these collections from which I
take the following summary of the more essential facts, omitting
much that is almost equally significant.

Locality A , near Honfleur. Both forms present, each sharply
and normally distinguished, without any intermediates. They

14 A. Lang, Die Bastarde von H. hortensis Mutter H. nemoralis L. Jena, G.
Fischer, 1908; with a fine coloured plate showing the varieties of the species and
their hybrids.

VARIATION AND LOCALITY 129

are thus found in many places. Coutagne instances Miiller's
observations in Denmark, his own series from the Jura, etc.

Locality B. Vonges (Cote d'Or), 242 hortensis taken at ran-
dom, showed 128 with light peristomes (either more or less pinkish
or quite white) and 114 with dark brown peristomes; together
with 26 nemomlis all with the usual brown peristomes.

Of the hortensis 50 were in ground-colour opalescens and I
roseus; and in shape 5 were umbilicatus.

Locality C, about 3 kilometres from B. There were found 35
hortensis , of which 20 had light peristomes and 15 brown; to-
gether with 7 nemoralis.

Of the hortensis none were opalescens; 18 were roseus and none
has the shape of umbilicatus.

Locality D, about 1,200 metres from B. 147 hortensis, of
which 4 had light peristomes and 143 had brown. No nemoralis
were found.

None of the hortensis were opalescens or roseus, but 30 were
umbilicatus.

In these localities intermediates of every grade existed be-
tween the well-characterised opalescens, roseus, or umbilicatus,
and the other forms, but there were no intergrades between the
other nemoralis and the smaller hortensis, about which there
was no hesitation. In the next locality a very different state
of things was found.

Locality E. Banks of the Yvette at Orsay (Seine-et-Oise).
The actual numbers are not given, but we are told that 58 per
cent, were hortensis, 33 per cent, nemoralis, and 9 per cent, inter-
mediate. As at Honfleur, the hortensis had white peristomes, and
the nemoralis brown. Coutagne's visits to this locality were
in 1878 and 1880, and he calls attention to the fact that Pascal
found similar intermediates in the same neighbourhood in 1873.

The two species, in Coutagne's view, when they occur to-
gether, can generally be sorted from each other with perfect
confidence, and it is only in exceptional localities that these
intermediates occur. Whether they are hybrids, or whether
sometimes the species in their variations transgress their usual
limitations is regarded both by Coutagne and by Lang as a

10

130 PROBLEMS OF GENETICS

question not yet answerable with certainty. Coutagne moreover
lays stress on the fact that although each species may be easily
known from the other in its own district, yet when shells from
different districts are brought together it is sometimes impossible
to sort them. He mentions an example of such casual inter-
mixture occurring under natural conditions on an island in the
Rhone, to which it may well be supposed that floods had brought
immigrants from miscellaneous localities. This population con-
tained a very large number of uncertain specimens, and as he
says, it was much as if he were to mix the shells from his 62 local-
ities, after which it would certainly be impossible to separate
the two species again.15

Further evidence is given in the same treatise as to other
examples of polymorphism, especially in the genus Anodonta, of
which Locard made 251 species for France alone. Here again
are cases like those already given, and many forms or "modes"
are found restricted to special localities, while occasionally
in the same locality dissimilar forms are found, collectively
forming a colony, without intermediates.

Taken as a whole the evidence shows the following conclusions
to be true. Local races, whether of animals or plants, may be
distinguished by characters which we are compelled to regard
as trivial, or again by features of such magnitude that if they
were known to us only as the characteristics of a uniform species
they would certainly be assumed without hesitation to be essential
for its maintenance. Local forms may be sharply differentiated
from the corresponding populations of other localities or they
may be connected with them by numbers of intermediates.
Not rarely also we find a fact which has always seemed to me of
special significance, that the peculiarity of the local population
or colony may show itself in a special liability to variation, and
this variability may show itself in one of many degrees, either
in the constant possession of a definite aberration, in a dimor-
phism, or in an extreme polymorphism.

At this stage attention should be called to two points.

15 With this evidence compare that given by A. Delcourt in his valuable
papers lately published relating to the variations of Notonecta. See especially
Bull. Set. Fr. Belg., 1909, XLIII, p. 443; and C. R. Soc. Biol., 1909, LXVI, p. 589.

VARIATION AND LOCALITY 131

First, that when the details of the geographical distribution of
any variable species are studied in that thorough and minute
fashion which is necessary for any true knowledge of the inter-
relations of the several forms, the conception of a species invented
by the popular expositions of Evolution under Selection is found
to be rarely if ever realised in nature.

A species in this generalised sense is an aggregate of indi-
viduals, none exactly alike, but varying round a normal type,
the characters of which are fixed in so far as they are adapted to
environmental exigency. In nature, however, the occurrence of
the varieties, and even the occurrence of the variability is
sporadic. In one place a population may be perfectly uniform.
In another it may be again uniform but distinct. In others
the two forms may occur together, sometimes with and sometimes
without intergrades. In some localities a sporadic variety
may be an element of the population, persisting through long
periods of time. In other localities there may be several such
aberrations occurring together which are absent elsewhere.

Secondly, I would remind the reader that in the light of genetic
analysis we know that intergrades, when they do occur, cannot
be assumed to represent conditions through which the species
must pass or has passed on its way to the extreme and definite
forms.

Often, perhaps generally, they are nothing but heterozygous
forms, and often also they are conditions corresponding with the
presence of factors in their reduction-stages.

A broad survey of the facts shows beyond question that it
is impossible to reconcile the mode of distribution of local forms
with any belief that they are on the whole adaptational. Their
peculiarities are occasionally the result of direct environmental
influence, as we shall hereafter notice in certain cases, but none
can attribute such sporadic and irregular phenomena to causes
uniformly acting.

Writers on systematics, especially those of former generations
often conjecture or assert that local distinctions are caused by
"differences of climate, soil, food, etc.," in vague general terms.
It is usually safe to assume that these remarks do not represent

132 PROBLEMS OF GENETICS

conclusions drawn from actual evidence, for only rarely can they
be translated into more precise language. So thoroughly have
the biological sciences become permeated with the belief that all
distinctions are dependent upon adaptation, that the mere
existence of definite distinctions is felt by many to be sufficient
ground to warrant an assumption that these distinctions are
directly or indirectly due to special local conditions. For
example, Dr. J. A. Allen, who has done so much careful and valu-
able work in delimiting the local forms of the United States
fauna, writes of the Ground Squirrels (Tamias)16 as follows: —

"From the extreme susceptibility of this plastic group to
the influences of environment, it is one of the most instructive
and fascinating groups among North American mammals. No
one can doubt its comparatively recent differentiation from a
common stock, and its dispersion from some common centre.
Whether the type originated at some point in North America,
or in the Northern part of Eurasia, it is perhaps idle to speculate,
but that it has increased, multiplied, spread, and become differ-
entiated to a wonderful degree in North America is beyond
question; as it is found from the Arctic regions to the high
mountain ranges of Central Mexico, and has developed some
twenty to thirty very palpable local phases."

"Some of them easily take rank as species, others as sub-
species. Probably a more striking illustration of evolution by
environment cannot be cited."

He proceeds to point out that the habits of these creatures
are such as lead to isolation. This may well be admitted, and
indeed no exception can possibly be taken to the passage as a
whole, save in the one respect that there is no real proof that
the local diversity is due to "evolution by environment" or an
indication of "susceptibility to the influences of environment."

Dr. Allen does indeed adduce the fact that California "ex-
tending through 800 miles of latitude, with numerous sharply
contrasted physiographic regions, has apparently no less than
six strongly differentiated forms, while the region east of the
Rocky Mountains from a little below the northern boundary of

" Allen, J. A., Bull. Amer. Mus. N. H., Ill, 1891, pp. 51-54.

VARIATION AND LOCALITY 133

the United States northward to the limit of trees — a slightly
diversified region of at least ten times the area of California — has
only one"! But when one comes to ask how the various forms
are adaptational, and how the influences of environment have
led to their production, only conjectures of a preliminary and
tentative character could be expected in reply. Desert forms
are no doubt pallid as in so many instances, and forest forms are
more fully coloured, and we may readily enough accept such facts
as indications of a connection between bodily features and the
conditions of life, but further than that no one can go; so that
when we find size, length of ears or of tail, the number of dorsal
stripes, the pattern of the colours, not to speak of differences in
the pigments themselves, all exhibiting large modifications, we
cannot refer these peculiarities to the causation of environmental
difference, save as a simple expression of faith. I incline far
more to agree with Gulick who, after years of study of the local
variations of the Achatinellidae, came to the conclusion that it
was useless to expect that such local differentiation can be
referred to adaptation in any sense.17 Even the most convinced
Selectionist must hesitate before such facts as those related by
A. G. Mayer regarding the distribution of Partula otaheitana, one
of these Achatinellidae. The island of Tahiti has been scored
by erosion so that a series of separated valleys radiate to the coast.
From four successive valleys Mayer collected the species, and
found that in the first (Tipaerui) valley all the shells were
dextral (115, containing 73 young); in the second valley
.(Fautaua) 54 per cent, of adults and 55.5 per cent, of the young
contained were sinistral; in the third valley (Hamuta) 69 per
cent, of adults and 73 per cent, of young contained in them were
sinistral; and lastly, in the fourth valley (Pirae) all the shells
(131, containing 62 young) were sinistral.18 In connection
with these observations I may mention the fact that in a certain
pond in the North of England19 the sinistral form of Limnaea

17 J. T. Gulick, Evolution, Racial and Habitudinal, Carnegie Institution, Pub-
lication No. 25, 1905.

18 A. G. Mayer, Mem. Mus. Comp. Anat. Harvard, Vol. XXVI, 1902, p. 117.
From the tables given I cannot ascertain the actual numbers from the two inter-
mediate valleys, but they were considerable.

19 To which I was very kindly guided by Mr. C. T. Trechmann.

134 PROBLEMS OF GENETICS

peregra has been known to occur for about fifty years. Visiting
it lately I found the left-handed shells to be about 3 per cent, of
the population. The species is the commonest British fresh-
water shell, but left-handed specimens are exceedingly rare.
Will anyone ask us to suppose that the persistence of a percentage
of this rarity in the same place is an indication of some specially
favouring circumstance in the waters of that pond? It is a
horse-pond to all appearances exactly like any other horse-pond ;
and I believe that in perfect confidence we may accept the
suggestion of common sense, which teaches us that there is
nothing particular in the circumstances which either calls such
varieties into existence or contributes in any direct way to their
survival. Had the phenomenon of local variation been studied
in detail before Darwin wrote, the attempt to make selection
responsible for fixity wherever found, could never have been
made. The proposition that not only the definiteness of local
forms but their variability also is sporadic, can be established
by countless illustrations taken from any group of either the
animal or the vegetable kingdoms. Only exceptionally can the
fixed differences be even suspected of contributing to adaptation,
and sporadic variability, which is a no less positive fact, must
manifestly lie outside the range of such suspicions. It is open
to any one to suggest speculatively that the persistence of
special varieties or of special variability in special places is an
indication that in those places the conditions of life are such
that the forms in question are tolerated though elsewhere
the same types are exterminated; but that consideration, even
if it could be proved to be well founded, is not one which lends
much force to the thesis that definiteness of type is a consequence
of Natural Selection. On the contrary, recourse to such reason-
ing implies the inevitable but very damaging admission that
the stringency of Selection is frequently so far relaxed that two
or more equally definite forms of the same species can persist
side by side. There is no doubt that this is the simple truth,
but when once that truth is perceived it is useless to invoke the
control of Selection as the factor to which definiteness of type
in general must be referred.

VARIATION AND LOCALITY 135

The genetic relations of local forms to each other cannot in
the absence of actual breeding experiments be often ascertained.
Standfuss formerly enunciated as a general principle that when
two forms co-exist in the same locality and are able to interbreed,
they do not produce intermediates ; but that when the forms are
geographically separated as local races, crosses between them
result in a series of intermediates.20 In this aphorism there is a
good deal of truth, but if in the light of Mendelian principles we
examine the two statements we see now that the first is in reality
only another way of saying that the distinctness of an aberrational
form co-existing with another is due to segregation, accompanied
by some degree of dominance of one type. Whether, however,
one geographically isolated race will give intermediates when
bred with another must depend entirely on the genetic physiology
of the special case, and no general rule can be laid down. It
may well be that, inasmuch as the distinctness of the variety is
maintained by isolation, the difference in factorial composition
between it and the representative form in another area is neither
simple nor sharp; but when two varieties co-exist, though inter-
breeding, it is now clear that their differences must depend on
the segregation of simple factors. Plainly such aberrations may
in one place co-exist with another type, and elsewhere be sep-
arated from it as local races.

Excellent illustrations of these two stages in evolution are
provided by the melanic varieties of British Lepidoptera. The
fact that black or blackish varieties of many species especially
of Geometridae have come into existence in recent years is well
known to British collectors, and it is not in dispute that they
have in several instances replaced the older type more or less
completely in certain districts. In the year 1900 the Evolution
Committee of the Royal Society instituted a collective inquiry
as to the contemporary distribution of these dark varieties. As
the change had happened within living memory and had greatly
progressed in recent years it was hoped that a record of the
existing distribution would serve as a point of departure for
future comparison. The records thus obtained were tabulated

w Standfuss, Handbuch d. palaarkt Gross-schmet, 1896, p. 321.

136 PROBLEMS OF GENETICS

by Mr. L. Doncaster.21 From that account and from the state-
ments n Barrett's British Lepidoptera22 this description of some
of the more notable cases is taken.

The most striking and familiar case is that of Amphidasys
betularia, of which only the ordinary type was known in any
locality until about 1848-1850, when the totally black var.
doubledayaria first appeared in the neighbourhood of Manchester.
This black form was subsequently recorded in Huddersfield
between 1860 and 1870; Kendal about 1870; Cannock Chase,
1878; Berkshire, 1885; Norfolk, Essex and Cambridge about
1892; Suffolk, 1894; London, 1897. For the Southern Counties
of England, except in the London district, there are still very few
records. It cannot of course be asserted positively that the
variety spread from its place of first appearance into the other
localities, and that it did not arise de novo in them, but there
can be little doubt that the process was one of colonisation.
On the European Continent the first records are from Hanover
in 1884, Belgium 1886 and 1894, Crefeld 188-, Berlin 1903,
Dresden about the same date.

As regards the increase of the variety we have the fact that
in Lancashire, Cheshire and the West Riding of Yorkshire the
black is now the prevalent form ; and in some places, as for example,
Huddersfield, the black alone is now found, though it was un-
known there till between 1860 and 1870. About 1870 at New-
port, Monmouth, the two forms were in about equal numbers,
but a few years later the type had almost vanished. Similarly
in Crefeld, where the black form was still very rare in the eighties,
it now forms about 50 per cent, of the population. In the
London district the black remains scarce and at the date of the
report it was still very scarce. From Ireland there is only one
record and there are hardly any from Scotland.

Boarmia repandata is another species which is behaving in a
somewhat similar way. Unlike betularia, however, the species
is a variable one, and has several colour-forms, amongst them
the banded var. conversaria, and many others. In addition

21 Ent. Rec., XVIII, No. 7, 1906.

28 This evidence was largely collected by Mr. G. T. Porritt, who has given
much attention to the subject.

VARIATION AND LOCALITY 137

to these there is a black form in the North of England which
seems to be spreading. In Huddersfield the black was first
recorded in 1888, and in 1900 20-25 per cent, were black. At
Rotherham the black or very dark are now prevalent and have
increased in the last 15 years. From the Midlands, East Anglia
and Southern Counties the returns show only the light and
medium forms.

Of Odontoptera bidentata several intergrading dark forms exist,
and these are found exclusively in the North and the Midlands.
Unicolorous blacks have been found recently in the Lancashire
mosses and at Wakefield. At Huddersfield 50 years ago the
light forms were prevalent, but now a rather dark brown, not
infrequently suffused with black, is the commonest. In Southern
Counties only light forms are known.

Phigalia pilosaria in South England is always light, but in
the North the prevalent form is darker. About 35 years ago
a form with unicolorous sooty fore-wings and dull grey hind
wings was first seen in Yorkshire and a similar form is now taken
regularly in South Wales.

In the following cases the dark varieties were found originally
only in the South.

Boarmia rhomboidaria gave rise about 40 years ago to a uni-
colorous smoky variety called perfumaria. This was at first
peculiar to the London district, but it has since been taken in
Birmingham and other large cities. More lately coal-black
specimens have been found at Norwich, and others similar but
hardly so dark were taken in the South of Scotland and at
Cannock Chase.

Eupithecia rectangulata is a similar case. Formerly the
light forms were prevalent but within sixty years they have
almost entirely been replaced in the South of London by a nearly
black form.

Tephrosia (Boarmia) consortaria and Tephrosia consonaria are
exceptionally interesting, for they have both given off dark
forms in the same wood near Maidstone, which is far from the
usual "centres of melanism." They were discovered in this
locality by Mr. E. Goodwin. That of consortaria is a dark

138 PROBLEMS OF GENETICS

grey, but that of consonaria is a full black, and nothing like
either has been found anywhere else.

These examples are all taken from the Geometridae but
others, though of a less conspicuous kind, could be given from
the Noctuidae or the Micro-Lepidoptera. Acronycta psi, for
instance, has a suffused form which is believed to be becoming
more frequent in the London district. Polia chi has two dark
forms, olivacea, a yellowish grey with dark markings, and suffusa
which is a darker, blackish-slate colour. Both occur in the North
of England, sometimes together, sometimes separately, or mixed
with the type and many intermediates. The distribution is
peculiarly irregular. At Huddersfield, where the very dark form
appeared suddenly about 1890, some 30 per cent, are said to be
now dark and about 6-7 per cent, very dark, but at Saddleworth,
12 miles away, only the pale forms occur.

Several questions of interest arise in regard to this evidence.
This progressive Melanism has arisen in certain families only,
and may be confined to certain species only, within those families.
As in almost all other examples in which variation has been much
observed, its incidence is capricious and specific. A collateral
line of inquiry relates to the degree of discontinuity which the
variation manifests. Here again there is no rule. Generally
speaking, in A. betularia, to take the case most fully studied, the
variation is discontinuous. Real intermediates between betu-
laria and doubledayaria are in most localities absent or rare.
The black spots of betularia may often be larger or more numerous
than in the normal, but this variation has nothing to do with
doubledayaria, and is not an intermediate stage towards it,
though sometimes wrongly so described. Doubledayaria owes its
characteristic appearance to a factor which blurs the surface
of the wings with a layer of black. Sometimes this blurring is
slighter than in the real doubledayaria, and these forms are real
intermediates. Occasionally the forewings alone are thus blurred.
These intermediates are clearly due to reduction-stages of the
doubledayaria factor, and are related to it as a blue mouse is to
a black, or a dutch rabbit to a self-colour. It cannot positively
be asserted that the full doubledayaria existed before the inter-

VARIATION AND LOCALITY 139

mediate, but it almost certainly did. In certain places as for
instance in Belgium, there is evidence that intermediates have
at various times been fairly abundant, but they have never be-
come common, nor are they known to exist in the absence of
doubledayaria. When the black variety and the light type breed
together they do not usually have intermediates among their
offspring, and the evidence is consistent with the view that the
black is a complete dominant. The same is probably true of
Tephrosia consonaria.

In some of the other species we know that the darkest forms
did not appear first. For example in Phigalia pilosaria and
Boarmia rhomboidaria dark forms existed and are believed to
have increased in number before the darkest made its appearance.
Hybernia progemmaria is said to have become darker gradually
both in Cheshire and in the West Riding, and a uniformly smoky
variety appeared in South Yorkshire less than 45 years ago which
has spread to neighbouring counties. The dark medium has
become the commonest form in Huddersfield district, where the
very dark variety is now about 20 per cent, of the population,
though the light form is still common.

Taking the evidence together we find it consistent with the
view that dark forms have appeared sporadically, in some species
the very dark appearing first and intermediates later, in others
the moderately dark came first and the darkest later in time. It
is practically certain that the change has in general come about
not by a gradual change supervening on the population at large,
but by the sporadic appearance of dark specimens as a new ele-
ment in the population, and strains derived from these dark
individuals have gradually superseded the normal type more or
less completely.

If it could be shown that these melanic novelties had a defi-
nite advantage in the struggle for existence they would provide
an instance of evolution proceeding much in the way which
Darwin contemplated. The whole process would differ from
that conceived by him as the normal method of evolution only
in so far as the change has come about with great rapidity and
in some instances largely by the appearance and success of dis-

140 PROBLEMS OF GENETICS

continuous varieties. The question, however, must be asked
whether the dark form can reasonably be supposed to have
an advantage by reason of their darkness. Some naturalists
believe that the darkness of the colours does thus definitely con-
tribute to their protection by making the insects less conspicuous
and thus more likely to escape the search of birds. In support
of this view it may be pointed out that it is in the manufacturing
districts of Lancashire and Yorkshire, and again in the London
area that the melanics have attained their greatest development.
Consistently with this argument also, it is in the neighbourhood of
Crefeld and Essen, the black country of Germany, that they have
chiefly established themselves on the Continent, and Phigalia
pilosaria in the black form is now at home in South Wales. Thus
superficially regarded, the evidence looks rather strong, but it
is difficult to apply the reasoning in detail. We have first the
difficulty that the black form of betularia for instance has estab-
lished itself in thoroughly rural districts, notably near King's
Lynn in Norfolk, and in the neighbourhood of Kendal and
Windermere. The black form of consonaria and the dark
consortaria appeared in a wood near Maidstone, far from town
smoke, and the black rhomb oidaria was first found at Norwich,
which, as towns go, is clean. Then again the spread of the
melanics is very irregular and unaccountable. The black pilo-
saria is found both in the West Riding and in the Swansea
district, but not yet elsewhere. It rapidly increased at Hudders-
field, but made no noticeable progress at Sheffield though re-
corded there for ten years. It is also a remarkable fact that no
similar melanic development has been observed in America,
and, so far as I am aware, comparable melanic varieties have not
appeared on the European continent except in the case of the
few sorts which possibly may have come from England.

The whole subject is beset with complications. It must
not be forgotten that in a few species of moths there is an obvious
and recognised conformity between the colours of the perfect
insect and that of the soil on which they live, comparable with
that which is so striking in the case of some Oedipodidae and
other grasshoppers. Of this phenomenon the clearest example

VARIATION AND LOCALITY 141

is Gnophos obscurata, which is a most variable species with many
local forms. Of these a well-known dark variety lives on the
peaty heaths of the New Forest and other districts, but on the
chalk hills of Kent, Sussex and Surrey various light varieties
are found, of which one is a bright silvery white, very near in
colour to the colour of a chalky bank. This case does not seem
to be one of direct environmental action,23 for Poulton found no
change induced by rearing larvae among either white or black
surrounding objects. No one however can doubt that there is
some indirect connection between the colour of the ground and
that of the moths.

To my mind there is a serious objection to the theory of pro-
tective resemblance in application to such a case as that of the
betularia forms, which arises from the fact that the black double-
dayaria is a fairly conspicuous insect anywhere except perhaps
on actually black materials, which are not common in any
locality. Tree trunks and walls are dirty in smoky districts but
they are not often black, and I doubt whether in the neighbour-
hood of Rotherham, for instance, which is one of the great
melanic centres, doubledayaria can be harder for a bird to find
than betularia would be. After all, too, many of the species
much affected are not urban insects. They live in country
places between the towns, and the general tone of these places
even in Lancashire and the West Riding is not very different
from that of similar places elsewhere. As against the objection
that the black varieties are much blacker than the case requires
it may be replied that we know nothing of the senses of birds,
and that perhaps to their eyes blackness does constitute a dis-
guise even though the surroundings are much less dark. This is
undeniable, but recourse to such an argument is dangerous; for
if the sight of the insect-eating birds is so dull that it does not
distinguish dark things from dingy grey, we cannot subsequently
regard the keen sight of birds as the sufficient control which has
led to the minute and detailed resemblance of many insects to
their surroundings. Th^se who see in such cases examples of

» Such direct action has of course been proved to occur in the case of several
dimorphic larvae (e. g., A. betularia, itself) and pupae.

142 PROBLEMS OF GENETICS

the omnipotence of Selection must frequently find themselves
in this dilemma.

Taking the evidence as a whole, we may say that it fairly
suggests the existence of some connection between modern urban
developments and the appearance and rise of the melanic vari-
eties. More than that we cannot yet affirm. It is a subject
in which problems open up on every side, and all of them are
profitable subjects for investigation. Unhappily such animals
are difficult to rear successfully in captivity for many generations,
owing to their extreme liability to disease. Not the least in-
teresting feature of the melanics is the fact that the black vari-
eties provide about the best and clearest example of a new dom-
inant factor attaching itself to a wild species in recent times.
None of the cases are satisfactorily recorded or analysed as yet,
but the evidence is clear that doubledayaria is a dominant to its
type, and in several other dark varieties, though the pigment
deposited is not black, the records show that the increased
amount of the pigment almost certainly is due to a positive factor.
Of this, Hemerophila abruptaria is a good example.24 There are
some irregularities in the results, but taken together they leave
little doubt that the dark brown variety is a dominant and the
light, yellowish brown a recessive.

A curious parallel to the rise of the melanic moths in England
is provided by the case of the Honey-creepers or Sugar-birds,
in certain West Indian islands.25 These birds of the genus
Coereba (Certhiola) range from Southern Mexico to the Northern
parts of South America and through the whole chain of the West
Indian islands and Bahamas except Cuba. There are numerous
local forms, and many of the islands have types peculiar to them-
selves, as is usual in such cases. Some of the types or species
range through several islands, but according to Austin Clark26
no island has more than one of them. Cory27 reckoned twelve

24 See Harris, Proc. Ent. Soc. London, 1904, p. Ixxii, and 1905, p. Ixiii; also
Hamling, Trans. City of London Ent. Soc., 1905, p. 5.

25 1 am indebted to Mr. Outram Bangs of the Harvard Museum for calling my
attention to this remarkable case.

26 Auk, 1889, VI, p. 219.

27 Ann. N. Y. Acad. Sci., 1878, I, p. 149.

VARIATION AND LOCALITY 143

such species within the Antillean region. They are small birds
about the size of a nuthatch with a general colouring of black,
yellow, and white. From the island of St. Vincent the Smith-
sonian Institution received in the late seventies of last century
several completely black specimens in addition to two of the usual
type of colouring. The black were described by W. N. Lawrence
as atrata, and those marked with the usual yellow and white
were called saccharina. The collector (Mr. F. A. Ober) reported
that the black form was common, and that the saccharina form
was rarer. Lawrence remarks, "Had there been only a single
example (of the black form) I should have considered it as prob-
ably a case of abnormal colouring, but it seems to be a represent-
ative form of the genus in this island." 28 There is of course no
doubt of the correctness of the view taken by Austin Clark that
" atrata" is a black variety. The black bird is in every respect,
other than colour, identical with saccharina, and it is even possible
to detect a greenish colour in the areas which would normally be
yellow, showing plainly enough the yellow pigment obscured by
the black.

We have next the interesting fact that like our melanic moths
the dark form is replacing the "type." At the time of Ober's
visit the type was already in a minority, but now it is nearly
or perhaps actually extinct, though the black form is one of the
commonest birds on the island. Austin Clark found no specimen
when he collected there in 1903-4, though formerly it was not
uncommon in the vicinity of Kingston and in the immediate
windward district of St. Vincent.

The Grenadines are geographically just south of St. Vincent,
though separated by a deep channel. In these islands no black
forms have yet been taken, but Grenada, the next island to the
south, has both normals and blacks. There are trifling dif-
ferences of size between the Grenada birds and those from St.
Vincent, the Grenada specimens being slightly smaller and for
this reason they have received distinct names, the form marked
with yellow and white being called Godmani (Cory) and the black,
Wellsi (Cory), but this merely introduces a useless complication.

* Ann. N. Y. Acad. Sci., 1878, I, p. 149.

144 PROBLEMS OF GENETICS

There is evidence that in Grenada, as in St. Vincent, the black is
gradually ousting the original type, but the process has not gone
so far as in St. Vincent. Austin Clark very properly compares
this case of the Sugar-birds with that of Papilio turnus, which as
is well-known, has a black female in the southern parts of its dis-
tribution, in addition to a female of the yellow type, but in the
Northern States the black female does not occur.

During the present year P. R. Lowe, who lately studied
Coerebas on a large scale in the West Indies, has published an
important paper on the subject.29 He calls attention to the
fact that Cory recently found a black form of Coereba on Los
Roques Islands, and he himself discovered another on the
Testigos Islands. Both localities are on the coast of Venezuela,
far from St. Vincent and Grenada. The whole problem is thus
further complicated by the fact that the black varieties have,
as we are almost driven to admit, arisen independently in remote
places. Improbable as this conclusion may be, it is still more
difficult to regard all the black forms as derived from one source.
For first, they present definite small differences from each other;
and secondly we have to remember a consideration of greater
importance, that the very fact that each island has its own type
must be accepted as proving that the localities are effectively
isolated from each other, and that migration must be a very
rare event.

The rarity of such illustrative cases is, I believe, more ap-
parent than real. It is probably due to the extreme reluctance
of systematists to admit that such things can be, and of course
to the almost complete absence of knowledge as to the genetic
behaviour of wild animals and plants. Only in such examples
as this of the Coereba, where colour constitutes the sole differ-
ence, or that of the moths which have been minutely studied by
many collectors, does the significance of the facts appear. The
arrangement of catalogues and collections is such that much
practical difficulty of a quite unnecessary kind is introduced. For
example, in this very case of Coereba, I find the British Museum
has a fine series from Grenada including 3 normals and 1 1 black,

29 Ibis, 1912, pp. 523-8.

VARIATION AND LOCALITY 145

and also 1 6 blacks from St. Vincent. If the black specimens from
Grenada were put with the normals which are almost certainly
nothing but a recessive form of the same bird, the variation would
strike the eye on even a superficial glance at the drawer. But
following the notions so naively expressed in the passage quoted
above from W. N. Lawrence, the blacks from Grenada are put
apart together with the other blacks from St. Vincent, though
two of them were shot on the same date as one of the normals.

CHAPTER VII

LOCAL DIFFERENTIATION. Continued

OVERLAPPING FORMS

The facts of the distribution of local forms on the whole are con-
sistent with the view that these forms come into existence by the
sporadic appearance of varieties in a population, rather than by
transformation of the population as a whole. Of such sporadic-
ally occurring varieties there are examples in great abundance,
though by the nature of the case it can be but rarely that we are
able to produce evidence of a previous type being actually super-
seded by the variety. When the two forms are found co-existing
in the same area they are usually recorded as one species if inter-
grades are observed, and as two species if the intergrades are
absent. On the other hand when two forms are found occupying
separate areas, when, that is, the process of replacement is com-
pleted in one of the areas, then forthwith each is named separately
either as species or subspecies. Successive observations carried
out through considerable periods of time would be necessary to
establish beyond question that the history proceeds in one way
rather than another. Such continuity of observation has for
the most part never been attempted. The kind of information
wanted has indeed only been lately recognized, and really critical
collecting is a thing of only the last few decades. The methods of
the older collectors, who aimed at bringing together a few typical
specimens of all distinct forms, are of little service in this class
of inquiry, which is better promoted by the indiscriminate col-
lection of large numbers of common forms from many localities.
When this has been done on a comprehensive scale we shall be
in a position to form much more confident judgments as to the
general theory of evolution.

Some little work of the kind has however been done and the
results are already of great value. Seeing that the differenti-
ation of local forms is only made possible by isolation, it neces-

146

OVERLAPPING FORMS 147

sarily happens that the collector finds one form in one locality
and another in a distinct locality, and there is no evidence as to
the behaviour which the two representative species might exhibit
if they came into touch with each other. In the most familiar
examples of such distinction each inhabits an island, completely
occupying it to the exclusion of any other similar form. It can
only be when the two representative species occupy parts of a
continental area connected with each other by regions habitable
for the organism in question, that there is a chance of seeing the
two forms in contact. Often also, even where this condition is
satisfied, the habits, social organisation, or some other special
cause may act as a barrier which prevents the distinguishable
forms from ever coming into such complete contact as to inter-
breed or to behave as a genetically continuous race. When
genetic continuity is ensured by a constant diffusion of the popu-
lation over the whole area which they inhabit there will mani-
festly be no formation of local races. The practical uniformity,
for example, of so many species of birds which inhabit widely
extended ranges of Western Europe is doubtless maintained by
such constant diffusion. When, as in the case of the Falcons,
many localities have peculiar forms, the fact may be taken as
conclusive evidence that there is little or no diffusion; and when
we find in such a species as the Goldfinch that in spite of mi-
gratory fluctuations there are nevertheless geographical races
fairly well differentiated, it may similarly be inferred that these
fluctuations habitually move up and down on paths which do
not intermingle. There are however a few examples of animals,
not given to much irregular wandering, which occupy a wide and
continuous range of diversified country and are differentiated as
local races in two or more districts, though the distinct races
meet in intervening areas. Of these the most notorious illus-
tration which has been investigated with any thoroughness is
that of the species of Colaptes (Woodpeckers) known in the United
States as Flickers. The study of the variations of these forms,
made by J. A. Allen1 is an admirable piece of work, with which

1 J. A. Allen, The North American Species of the Genus Colaptes, Considered with
Special Reference to the Relationships of C. auratus and C. cafer. Bull. Am. Mus.
Nat. Hist., IV, 1892.

148 PROBLEMS OF GENETICS

every student of variation and evolutionary problems should
make himself familiar. The two forms with which we are most
concerned are known as C. auratus and C. cafer, and are very
strikingly different in appearance. In size, proportions, general
pattern of colouration, habits, and notes, the two are alike, but
they differ in the following seven respects as stated by Allen.

Auratus Cafer

1. Quills yellow. I. Quills red.

2. Male with a black malar stripe. 2. Male with a red malar stripe.

3. Adult female with no malar stripe. 3. Adult female with usually a brown

malar stripe.

4. A scarlet nuchal crescent in both sexes. 4. No nuchal crescent in either sex.

5. Throat and fore neck brown. 5. Throat and fore neck grey.

6. Whole top of head and hind neck 6. Whole top of neck and hind neck

grey. brown.

7. General plumage with an olivaceous 7. General plumage with a rufescent

cast. cast.

These differences are illustrated in the accompanying coloured
plate, which has been most kindly prepared for me under the in-
structions of Dr. F. M. Chapman of the American Museum of
Natural History. Before going further it is worth considering the
nature of these differences a little more closely. All but the last
are large differences which no one would overlook even in a hasty
glance at the birds. If the only distinction lay in the colour of the
quills we might feel fairly sure that auratus was a recessive form
of cafer, and so probably it is in this respect. Similarly the black
malar stripe of auratus is in all probability recessive to the red
malar stripe of cafer and I imagine the pigments concerned are
comparable with those in the Gouldian Finch (Poephila gouldiae)
of Australia. Both sexes in that species may have the head black,
red, or, less often, yellow, and though it is not any longer in
question that birds may breed in either plumage, I believe that
the young are always black-headed and I imagine that those
which become red-headed possess a dominant factor absent from
the permanently black-headed birds.2 Yellow as a recessive

2 For a case in which a red-headed female X a black-headed male gave a
black-headed female and a red-headed male, see Avian Mag., N. S., IV, pp. 49
and 329.

OVERLAPPING FORMS 149

form of a red is certainly very common, but red and black as
variants of the same pigment are less usual. In the Gouldian
Finch we seem to have a case where a pigment can assume all
three forms. It would be interesting to know whether the red
of the malar stripes in Colaptes is a pigment of the same nature
as the red of the quills. Both in Colaptes and in Poephila
gouldiae I have seen specimens intermediate between the black
and the red, and the appearance of the part affected was exactly
alike in the two cases, red feathers coming up among the black
ones, and many feathers containing both red and black pigments
mixed together.

The development of the scarlet nuchal crescent in auratus
and the absence of this conspicuous mark in cafer constitute from
the physiological point of view the most remarkable pair of dif-
ferences. When the red crescent is not formed, the feathers
which would bear it are exactly like the rest, and no special
pigment is visible in them which one can regard as ready to be
modified into red. If the crescent is due to a factor it must
therefore be supposed that this factor has the power of modifying
the pigment of the neck in one special place alone. Dr. W. D.
Miller called my attention to the fact that a similar variation
occurs in another American woodpecker, the Sapsucker, Sphy-
ropicus varius?

I do not suggest that such variations are without parallel:
indeed in P. gouldiae the factor which turns the black of the head
into scarlet affects one special region of the black only, being
sharply distinct from the unmodified black of the throat. These
regions of the head are however often the seat of special colours
in birds.4 So also may be instanced the variety of the Common

8 The other variations of this bird are also interesting and important. The
normal male has a red head and a red throat. The female has a red head and a
white throat, but varieties of the female are known with a black head, thus again
illustrating the change from black to red. It should be noted that this is not a
mere retention of a juvenile character, but, as the birds mature, the red feathers
come up, or as an exception, the black. There is also a western species, ruber,
in which both sexes have a great extension of red, and are alike. The male of
nuchalis intergrades with this type, but the female does not.

4 Dr. W. Brewster, for example, has a remarkable specimen of the Teal (Nettion
carolinense) with a white collar strongly developed at the front and sides of the
neck, in a place where the normal has no such mark.

150 PROBLEMS OF GENETICS

Guillemot (Uria troile) which has a white line round the eyes and
at the sides of the head where the normal has no such mark; but
this line is formed in a very special place, the groove joining the
eye to the ear, whereas the feathers of the nuchal crescent are
not ostensibly distinguished from those adjacent.5

The transposition of the brown and the grey on the back and
front of the neck also constitutes a very remarkable difference.
If either grey or brown depends on a factor then it must be sup-
posed that auratus has one of these factors and cafer the other.

From these several considerations it is quite clear that if
auratus and cafer are modifications of the same type produced
by presence or absence of factors, several independent elements
must be concerned, and to unravel their inter-relations would be
most difficult even if it were possible to breed the types under ob-
servation, which is of course quite beyond present possibilities.

The distribution of the two is as follows. On the east side of
the Continent C. auratus, relatively pure, occupies the whole of
Canada and the States from the North to Galveston. Westward
it extends across the whole continent in the more northern
region to Alaska, but in its pure form it only reaches down the
Pacific coast to about the northern border of British Columbia.
Its southern and western limit is thus roughly a line drawn from
north of Vancouver, southeast to North Dakota and then south
to Galveston. C. cafer in the comparatively pure form inhabits
Mexico, Arizona, California (except Lower California and the
opposite coast), central and western Nevada, Utah, Oregon, and
is bounded on the east by a line drawn from the Pacific south of
Washington, south and eastward through Colorado to the mouth

B This variety is spoken of as the Ringed Guillemot and is sometimes regarded
as a distinct species to which the name ringvia was given by Briinnich. In sup-
port of this view Dr. William Brewster, to whom I am indebted for much assist-
ance in regard to the variation of birds, called my attention to observations of his
own and also of Maynard's, that the ringed birds were sometimes mated together,
though in a small minority (see Brewster, Proc. Boston Soc. N. H., XXII, 1883, p.
410). It would however be possible to produce many instances of varieties mated
together though surrounded by a typical population (e. g., two varying Black-
birds, Zoologist, p. 2765; two varying Nightjars, ibid., p. 5278). I am inclined to
believe that in nature matings between brothers and sisters are frequent in many
species of animals, and that the production of sporadically varying colonies is
thus greatly assisted.

OVERLAPPING FORMS 151

of the Rio Grande or the Gulf of Mexico. Between the two
lines thus roughly denned is a band of country about 1 ,200-1 ,300
miles long and 300-400 miles wide, which contains some normal
birds of each type, but chiefly birds exhibiting the characters of
both, mixed together in various and irregular ways. Even in
the areas occupied by the pure forms occasional birds are re-
corded with more or less indication of characteristics of the other
form, but within the area in which the two forms are conterminous,
the mixed birds are in the majority. The condition of these birds
of mixed character is described by Allen as follows :

"As has been long known — indeed, as shown by Baird in
1858 — the 'intermediates' or 'hybrids' present ever-varying
combinations of the characters of the two birds, from individuals
of C. auratus presenting only the slightest traces of the char-
acters of C. cafer, or, conversely — individuals of C. cafer present-
ing only the slighest traces of the characters of C. auratus —
to birds in which the characters of the two are about equally
blended. Thus we may have C. auratus with merely a few red
feathers in the black malar stripe, or with the quills merely
slightly flushed with orange, or C. cafer with either merely a few
black feathers in the red malar stripe, or a few red feathers at
the sides of the nape, or an incipient, barely traceable scarlet
nuchal crescent. Where the blending of the characters is more
strongly marked, the quills may be orange-yellow or orange-red,
or of any shade between yellow and red, with the other features
of the two birds about equally blended. But such examples are
exceptional, an unsymmetrical blending being the rule, the two
sides of the same bird being often unlike. The quills of the tail,
for example, may be part red and part yellow, the number of
yellow or red feathers varying in different individuals, and very
often in the opposite sides of the tail in the same bird. The
same irregularity occurs also, but apparently less frequently,
in the quills of the wings. In such cases the quills may be mostly
yellow with a few red or orange quills intermixed, or red with a
similar mixture of yellow. A bird may have the general coloura-
tion of true cafer combined with a well-developed nuchal crescent,
or nearly pure auratus with the red malar stripes of a cafer.

152 PROBLEMS OF GENETICS

Sometimes the body plumage is that of C. auratus with the head
nearly as in pure cafer, or exactly the reverse may occur. Or we
may have the general plumage as in cafer with the throat and
crown as in auratus, and the malar stripe either red or black,
or mixed red and black, and so on in almost endless variations,
it being rare to find, even in birds of the same nest, two indi-
viduals alike in all their features of colouration. Usually the
first trace of cafer seen in auratus manifests itself as a mixture of
red in the black malar stripe, either as a few red feathers, or as a
tipping of the black feathers with red, or with merely the basal
portion of the feathers red. Sometimes, however, there is a
mixture of orange or reddish quills, while the malar stripe remains
normal. In C. cafer the traces of auratus are usually shown by a
tendency to an incipient nuchal crescent, represented often by
merely a few red-tipped feathers on the sides of the nape; at
other times by a slight mixture of black in the red malar stripe."
Such a state of things accords very imperfectly with expecta-
tions under any received theory of Evolution. As in some of
the instances discussed in the first chapter we have here two
fairly definite forms, nearly allied, which on any evolutionary
hypothesis must have been evolved either the one from the other,
or both from a third form at a time not very remote from the
present, as time must be measured in evolution. Yet though
intermediates exist in some quantity, no one can for a moment
suggest that they are that definite intermediate from which
auratus and cafer descend in common. One cannot imagine that
the immediate ancestor of these birds was a mosaic, made up of
asymmetrical patches of each sort: but that is what many of the
intermediates are. It is not much easier to suppose the ancestor
to have been a nondescript, with a compromise between the
developed characters of each, with quills buff, malar stripes
neither black nor red, with a trace of nuchal crescent, and so on.
Such Frankenstein-monsters have played, a considerable part in
the imaginations of evolutionary philosophers, but if it were
true that there was once a population of these monsters capable
of successful existence, surely they should now be found as a
population occupying the neutral zone between the two modern

OVERLAPPING FORMS 153

forms. Yet, though much remains to be done in clearing up the
facts, one thing is certain, namely that the neutral zone has not a
definite and normally intermediate population, but on the con-
trary it is peopled by fragments of the two definite types and
miscellaneous mongrels between them.

On the other hand, one cannot readily suppose that either
form was the parent of the other. The process must have
involved both addition and loss of factors, for whatever hypoth-
esis be adopted, such changes must be supposed to have occurred.
A careful statistical tabulation of the way in which the characters
are distributed in the population of the mixed zone would be
of great value, and till that has been done there is little that can
be said with certainty as to the genetics of these characters.
In the collection of Dr. Bishop of New Haven I was very kindly
allowed to examine a sample, all taken at random, near together,
in Saskatchewan. There were females 4 adult, 2 young; males
4 adult and 5 young. This number, though of course insufficient,
is enough to give some guide as to the degree of definiteness which
the characters generally show in their variations. Of the 15
birds, 8 had simply yellow quills; 2 had red; I was almost red
but had one yellow tail-quill; 3 were intermediate and I was
buff. As regards the malar patch, which can only be determined
properly in the adult males, I was red, I was approximately red,
2 intermediate. As to nuchal crescent 4 females had none, 2
females very slight; 7 males had it, I had only a slight crescent,
and I had none. In point of quills therefore 10 were definite
out of 15; in point of crescent, n were definite out of 15; and in
point of malar patch I only was definite out of 4. The last is a
feature directly dependent on age and so counts for less, but as
regards the other two features there is some indication that the
factors show definiteness in their behaviour. It must be re-
membered that we have no knowledge what the heterozygous
form may be, and in the case of red and yellow it is probably a
reddish buff. The patch-works are no doubt to be compared with
other well-known pied forms, and in these we must suppose the
active factor broken up, which it probably can be very easily.
The asymmetry, which Allen notices as so marked a feature, in the

154 PROBLEMS OF GENETICS

distribution of the red and yellow quills of the tail especially,
recalls that of the black markings in the pied Canaries. As is well
known to students of variations some pigment-factors in some
animals are apparently uncontrolled by symmetry, while in other
specific cases symmetry is the rule. On the other hand the
blackness or redness of the malar patches is, I think, as a rule
nearly symmetrical. It should be mentioned that two of Dr.
Bishop's young birds belonged to the same nest, one a female
with red quills, the other a male with yellow. Both are without
crescent.

As to the question whether certain combinations of characters
occur with special frequency, the evidence is insufficient to give a
definite answer. Among all the birds I have seen in America
or in England I have not yet found one having the malar patches
black without any nuchal crescent. Of Dr. Bishop's 8 adults
not one, however, showed the combination of the three chief
features normal for auratus or for cafer.

Besides the two forms that we have hitherto considered,
several other local types exist, and these throw some further
light on the problem. Of these the most important in this
connexion is chrysoides, which inhabits the whole of southern
California and the mainland opposite. This remarkable form
is as Allen says, very different from auratus except that it has
the quills yellow like auratus, not red like cafer. So that we
find here in the extreme west of the whole distribution a type
agreeing in one of its chief features with the eastern type. Be-
tween this and cafer intergrades have, according to Allen, not
been found. The relations of this chrysoides are, Allen thinks,
rather with mexicanoides , a southern, smaller race with colours
more intense, which inhabits Guatemala, but however that may
be, it must be regarded as a cafer which has lost its red quills.
The island of Guadeloupe off Lower California has an island
form. Beyond the other side of the continent there is also an
island form of auratus, inhabiting Cuba, so that clearly the yellow
quills can extend into the tropics.

The above account is in many respects incomplete, but it
suffices to give an outline of the chief facts. The whole problem

OVERLAPPING FORMS 155

is complicated by the undoubted effects of an uncertain amount
of migration, and in many, perhaps all, districts, the winter
population differs from the summer population of the same
localities. The existence of these seasonal ebbs and flows is
now well known to ornithologists, and most of the bird species
of temperate regions are subject to them.

Difficult as it may be to conceive the actual process of origin
of the two types auratus and cafer, it is I think still harder to
suggest any possible circumstance which can have determined
their development as distinct races, or which can maintain
that distinctness when created. Some will no doubt be disposed
to appeal once more to our ignorance and suggest that if we only
knew more we should see that the yellow quills, the black
"moustache" and the red crescent, specially qualify auratus
for the north and eastern region, and the red quills, red "mou-
stache" and absence of crescent fit cafer to the conditions of
its homes. Each can judge for himself, but my own view is
that this is a vain delusion, and that to cherish it merely blunts
the receptivity of the mind, which if unoccupied with such fancies
would be more ready to perceive the truth when at last it shall
appear. Think of the range of conditions prevailing in the
country occupied by auratus — a triangle with its apex in Florida
and its base the whole Arctic region of North America. Is it
seriously suggested that there is some element common to the
"conditions" of such an area which demands a nuchal crescent
in the Flickers, though the birds of the cafer area, almost equally
varied, can dispense with the same character? Curiously enough,
the geographical variation of Sphyropicus varius, another though
a very different Woodpecker6 shows that conversely the nuchal
crescent can be dispensed with in the Eastern form though it
is assumed by the Western.7

8 The Sap-suckers feed on trees and somewhat resemble our Spotted Wood-
peckers in general appearance. Colaptes feeds on the ground and corresponds
perhaps rather with the European Green Woodpecker.

7 For an introduction to this example I am indebted to Mr. W. D. Miller of
the American Museum of Natural History. Some account of the facts is given by
Baird, Brewer, and Ridgway (A Hist, of N. Amer. Birds, 1874, II, pp. 540, 544,
etc.). 5. -varius occupies the whole country in suitable places from the Atlantic
to the eastern slopes of the Rockies, and all Mexico to Guatemala. 5. nuchalis was

156 PROBLEMS OF GENETICS

Allen points out the interesting additional fact that super-
posed upon each of the two distinct forms, auratus and cafer,
are many geographical variations which can very naturally be
regarded as climatic. Each decreases in size from the North
southward, as so many species do.8 They become paler in the
arid plains, and show the ordinary phases which are seen in
other birds having the same distribution. Such differences
we may well suppose to be determined directly or indirectly, by
environment, and we may anticipate with fuller knowledge it
will be possible to distinguish variations of this nature as in the
broad sense environmental, from the larger differences separating
the two main types of Colaptes, which I surmise are altogether
independent of such influences.

It is generally supposed that phenomena like those now so
well established in the case of Colaptes are very exceptional, and
as has already been stated a number of circumstances must
combine in order that they may be produced. I suspect however
that the examples are more numerous than is commonly thought.
In all likelihood the three forms Sphyropicus varius, nuchalis
and ruber are in a very similar condition though the details
have not, so far as I know, been worked out. A complex example
which is closely parallel to the case of Colaptes was described by
F M. Chapman9 at the same date as Allen's work. This is the
case of Quiscalus, the Crackles, which in the North American
Continent have three fairly distinct forms which Chapman speaks
of as Q. aeneus, Q. quiscula, and Q. quiscula aglaeus. The birds
are all, so far as pigment is concerned, dark blackish brown, but
the head and mantle have superposed a metallic sheen of inter-
ference-colours which in the various forms take different tints,

first known from the Southern Rockies only, but many were afterwards taken in
Utah. 5. ruber is restricted to the Pacific coast. In Ridgway's opinion all three
are geographical forms of one species. In ruber the sexes are alike having both a
great extension of the red in the throat, and a red crescent. The male of nuchalis
grades to the ruber form, but the female does not. This female has some red in
the throat like the male of varius, whereas the female of varius has a whitish throat.

8 Not only vertebrates but the marine Crustacea and Mollusca illustrate this
curious "principle" of variation, as Canon Norman formerly pointed out to me
with abundant illustrations. There are of course cases to the contrary also.

9 Chapman, F. M., Bull. Amer. Mus., IV, 1892, p. i; see also Ridgway, Birds
of North and Middle America, 1902, Part II, p. 214.

OVERLAPPING FORMS 157

bluish green, bronze green, or bronze purple. The details are
complicated and difficult to appreciate without actual specimens,
but the two common types are sufficiently distinct. The birds
inhabit the whole area east of the Rockies, quiscula aglaeus oc-
cupying Florida and the Southern States southwest of a band
of country about a hundred miles broad extending roughly from
Connecticut to the mouth of the Mississippi; and aeneus taking
the area north and west of this band. In discussing this case
Chapman expresses the same view as Allen does in the Colaptes
case, that there are two distinct populations, substantially fixed,
and that the band of country in which they meet each other
has a mongrel population, with no consistent type, but showing
miscellaneous combinations of the character of the two chief
types.

The warblers of the genus Helminthophila provide another
illustration which has points of special interest. The two chief
species are H. pinus, which has a yellow mantle and lower parts,
white bars on the wings, a black patch behind the eyes and a
broad black mark on the throat ; and H. chrysoptera with dark grey
mantle and pale whitish grey lower parts, yellow bars on the
wings, and grey marks on cheeks and throat where pinus has
black. These two birds are exceeding distinct, and in addition
their songs are quite unlike. H. pinus ranges through the eastern
United States up to Connecticut and Iowa. H. chrysoptera is a
northern form extending down to Connecticut and New Jersey.
Both are migrants.

In these two States, where the two types overlap, certain
forms have been repeatedly found which have been described as
two distinct species, Lawrencei and leucobronchialis. Dr. L. B.
Bishop and Mr. Brewster showed me two long series of Hel-
minthophila containing various intergrades between the four
named kinds, and details regarding these may be found in
Chapman's North American Warblers and in Dr. Bishop's paper
in Auk, 1905, XXII. Though the characters evidently break
up to some extent, the series can be represented as due to re-
combinations of definite factors more easily than the others
which I have described. The differentiating characters are:

158 PROBLEMS OF GENETICS

Pinus Chrysoptera

1. Mantle and lower parts yellow (Y1). i. Mantle and lower parts grey (y1).

2. Wing-bars white (y2). 2. Wing-bars yellow (Y2).

3. Cheek and throat not black (b). 3. Cheek and throat black (B).

The grey pigment of the mantle is common to both, but is
masked by the yellow in pinus, the net result being an olive-
green.10

I am much indebted to Dr. F. M. Chapman for the loan of
the coloured plate in which these distinctions are shown. It first
appeared in his book, North American Warblers.

We cannot tell whether yellow or not-yellow is due to the
presence of a factor, but we may suppose that one or other
gives the special colour to the parts. The black of character 3
is no doubt a dominant. Thus pinus becomes Yxy2b and chry-
soptera in y1Y2B. The Lawrencei which has the underparts
yellow, wing-bars white, and black patches is Yxy2B and leuco-
bronchialis which has mantle and underparts not-yellow, wing-
bars yellow and no black patches is y1Y2b. This representation,
it should be clearly understood, is tentative and approximate
only. The characters are not really sharp, for there is much
grading ; but allowing for the effects of heterozygosis and for some
actual breaking-up of factors I believe it gives a fairly correct
view of the case. In particular we can see how it meets the dif-
ficulty which Chapman felt in accepting leucobronchialis as in
any sense derived from pinus which has a yellow breast, and
chrysoptera which has a black throat, seeing that leucobronchialis
has neither. We now recognize at once that this form could be
produced by ordinary re-combination of the absence of Y1 with
the absence of B.

I note also with great interest that the modern observers
agree that the so-called hybrids may have the song either of
the one species, or of the other, or a song intermediate between
the two. It may also be added that these two types have several

10 It would aid greatly in factorial analysis if the descriptive term "green"
could be avoided in application to cases where the green effect is due only to a mix-
ture of black and yellow pigments. The absence of yellow is the sole difference
between the mantle and underparts of pinus and chrysoptera.

FIG. i. fltlminthophila pinus, male.

FIG. 2. fIdmi>:lJiophila pinus, female.

FIG. 3. "Lawrence's Warbler," male; one of the integrading forms.

FIG. 4. "Brewster's Warbler," male; another of the intergrading forms

FIG. 5. Helminthopkila chrysoptera, male-.

FIG. 6. Helminthophila chrysoptera, female.

L

if

f.C I/

158 PROBLEMS OF GENETICS

Pinus Chrysopiera

z. Mantle and lower parts yellow (Y1). z. Mantle and lower parts grey (y1).

a. Wing-bars white (y*). 2. Wing-bars yellow (Y1).

3. Cheek and throat not black (b). 3. Cheek and throat black (B).

The grey pigment of the mantle is common to both, but is
masked by the yellow in pinus, the net result being an olive-
green.10

I am much indebted to Dr. F. M. Chapman for the loan of
the coloured plate in which these distinctions are shown. It first
appeared in his book, North American Warblers.

We cannot tell whether yellow or not-yellow is due to the

presence of a factor, but we may suppose that one or other

gives the special colour to the parts. The black of character 3

•Moubt a dominant. Thus pinus becomes Yly2b and chry-

The Laivrencei which has, the underparts

white, and

.anno* anH>sis*m srf* to one ;9iBm -.-wid-i

i arfi to liftoius ;<A&m ".-wld -r:77 J

.01^

.g .

.^osis ana for some

,d .01*5

airy correct

se. In particular we can see how it meets the dif-
ficulty which Chapman felt in accepting leucobronchialis as in
any sense der which has a yellow breast, and

chrysoptcr throat, seeing that leucobronchialis

has neither. We now recognize at once that this form could be
produced by ordinary re-combination of the absence of Y1 with
the absence of B.

I note also with great interest that the modern observers

that the so-called hybrids may have the song either of

the one species, or of the other, or a song intermediate between

the two. It may also be added that these two types have several

would aid greatly in factorial analysis if the descriptive term "green"
could be avoided in application to cases where the green effect is due only to a mix-
ture of black and yellow pigments. The absence of yellow is the sole difference
•* en the mantle and underparts of pinus and chrysoptera.

OVERLAPPING FORMS 159

times been seen, in the breeding season, paired with each other
or with one of the other combinations.

Allen11 has described another excellent American example,
the Tits of the group Baeolophus bicolor -atricristatus. The form
bicolor belongs to the eastern States and ranges from the Atlantic
coast to the Great Plains, and atricristatus, of east Mexico, ex-
tends from Vera Cruz to central Texas. In southern and
central Texas the breeding ranges adjoin, and in this country
various intermediates occur. The chief types differ in two main
points.

B. bicolor B. atricristatus

Forehead varies from deep black to dull Forehead white to huffish white.

black, suffused with rusty brown.

Crown and crest grey, slightly darker Crown and crest black, abruptly con-

than the black. trasting with the back.

The intergrades between the two have, as usual, received specific
names. A detailed description is given by Allen, from which
it appears that the gradation is very complete. In one case a
series of 16 adults were all intermediates. It is not stated whether
the collector took these at random, but from the local lists it is
clear that the types are found not far away from the place where
the intergrades were shot.

Another very striking case is that of the Tanagers, of the
genus Rhamphocoelus. In this group there are several local forms
which are related to each other in remarkable ways. The forms
known as passerinii and icteronotus exhibit the clearest phenomena
of intergradation. The species passerinii has a brilliant scarlet
and black male, and it inhabits Honduras and Nicaragua.
Proceeding southwards along the isthmus we find next costari-
censis which has a male like that of passerinii (but a female
with more orange than the olive-grey female of passerinii).
Next we come to Panama which is occupied by icteronotus,
sharply distinguished from passerinii by the fact that the scarlet
is replaced by lemon-yellow. This same icteronotus occurs again
as a pure type in Ecuador and many other parts of South America;
but Colombia, between Panama and Ecuador, contains scarlets
like passerinii, yellows like icteronotus, and various intergrades

11 Bull. Amer. Mus. Nat. Hist., XXIII, 1907, P- 46?-

160 PROBLEMS OF GENETICS

of several shades of orange. The passerinii males from Nica-
ragua are indistinguishable from those of Colombia, and the
icteronotus of Ecuador are the same as those in Panama. The
orange intergrades, doubtless heterozygous forms, though col-
lected at the same locality (Medellin in Colombia) as several
pure yellows and pure scarlets, are in the British Museum series
sorted out as a separate species under the name chrysonotus!
Complications are introduced by the relations of these forms to
another named type, flammigerus, but we may for our purpose
leave that out of consideration, and say that the order of geo-
graphical sequence from Honduras to Ecuador is (i) scarlet,
(2) yellow, (3) mixture of types, scarlet, yellow, orange, (4)
yellow.

Similar examples exist in the birds of the old world, but I do
not know of any that have been studied so fully as those of
America. The best known is that of the two Rollers, Coracias
indicus which spreads from Asia Minor through Persia, Balu-
chistan, the Indian Peninsula and Ceylon, and affinis which
ranges from Nepal, through Assam, Tenasserim and the Indo-
Chinese countries. The two types are very different and may be
distinguished as follows:

C. indicus C. affinis

Mantle drab brown-chestnut. Dark olive-green.

Breast chestnut. Dull purple brown.

Throat purplish, streaked with white. Purple, streaked with blue.

Upper tail-coverts indigo. Turquoise.

The wings are the same in both. In the provinces of Nepal,
Sikhim, and Darjiling the two species coexist, with the result
that intergrades have been frequently recorded. The line of
intergradation extends to the coast, and birds showing various
combinations of the two types from the Calcutta district exist
in collections.12 The case is interesting inasmuch as like that of
Quiscalus it shows a series of combinations of various metallic
colours. Some of these are probably evoked by the development
of pigment behind striations or other interferences already exist-
ing, but in the present state of knowledge it would be quite im-

12 References on this subject will be found in Brit. Mus. Cat. Birds, XVII, p. 13.

OVERLAPPING FORMS 161

possible to suggest what the actual factors producing these ap-
pearances may be.

There are, naturally, many other cases among birds which
are suspected of being in reality comparable, but in most of them
the evidence is still inadequate. Among Lepidoptera also there
are a few of these ; perhaps the most striking is that of Basilarchia
"proserpina."1* The genus is well known to European col-
lectors under the name LimenitiSj of which we in England have
one species, L. sibylla, the "White Admiral." A species very
like sibylla in general appearance is common in the northern
parts of the United States, ranging through Canada and Northern
New England, but rarely south of Boston. This species has
the conspicuous white bands across both wings like our sibylla.

There is also a more Southern type known as astyanax,
which is very different in its appearance, being without the white
bands and having a broad irroration of blue scales on the posterior
border of the hind wings. The two are so distinct that one would
not be tempted to suspect any very close relation between them.
In its distribution astyanax is described by Field as replacing
arthemis south of latitude 42°. About Boston it is much more
common than arthemis.

The two forms encroach but little on each other's territory,
but where they do coexist, a third form, known as proserpina, is
found which is almost intermediate, with the white bands much re-
duced. There is now no doubt that this proserpina is a hetero-
zygous form, resulting from a combination of the characters of
arthemis and astyanax. Field succeeded in rearing a brood of 16
from a proserpina mother caught wild which laid 31 eggs, and of
these, nine (five males, four females) resembled the mother,
being proserpina, and seven (four males, three females) were
arthemis. There can be no question therefore that the mother
had been fertilised by a male arthemis and that no-white-band
is a factor partially dominant over the white band. Another
point of interest which Field observed was that the proserpina
female refused to lay on birch, poplar or willow, but accepted

13 For these facts I am indebted to Mr. W. L. W. Field, who has lately published
an account of his observations and experiments. See especially, Psyche,
XVII, No. 3, where full references to previous publications are given.

12

162 PROBLEMS OF GENETICS

wild cherry (Prunus serotina) a species on which astyanax can
live, though that tree is not known to be eaten by arthemis.
Incidentally also the observations show that sterility cannot be
supposed to be the bar which maintains the distinctness of
arthemis and astyanax.

In this connection Papilio oregonia and bairdii should be
mentioned.14 P. oregonia is one of the numerous forms like
machaon, but rather paler. It is a northern insect, inhabiting
British Colombia east of the Cascade Range, and reaching to
Colorado. P. bairdii is a much darker butterfly, representing
the asterias group of the genus Papilio. Like asterias it has the
abdomen spotted at the sides, not banded as in the machaon
group. It belongs to Arizona and Utah extending into Colorado.
From Colorado the form brucei is described, more or less inter-
mediate, like bairdii but with the abdomen banded as in oregonia*
W. H. Edwards records the results of rearing the offspring of the
bairdii-\ike and of the oregonia-\ike mothers. Each was found
able to have offspring of both kinds, that is to say, bairdii
females gave both forms, and oregonia females gave both forms.
It is not possible to say which is dominant, since the fathers were
unknown. On general grounds one may expect that the bairdii
form will be found to dominate, but this is quite doubtful.

From this particular discussion I omit reference to those
examples in which the permanently established types are ob-
viously associated with special conditions of life. Where con-
siderable climatic differences exist between localities, or when we
pass from South to North, or from the plains into Alpine levels
we often find that in correspondence with the change of climate
there is a change in the characteristics of a species common to
both. When I say "species " in such a connection I am obviously
using the term in the inclusive sense. Some would prefer to
say that in the two sets of conditions two representative species
exist. Whichever expression be preferred it is plain that such
examples present another phase of the problem we have been
just considering, and in them also we have an opportunity of

14 For the facts and further references see W. H. Edwards, Butterflies of N.
America, 2d series, Papilio VII and X; 3d series, 1897, Papilio IV, Can. Entom.t
1895, XXVII, p. 239.

OVERLAPPING FORMS 163

observing the consequences of the overlap of two closely related
types, but there are advantages in considering them separately.
In the examples hitherto given, with the possible exception of
the Papilios,15 the two fixed types severally range over so ex-
tensive a region that it may fairly be supposed that in the dif-
ferent parts they are subject to considerable diversities of climate.
There is no outstanding difference that we know distinguishing
the habitats of the two forms; but in comparing Alpine with
Lowland forms, or essentially northern with essentially southern
forms we do know an external circumstance, temperature, that
may reasonably be supposed to have an influence, direct or indi-
rect, on the population.

15 1 think this case is fairly included because the machaon type is so widespread
that it cannot be regarded as a product of a Northern climate, nor can asterias be
claimed as especially a warm country form, seeing that brevicauda, which is scarcely
distinguishable from asterias, inhabits Newfoundland (having a curious phase
there in which the yellow is largely replaced by red).

CHAPTER VIII

LOCALLY DIFFERENTIATED FORMS. Continued

CLIMATIC VARIETIES

In this chapter we will examine certain cases which illustrate
phenomena comparable with those just considered, though as
I have already indicated, they form to some extent a special
group. The outstanding fact that emerges prominently from
the study of the local forms is that when two definite types,
nearly allied, and capable of interbreeding with production of
fertile offspring, meet together in the region where their dis-
tributions overlap, though intergrades are habitually found,
there is no normally or uniformly intermediate population oc-
cupying the area of intergradation. Such phenomena as these
must, I think, be admitted to have great weight in any attempt
to construct a theory of evolution. True we must hesitate in
asserting their positive significance, but I see no escape from the
conclusion that they throw grave doubt on conventional views.
Again and again the same question presents itself. If A and B
lately emerged from a common form why is that common form
so utterly lost that it does not even maintain itself in the region
of overlapping? Almost equally difficult is it, in the cases which
I have numerated, to apply concrete suggestions based on any
factorial scheme. We may see that in Heliconius erato the type
with the red mark on the hind wing probably contains a dominant
factor, and that where the red mark is absent the metallic colours
are exposed ; and that similarly the green metallic colour may have
another factor which distinguishes it from the blue. In this way
we can fairly easily represent the various types of erato on a
factorial system as the result of the various possible combinations
of two pairs of factors. But there we stop, and we are quite
unable to suggest any reason why one area should have the red
and the green type while another should have the blue also. So
again with Colaptes or the Warblers. By application of a fac-

164

CLIMATIC VARIETIES 165

torial system, admittedly in a somewhat lax fashion, the genetic
interrelations of the types can be represented; but how it comes
about that each type maintains a high degree of integrity in
its own region we can only imagine. Each has in actual fact
a stability which the intermediate forms have not, but we cannot
yet analyse the nature of that stability. Mendelian conceptions
show us how by segregation the integrity of the factors can be in
some degree maintained, but not why certain combinations of
factors should be exceptionally stable. All that is left us to
fall back on is the old unsatisfying suggestions that some com-
binations may have greater viability than others, that there
may be a tendency for like to mate with like, and so forth.

These difficulties acquire more than ordinary force in those
cases in which the two fixed types inhabit regions differing in
some respect so obvious and definite that we are compelled to
regard each type as climatic and as specially adapted to the
conditions. When for example an animal has a distinct type
never met with except in Arctic or Alpine conditions, and another
type proper to the plains and temperate regions, what are the
characteristics of the population of intermediate latitudes or
at intermediate levels? Some of the examples discussed in the
last chapter may be instances of this very nature, but even if
they are not, others are forthcoming which certainly are. The
evidence of these cases leads to the suspicion that with further
knowledge they will be found to consist of two classes, some in
which the observer as he passes from the one climate to the other
will find the intermediate area actually occupied by a population
of intermediate character, and others in which, though we may
presume the maintenance of intermediate conditions in the tran-
sitional area, there is no definite transitional population. This
interrupted or discontinuous distribution seems, so far as I
have means of judging, to be by far the more common of the two.
I do not doubt that by sufficient search individuals representing
every or almost every transitional form can be found, but it is
apparently rare that populations corresponding to these several
grades can be seen. The question has in few if any cases been
studied with precision sufficient to provide a positive answer;

166 PROBLEMS OF GENETICS

but I suspect that real and complete continuity, in the sense
thus denned, will only be found where the character of the local
populations depends directly on the conditions of life, and shows
an immediate response to changes in them apart from that post-
poned response which we suppose to be achieved by selection.
Obviously the character must be one, like size for instance,
capable of sensibly complete gradation.

The only example I have met with of the phenomenon of
anything like a complete intergradation between local types
really distinct in kind is that provided by the butterfly Pararge
egeria. It is well known to entomologists that this insect exists
in two very different types, a northern one, the "Speckled Wood "
of England, in which the spots are a pale whitish yellow, and a
southern type having the full fulvous colour that we know as
characteristic of megaera, the "Gatekeeper." It appears that
Linnaeus gave the name egeria to the southern type,1 and our
own is now called egerides. Broadly speaking, so far as Great
Britain, France, and the Spanish Peninsula are concerned, the
tawny-coloured egeria occupies Spain and western France up
to the latitude of Poitiers and the pale yellow egerides extends from
Scotland, where it has a scanty distribution, through southern
England, where in suitable localities it is common, and the north
of France to Paris.2 The two types when placed side by side
are strikingly different from each other, and are an excellent
illustration of what is meant by climatic variation. The insect
is not a great traveller and probably scarcely ever wanders far
from its home. It should therefore be possible by collecting
from north to south to find out how the transition is effected,
whether suddenly or gradually. This at various times I have
endeavoured to do, but I am still without exact information as
to the population in certain critical areas. In addition to the
information derived from specimens which I have collected or
seen in the collections of others there is a good account of the
general distribution in Europe given by the Speyers,3 who evi-

1 Often referred to by older writers as Meone, Esper's name.

2 There are also two distinct island forms, unlike the European, Xiphia of
Madeira, and a smaller variety, Xiphioides of Canary. See especially, Baker,
G. T., Trans. Ent. Soc. London, 1891, p. 292.

8 Speyer, Adolf, and August. Verbreitung der Schmetterlinge, 1858, I, p. 217.

CLIMATIC VARIETIES 167

dently paid more attention to the subject than most lepidopterists
have done, and many more recent records. In particular
Oberthiir4 has published many details as to the distribution in
western France and I am especially indebted to Mr. H. Rowland-
Brown for a long series of notes as to the distribution in France
generally, and to Mr. H. E. Page and Dr. T. A. Chapman, Mr.
Oberthur Prof. Arrigoni degli Oddi, Mr. H. Williams and other
correspondents, for showing me forms from many localities. The
butterfly is attached for the most part to woods of deciduous trees
and to country abounding in tall hedges or rough scrub. It is not
usually to be found in highly cultivated districts or in very dry
regions. Hence there is necessarily some want of continuity in
the distribution at the present time and I should think a mile or
two of arable land without big hedges would constitute a barrier
hardly ever passed. The larva feeds on several coarse grasses,
especially Dactylis glomerata. Barrett mentions also Triticum
repens. In this country the winter is usually passed in the larval
stage, but I have found that in captivity, at least, there is much
irregularity. The larvae feed whenever the weather is not very
cold and may pupate, but if sharp cold comes on when they are pu-
pating or nearly full-grown they often get killed unless protected.

Some writers speak of a difference between the early and
later broods, but I have never noticed this, and I do not think
that the general tone of the yellow is affected by the seasons
(see Tutt, Ent. Rec., IX, 1897, p. 37).5

Beginning at the south of Spain the thoroughly fulvous type
egeria is common at Gibraltar in the Cork woods, at Granada,
and doubtless generally. Lederer is said to have found only
this type in Spain (Speyer), and though I have no precise in-
formation as to other places in the Peninsula north of Jaen I feel
tolerably sure that there is no change from south to north.8

4 Lepid. Compare, fsc. Ill, p. 372.

6 Mr. Rowland-Brown has called my attention to a statement by Dr. Vaillantin
(Petites Nouv. Enl., II, 235) that in Indre-et-Cher the first brood is of the northern
type and the second of the southern. My experience is that in captivity these
distinctions do not occur, and I have true egeria as first brood from Vienne and as
the late brood from the Landes. I never collected in Indre-et-Cher.

6 1 have since seen true egeria from Ferrol in the extreme northwest, which was
in Mr. Tutt s collection.

168 PROBLEMS OF GENETICS

Immediately north of the Pyrenees we still meet egeria exclusively,
and up to Poitiers at least there is no noticeable change. But
somewhere between Poitiers and the bottom of the Loire valley
at Tours, the genuine southern type comes to an end, and the
whole population begins at the Loire to be of an intermediate
type, easy to distinguish both from egeria and from egerides. As
to the exact condition of the species in the fifty miles separating
St. Savin on the Vienne from places on the Loire I have no ade-
quate information. I have only one small sample from there,
but it does contain insects both of the southern and intermediate
types taken on the same day, in a wood near Preuilly. Oberthur
also states that at Nantes the true southern form exists in com-
pany with the northern. From this I infer that the southern
form extends up the coast further than it does inland, but I
imagine the representative spoken of as northern would be of
usual Brittany or intermediate type.

The Vienne river joins the Loire, so the true southern type
reaches over into the basin of the Loire. From the Loire (Tours,
Corm6ry) north to Calvados (Balleroy) only the intermediate
is found, so far as I know, and the same type extends over
Brittany.7 In general, however, the woods near Paris have the
thoroughly northern type egerides, but at St. Germain-en-Laye
and at Etampes (Oberthur) the population approaches the
intermediate type.

On the whole the intermediate type is certainly less homo-
geneous than either of the extremes, and females with the two
central spots either paler or more fulvous than the rest are not
uncommon, but I have never taken one on the Loire or in Brit-
tany which I should class with either of the extreme types.

Before speaking of the distribution in other parts of France
and in Europe generally I will briefly state the results of my breed-
ing experiments. The work was done many years ago before
we had the Mendelian clue, and it is greatly to be hoped that
some one will find opportunities of repeating it. Crossing the
English and the thoroughly southern type the families produced

7 Mr. G. Wheeler kindly showed me a series identical with this type, from
Guernsey, and others from near Laon.

CLIMATIC VARIETIES 169

agree entirely with the intermediates of Brittany and the Loire.
Reciprocals are alike. Of F2 I only succeeded in raising very
few and of those that I had (about 30) nearly all were intermediate
in character, though perhaps rather less uniform than FI. One
family alone, containing only 4 specimens, had one egerides,
and three fulvous intermediates. As the case stands alone I
hesitate whether or not to suppose it due to some mistake. More-
over from FI crossed back with the respective parental types I
had fairly long series, especially from FI X the southern type, and
looking at these families I cannot see any clear evidence of segre-
gation. On the contrary, I think that though there are slight
irregularities, they would, taken as a whole, be classed as coming
between the intermediate type and the extreme form used as the
second parent. This at least is true when the second parent was
of the southern type.

On this evidence I have regarded the case as one in which
there is no good evidence of segregation and as conforming most
nearly with the conventional view of gradual transition in re-
sponse to climatic influences. Such influence must however be
indirect; for I reared five generations of the northern type in
England, and these, though they included several abnormal-
looking specimens in the last generation and then died out, did
not show any noticeable change from the fulvous colour of the
wild type. Merrifield8 also found that heat applied to pupae
of the northern type produced no approach to the southern
type.

Looking at the facts now in the light of more experience it
seems to me just possible that the case may be one in which, as
in Nilson-Ehle's Wheats, the dominant differs from the re-
cessive in having two pairs of factors with similar effects. The
fulvous type for example may have two or more elements in
separate pairs which together produce the full effect, and the
intermediate may have one of these. If this were so, some
segregation should of course eventually be observable, but the
proportion of the various fulvous and fulvous-intermediate
individuals would be large, and the reappearance of actual repre-

8 Ent. Rec., V, 1894, p. 134.

170 PROBLEMS OF GENETICS

sentatives of the northern type might be rare. I admit that this
is a somewhat strained interpretation of the facts, and as yet it
is not entitled to serious consideration. Nevertheless I am led
to form some such expectation partly from the great difficulty
in the way of any other, partly from the evidence of the small
mixed sample found at Preuilly and partly from the statements
given by Oberthiir. There are moreover other features in the
general distribution of the species which make it improbable
that the dependence on climate can after all be so close. Pub-
lished lists are unfortunately of little use in deciding which form
occurs at a particular place, because, since the name Meone has
ceased to be used for the southern form, there is no complete
unanimity among authors as to the application of the names
egeria and egerides, and unless more particulars are given, either
name may be used for either form. Besides this, difficulty arises
from the fact that the intermediate type is not generally dis-
tinguished at all, and English collectors finding it, may easily
record it as the southern type. From Staudinger's note on the
distribution, I gather that he, on the contrary, reckoned the
intermediate with the northern type, as do the Speyers also.
The late Mr. J. W. Tutt was careful to distinguish the three
forms and has left several useful records. Easy therefore as it
might seem to be to make out the distribution of such a familiar
insect in its various modifications, there are serious practical
difficulties, and until long series are brought together with this
special object in view many obscurities will remain.

With only the series from England, the west of France, and
Spain before one it would be easy to regard the successive series
of tones as a fair measure of climate ; the brighter the colour, the
hotter might one expect the locality to be. Such rough corre-
spondence is often to be observed in butterflies and birds. It
becomes impossible to take these simple views in the light of
more complete knowledge. Beginning with France the fulvous
egeria occupies the lower valley of the Rhone, probably from well
above Lyon, though I have no exact information respecting the
country above Avignon. According to Speyer it also takes the
department of Lozere. The same authority says that Puy-de-

CLIMATIC VARIETIES 171

Dome has "egeria" meaning perhaps the intermediate form,
with the fulvous form much less commonly. Next comes the
curious fact that though the Lower Rhone (Avignon, Tarascon,
Nimes) has the true fulvous form, Hyeres, Cannes, Grasse, Nice,
Digne, and Alassio have the intermediate. Savoy has the inter-
mediate (Chambery) and even egerides perhaps, though in the
same latitude on the west of France there is nothing but the
fulvous type. At Chalseul and Besangon (Doubs) the ordinary
northern type is found. Switzerland generally, I believe, has the
northern type, but Staudinger gives egeria for Valais and the
intermediate occurs in Vaud.9 The south side of the Alps has
probably colonies of the pale egerides, and of intermediates.
Orta, with a very hot summer, has the English type (Tutt, Ent.
Rec., XII, 1900, p. 328). Locarno has the intermediate (ibid.,
XV, 1903, p. 321). North Italy in general and western Pied-
mont have the intermediate; but further south egeria begins,
at what region I do not know. Speyer gives on his own authority
the remarkable statement that at Florence both extremes occur,
but chiefly intermediates between the two. Mr. R. Verity
however kindly informs me that in his experience this is not so,
and that neither the real southern type nor the northern occur
there. Sardinia, Sicily, Crete all have the southern type.
Greece probably has various types. Staudinger (Hor. Ross., VII,
1870, p. 78) says intermediates resembling Nice types common
everywhere, but from "Greece" the British Museum has a series
that would pass for English specimens; and the same type occurs
near Constantinople. The island of Corfu has a pale inter-
mediate, distinct from egerides but approaching it. In Roumania
all three forms are recorded from various places : egeria in the
Dobrutscha; not quite typical (presumably an intermediate)
at Bukharest; intermediate in various mountainous localities
as well as in Macedonia and Dalmatia; but egerides in Azuga
at about 3,000 feet.10 Hungary has the true egerides also.
(Cf . Caradja, Deut. Ent. Z/., IX, p. 58.) Mathew records the same

9 Mr. Wheeler has some pale but rather worn specimens from the Rhone Valley
at Vernayaz.

10 See Fleck, E., Die Macrolep. Rumaniens, Bui. Soc. Sciinte, VIII, 1899,
p. 720.

172 PROBLEMS OF GENETICS

from Gallipoli (E. M. M., 1881, p. 95). Staudinger does not
distinguish the intermediates from the northern, but he gives
"egerides" for Armenia and Fergana (Central Asia). As against
the mere proximity of a great mountain chain being the influence
which keeps the Riviera population intermediate may be
mentioned the fact that the northern foothills of the Pyrenees
have the pure southern type, and the climate of Cambo must
surely be far cooler than that of Nice. The exact locality of
the Greek specimens is not given, but there can be no part of
Greece which is not much hotter in summer than Brittany, or
Calvados, which have the intermediate, not the English type.
In face of these facts it can scarcely be maintained that
average temperature is the efficient cause of the particular tone
of colour which the butterfly shows 'in a given region. Never-
theless it is clear that climate counts for much in determining
the distribution. It is noticeable that though the pale egerides
can be established in a warm climate we never find egeria in cold
climates, and even the intermediate is not found in places that
have a hard winter. I suspect that the distribution of the
broods through the year and the condition of the animal at the
onset of hard frost are features which really determine whether
a strain can live in a particular place or not. Though the truth
of the suggestion cannot be tested by experiments in captivity,
which at once introduce disturbances, I incline to the idea that
egeria has not got the right periodicity for northern climates.
If it could arrange its life so that the population consisted either
of young larvae, or perhaps of thoroughly formed pupae11 at
the onset of winter, it might, for any obvious reason to the
contrary, be able to live in England. It is irregularly "poly-
voltine," as the silk-worm breeders say, and as soon as a little
warmth encourages it, a new generation starts into being, which
if the frost comes at an untimely moment, is immediately

11 My experience agrees with that of Mr. H. Williams (Ent. Rec., VIII, 1896,
p. 181) that pupae, well-formed, can stand considerable frost; but I used to find
that half -grown larvae usually died if unprotected, and I believe that larvae which
attempted to pupate in warm autumn weather and then got caught by frosts,
always died. Small larvae which can creep into shelter at the bottom of the plants
survived, and I expect that in the north the winter is usually passed in that state
(see also Merrifield, F., Ent. Rec., VIII, 1896, p. 168, and Carpenter, J. H., ibid.).

CLIMATIC VARIETIES 173

destroyed. Many species are continually throwing off indi-
viduals which feed up fast12 and emerge at once if the temperature
permits, and I imagine a species of Satyrid wholly or largely
represented by such individuals could scarcely survive in a
country which had a hard winter. For such a climate some
definite periodicity in the appearance of the broods may well be
indispensable. But assuming that egeria is cut off from cold
climates for such a reason, there is nothing yet to connect these
habits with the fulvous colour, and until breeding can be carried
out on a satisfactory scale there is no more to be said.

From time to time records appear of individual specimens
more or less fulvous being caught in southern England, especially
in the New Forest.13 It would be interesting to know what
offspring such individuals might produce. From the evidence
now given some notion both of the strength and the weakness
of the case considered as one of continuous climatic variation
can be formed. I know no other equally satisfactory. Whether
or not definite mixture of the intermediates with either of the
extremes will be proved to occur, the case differs materially from
those considered in the last chapter in the fact that at all events
there is no general overlapping of forms. In a species so little
given to wandering, overlapping could indeed scarcely be expected
to occur. It is this circumstance which makes the species
preeminently suitable as a subject for the study of climatic
influences, and I trust that entomologists with the right oppor-
tunities may be disposed to explore the facts further.

Just as many species, like egeria, have varieties which can
be regarded as adapted to northern and southern regions, so
there are also several which have lowland and Alpine forms quite
distinct from each other. Every such case presents an example
of the problem we have been considering. As the collector
passes from the plains to the Alpine region, how will he find the

u Some most unlikely species do this. I once had a larva of Parnassius delizis,
found at about 5,500 feet, which emerged late in the autumn (in October I believe),
a season at which it must have perished in its own country.

13 See, for examples, Barrett, G. C., Lepidoptera of the Brit. Islands, I. 1893,
p 229; also Grover, W., Ent. Rec., IX, 1897, p. 314; Williams, H., Proc. Ent.Soc.,
1898, who reared several specimens from the New Forest which would pass for
Bretons, though the rest of the family were true egerides.

174 PROBLEMS OF GENETICS

transition from one form to the other effected? Does the low-
land form give place to the Alpine form suddenly, with a region
in which the two are mixed, or will he find a zone inhabited by
an intermediate population? I have spent a good deal of time
examining the facts in the case of Pieris napi and its Alpine
female variety bryoniae, and though there are many compli-
cations which still have to be cleared up, no doubt is possible
as to the main lines of the answer. If in any valley in the Alps
inhabited by both napi and bryoniae the collector catches every
specimen he can, beginning at the bottom and working up to
7,000 feet, he will at first get nothing but napi. At about 2,500
feet, he may catch an occasional bryoniae flying with the napi.
After 3,000 feet napi usually ceases, and only bryoniae are found.
As an exception a colony of napi may be met with at much
greater heights. I once found them in numbers at about 6,000
feet.14 Not only were they free from any trace of modification
in the direction of bryoniae, but they were of the thoroughly
southern type of napi, being a late brood of that large and very
pale kind (meridionalis) almost destitute both of dark veining
above and of green veining below, which are common on the
shores of Lago Maggiore and in other hot southern localities.
Not far off at the same level were typical bryoniae in fair abund-
ance. Occasionally an intermediate may be met with. I have
taken a few, for example, at Macugnaga and at Fobello. These,
however, in my experience are rarities in the Alps. Fleck15
gives notes on the distribution in Roumania which shows the
same state of things. The lowland form is not transformed
though found at great heights, and at Azuga (nearly 3,000 feet)
bryoniae occurs with only occasional "flavescens," viz., inter-
mediates of the second brood.

If this were all the evidence we should be satisfied that the
lowland and Alpine types keep practically distinct, overlapping
occasionally, but rarely interbreeding. The problem would
remain, how is the distinctness of the two types maintained in
the region of overlapping? Nowadays, I suppose, we should

14 Above the Tosa falls.

16 Bui. Soc. Sciinte, VIII, 1899, p. 691.

CLIMATIC VARIETIES 175

incline to answer this question by reference to segregation, and
perhaps by an appeal to selective mating. The suggestion that
segregation does take place is certainly true to some extent.
There are, however, difficulties in the way, and the whole subject
is one of great complexity. My own experiments were made in
pre-Mendelian times and were not arranged with the simplicity
which we now know to be essential. The results are neither
extensive enough nor clear enough to settle the many collateral
questions which have to be considered, and the work ought to
be done again. Nevertheless, some notes of the observations
may have a suggestive value.

When I began, I did not sufficiently appreciate that the
"napi" group, omitting the North American forms, and the
Asiatic representatives, has at least three chief types in western
Europe. The differences we have to deal with are manifested
by the females only, so in this account particulars as to the males
are omitted for the most part. These are (i) our own British
napi; (2) the form found in the south, from the Loire downwards,
and in the Italian Alps, which I think may be spoken of as
meridionalis; (3) bryoniae, which is a form clearly recognizable
in the female only, and is found only in the arctic regions and
in the Alps above 2,500 feet. The first two have several broods,
two, three, or more, according to opportunity, and the first
brood is different from the later ones. In napi the markings on
the upper surface are a dark grey but in meridionalis they are a
pale silvery grey and much less extensive. In the later broods
of napi there is much less general irroration of the veins, and the
spots stand out as more defined and blacker. These differences
vary greatly in degree of emphasis. In meridionalis the later
broods are entirely different from the first. Instead of having
silvery markings they have the ground colour quite white, with
the spots large and a full black. On the under side of the hind
wings the usual green veins are almost absent, and I have seen
individuals which could scarcely be distinguished from rapae.
To these later broods the term napaeae is sometimes applied,
but I here use meridionalis for the southern race in general as
applicable to all broods.

176 PROBLEMS OF GENETICS

The female bryoniae is totally unlike the others. The ground
colour is a full yellow, and each nervure is thickly irrorated with
a brown pigment often spreading so far as to hide the ground
almost entirely in the forewings. The males corresponding with
these females are not certainly distinguishable from those of our
own napi. Both sexes have the green veining of the underside
of the hind wing fully developed, rather more than is usual in the
lowland races, but this is not really diagnostic of the variety.
The first serious difficulty arises in regard to the second brood
of bryoniae. It is stated that there is only one brood,16 but I feel
fairly sure that a second brood is sometimes produced, and that
the females with a yellow ground and diminished irroration of the
veins, not very uncommon in the Italian Alps in July to August,
are generally representatives of it. Such insects would of course
be classed with bryoniae in collections.

My experiments began with eggs of true bryoniae females
caught at about 2,500 feet early in July. These emerged in
August— September as intermediates with yellow ground and
about half as much black on the upper surface as bryoniae.
They are exactly like the intermediates usually found in nature
and in the light of later experience I regard them as natural FI
forms, and I think the mothers had been fertilised by napi males,
though I admit that in view of the rarity of natural intermediates
there is a difficulty in this suggestion. Three of these females
were mated with males raised from thorough meridionalis
females, and three families were produced. Two of them
showed distinct evidence of segregation, some being yellow and
some white with various intergrades, some being no blacker than
meridionalis and some ranging up to a dark intermediate type.
Part emerged in the same autumn; and part overwintered, emerg-
ing as the spring meridionalis or as the peculiar type which I after-
wards learnt to know as the spring FI form. The distinctions
were fairly sharp between the several forms. But the offspring
of the third female gave a series practically continuous from

16 The fact that Weismann by heating pupae obtained only one autumn speci-
men seems to me to show rather that a second brood can be produced than that it
cannot, which is the inference usually drawn.

CLIMATIC VARIETIES 177

meridionalis to the FI type. The work of subsequent years
gave results similarly irregular which could only be described
adequately at great length. The outcome may however be
summed up in the statement that there is evidence that both
the yellow ground and the dark veining are due to factors, but
that there are several of these and that imperfect segregation
is not uncommon, producing various reduction-stages. The
yellow ground may be due to one factor, and the several shades
may be the result of irregularities in dominance , but the black
markings when fully developed cannot I think be the result of
less than three factors, one for the basal darkening, one for general
irroration, and one for the margins. Probably also the enlarge-
ment of the spots is produced by a fourth factor.

There was not, in my experience any great difficulty in getting
the various forms to pair in captivity. Some attempts were
made to see whether individuals of either type selected mates
of their own type in preference to those of the other, but the
results were inconclusive. There were some indications of such
a preference; though, from the impossibility of judging how much
of this may be due to other circumstances, I could not come to
a positive conclusion on the rather meagre evidence.

Recently Schima17 has given a careful and detailed account
of all the forms found in Lower Austria which he enumerates
under 14 distinct varietal names. He gives full references to
previous accounts, especially to the beautiful plates lately pub-
lished by Roger Verity.18 Examination of these and of my own
specimens strongly suggests that the several forms are due to
the recombination of the factors I have named. Among those
which I have bred are representatives of most if not all the types
enumerated by Schima in addition to other curious forms. For
example I have bryoniae markings on a ground practically white;
the dark veins with spots almost obsolete; meridionalis on a
yellow ground; the intermediate amount of black on a white
ground, etc. The last-named may occur wild and I have one
from Macugnaga as well as one given me by Mr. F. Gayner from
Lulea (Lapmark).

17 Schima, K., Verh. Zool. boi. Ges. Wien, LX, 1910, p. 268.

18 Rhopalocera Palaearctica, Florence, 1905-11, especially PI. XXXII.
13

178 PROBLEMS OF GENETICS

To obtain really exact knowledge of the number of factors
and their properties it would be necessary to repeat the work.
After the beginning, I made a mistake in using British napi
instead of meridionalis and the results were much confused
thereby. The contrast between meridionalis and the various
dark forms is much greater and classification of the types would
have been therefore easier. The British form is presumably
meridionalis plus the factor for the basal pigmentation. The
problem is greatly complicated by the differentiation of the
seasonal forms. The first point to be determined is whether
bryoniae is capable of producing a second brood when it is thor-
oughly pure-bred, and whether such a second brood is, as I
suspect, normally intermediate in character.

In the Alps generally there is no definitely intermediate
population; nor I believe, is any such population met with in
the north where the arctic bryoniae meets napi, but as to this
I have no precise information. One curious fact, however, must
be mentioned, namely that there is a population that can prob-
ably be so described with fairness established at Modling near
Vienna. This is not in any sense an Alpine locality, and does
not, as I am told, differ in any obvious way from the other sub-
urbs of Vienna. Dr. H. Przibram was so good as to send me a
set taken at this place, representing a second brood, and they
were decidedly heterogeneous, ranging from an intermediate
form such as bryoniae fertilised by napi usually produces, to a
light yellowish second-brood type with little dark pigment.
There are also two actual bryoniae. Whether true napi also
occur there I do not know, but I have no doubt they do. It
would be well worth while to investigate the Modling population
statistically, and to breed from the intermediates which might
not impossibly prove to be heterozygotes. There are also records
of such intermediates being occasionally found in some parts of
Ireland, in the north of Scotland, and in south Wales,19 but I do
not know of any regular colony of these forms. We can scarcely
avoid the inference that one or more of the factors which make
up bryoniae may be carried by these intermediates. It is not

19 See figures in Barrett, G. C., Lepidoptera of Brit. Islands, I, pt. 3, p. 25.

CLIMATIC VARIETIES 179

clear why their interbreeding does not produce actual bryoniae
occasionally. If this occurred, the probability is that the fact
would be known to collectors, at least in the British localities.
The absence of true bryoniae must, I think, be taken to mean
that some essential factor is absent from these intermediates.

To sum up the evidence, the facts that are clear may be thus
enumerated :

1. Napi and bryoniae, or in the Italian Alps, napaeae and
bryoniae frequently meet each other.

2. They cross without difficulty, producing fertile offspring.

3. But in the levels at which they overlap there is no inter-
mediate population, and only occasional intermediate individuals.

4. In certain parts of the distribution of napi similar inter-
mediates sometimes occur, and at one place (Modling) they are
so frequent as apparently to constitute a colony.

5. As to the genetic relations of the two forms there is no
complete certainty. Indications of segregation have been ob-
served in some cases, but there are several factors concerned and
they are liable to some disintegration.

Another form in which I tried to investigate the same problem
is Coenonympha arcania, which has one Alpine form known as
darwiniana, and another, satyrion. In calling satyrion a form of
arcania I follow Staudinger and other authorities, but I have
never been quite satisfied that it should be so regarded. The
differences between arcania. and darwiniana are essentially dif-
ferences of degree; C. arcania occurs in places where there is
cover, and reaches up the valleys usually as high as the mixed
woods of deciduous trees, which is about 2,500 feet. The variety
darwiniana, on the contrary, is an insect of treeless hillsides, and
I regard it as a dwarf and possibly a stunted form. It would not
greatly surprise me to find that with the application of good
conditions arcania could be raised from darwiniana eggs, or that
if arcania larvae were starved they might give rise to darwiniana
butterflies. I have been unsuccessful in trying to rear the species,
having lost the larvae by disease. Usually one does not catch
arcania and darwiniana on the same ground, and as Festuca ovina
— a typically hill-side grass — is a common food-plant of dar-

i8o PROBLEMS OF GENETICS

winiana there can be little doubt that arcania feeds on some other
grass, probably woodland species. Colonies of arcania of varying
size and brightness are commonly found, and though a sample of
arcania, finely grown, from a warm Italian wood, presents a
striking contrast with darwiniana from an Alpine pasture, one
certainly may get samples which fill all the gradations. Gen-
erally the sample from a given locality is fairly homogeneous.

Of satyrion I have little personal experience. I only twice
found it, namely at Zinal, and at Hallstatt in Austria, but it
occurs at Zermatt, Arolla, and in several Swiss localities above
5,000 feet, and I understand that it is the typical Alpine form in
the Engadine. With its darkened colour and reduced size it
might well be expected to be a still further stunted form of
darwiniana. Yet I have never found the one succeed to the
other at the higher levels. If darwiniana appears when Alpine
conditions are reached in a valley it will be met with up to the
highest level at which such butterflies live. Tutt was of opinion
that satyrion is a distinct species.20 I once, at the top of the Vor-
derrheinthal caught a sample of darwiniana a few of which (males)
were so dark and had the eye spots so poorly developed that they
looked like transitions to satyrion. Otherwise I never found
any such transitional forms and they are certainly exceptional.
There is further a record21 of satyrion having been taken flying
with arcania. This was near Susa, at about 2,000 feet I infer.
Mr. H. E. Page has similar specimens from Caud and from St.
Anton (Arlberg). The females, however, both of mine and of
Mr. Page's samples are a pale brown, quite unlike the females
both of arcania and of the dark Zinal satyrion. The difficulty
thus raised has not I think yet been considered by the authorities,
and it is possible that the Alpine forms of arcania are in reality
three, not two.

The evidence taken together suggests, I think, that darwin-
iana is related to arcania much as so many of the Alpine varieties

20 Tutt, J. W., Ent. Rec., XVIII, 1905, p. 5. In the same place he states that
on the Mendel Pass arcania "runs into" darwiniana and that in the Tyrolean
localities the transition is especially evident. Wheeler (ibid., XIII, 1901, p. 121)
expresses the contrary opinion, that satyrion does grade to arcania.

21 H. Rowland-Brown, Ent. Rec., XI, 1899, p. 293.

CLIMATIC VARIETIES 181

of plants are to the well-developed individuals of the lower
levels. I do not anticipate that factorial differences will be
found in these insects, and it is by no means impossible that the
distinctions between them are the direct consequences of altered
conditions. The relations of arcania to satyrion are more doubt-
ful, and in that case a factorial difference may at least be sus-
pected.

The species of the genus Setina have Alpine forms which
agree in possessing a characteristic extension of the black pigment
to form radiating junctions between the spots on the wings.
Speyer, who discussed the interrelations of these forms in detail,22
lays stress on the absence of genuine transitional forms between
aurita and the variety ramosa. Both are mountain insects but
ramosa extends to levels higher than that at which aurita ceases,
which is about 4,000 feet. The two forms are often found flying
together. Speyer says that his brother searched diligently for
transitional forms at the level of overlapping, but found none,
so that at least they may be regarded as rare. The variety
ramosa is not infrequent at much lower levels (e. g., Chiavenna,
1,020 feet; Reussthal, 1,500 feet) and extends as high as the
permanent snows. In the British Museum collection, however,
I have seen several that I should regard as transitional. Speyer
perhaps would have classed as ramosa all in which the spots of
the central field were united, and it is by no means unlikely that
breeding would prove such individuals to be heterozygous.23

22 Speyer, Stettiner, Ent. Ztg., XXXI, 1870, p. 63.

23 In regard to the closely analogous case of Spilosoma lubricipeda, Standfuss
makes a similar statement. He bred the type on a large scale with the radiate form
which he calls intermedia, and says that in four years of miscellaneous crossing he
never obtained really transitional forms. Nevertheless after examining large series,
especially those of Mr. W. H. B. Fletcher, I came to the conclusion that several
might be so classed, but I am quite prepared to find that such specimens are hetero-
zygous. (See Standfuss, Handb. d. Gross- Schmet., 1896, p. 307.) It is by no means
unlikely that various dark forms of lubricipeda correspond with a progressive series
of factorial additions. Many of the stages have been named, and of these the most
definite are the intermedia of Standfuss (probably = eboraci of Tugwell) and the
very dark Zatima of Heligoland, in which only the thorax, the nervures and a small
field in the forewings remain yellow. A form was bred by Deschange from Zatima
in which even the field in the forewing is obliterated. The exact circumstances in
which Zatima occurs in Heligoland would be worthy of special investigation, for the

1 82 PROBLEMS OF GENETICS

There can scarcely be a doubt that the distinction between
aurita and ramosa is factorial, the radiate ramosa probably having
the factor for striping. In support of this view may be men-
tioned the observation of Boisduval,24 respecting a gynandro-
morphous individual, which was aurita male on one side, and
ramosa female on the other. Speyer makes another excellent
comment. He points out that the simple notion that the radi-
ation is a mere extension of pigmentation consequent on the
climate of the higher levels, will not fit the facts very easily,
because the size of the spots varies greatly in aurita itself at any
level, and lowland specimens may actually have more black
confined to the spots alone than some ramosa possess on spots and
lines combined.25

The two Salamanders, S. maculosa and its Alpine form atra,
might not improbably furnish evidence bearing on the same
problem. The two are of course very distinct, not merely in
colour (maculosa being spotted with yellow or orange while atra
is entirely black) but also in the mode of reproduction, a feature
to which reference will be made in the next chapter. I cannot,
however, find any evidence as to the overlapping of the two forms.
S. atra occurs from about 3,000 feet or somewhat less, and reaches
great elevations in the Eastern Alps, but I do not know if the
two forms ever occur in the same localities. Leydig,26 Boulenger,27
and most modern authorities regard the two types as distinct
species, but they are in any case closely allied, and it would be of
interest to have exact knowledge of their geographical delimi-
tations.

The reader who has considered the cases adduced will ap-
preciate the difficulties which must be faced in any attempt to

normal lubricipeda in also found on the island. For references as to the British
occurrences see especially, Hewett, W., Naturalist, 1894, p. 353. As to Zatima see
especially Krancher, Soc. Ent., II, 1887-8, p. 26. I am indebted to Dr. Hartlaub
for information as to the Heligoland types.

24 Boisduval, Bull. Soc. Ent. Fr., Ill, 1834, p. 5.

25 The systematics of Setina have been much controverted, but no one I believe
doubts that aurita and ramosa are forms of one species. See also Chapman, A. T.,
Ent. Rec., XIII, 1901, p. 139.

™Arch. Naturg., 33, 1867, p. 116.

27 Brit. Mus. Cat., Batrachia Gradientia, 1882.

CLIMATIC VARIETIES 183

account for the facts in a rational way. As always in a problem
of Evolution, two separate questions have to be answered.
First how did the form under consideration come into existence,
and secondly, how did it succeed in maintaining itself so as to
become a race? The evidence from the local forms, though very
far from giving complete answers to either of these questions
definitely refutes the popular notion that a new race comes into
existence by transformation of an older race. If a gradual mass-
transformation of this kind took place we should certainly expect
that when two types, nearly allied and capable of interbreeding,
overlap each other in their geographical distribution, a normally
intermediate population would exist. If each type can main-
tain itself, and if each came into existence by gradual transfor-
mation, then there must have been an intermediate capable of
existing and maintaining itself as a population; and if this had
ever been, surely in the region of overlapping, that intermediate
population should continue. Especially should such a population
be found when the two extreme types are adaptational forms and
the region of overlap is a region of intermediate conditions.
But of the examples we have examined there is only one, that of
Pararge egeria and egerides, which can at all be so interpreted,
and even in that case it is not impossible that more minute ob-
servation would reveal discontinuity between the extremes
and the admittedly normal intermediate population. Granting
provisionally however that this example, as it stands, is con-
sistent with the conventional theory of evolution, I know not
where we should look for another case equally good. When the
distinctions are produced by direct influence of conditions oper-
ating during the lifetime of the individuals, examples of inter-
mediate populations occupying the areas of intermediate con-
ditions can no doubt be produced. Many turf -like Alpine
plants, for instance, if protected from exposure and properly
nourished can grow as large as those of the same species found in
the valleys, and in the case of such quantitative effects, inter-
mediate conditions can doubtless produce intermediate characters.
Even these examples however are not very abundant, and
often the intermediate locality has not a form intermediate

184 PROBLEMS OF GENETICS

between those of the two extreme localities, but some third
form distinct from either. This is the case for instance in the
fauna of brackish waters. We are taught to believe that the
fresh water fauna was evolved from the marine fauna, which
it well may have been; but as students of Crustacea and Mollusca
know familiarly, the brackish water forms are not as a rule inter-
mediates between fresh water species and sea species, but more
usually they are special forms belonging to the brackish waters,
with the peculiar property that they can tolerate a great range of
conditions, and live without ostensible variation in waters of
most various compositions and densities, which very few marine
or fresh water species are able to do.

Sometimes the distinction between local races, as in Rham-
phocoelus passerinii and icteronotus may be regarded with con-
fidence as due to one simple Mendelian factor possessed by one
race and absent from the other, but I think, more often, as in
Colaptes or in the varieties of Pieris napi, the existence of several
distinct factors is to be inferred. As we have seen, the races
of Colaptes show almost beyond doubt that in different areas at
least three distinct factorial combinations can be perpetuated
as races.

In the distribution of variability we find, I think, some hint
as to the steps by which the phenomena under consideration
have come to their present stage, and I am disposed to regard
the facts so well attested in the case of our own melanic moths
as a true indication of the process. Following this indication
we should regard the change in the character of a population
as beginning sporadically, by the appearance of varying indi-
viduals, possibly only one varying individual, in, it may be, one
place only. As to why a variety should increase in numbers we
have nothing but mere speculation to offer, and for the present
we must simply recognise the fact that it may. That such sur-
vival and replacement may reasonably be taken as an indication
that the replacing race has some superior power of holding its
own I am quite disposed to admit. Nevertheless it seems in
the highest degree unlikely that the outward and perceptible
character or characters which we recognise as differentiating the

CLIMATIC VARIETIES 185

race should be the actual features which contribute effectively
to that result.

In discussions of geographical distribution in relation to
problems of origin it is generally said that very nearly allied
species usually occupy distinct areas, while other competent
observers state the exact contrary. Lately, for example, Dr.
R. G. Leavitt28 has published an important collection of evidence
upholding the latter proposition, taken chiefly from the botanical
side, showing how in numerous genera two or more closely allied
species coexist, frequently without intermediates, in the same
localities, and may even be thus found in company throughout
their distribution. The difference of opinion evidently arises
from a confusion as to the sense in which the term "species'*
is understood and applied. Leavitt, for example, is avowedly
following Jordan and, among moderns, Sargent, in applying
a close analysis, and denoting as species all forms which are
distinct and breed true. Against this use of the term I know
no valid objection29 but it must be obvious that if others follow
a different practice confusion may result when observations are
summarised in general statements. We will consider this subject
again in another place, but here it may be sufficient to say that
there can scarcely now be a doubt that numbers of these associ-
ated species, such as Jordan discriminated, represent various
combinations of the presence and absence of Mendelian factors.
This does not in any way weaken the argument which Leavitt
founds upon the facts, namely, that the observed distribution
of these forms is consistent with the supposition of an evolution
largely discontinuous.

On the other hand, those who have come to the opinion that
nearly allied species generally occupy distinct ground are pre-
sumably more impressed by the characters differentiating the
geographically distinct or adaptational races, seeing that genuine
intermediates between them are less commonly found. Those
geographical races may no doubt contain various differentiated
forms; but when all live together, occasional intermediates are

28 The Geographical Distribution of nearly related Species. Amer. Nat., XL I .
1907, p. 207.

29 See later, p. 242.

186 PROBLEMS OF GENETICS

usually to be found even in the case of characters habitually
segregating. These segregating forms Jordan would certainly
have determined as species, and it must be conceded that no
physiological definition has yet been drawn which consistently
excludes them.

CHAPTER IX

THE EFFECTS OF CHANGED CONDITIONS: ADAPTATION

In the attempt to conceive a process by which Evolution
may have come about, the first phenomenon to be recognized
and accounted for is specific difference. With that recognition
the outline of the problem is defined. The second prerogative
fact is adaptation. Forms of life are on the whole divided into
species, and these species on the whole are adapted and fit the
places in which they live. To many students of Evolution,
adaptation has proved so much more interesting and impressive
than specific diversity that they have preferred it to the first
place in their considerations.

Whether this is, as I believe, an inversion of the logical order
or not, there is one most serious practical objection to such
preference, that whereas specific diversity is a subject which
can be investigated both by the study of variation and by the
analytical apparatus which modern genetic science has developed,
we have no very effectual means of directly attacking the problems
of Adaptation.

The absence of any definite progress in genetics in the last
century was in great measure due to the exclusive prominence
given to the problem of Adaptation. Almost all debates on
heredity centered in that part of the subject. No one disputes
that the adaptation of organisms to their surroundings is one
of the great problems of nature, but it is not the primary problem
of descent. Moreover, until the normal and undisturbed course
of descent under uniform conditions is ascertained with some
exactness, it is useless to attempt a survey of the consequences
of external interference; nor as a rule can it be even possible to
decide with much confidence whether such interferences have or
have not definite consequences. Those, for example, who de-
bated with enthusiasm whether acquired characters are or are
not transmitted were constantly engaged in discussing occur-

187

1 88 PROBLEMS OF GENETICS

rences which we now know to be ordinary features of descent
under uniform conditions, and the origin of variations which
were certainly not caused directly by circumstances at all. In
the absence of any factorial analysis, or of any conception of what
factorial composition means and implies, no one knew what
varieties might be expected from given parents. The appearance
of any recessive variety was claimed as a consequence of some
treatment which might have been applied to the parents. There
was no possible standard of evidence or means of controlling it,
and thus the discussion was singularly unfruitful. Before we can
tell how the course of descent has departed from the normal, we
must know what the normal would have been if we had let alone.
We are still far from having such knowledge in adequate measure,
but it does now exist in some degree, and we are steadily approach-
ing a position from which we shall be able to form fairly sound
estimates of the true significance of evidence for or against the
proposition that environmental treatment can produce positive
disturbances in the physiological course of descent.

Thus described, the field for consideration is very wide.
Though the effects of changed conditions were especially studied
in the hope of solving the problem of adaptation by direct ob-
servation, that, as all are now agreed, is but a part of a more
general question. We must ask not only do changed conditions
produce an adaptative response on the part of the offspring, but
whether they produce any response on the part of the offspring
at all. It is not in doubt that by violent means, such as starvation
or poisoning of the reproductive cells, effects of a kind, stunting
and deformity for instance, can be made evident, just as similar
effects may follow similar treatment during embryonic or larval
life. Apart from interferences of this class, are there any that
may be reasonably invoked as modifying the course of inher-
itance?

No epitome of the older evidence for the inheritance of
adaptative changes is here required. That has often been col-
lected, especially by Weismann, who exposed its weaknesses so
thoroughly as to carry conviction to most minds, and showed
that whether the phenomenon occurs or not, no one can yet prove

ADAPTATION 189

that it does. Belief in these transmissions, after being almost
universally held, was with singular unanimity abandoned. This
change in opinion, though doing credit to the faith of the scientific
community in evidential reasoning, is the more remarkable
inasmuch as the strength of the idea was not derived from the
minute amounts of supposed facts now demolished. On the
contrary, it was really an instinctive deduction from a wide
superficial acquaintance with the properties of animals and
plants. They can accommodate themselves to circumstances.
They do make responses sometimes marvellously appropriate
to demands for which they can scarcely have been prepared.
What more natural than to suppose that the permanent adapta-
tions have been achieved by inherited summation of such re-
sponses? No one had actually been driven to believe in the
inheritance of adaptative changes because bitches which had
been docked had been known to give birth to tailless puppies,
or because certain wheat in Norway was alleged to have become
acclimatized in a few generations. Evidence of this kind was
collected and produced rather as an ornamental appendix to a
proposition already accepted, and held to be plainly demon-
strated by the facts of nature. Looked at indeed in that pre-
liminary and uncritical way, the case is simply overwhelming.
Those who desire to see how strong it is should turn to Samuel
Butler's Life and Habit, and even if in reading they reiterate to
themselves that no experimental evidence exists in support of
the propositions advanced, the misgiving that none the less they
may be true is likely to remain. Making every deduction for
the fact that the wonders of adaptation have been grossly ex-
aggerated, and that marvels of fitness and correspondence be-
tween means and ends have grown out of mere anthropomorphic
speculations, there is much more left to be accounted for than
can at all comfortably be accepted as the product of happy
accidents. So oppressive are these difficulties that we can scarcely
blame those who imagine that the study of heredity is primarily
directed to the problem of the transmission of acquired characters,
a preconception still almost universal among the laity.

But since the belief in transmission of acquired adaptations

190 PROBLEMS OF GENETICS

arose from preconception rather than from evidence, it is worth
observing that, rightly considered, the probability should surely
be the other way. For the adaptations relate to every variety
of exigency. To supply themselves with food, to find it, to seize
and digest it, to protect themselves from predatory enemies
whether by offence or defence, to counter-balance the changes
of temperature, or pressure, to provide for mechanical strains,
to obtain immunity from poison and from invading organisms,
to bring the sexual elements into contact, to ensure the dis-
tribution of the type; all these and many more are accomplished
by organisms in a thousand most diverse and alternative methods.
Those are the 'things that are hard to imagine as produced by
any concatenation of natural events; but the suggestions that
organisms had had from the beginning innate in them a power
of modifying themselves, their organs and their instincts so as
to meet these multifarious requirements does not materially
differ from the more overt appeals to supernatural intervention.

The conception, originally introduced by Hering and inde-
pendently by S. Butler, that adaptation is a consequence or
product of accumulated memory was of late revived by Semon
and has been received with some approval, especially by F.
Darwin. I see nothing fantastic in the notion that memory
may be unconsciously preserved with the same continuity that
the protoplasmic basis of life possesses. That idea, though
purely speculative and, as yet, incapable of proof or disproof
contains nothing which our experience of matter or of life at all
refutes. On the contrary, we probably do well to retain the
suggestion as a clue that may some day be of service. But if
adaptation is to be the product of these accumulated experiences,
they must in some way be translated into terms of physiological and
structural change, a process frankly inconceivable.

To attempt any representation of heredity as a product of
memory is, moreover, to substitute the more obscure for the
less. Both are now inscrutable ; but while we may not unreason-
ably aspire to analyse heredity into simpler components by or-
dinary methods of research, the case of memory is altogether
different. Memory is a mystery as deep as any that even psy-

ADAPTATION 191

chology can propound. Philosophers might perhaps encourage
themselves to attack the problem of the nature of memory by
reflecting that after all the process may in some of its aspects
be comparable with that of inheritance, but the student of gen-
etics, as long as he can keep in close touch with a profitable basis
of material fact, will scarcely be tempted to look for inspiration
in psychical analogies.

For a summary of the recent evidence I may refer the reader
to Semon's paper1 where he will find a collection of these obser-
vations described from the standpoint of a convinced believer.
At the outset one cannot help being struck by the fact that of
the instances alleged, very few, even if authentic, show the trans-
mission of acquired modifications which can in any sense be re-
garded as adaptative, and many are examples not so much of a
transmission of characters produced in the parents as of variation
induced in the offspring as a consequence of treatment to which
the parents were submitted, the parents themselves remaining
apparently unmodified. No one questions the great importance
of evidence of this latter class as touching the problem of the
causes of variation, but it is not obvious why it is introduced in
support of the thesis that acquired characters are inherited.

It is most difficult to form a clear judgment of the value of
the evidence as a whole. To doubt the validity of testimony
put forward by reputable authors is to incur a charge of obstinacy
or caprice; nevertheless in matters of this kind, where the alleged
phenomena are, if genuine, of such exceptional significance, belief
should only be extended to evidence after every possible source
of doubt has been excluded. We believe such things when we
must, but not before. At the very least we are entitled to require
that confirmatory evidence should be forthcoming from inde-
pendent witnesses. So far as I have seen, this requirement is
satisfied in scarcely any of the examples that have been lately
published, and until it is, judgment may reasonably be suspended.

In some cases, however, the facts are not doubtful. Stand-
fuss, by subjecting pupae of Vanessa urticae to cold, produced

1 Semon, R., Der Stand der Frage nachder Vererbung erworbener Eigenschaften,
published in Fortschr. der naturw. Forschung., Bd. n, 1910.

I92 PROBLEMS OF GENETICS

the now well-known temperature-aberrations in which the dark
pigment is greatly extended. He put together in a breeding-cage
32 males and 10 females showing this modification in various
degrees. Two of these females died without leaving young.
Seven produced exclusively normal offspring. From the eighth
female 43 butterflies were bred, and of these there were four (all
males) which to a greater or less extent exhibited the aberrational
form.2 The mother of this family was the most abnormal of the
10 females originally put in.

Fischer's experiment with Aretia caja was on similar lines.
From pupae which had been frozen almost all the moths which
emerged showed aberrational markings. A pair of these mated
and produced 173 young which pupated. Those which emerged
early were all normal, but of those which emerged late, 17 had
in various degrees abnormal markings like those of the parents.3
In neither of these examples is there any question as to the facts.
Both observers have great experience and give full details of their
work.

As regards Vanessa urticae, however, it must be recalled that
Fischer himself showed that in Nymphalids somewhat similar
aberrations could be produced both by heat and by cold, and
even by centrifuging the pupae. Frl. von Linden produced a
transitional form of the same aberration in V. urticae by the
action of carbonic acid gas.4 It is highly probable that the ap-
pearance is due to a morbid change, perhaps an arrest of develop-
ment, which may be brought about by a great diversity of causes.
In the experiments the cause probably was a diseased condition
of the tissues of the mother herself. She had been subjected to
freezing sufficiently severe to prevent the proper development of
the pigments and some of the ovarian cells presumably suffered
also. It will be observed that the only specimens which were
affected were the offspring of the most abnormal female, and of
them only four out of forty- three showed any change.

The same interpretation probably applies to the cases in

2 Standfuss, M., Denks. Schweiz. naturf. Ges., XXXVI, 1898, p. 32.
•Fischer, E., Allg. Ztschr. f. Entomologie, Bd. VI, 1901.

4 Out of 12 pupae treated 8 died and of the 4 survivors, one only was affected.
See M. v. Linden, Archiv. Rassen. u. Gesells., 1904, I.

ADAPTATION 193

Arctia caja. In this species the markings are well known to
be liable to great variation. As Barrett says, even in nature
individuals are rarely quite alike, and an immense number of
strange forms occur in collections.5 These are greatly sought
after by some collectors, especially in England, where they fetch
high prices at auctions, and it is notorious that most of them come
from Lancashire and the West Riding of Yorkshire. It is com-
monly supposed that the breeders of that district subject them to
abnormal conditions, and especially to unnatural feeding, but
I know no clear evidence that this is true. From whatever cause
it is certain that the natural pattern is, in some strains at all
events, very easily disturbed.

The elaborate experiments of Schroder with Abraxas grossu-
lariata are difficult to follow and are complicated by the fact
that the series which was submitted to abnormal temperatures
was derived from an abnormal original pair. From the evidence
given it is not clear to me whether the temperature had a distinct
effect. This insect, like A rctia caja, produces an immense number
of variations (especially in the amount of the black pigment)
and as most of these are, I believe, reared in domestication for
sale, it is highly probable that the species is easily influenced
by cultural conditions.

Schroder describes two other experiments which have been
accepted by Semon and other supporters of the view that acquired
characters are transmitted. In the first, Phratora vitellinae, a
phytophagous beetle living on the undersides of leaves, was used.
It naturally feeds on Salix fragilis, a species without a felt, or
tomentum, on the underside of the leaves. Larvae were trans-
ferred to another willow (near S. viminalis) which has the under-
sides of the leaves felted. The larvae took readily to the new
food, pushing the tomentum before them as they gnawed the
leaves. They came to maturity and when they were about to
lay their eggs they were given a free choice between S. fragilis
and the tomentose species. The greater number of ovipositions,

6 For illustrations see Oberthur's Etudes d'Entom., 1896, where many of these
curious aberrations are represented; also Barrett, Lepid. Brit. Islands, II, pp. 71
and 72.

14

194 PROBLEMS OF GENETICS

£19, took place on fragilis, and there were 127 on the tohientose
bush, which we are told was six times as large as the fragilis.
The larvae from fragilis were next put on the tomentose species
and reared on it. When they became imagines they were simi-
larly given their choice, with the result that there were 104
ovipositions on the tomentose species and only 83 on fragilis.
In the next generations there were 48 ovipositions on the tomen-
tose and ii on fragilis. Finally the fourth generation made
15 ovipositions on the tomentose and none on fragilis.

The difficulty about such experiments is obviously that one
has no assurance that the change of instinct, in so far as there
is any, may not be a mere consequence of the captivity. It
must, besides, be extremely difficult to arrange the experiment so
that there is really an equal choice between the two bushes, when
one stands beside the other. Przibram, in quoting this case,
considers that as the tomentose bush was about six times as
large as the fragilis , some indication of the relative attractiveness
of the two may be obtained by dividing the ovipositions on the
larger bush by six, but I imagine the matter must be much more
complex.

Schroder's second example is not more convincing, in my
opinion, though Semon regards it as one of the most important
pieces of evidence. It concerns a leaf-rolling moth, Gracilaria
stigmatella, the larva of which is said normally to make its house
by bending over the tips of the sallow leaves on which it feeds.
Schroder placed larvae on leaves from which the tips had been
cut, and these larvae made their houses by rolling over the sides
of the leaves. Their offspring were again fed on leaves without
tips, and as before, they rolled in the leaf-margins either on one
side or both. The offspring of this second generation were then
fed on entire leaves. There were 19 houses made by these (?I9)
larvae, and of them 15 were normal, made by folding down the
tips of the leaves, while 4 were abnormal, made by rolling in the
leaf-margins. Schroder says that in nature he has only twice
seen abnormal houses ; but it is clearly essential not only that the
frequency of such variability in nature should be thoroughly
examined, but also that we should know whether when the species

ADAPTATION 195

is bred in captivity these irregularities of behaviour do or do not
occur when the larvae are fed on uninjured leaves.

The famous case of Schiibeler's wheat is revived by Semon.
The story will be familiar to most readers of the literature of the
subject. Briefly it is that annuals, especially wheat and maize,
raised from seed in Central Europe take more time in coming
to maturity and ripening than similar plants raised in Norway,
where the summer days are much longer. The received account
is that he imported seed especially of maize and of wheat from
Central Europe to Norway and found that in successive years
the period of growth and ripening was increasingly reduced.
After two generations seed of the accelerated wheat was sent
back to Breslau where it was grown, and was found to ripen rather
more slowly than in Norway, but much more quickly than the
original stock had done. The facts recorded by Schlibeler6 are
that he received seed from Eldena, which is on the Baltic near
Greifswald. The variety is described as " 100 tdgiger Sommer
Weizen" but no more exact record of its behaviour in Germany
is given. This wheat, grown at Christiania in 1857, took 103
days to harvest. Its seed was again grown in Christiania in 1858,
and took 93 days, and sown again in 1859 it took only 75 days, 28
days less than in the first year of cultivation in Norway. Seed of
the 1858 crop was sent to Breslau, and grown there by Roedelius
in 1 859; it took 80 days. Evidently before such a record can be
used as proving an inheritance of acquired characters numbers of
particulars should be forthcoming. The view that Johannsen
has taken is that the result was probably due to unconscious
selection of the earlier individuals among a population consisting
of many types of various compositions. Some effect may no
doubt be ascribed to that cause, but I cannot think that alone
it would account for the results. My impression is rather that
they were produced by differences in the cultivation and especially
in the seasons. Research of an elaborate character would be
necessary in order to eliminate the various sources of error, and
nothing of the kind has been done; nor does Semon allude to these
difficulties in prominently adducing Schiibeler's evidence. A

6 Schiibeler, F. C., Die Culturpflanzen Norwegens, 1862, especially pp. 24 and 28.

196 PROBLEMS OF GENETICS

difference of even three weeks in time of harvesting may easily
be due to variation in the season. It would in any case be dif-
ficult to analyse the meteorological conditions, and to decide how
much effect in postponing or accelerating the harvest might be
due to cold days, to cloudy days, to wet weather, to fluctuations
in average temperature, to hot days, and other such incidents
occurring at the different periods of growth, even if they were
specially watched while the experiments were in progress, and
at this distance of time such analysis is practically impossible.
Without careful simultaneous control-experiments this evi-
dence is almost worthless. The director of the Meteorological
Office7 has, however, kindly sent me some details of the weather
at Breslau from 1857 to 1860, and I notice that as a matter of
fact July, 1859, was an exceptionally hot month, having an average
0/2.67° C- above the mean for the twenty years 1848-1867. June
in that year was slightly (0.31° C.) below the mean and May
slightly above it (0.18° C.). August was also abnormally hot,
2.35° C. above the average. The Breslau wheat was sown on
May ip and harvested on August 6. There was a cold spell from
May II to 14, which this wheat escaped, as it was sown on May
19. In the other years the cold spell came much later. These
elements of the weather may possibly have done something to
hurry the ripening in 1859. It is unfortunate that we are not
told how long similar wheat from Breslau seed took to ripen in
that year.

As regards the Norway cultivations we have the average
monthly temperatures recorded by Schiibeler, though he does
not discuss them in connection with this special problem. It is
quite clear that 1857, in which the period was 103 days, was an
exceptionally cold summer, especially as regards the months of
June and July, but though there was, so far as the temperature

1 1 am obliged to him and to Dr. E. Gold for much trouble taken to answer
my questions. Some idea of the kind of weather indicated by an average of 2.7°6
C. above the mean may be got from a comparison with the year 1911, which most
people will remember as one of the hottest summers they have known. The
July of that year was in east and southeast England about 4° F. above the mean
but 2.67 C. means about 4.8° F. above the mean. At Greenwich July, 1859, was
about 6.5° F. above the average.

ADAPTATION 197

records go, no great difference between 1858 and 1859, the year
1859, in which the period of ripening was the shortest, was some-
what colder in Norway than 1858. But we have the further
difficulty that there were ten days difference in sowing, for in
1858 the sowing was made on May 14, and in 1859 on May 24.
With all these possibilities uncontrolled, and indeed unconsidered,
I am surprised that Semon should claim these experiments as one
of the chief supports for his views.

Schiibeler's other allegations respecting the influence of
climate on plants grown in various places and especially at dif-
ferent elevations in Norway have been destructively criticised by
Wille8 to whose paper readers interested in the subject should
refer.

Before the appearance of Wille's criticisms Wettstein9 made
a favourable reference to Schiibeler's work, accepting his con-
clusion. He states also that he has himself made analogous
experiments with flax, finding that the length of the period of
development and a series of morphological characters show an
adaptation to local conditions, and that on transference of seed
to other conditions the previous effects are maintained. No
details, however, are given, and I do not know if anything more
on the subject has appeared since. The other examples cited
by Wettstein, such as the observations of Cieslar on forest-trees
and those of Jakowatz on gentians seem to me open to all the
usual objections applicable to evidence of this kind. Such work,
to be of any value for the purpose to which it is applied, must be
preceded by a study of the normal heredity and of the variations
of the species.

Most of the recent writers (Semon, Przibram, etc.) on the
inheritance of acquired characters accept the story of Brown-
S6quard's guinea pigs, which are said to have inherited a liability
to peculiar epileptiform attacks induced in their parents by various
nervous lesions.

The question has been often debated and several observers
have repeated the experiments with varying results, some failing

s Wille, N., BioL Cbltt., XXV, 1905, p. 521.

9 Wettstein, R. von, Der Neo-marckismus u. seine Beziehungen zum Danvinis-
mus, Jena, 1903-

198 PROBLEMS OF GENETICS

to confirm Brown-S6quard, others finding evidence which in
various degrees supported his conclusions. Recently a new and
especially valuable paper has been published by Mr. T. Graham
Brown10 which goes far towards settling this outstanding question.
He states that "the Brown-S6quard phenomenon is nothing more
or less than a specific instance of the scratch-reflex," and it is
due to a raised excitability of the mechanism of this reflex. This
raised excitability is the character acquired as a consequence,
for instance, of the removal of part of one great sciatic nerve.
The nature of this raised excitability and its causation are dis-
cussed and elucidated, but this part of the work is not essential
to the present consideration. Mr. Graham Brown in his summary
of conclusions remarks that it is very difficult to see how this con-
dition of raised excitability can be transmitted to the offspring,
and this comment which might be made in reference to any of
the alleged cases certainly applies with special cogency to the
present example.

He then calls special attention to three observations:

1. That guinea pigs which had a " trophic " change in the
foot, as a result of division of the great sciatic nerve, have re-
peatedly been seen to nibble the feet of other guinea pigs which
had this change in the foot from the same causes.

2. That accidental injury to the toes may be followed by the
Brown-S6quard phenomenon in an otherwise normal animal.

3. That in several instances the young of guinea pigs which
exhibited the phenomenon have been noticed to have one or more
toes eaten off by the mother.

Brown-Sequard noticed that almost all his animals in which
the great sciatic was divided acquired the " epilepsy " and
nibbled those parts of their feet in which sensation had been lost.
Of the offspring of such animals he found that a very small pro-
portion exhibited a malformation of the feet, and of these some
showed the " epilepsy." The proportion which showed the
" epilepsy " was one to two per cent, of the offspring.

Morgan11 is quoted by Graham Brown as having suggested

10 T. Graham Brown, Proc. Roy. Soc., 1912, vol. 84, B, p. 555. This paper gives
full reference to the previous literature of the subject.

11 Morgan, T. H., Evolution and Adaptation, New York, 1903.

ADAPTATION 199

that the loss of toes in the offspring may have been due to mu-
tilation by the mother, following his experience in a case in which
the tails of mice in succeeding litters were thus devoured, and
there can be little doubt that in this suggestion lies the clue to
the explanation of the whole mystery. Graham Brown concludes
that it may be supposed with every degree of probability that
the " transmission " was due to injuries inflicted upon the young
by their parents. With this conclusion most people will now be
disposed to agree, and we may hope that we shall hear the last
of this curious myth — to the elucidation of which a vast quan-
tity of research has been devoted.

The series of experiments made by Kammerer with various
Amphibia have attracted much attention and have been ac-
claimed by Semon and other believers in the transmission of
acquired characters as giving proof of the truth of their views.
With respect to these observations the chief comment to be made
is that they are as yet unconfirmed. Many of the results that
are described, it is scarcely necessary to say, will strike most
readers as very improbable; but coming from a man of Dr.
Kammerer 's wide experience, and accepted as they are by Dr.
Przibram, under whose auspices the work was done in the Bi-
ologische Vesuchsanstalt at Vienna, the published accounts are
worthy of the most respectful attention.

The evidence relates chiefly to three distinct groups of oc-
currences :

1. Modification in Alytes obstetricans, the Midwife Toad,
affecting both the structure and the mode of reproduction, in-
duced by compulsory change of habits.

2. Modification in the mode of reproduction of Salamandra
atra and maculosa induced by compulsory change of habits.

3. Modification in the colour of Salamandra maculosa induced
by change in the colour of the soil on which the animals were kept.

I. I will take first the case of Alytes,12 because it is the most

12 Kammerer's chief paper on this subject is in Arch. f. Entwm., 1909, XXVIII,
p. 447, and it is to this that the paginal references in the present text relate. His
previous paper appeared, ibid., 1906, XXII, p. 48. An account of his further ex-
periments with Alytes is given in Natur, 1909-10, Heft 6, p. 95.

200 PROBLEMS OF GENETICS

definite example, and because it is the case which most readily
admits of repetition and verification.

The habits of Alytes obstetricans are well known. The
animals copulate on land. As the strings of eggs leave the
female they are entangled by the hind legs of the male, and being
adhesive they stick to him and undergo their development at-
tached to his back and legs. The number of eggs varies from 18
to 86, a number much smaller than is usual in toads and frogs
which lay their eggs in water. The eggs are large and full of yolk.

There are two breeding seasons, one about April and the other
about September, and a winter hibernation. Not only animals
brought in from outside, but their offspring reared in domestica-
tion maintain these normal habits in confinement, if the temper-
ature does not exceed 17° C. (pp. 499 and 534).

If, however, the temperature be artificially raised and kept
at 25-30° C., the males do not attach the eggs to themselves
when spawning occurs on land but let them lie. The adhesion
of the eggs is said to be hindered by the comparatively rapid
drying of their surfaces.

More usually in the high temperatures the animals take to
the water and copulate there. The eggs are ejected into the
water, and as their gelatinous coverings immediately swell up,
they do not stick to the males.

The offspring thus derived from the parents subjected to
heat for one breeding-period only, whether they were laid in water
or on land, did not show departures from the normal type.

Kammerer states next, however, that in subsequent breeding-
periods the same parents frequently take to the water to breed,
though they have become quite accustomed to the heated
chamber; and furthermore that if such animals, having thus lost
their instinct to brood their young, be transferred to ordinary
temperatures they do not readily reassume their normal habits,
but for several breeding seasons — at least four — will take to
the water. These parents lay from 90 to 115 eggs, which are
small and contain little yolk, and the larvae, on hatching, breathe
with their embryonic gills until they are absorbed instead of
being broken off as normally.

ADAPTATION 201

The offspring thus abnormally developed when they mature
are said never to brood their eggs. If they are derived from
the earlier spawnings of their parents, before, that is to say, the
parents had been submitted to the changed conditions long enough
to transmit their effects, they lay on land ; but if they are derived
from the later spawnings, they lay in the water. These changes
of habit are manifested without the continued application of
the abnormal experimental conditions, and, as I understand the
account, in normal conditions of temperature.

If the abnormal experimental conditions are continued, the
toads always lay in water, and their eggs become progressively
smaller and more numerous. The larvae in the fourth generation
acquire three pairs of gills instead of one pair, and are in other
respects also different from the normal form.

Respecting the Alytes bred in this way Kammerer makes the
very striking statement that the males in the third generation
(p- 535) have roughened swellings on their thumbs and that in the
fourth generation (pp. 516 and 535) these swellings develop black
pigment. Together with the appearance of this secondary sexual
character there is hypertrophy of the muscles of the fore-arm.
To my mind this is the critical observation. If it can be sub-
stantiated it would go far towards proving Kammerer's case.
Alytes, among toads and frogs, is peculiar in that the males do
not develop these lumps in the breeding season, and the fact
may no doubt be taken to be correlated with the breeding habits,
copulation occurring on land and not in water as is usual with
Batrachians. It is to be expressly noticed that these lumps on
the thumbs or arms of male toads and frogs are not merely pig-
mented swellings, but are pads bearing numerous minute horny
black spines, which are used in holding the females in the water.
The figures which Kammerer gives (Taf. XVI, figs. 26 and 260)
are quite inadequate, and as they merely indicate a dark patch
on the thumbs it is not possible to form any opinion as to the
nature of the structure they represent.

The systematists who have made a special sudy of Batrachia
appear to be agreed that Alytes in nature does not have these
structures; and when individuals possessing them can be pro-

202 PROBLEMS OF GENETICS

duced for inspection it will, I think be time to examine the evi-
dence for the inheritance of acquired characters more seriously.
I wrote to Dr. Kammerer in July, 1910, asking him for the loan
of such a specimen13 and on visiting the Biologische Versuchsan-
stalt in September of the same year I made the same request, but
hitherto none has been produced. In matters of this kind much
generally depends on interpretations made at the time of ob-
servation; here, however, is an example which could readily
be attested by preserved material. I notice with some surprise
that in a later publication on the same subject no reference to the
development of these structures is made (see below) .

The statements here given represent but a small part of
Kammerer 's papers on the subject. He gives much further
information as to the course of the experiments, especially in
regard to the fate of the eggs laid on land and the aberrations
induced in them by treatment. The ramifications of the ex-
periments are, however, very difficult to follow, and as I am not
sure that I have always understood them I must refer the reader
to the original.

More recently Kammerer has published14 a most curious
account of experiments in crossing his modified and abnormal
Alytes, derived from the water-eggs, with normal individuals.

In the first case the cross was made between a normal female
and an abnormal male. The offspring were normal in their
habits. In the next generation bred from these almost exactly
a quarter showed the abnormal instinct.

The reciprocal cross was made between an abnormal female
and a normal male. In this case the offspring were abnormal in
their behaviour; but the second generation bred from them
showed three quarters abnormal and one quarter normal.

Certain details as to numbers and sexes of the various families
bred in the course of this amazing experiment are given in a

13 In reply to my letter Dr. Kammerer who was then away from home very
kindly replied that he was not quite sure whether he had killed specimens of Alytes
with " Brunftschwielen " or whether he only had living males of the fourth genera-
tion, but that he would send illustrative material.

14 Kammerer, P., Natur, 12 December, 1909, Heft 6, p. 95, repeated in 12
Flugschrift d. Deutsch Ges. f. Ziichtungskunde, Berlin, 1910.

ADAPTATION 203

subsequent publication.15 This later paper goes somewhat fully
into the question of the difference in behaviour between the
normal and modified individuals, describing the ways in which
the males and females possessing the acquired character could be
recognised from the males and females which were normal, but
in this account I find no reference to the development of the
" Brunftschwielen " — the horny pads on the hands of the males.
As these structures would be of special value in such a diagnosis
the omission of any allusion to them calls for explanation.
Kammerer claims the evidence as proof of Mendelian segregation
in regard to an acquired character, the first example recorded.
Pending a repetition of the experiments there is no more to be
said.

2 . The Mode of Reproduction of Salamandra atra and maculosa.1*
— Salamandra maculosa, the common lowland form, with yellow
bands or spots, deposits its young in water, generally as gill-
bearing tadpoles, with a wide, swimming tail, though occasionally
they are born still enclosed in the egg-capsule out of which they
soon hatch. Spawning extends over a considerable period,
often many weeks, and during the season one female may bear
more than 50 young.

S. atra, the black Alpine form, produces its young on land.
They are born without gills, ready to breathe air, and with the
rounded tail of the adult. These differences may, as Kammerer
says, naturally be regarded as adaptations to the Alpine condi-
tions. Moreover, the female bears only two young in a season,
and this reduction in the number must be taken to be a conse-
quence or condition of viviparity. There are many eggs in the
ovary, but all except the two which are destined to develop
degenerate and form a yolk-material on which these two sur-
vivors feed.

Kammerer gives a long account of the various conditions to
which he subjected both species. The treatment was complicated

16 Festschrift zum Andenken an Gregor Mendel, being vol. XLIX of the Ver h.
Naturf. Ver. in Brunn, 1911, p. 98.

16 Kammerer's chief papers on this subject are Archiv fur Entwm., XVII,
1904, and ibid., XXV, 1907. An epitome of results is also given by him in 12
Flugschrift d. Deutsch. Ges. f. Ziichtungskunde, Berlin, 1910.

204 PROBLEMS OF GENETICS

in many ways, but the essential statements are, as regards
S. maculosa, that when no water was provided in which the young
might be born, they were dropped on land, larger and in a later
stage of development and of a darker colour than is normal; that
the larvae so born gradually diminished in number until only
two were deposited in each breeding-period; that dissection
showed that the other ova degenerated to form a yolk-material.
The larvae so produced reached maturity. The summary of
results describes their behaviour, stating that they produced:

(a) In water, either (i) very advanced, large-headed larvae
45 mm. long (instead of 25-30 mm.) with gills already reduced,
which had awkward, embryo-like movements, and in some few
days metamorphosed into small perfect salamanders; or (2)
moderately advanced, properly proportioned larvae, 40-41 mm.
long, provided with large gills of (at first) intrauterine character,
which were reduced during aquatic life.

(b) On land, small (26 mm. long) larvae with rudimentary
gills, having the body rounded instead of being flattened from
above downwards, and an elongated narrow head, which were
unable to live in deep water. These larvae changed to the sala-
mander colour in 10-12 days, and after four weeks metamor-
phosed into salamanders 29 mm. long.

(c) In the foregoing cases the experimental conditions were
not continued, or in other words, basins of water were provided
in which they could spawn. But if the experimental conditions
are continued, these Salamandra maculosa which were born
newt-like (viz., not in a larval condition), are themselves newt-
bearing from the first time they give birth, using the dry land,
and bringing forth only two young, the normal number for the
births of 5. atra. These young are 40-41 mm. long, and are
dark-coloured, resembling greatly the normal new-born S. atra.

This epitome of the observations illustrating the inheritance
of acquired characters has been very widely quoted, and may
not unnaturally be taken to summarize a wide experience of
the modified animals. Reference to the details given in the
same paper shows that, as alleged, each of the four types of be-
haviour enumerated was witnessed once only in the case of each

ADAPTATION 205

of four females, no two agreeing with each other. As to the
number of the males or their habits nothing is said. The first
female, a (i), bore five young; the second, a (2), bore two, of
which one was a partial albino; the third, &, produced four young;
and the fourth, c, two as already stated.

In the case of c the details show that the female gave birth
immediately after being transferred from the open-air terrarium
to one indoors, which contained no basin of water. This is the
example of the consequences which follow on a continuance of
the experimental conditions.17

As regards S. atra the converse is reported. Various means
were used to induce them to eject their young prematurely in
water, such as massaging the sides of the mothers, or raising
the temperature to 25° or 30° C., with various degrees of success.
But afterwards it was found that specimens collected wild at an
elevation of about 1,000 metres responded to much simpler
treatment, and gave birth prematurely in water when they were
kept in a large shallow basin of water not so deep but that they
could everywhere touch the bottom with their feet and keep their
heads above the surface. With specimens collected at higher
elevations this treatment was inoperative, and the suggestion is
made that S. atra at the lower confines of its habitat partakes
more of the nature of maculosa than do the individuals from
greater heights; for Kammerer argues that pools suitable for
breeding must be more uncommon at those elevations than they
are lower down.

In the earlier paper18 Kammerer states that newly caught
females of S. atra often give birth in the water, and show an
undoubted preference for doing so. He describes also how he
once saw several females, wild in their natural habitat, lay their
young in a rain-puddle at 1,800 metres elevation, but the larvae
thus born were fully formed.

When the deposition of the young as larvae has become

17 " Bei Fortdauer der Versuchsbedingungen sind ah Vollmolche geborene Sala-
mandra maculosa gleich bei der ersten Geburt abermals voll molchgebarend, benutzen
zum Geburtsakt das trockene Land, und zwar unter Erreichung der (bei Salamandra
atra normalen) Embryonen-Zweizahl," Kammerer, 1907, p. 49.

18 1904, p. 56.

206 PROBLEMS OF GENETICS

" habitual "19 with S. atra, three to nine larvae may be produced
at one spawning period, from 35 to 45 mm. long, with gills at
most 8 mm. long, and a tail-fin 2-3 mm. broad. Such larvae
are generally coffee-brown, or grey (instead of black), and show
other minor differences.

The summary states that when grown to maturity they be-
come in their turn larva-bearing, and go into the water to bring
forth. Their young are more than two (3 to 5 being the numbers
observed) with a length of 33-40 mm. or of 21-23 mm- at birth.
They are light grey, spotted (mottled with lighter and darker
colour), have relatively short gills (8 to 9 mm. at most) and a
broad tail-fin (3 mm. wide). At metamorphosis they are rela-
tively long (44 mm.) and one of them had some yellow pigment.

Here again this summary is, as a matter of fact, describing
the behaviour of two mothers, of which one produced three, and
the other five young.

To my mind these experiments suggest that the reproductive
habits of both species, if closely observed, will be found to be
subject to considerable variation, and I think it not impossible
that each species is, especially in confinement, capable of being a
good deal deflected from its normal behaviour. Moreover, there
seems to me no great improbability in the idea that there is an
interdependence between the number of young and the stage of
maturity in which they are born. But, at the same time, the case
as told by Kammerer strikes me as proving too much. If each
species is so sensitive to conditions that the normal procedure
is gravely modified in one generation, and if that modification
can reappear in a pronounced form in the next generation without
a renewal* of the disturbing conditions, it becomes extremely
difficult to understand how the regularity which each species is
believed to display in nature can be maintained. Surely both
species might be expected to be in confusion. From a passage
in Kammerer's earlier paper (1904, p. 55) on the subject, I infer
that he also would expect considerable irregularity in the natural
behaviour, but that he has not investigated the point.20

19 Throughout Kammerer's papers this is used almost as a technical term.
It means, I presume, that the feature was manifested more than once.

20 It should be stated that the papers contain a quantity of detail, especially

ADAPTATION 207

3. Modification of the Colour of Salamandra maculosa induced
by Change in the Colour of the Soil on which the Animals were
kept. — Kammerer speaks of this as the most convincing of all
his experiments on the transmission of acquired characters. So
far, however, no full account of them has been published.21 The
statement is that when salamanders are kept in yellow surround-
ings the yellow markings gradually in the course of years increase
in amount relatively to the black ground colour. Conversely by
keeping the animals on black garden soil, the yellow may be
greatly diminished in quantity until it largely disappears. (The
account in Natur adds that very moist conditions also favour the
increase of yellow, and that with less moist conditions the yellow
diminishes.) From each kind, the (induced) yellower and the
(induced) blacker, a second generation was raised, on soil of
neutral colour, and each family was later divided into two parts,
half being put on black and half on yellow ground.

As regards the offspring of those which had lived on black
soil no positive result had been reached up to the date of publica-
tion, but it is stated that these young resembled their parents
in having the yellow distributed in irregular spots.

As regards the offspring of those which had lived on yellow
soil the account follows up the story of that part of the offspring
which were put on yellow soil again. It is stated that these, though
derived from parents with irregular spots, developed the yellow
as longitudinal bands.

This account is given with slight differences of expression in
the three places to which I have referred. On returning from
Vienna in 1910 I consulted Mr. G. A. Boulenger in reference to
the subject, and he very kindly showed me the fine series from
many localities in the British Museum, and pointed out that in
nature the colour- varieties can be grouped into two distinct types,

descriptive of the state of the larvae, which I have not attempted to represent, but
the account here given contains all that seemed essential to an understanding of
the more important features of the account.

21 The first appeared in Natur, 1909-10, Heft 6, p. 94; and the second, which
contains coloured plates of the animals, in the lecture already referrred to, 12
Flugschr. d. Deut. Ges.f. Zilchtungkunde, Berlin, 1910, p. 26. In the paper in Mendel
Festschrift, 1911, the subject is continued, but no more is added as to this part of
the experiment.

eo8 PROBLEMS OF GENETICS

one in which the yellow of the body is irregularly distributed in
spots and one in which this yellow is arranged for the most part
in two longitudinal bands which may be continuous or inter-
rupted. The spotted form is, as he showed me, an eastern variety,
and the striped form belongs to western Europe. Mr. E. G.
Boulenger22 has since published a careful account of the distri-
bution of the two forms. The spotted he regards as the typical
form, var. typica, and for the striped he uses the name var.
taeniata. The typical form occupies eastern Europe in general,
including Austria and Italy, extending as far west as parts of
eastern France. The var. taeniata is found all over France,
excepting parts of the eastern border, Belgium and western
Germany, Spain and Portugal. Of the very large series examined
there was only one specimen (Lausanne) which could not with
confidence be referred to one or other of the two varieties.
Mr. E. G. Boulenger points out that both varieties inhabit very
large areas, and live on soils of most different colours and com-
positions. Both are liable to variations in the amount and the
shade of the yellow, but that any suggestion that taeniata belongs
especially to yellow soils and typica to black soils is altogether
inadmissible. He expresses surprise that Kammerer should not
allude to these peculiarities in the geographical distribution of
the two forms. He suggests further that it is more likely that
some mistake occurred in Kammerer's observations than that the
east European typica should, in the course of a generation, have
been transformed into the west European taeniata by the influ-
ence of yellow clay soil.

In his last paper on the subject Kammerer states incidentally23
that he has found the striped form recessive to the spotted. No
evidence for this statement is given, and I have not found any
other reference to crosses effected between the two natural types.
If, however, this representation is correct, it is conceivable that
the production of taeniata from typica was in fact the re-ap-
pearance of a recessive form. The plate which Kammerer gives
in illustration of his modified parent figures a single animal at
four stages, and though it is certainly more like the spotted than

ME. G. Boulenger, Proc. Zool. Soc.t 191 1, p. 323-
28 Mendel Festschrift, 1911, p. 84.

ADAPTATION 209

the striped form, it has a certain suggestion of the striped arrange-
ment, such as I can well imagine being produced in the hetero-
zygote.24

In continuation25 of the experiments on the colour of S. macu-
losa Kammerer publishes an account of elaborate experiments
in grafting ovaries of the various forms, modified and unmodified,
into each other, and describes the offspring which followed.
Before pursuing this part of the inquiry I am disposed to wait
until the earlier steps have been made much more secure than
they yet are.

More recently Kammerer has published similar statements in
regard to the inheritance of characters induced in various lizards
by keeping them in abnormal temperatures, high and low. The
changes induced affected in some species the colours, in others
the reproductive habits. Respecting these examples I feel the
same scepticism that I have indicated in regard to the others,
somewhat heightened by the fact that insufficient evidence is
given both regarding the behaviour of these various species in
captivity when not subjected to abnormal temperatures, and
in the wild state.

Respecting this part of the evidence Mr. G. A. Boulenger has
lately published a criticism26 from which I extract the following
passages. Referring to a previous note27 on the question of the
melanism of the various insular forms of Lacerta muralis he
writes: " I also alluded (/. c.) to the theories that have been
propounded to explain the melanism of various insular forms.
This is a subject which has been lately taken up by Dr. Kam-
merer at the Biologische Versuchsanstalt in Vienna, and he claims
to have produced nigrinos artificially by a very strong elevation
of the temperature, accompanied by extreme dryness. Dr.
Werner28 has already opposed his own experiments to those of
Kammerer, artificial melanism having been produced by him in
Lacerta oxycephala by keeping two very light specimens from

24 12 Flugschrift. Detit. Ges. Ziichtungskunde, 1910, Fig. 15, P. Reihe.

25 Mendel Festschrift, 1911, p. 83.

26 Field, 1912, 30 March.

27 Ibid., 1904, p. 863.

28 Mitth. Naturw. Ver. a. d. Univ. Wien, 1908, p. 53.

IS

2io PROBLEMS OF GENETICS

Ragusa for a whole summer in very damp conditions. Neither is
Kammerer's theory in accordance with the distribution of the
black lizards, as pointed out by Werner. Kammerer also finds
that those forms which are known to produce melanic races in a
state of nature, lend themselves more readily than the others
to the success of his experiments. But he shows himself misin-
formed when he states that the variety called Lacerta fiumana
belongs to the category of those of which black forms are not
known. He overlooks the fact, first pointed out by Scherer in
1904, and which I can confirm, that the black lizard from Meli-
sello near Lissa in the Adriatic is unquestionably derived from
the lizard from Lissa, which he correctly regards as not separable
from L. fiumana. . . ."

" Another colour modification which Dr. Kammerer states
that he obtained by raising the temperature is the assumption by
the female of the typical Lacerta muralis of the bright red colour
of the lower parts which often distinguishes the male from the
female, and which was not shown by the individuals of the latter
sex kept by him under normal conditions. He quotes various
authorities to show that the lower parts are never red in the
females, but he has omitted to consult others who say the con-
trary. Thus Bedriaga (1878 and 1879) remarks that a so-called
var. rubriventris of the typical wall lizard has the lower parts red
in both sexes."29

In reading such papers as those of Semon or Kammerer the
thought uppermost in my mind is that to. multiply illustrations
of supposed transmission of acquired characters is of little use
until some one example has been thoroughly investigated. If
we had certain assurance that even a single unimpeachable case
could be repeated at will, the whole matter would assume a more
serious aspect. If, for instance, Kammerer were able to show us
Alytes males with horny pads on their hands, it would be some-
thing tangible; still more, if the experiment were repeated by
others until no doubt remained that the offspring of Alytes which
had bred in water for some three generations did acquire these

29 As to the variations of Lacerta muralis in Western Europe and North Africa
see Boulenger, G. A., Trans. Zool. Soc., 1905, vol. XVII, p. 351.

ADAPTATION 211

pads and that they could transmit these novelties to descendants
raised in normal conditions. Till evidence of this kind is pub-
lished by at least two independent observers investigating similar
material, I find it easier to believe that mistakes of observation or
of interpretation have been made than that any genuine trans-
mission of acquired characters has been witnessed.

Meanwhile there is no denying that the origin of adaptational
features is a very grave difficulty. With the lapse of time since
evolutionary conceptions have become a universal subject of study
that difficulty has, so far as I see, been in nowise diminished.
But I find nothing in the evidence recently put forward which
justifies departure from the agnostic position which most of us
have felt obliged to assume.30

APPENDIX TO CHAPTER IX.

Professor G. Klebs, as is well known to students of evolution-
ary phenomena, has for several years been engaged in investi-
gations relating to the inheritance of acquired characters. In
his many publications on the subject the issue has always been
represented as more or less uncertain.

Desiring to know how the matter now stands according to
Professor Klebs' present judgment I wrote to him asking him to
favour me with a brief general statement. This he most kindly
sent in a letter dated 8th July, 1912.

As such a statement will be read with the greatest interest
by all who are watching the progress of these studies I obtained
permission to publish it as follows:

8. Juli 1912

Ihre liebenswurdige Anfrage will ich sehr gern beantworten,
obwohl ich sie nicht so beantworten kann wie ich erwiinschte.
Ihr Skepticismus in der Frage der Uebertragung erworbener
Charactere auf die Nachkommen ist nur zu berechtigt. Meine
Versuche mit Veronica sind nicht beweisend, da es mir bisher
nicht gelungen ist eine einigermasse konstante Varietat mit
verlaubten Inflorescenze zu erzeugen. In Bezug auf mein
Semper vivum bin ich allerdings noch heute der Meinung dass

80 As to the experiments of Klebs relating to the transmission of acquired char-
acters, see Appendix.

212 PROBLEMS OF GENETICS

die starke kunstliche Veranderung der Bliite einen Einfluss auf
einzelnen Nachkommen gehabt hat. Ich habe seither nichts
dariiber veroff entlicht : die Mehrzahl der anormalen gefiillten
Bliiten war leider steril. Von einem weniger veranderten Ex-
emplar erhielt ich einige Samlinge, aber sie haben noch nicht
gebliiht. Es kann sich in diesem Falle nur um eine Nachwirkung
in der ersten Generation handeln, vergleichbar jenen Fallen in
denen Samen von Baumen aus den hohen Alpen in der Ebene
gewisse Nachwirkungen zeigen. Aber es ist bisher kein sicherer
Fall bekannt in den der kunstliche herbeigefiihrte Charakter
mehrere Generationen hindurch unter der gewohnlichen "normalen"
Bedingungen iibertragen worden ist.

Auf der andere Seite sind diese negativen Resultaten nicht
entscheidend. Denn wie wenig ist in dieser Beziehung iiber-
haupt ernstlich versucht worden! Und zweifellos geht die
Sache nicht so einfach.

Ich versuche es mit anderen Pflanzen weil ich der Meinung
bin dass es moglich sein musse wenigstens solche neuen Varie-
taten zu erzeugen, wie sie die Gartenvarietaten entsprechen.

Aber bis jetzt leider sind die Versuche nicht gelungen, weder
mir noch irgend einem anderen.

CHAPTER X

EFFECTS OF CHANGED CONDITIONS CONTINUED
THE CAUSES OF GENETIC VARIATION

IN the last chapter we examined some of the evidence offered
in support of the belief that adaptation in highly organised forms
is a consequence of the inheritance of adaptative changes induced
by the influence of external conditions. The state of knowledge
of this whole subject is, as I have said, most unsatisfactory,
chiefly for the reason that in none of the cases which are alleged
to show a positive result have two observers been over the
same ground, or as yet confirmed each other. In the wider
consideration respecting the causes of variation at large we find
ourselves still in the same difficulty. The study has thus far
proved sadly unfruitful. In spite of the considerable efforts
lately made by many observers to induce genetic variation in
highly organised plants or animals, and though successes have
occasionally been announced, I do not know a single case which
has been established and confirmed in such a way that we could
with confidence expect to witness the alleged phenomena if we
were to repeat the experiment. Abundant illustrations are
available in which individuals exposed to novel conditions mani-
fest considerable changes in characters or properties, but as yet
there is no certain means of determining that germ-cells of a new
type shall be formed.

Of the direct effect of conditions the lower organisms, espe-
cially bacteria, offer the best examples, the alterations of virulence
which can be produced in so many distinct ways being the most
striking and familiar. That attenuation of virulence can be
produced by high temperatures or by exposure to chemical
agents, and that this diminution in virulence may remain perma-
nent is, from our point of view, not surprising ; but the fact that
in many cases the full virulence can by suitable cultivation be

213

214 PROBLEMS OF GENETICS

restored is difficult to understand. Similar variations have been
observed in power of pigment production and other properties.

These phenomena naturally raise the question whether any
cases of apparent loss of factors in higher forms may be com-
parable.

The subject of variations in the lower organisms and their
dependence on conditions is a highly special one, and I have no
knowledge which can justify me in offering any discussion of
them, but I understand that hitherto little beyond empirical
recognition of the phenomena has been attempted. A useful
summary of observations made by many investigators was lately
published by Hans Pringsheim,1 who enumerates the different
agencies which have been observed to produce modifications,
and the various ways in which these changes are manifested.
One of the most comprehensive studies of the subject from the
genetic point of view is that made by F. Wolf.2 In his extensive
cultivations of Bacillus prodigiosus, Staphylococcus pyo genes and
Myxococcus he succeeded in producing many strains with modi-
fied properties. In most of these the modifications arose in
consequence of the application of high or low temperatures or of
the addition of various chemical substances to the culture-media.
Some of the variations, which are for the most part in the powers
of pigment-formation, persisted when the strains were returned
to normal conditions, and others did not. In reference especially
to the variations witnessed in the Cocci the reader should consult
the critical account of variation in that group published by the
Winslows,3 where much information on the subject is to be
found. The authors attempted to determine the systematic
relationships of the several forms, as far as possible, by the
application of statistical methods. The result is interesting as
showing that the problem of species in its main features is pre-
sented by these organisms in a form identical with that which
we know so well in the higher animals and plants, whatever

1 Pringsheim, H., Die Variabilitdt niederer Organismen, Berlin, 1910.

2 F. Wolf, Modifikationen u. Mutationen von Bakterien, Zts. F. indukt. Abstam.
u. Vererbungslehre, II, 1909, p. 90.

3 Winslow, C. E. A. and A. R.t Systematic Relationships of the Coccaceae.
New York. 1909.

CAUSES OF GENETIC VARIATION 215

properties be selected as the diagnostic characters. There are
many types perfectly distinct and others which intergrade.
Some of the types change greatly with conditions while others do
not. This is exactly what we encounter whenever we study the
problem of species on an extended scale among the higher forms
of life.

There is now practically complete agreement among bac-
teriologists that the observations made first by Massini on the
change in color of Bacterium coli mutabile grown in Endo's
medium, associated with the acquisition of the power to ferment
lactose, are perfectly reliable and free from possibilities of mis-
take. The work has been extended and confirmed by many
workers, especially R. Miiller, who finds that this bacterium can
similarly acquire and maintain the power to ferment other
sugars. A careful account of the whole subject written by Miiller
for the information of biologists will be found in Zts.fur Abstam-
mungsl., VIII, 1912. After discussing the biological significance
of the facts, he concludes with a caution to the effect that bacteria
are so different from all other living things that generalizations
from their behavior must not be indiscriminately applied to
animals and plants.

In all work with this class of material there is obviously
danger of error through foreign infection of the cultures, but
there can be no doubt that though some of the "mutations"
recorded may be due to this cause, the majority of the instances
observed under stringent conditions are genuine.

Another and equally serious difficulty besetting work with
bacteria and fungi cultivated from spores is that the appearance
of variation may in reality be due to the selection of a special
strain previously living masked among other strains. This
possibility must be remembered especially in those instances
wrhich are claimed as exemplifying the effects of acclimatisation.
Manifestly this consideration can be urged with most force when
the strain which gave rise to the novelty was not raised from a
single individual spore. Moreover, when once the possibility of
spontaneous variation is admitted, it must be difficult to be quite
confident that any given variation observed is in reality due to

216 PROBLEMS OF GENETICS

the novel conditions applied, and as I understand the evidence,
the appearance of the mutational forms does not with any
regularity follow upon the application of the changed conditions.

Researches into the variation of these lower forms will, no
doubt, be continued on a comprehensive scale. So long as the
instances recorded are each isolated examples it is impossible to
know what value they possess. If they could be coordinated in
such a way as to provide some general conception of the types of
variation in properties to which bacteria, or any considerable
group of them, are habitually liable, the knowledge might
greatly advance the elucidation of genetic problems.

Of mutational changes directly produced with regularity in
micro-organisms by treatment, the experiments with trypano-
somes provide some of the clearest examples. A summary of
the evidence was lately published by Dobell,4 from which the
present account is taken. The most definite fact of this kind
established is that certain dyes introduced into the blood of the
host have the effect of destroying the small organ known as the
" kineto-nucleus " in the trypanosomes. The trypanosomes thus
altered continue to breed, and give rise to races destitute of
kinetonuclei. This observation was originally made by Wer-
bitzki and has been confirmed by several observers. The exact
way in which this alteration is effected in the trypanosomes is not
quite definitely made out, but there is good reason for supposing
that the dyes have a direct and specific action upon the kineto-
nucleus itself, and circumstances make it improbable that in
some division a daughter-organism without that body is pro-
duced, or that any selection of a pre-existing defective variety
occurs.

Ehrlich has suggested with great probability that the dyes
which possess this action owe it to the fact that they have the
particular chemical linkage which he calls "ortho-quinoid." In
outward respects, such as motility and general appearance, the
modified organisms are unchanged, but their virulence is di-
minished. As regards the possibility of the defective strain re-

4C. C. Dobell, Jour. Genetics, 1912, II, p. 201, where full references are given.
Still more recently the same author has contributed an excellent summary of
the evidence relating to bacteria (ibid., II. 1913, p. 325).

CAUSES OF GENETIC VARIATION 217

acquiring the kinetonucleus, Werbitzki states that in one case
passage through 50 animals and treatment with dyes left the
strain unaltered; but that in another case at the sixteenth
passage 7 per cent, of the trypanosomes were found to have
re-acquired the organ, and in subsequent passages the percentage
increased, until at the twenty-seventh passage practically all had
re-acquired it. Kudicke, however, in similar experiments did not
succeed in causing re-acquisition by transplantation.

By the action of various drugs and antibodies races of try-
panosomes resistant to those substances have been obtained.
These breed true, at least when kept in the same species of animal
in which the resistance was acquired. As to whether change of
virulence is produced by passage through certain animals or not,
there is as yet no general agreement.

Other changes, especially in size and some points of structure,
are said to occur when certain trypanosomes proper to mammals
are passed through cold-blooded vertebrates (Wendelstadt and
Fellmer), and it is stated that these changes persist, but the
observations have not yet been confirmed.

Experiments lately conducted by Woltereck with Daphnia
are interesting as having given a definite positive result, in so far,
at least, as the ova were affected by conditions before leaving the
bodies of the parent individuals. The observations relate to
the offspring resulting from partheno genetic eggs. Females
bearing ephippia (fertilised eggs) were isolated until the ephippia
were dropped, and in this way the offspring of fertilisation were
excluded. Males, of course, appeared from time to time in the
cultures, but as fertilised eggs were rejected, their presence did
not disturb the result. The most remarkable observations
related to Daphnia longispina.

This species as found in the lower lake at Lunz had the front
end of the body blunt and nearly round in profile; but on being
cultivated in a warm temperature and with abundant nourish-
ment the front end of the body became produced into an elon-
gated "helmet," as Woltereck calls it. Experiment showed that
the change was primarily due to the abundance of food, and owing
to temperature in a subordinate degree.

'2i8 PROBLEMS OF GENETICS

This distinction arose as soon as the species was taken into
the hothouse, but when the modified individuals were put back
into the original conditions, a lower temperature and scanty
food-supply, the next generation returned to their original form.
After being cultivated for two years and about 40 generations in
the more favourable conditions, when similarly put back into
the lower temperature with scanty food the first generation born
in these conditions was helmeted like the modified parents.
Woltereck is of opinion that the ova were still unformed at the
time the parents were put back, and the influence of the favour-
able conditions upon the unformed ova he speaks of as a "prae-
induction." The effect never extended beyond the one gener-
ation, after which the strain returned to its original state.

The fact that the influence on the offspring was not mani-
fested at first led Woltereck to expect that by more prolonged
cultivation in the favourable conditions a further extension of
this influence would be produced, but this expectation was never
fulfilled, though the attempt was made again and again.

Similar experiments were made with Hyalodaphnia cucullata,
which is far more sensitive to cultural influences, and in nature
manifests a considerable elongation of the helmet as a seasonal
modification, but the results were essentially the same as in the
preceding case, no modification extending beyond the first
generation born after the restoration to normal conditions?

The only criticism of these extremely interesting results which
suggests itself is that perhaps the original appearance of the
modification was not in reality due to an accumulated effect of
the conditions, but to some change in the conditions themselves
which was not noticed. It is difficult to see how length of
time or even the lapse of several generations could have so specific
an effect on the race. It is no doubt often vaguely supposed
by many that a long period of time may be necessary for the
effect of climate or of other environmental conditions to be
produced in an organism which does not thus respond at first.
I have never been able to see any reason for this opinion nor how

5 See Woltereck, Verh. d. Deut. ZooL Ges., 1909, p. no; and 1911, p. 142. This
is a subject which can only be properly appreciated on reference to the original
papers. Several complications are involved to which I have not here alluded.

CAUSES OF GENETIC VARIATION 219

it is to be translated into terms of physiological fact, and I
imagine that in those cases in which the lapse of time is really
required for the production of an effect, the influence of the
prolongation is rather on the conditions than on the organisms.
The response of the organisms thus probably indicates not that
the creature is at length feeling the effects because of their
accumulated action on itself, but that the conditions have at
length ripened.

As this sheet is passing through the press Agar has published6
an abstract of evidence as to another comparable case in a par-
thenogenetic strain in the daphnid, Simocephalus vetulus. When
fed on certain abnormal foods the shape of the body is changed,
the edges of the carapace being rolled backwards so as to expose
the appendages. The offspring of animals thus modified showed
similar modification in the first, and to a very slight degree, in
the second generation, though the original mothers were removed
to normal conditions before their eggs were laid. In the third
generation there was "a very pronounced reaction in the opposite
direction." Agar suggests that the change may be due to some
toxin-like substances, carried on passively by the egg into the
next generation, against which the protoplasm eventually pro-
duces an anti-body.

The experiments which have been in recent years regarded
by evolutionary writers as the most conclusive proof that direct
environmental action may produce germinal variation are those
of Professor W. L. Tower, of Chicago, on Leptinotarsa, the
potato beetles. This work has attained considerable celebrity
and has been generally accepted as making a definite extension
of knowledge. After frequently reading Tower's papers and
after having been privileged to see some of the experiments in
progress (in 1907) I am still in doubt as to the weight which
should be assigned to this contribution.

The work is described in two chief publications, the first of
which appeared in I9o6.7 This treatise contains a vast amount
of information about numerous species and varieties of these

6Proc. Roy. Soc., B, Vol. 86, 1913, p. 113.

7 An Investigation of Evolution in Chrysomelid Beetles of the Genus Leptinotarsa,
Carnegie Publications, 1906, No. 48.

220 PROBLEMS OF GENETICS

beetles which the author has observed and bred in many parts of
their distribution throughout the United States, Mexico and
Central America. The part of the book which has naturally
excited the greatest interest is that in which Tower states that
by subjecting the beetles to change in temperature and moisture,
he caused them to produce offspring quite unlike themselves,
which in several cases bred true.

It is much to be regretted that the author did not happen to
become acquainted with Mendelian analysis at an earlier stage
in the investigation. The evidence might then have been
handled in a much more orderly and comprehensive way, and a
watch would have been kept for several possibilities of error.

The headquarters of the genus is evidently as Tower states,
in Mexico and the adjoining countries. In this region there is
a great profusion of forms, some very local, some as for instance
the well-known decemlineata* more widely spread. The dis-
tinctions are almost all found in peculiarities of colour and
pattern, and the limits of species are even more indefinable than
is usual in multiform animals. Tower arranges the various types
into seven groups of which the one most studied is that which
he calls the lineata group. To this group belong all the forms
to which reference is here made, and, as I understand, they differ
among themselves entirely in size, colour and pattern. There
is no suggestion of infertility in the crosses made between the
several forms of the lineata group ; in fact they present, like many
Chrysomelidae, a good example of what most of us would now
call a polymorphic species, consisting of many types, some found
existing in the same locality, others being geographically isolated.

A series of experiments was devoted to the attempt to fix
strains corresponding to the extremes of continuous variations.
For example, those with most black pigment and those with
least black taken from a population continuously varying in this
respect, were separately bred; but almost always the selection
led to no sensible change in the position of the mean of the popu-

8 This is the famous Colorado beetle or potato-bug, which has caused such serious
destruction in potato crops. There seems to be no doubt that this insect, formerly
unknown in the eastern States, made its way east along the mining trails when the
west was opened up.

CAUSES OF GENETIC VARIATION 221

lation. The variations in these cases were evidently fluctu-
ational. In some instances, however, real genetic differences
were met with, and strains exhibiting them were, as usual,
rapidly fixed.

Tower points out that several of the varieties (or species, as
he prefers to call them) were obviously recessive to decemlineata.
This is most clearly demonstrated in the case of the form called
pallida, which is a pale depauperated-looking creature, with the
orange of the thorax almost white and the eyes devoid of pig-
ment.9 This form behaved as an ordinary Mendelian recessive,
breeding true whenever it appeared in the cultures, or when
individuals found wild were studied in captivity. A black form
which Tower names melanicum was similarly shown to be a
Mendelian recessive. Wild specimens of this variety of opposite
sexes were not found simultaneously in nature, and there was thus
no opportunity of breeding them together, but the hereditary
behaviour was seen in the F2 generation from a melanicum found
coupled with decemlineata. Experiments also occurred giving
indication that a variety with the stripes anastomosing in pairs
(tortuosa), was another recessive, and that a variety — called
" rubri-vittata" — gave an intermediate FI with subsequent
segregation. All these are forms of decemlineata Stal.

Similar observations were made regarding forms recessive to
multitaeniata Stal. Of these two were thrown by multitaeniata
itself, namely a form named by Stal melanothorax, and regarded
by him as a species, and one which Tower names rubicunda n. sp.
The facts proving the recessive behaviour of their several forms
will be found in the following places in Tower's book:

pallida, pp. 273-278.

melanicum, p. 279.

tortuosa, p. 280.

rubrivittata, pp. 280-281.

melanothorax and rubicunda, pp. 283-285.

Following this evidence of recessive nature of the six forms

9 This is indicated in the coloured plate, but I have not found any explicit state-
ment to this effect in the text, and am not sure if the absence of pigment was re-
garded as complete.

222 PROBLEMS OF GENETICS

enumerated, Tower describes experiments showing, as he believes,
that some of them may be caused to appear by applying special
treatment to the parents during the "growth and fertilisation"
(p. 287) of the eggs. The most striking example is that in which
4 males and 4 females of decemlineata were kept very hot (average
35° C.) and dry, and at low atmospheric pressure (19-21 inches).
The eggs laid were restored to natural conditions. These gave
506 larvae, from which emerged 14 normal, 82 pallida and 2
" immaculo-thorax " viz., without pigment on the pronotum.
The account of the rest of the experiment is somewhat involved,
but I understand that the pallida, of which two only survived,
behaved as normal recessives when bred to the type: also that
the parents, after having laid the eggs whose history has been
given, were restored to normal conditions and laid 319 eggs which
gave 6 1 normals.

In another case normal parents laid 409 eggs in the hot and
dry conditions, and on restoration to normal conditions, the
same parents laid 840 eggs. Then 409 eggs gave 64 adults as
follows :

Males Females

decemlineata 12 8

pallida 10 13

immaculothorax 2 3

albida 9 7

13 3i

The 840 eggs laid in normal conditions gave 123 normal decem-
lineata.

Similar experiments were made with multitaeniata and gave
comparable results, the two recessives (melanothorax, rubicunda)
being produced in large numbers when the parents were subjected
to heat, but in this case the atmosphere was kept saturated with
moisture, instead of dry, as in the previous instance. The same
parents transferred to normal conditions gave normals only.

Lastly the form undecimlineata was exposed "to an extreme
stimulus of high temperature, 10° C. above the average," and a
dry atmosphere, with the result that from 190 eggs there emerged
II beetles, all of the form angustovittata Jacoby, which subse-
quently bred true to that type (see p. 295).

CAUSES OF GENETIC VARIATION 223

In the results of these experiments, as described, there is one
feature which I regard as quite unaccountable. Tower makes no
comment upon it. Indeed, from the general tenour of the paper,
I infer, not only that he does not perceive that he is recounting
anything contrary to usual experience, but rather that he regards
the result as conforming to expectations previously formed.
The point in question is the genetic behaviour of the dominant
normals produced under the abnormal conditions. These
normals were the result of the breeding of parents declared to be
at the same time giving off many recessive gametes. Some of
these normals must be expected therefore to be heterozygous
unless some selective fertilisation occurs. Nevertheless in every
case they and their offspring are reported to have continually
bred true. I allude especially to the tables given on pp. 288, 289,
292, and 293. Tower does not mention any misgiving about
this result, and I think he regards himself as recounting phe-
nomena in general harmony with the ideas of mutation expressed
by De Vries. This they may be; but to anyone familiar with
analytical breeding the course of these experiments must seem so
surprising as to call for most careful, independent confirmation.

In I9io10 Tower published an account of further experiments
with Leptinotarsa. The work described related to two subjects.
Crosses were made between three forms, undecimlineata Stal,
signaticollis Stal and "diver so," named by Tower as a new
species. The distinctions between these three depend partly on
characters of the adults and partly on those of the larvae. The
adults of undecimlineata and diver sa have the elytra striped, but
the elytra of signaticollis are unstriped. The larvae of sig-
naticollis and of diver sa are yellow, but those of undecimlineata
are white.11 Moreover, in signaticollis and diversa the black in-
creases in the third stage of the larvae to form transverse bands
which are absent in undecimlineata. The general course of the
experiments shows that these differences may be approximately

10 Biol. Bull., XVIII, 1910, p. 285.

11 This description does not quite agree with the representation of the larvae
in PI. 17 of the book Evolution in the Genus Leptinotarsa for there the larva of
undecimlineata is shown as white in the second stage, but yellowish in the third
stage; perhaps there is an error in printing.

224 PROBLEMS OF GENETICS

represented as due to the action of three factors, any of which
may be independently present or absent. The stripings of the
elytra and of the larvae are each due to a separate factor. As
regards the distinction between the yellow and the white larvae
the evidence does not prove that there is decided dominance of
either colour and I infer that the heterozygotes are often inter-
mediate.

The chief contribution which this new paper claims to make
relates to differences in the results which ensue from crosses
effected between these three types at different average tempera-
tures.

We are first concerned with four experiments which I number

(I), (2), (3), (4):.

1. Signaticollis 9 X diver so, o71 bred at an average tempera-
ture of 80° F. by day and 75° F. by night, gave two groups in
about equal numbers. The first (49) was pure signaticollis and
bred true. The second (53) was of an intermediate type, which
on being bred together gave the typical Mendelian result — I sig.:
2 intermediate: I div.

2. Next, as the account originally stood in the published
paper, we are told that sig 9 X div c? bred together at a day-
temp, average 75° F. and night average 50° F. gave an inter-
mediate only, which subsequently produced a normal 1:2:1
ratio. The two crosses were repeated eleven times with identical
results.

In a further experiment (3) signaticollis 9 X diver sa <? were
bred under the same conditions as those used in expt. (i). They
again gave sig. and intermediates as before in fairly equal numbers.
The sig. as before bred true, and the intermediate gave I : 2 : I ,
all exactly as in expt. (i).

In expt. (4) the same parents used in (3) were again mated
under conditions of expt. (2) at the lower temperature, and this
time gave signaticollis exclusively, which bred true for four
generations. This experiment was repeated seven times with
uniform results.

Diagrams are given representing all these histories in graphic
fashion.

CAUSES OF GENETIC VARIATION 225

From these observations, Tower concludes that the determi-
nation of dominance, and the ensuing type of behaviour, is clearly
a function of the conditions incident upon the combining germ
plasms.

It will be observed that expts. (i) and (3) gave identical
results but (2) and (4), though much the same conditions were
applied, are at variance, for (2) gave all intermediates, while
(4) gave all signaticollis. In Amer. Nat., XLIV, 1910, p. 747,
Professor T. D. A. Cockerell commented on this paper of Tower's
and pointed out that there must be an error somewhere, for when
he discusses these experiments Tower speaks of (2) and (4) as
confirming each other. To this Tower replied12 that there had
been a mistake. He states that in preparing the paper "certain
minor experiments were taken from a larger series and combined
to illustrate a general point in the behaviour of alternative
characters in inheritance," and that expt. (2) was introduced
inadvertently in place of another which he desires to substitute.
In this, which I number (5), signaticollis 9 X diversa cf from
exactly the same stocks as those used in (i), were mated at the
lower temperatures specified for (2), day average 75° F., night
average 50° F. These gave all of the signaticollis type with a
narrow range of variability, which bred true, in some cases to F«.
Tower says he has repeated this experiment six times with identi-
cal results.

Nevertheless he proceeds to say that the description of expt.
(2), which was repeated eleven times with identical results, was
correct "as far as given." That experiment was " from a second
series of cultures parallel to the one given, but in which there are
other factors involved, which in H. 410 [my (2)] are productive
of a typical Mendelian behaviour." He adds he does" not care
at this time to make any statement of what these factors are,
nor of their relations to the behaviours given in the H. 409, H. 41 1 ,
H. 409/11 series [my (i), (5) and (3)-(4)] which are the simplest
and most easily presented series obtained in the crossing of
signaticollis and diversa."

Professor Cockerell's intervention has thus elicited the fact

« Biol. Bull., XX, 1910, p. 67.
16

226 PROBLEMS OF GENETICS

that we have as yet only a small selected part of the evidence
before us, even as concerning the effect of temperature on the
cross between signaticollis 9 X diver sa cf . We learn that at
the lower temperatures the result was eleven times the expected
one, and six times an unexpected one ; further, that we owe it to
the author's inadvertence that we have come to hear of the
expected result at all, and that though he knows the factors
which determine the discrepancy, he declines for the present to
name them. In these circumstances we can scarcely venture as
yet to estimate the significance of these records.

The paper goes on to recount somewhat comparable, but more
complex instances in which the descent of the colour of adults and
of larvae was affected by temperature in crosses between un-
decimlineata and signaticollis. As they stand the results are
very striking and unexpected, but I think, in view of what has
been admitted respecting the former part of the paper, full dis-
cussion may be postponed till confirmation is forthcoming.

One feature, however, calls for remark. This second paper is
written apparently without any reference to the discoveries
related by Tower in his previous book, to which no allusion is
made. This is most noticeable in the case of an experiment in
which ( p. 296, H. yooA ) undecimlineata 9 (the dominant)
was mated to signaticollis o71 with the result that all the offspring
were undecimlineata and bred true to that type (Parthenogenesis
was tested for, but never found to occur). This experiment was
made at a temperature averaging 95° F. =1= 3.5° by day and 89° F.
=t= 4.8° by night, and in a humidity given as 84 per cent, by day
and 100 per cent, by night; but in the previous book (p. 294) we
are told that pure undecimlineata bred together "under an ex-
treme stimulus of high temperature, 10° C. above the average"
and a relative humidity of 40 per cent, gave n beetles only,
all angustovittata. But reference to the Plate 16, Fig. 2, shows
that angustovittata must be exceedingly like signaticollis, having,
like it, the elytral stripes obsolete, and if there is any marked
difference at all, it can only be in the larvae. It seems strange
that if undecimlineata really gives off ova of this recessive type at
high temperatures, the fact should not be alluded to in connection

CAUSES OF GENETIC VARIATION 227

with expt. H. 7OoA, where, as the father was signaticollis, having
the same recessive character, their appearance might have been
expected not to pass unobserved. The temperature in the
older experiment is, of course, not given with the great accuracy
used in the second , and it may have been higher still. The humid-
ity also was widely different. Still, in discussing the phenomena
we should expect some reference to the very remarkable and
closely cognate discovery which Tower himself had previously
reported in regard to the same species.13

The hesitation which I had come to feel respecting these two
publications of Tower's has been, I confess, increased by the
appearance of a destructive criticism by Gortner14 who has ex-
amined the parts of Chapter III of Tower's book, in which he
discusses at some length the chemistry of the pigments in Lep-
tinotarsa and other animals. As Gortner has shown, this dis-
cussion, though offered with every show of confidence, exhibits
such elementary ignorance, both of the special subject and of
chemistry in general, that it cannot be taken into serious con-
sideration.

Some observations made by Dr. W. T. Macdougal15 have
also been interpreted as showing the actual causation of genetic
variation by chemical treatment. Of these perhaps the least open
to objection were the experiments with Raimannia odorata, a
Patagonian plant closely allied to Oenothera. The ovaries were
injected with various substances and from some of the seeds
which subsequently formed in them a remarkable new variety
was raised. This varying or mutational form was strikingly
different from the parental type, with which it was not con-
nected by any intergradational forms, and it bred true. It made

11 As to the interrelations of these three forms, Tower states (1906, p. 18) that
angustavittata, which he reared from undecimlineata, is intermediate between it
and signaticollis. Compare Stal, "Monogr. des Chrysomelides," 1862, p. 163; and
Jacoby, Biol. Centr. Amer. Celeopt., vi, Pt. i, p. 234, PI. xiii, fig. 20; Tab. 41, fig.
15; ibid., Suppl., p. 253. All these forms are evidently very closely related, and
the delimitation of species is quite arbitrary. Jacoby indeed suggests that unde-
cimlineata may be a variety of decemlineata.

14 Gortner, Amer. Nat., Dec., 1911, XLV, p. 743.

16 Mutations, Variations, and Relationships of the Oenotheras, Carnegie Institu-
tion Publication No. 81, 1907, pp. 61-64.

228 PROBLEMS OF GENETICS

no rosette, growing to a much smaller size than the parent, and
was totally glabrous instead of being very hairy as the parental
type is. I was shown specimens of these plants by the kindness
of Dr. Britton in the Bronx Park Botanic Garden in 1907 and
can testify to their very remarkable peculiarities. They had a
somewhat weakly look, and might at first sight be thought to be
a pathological product, but they had bred true for several
generations. From the evidence, however, I am by no means
satisfied that their original appearance was a consequence of
the treatment applied. This treatment was of a most miscel-
laneous description. Two of the mutants came from an ovary
which had been treated with a ten per cent, sugar solution. Ten
came from one into which a o.i per cent, solution of calcium
nitrate had been injected. One was from a capsule which "had
been exposed to the action of a radium pencil." Macdougal
speaks of these results as decisive, but clearly before such evidence
can be admitted even for consideration it must be shown by con-
trol experiments that the individual plants which threw the
mutant were themselves breeding true in ordinary circumstances.
Nothing is more likely than that the mutant was an ordinary
recessive. I may add that Mr. R. H. Compton made a number
of experiments with Raimannia odorata, raised from seeds kindly
given me by Dr. Britton, injecting the ovaries with a variety of
substances, including those named by Macdougal; but though a
numerous progeny was raised from the ovaries treated, all were
normal. Macdougal relates also that some mutational forms
came from ovaries of Oenothera Lamarckiana exposed to radium
pencils, and also from Oenothera biennis injected with zinc sul-
phate a peculiar mutant was raised, but taking into account the
frequency of these occurrences in those species, he very properly
regarded this evidence as of doubtful application. In a later
paper,16 however, he has returned to the subject and affirms his
conviction that the appearance of a mutant among seedlings
raised from an ovary of Oenothera biennis treated with zinc
sulphate was really a consequence of the injection, saying that

16 Macdougal, D. T., "Alterations in Heredity induced by Ovarial Treat-
ments," Bot. Gaz., vol. 51, 1911, p. 241.

CAUSES OF GENETIC VARIATION 229

the variation previously observed in the species was afterwards
shown to be due to fungoid disease. The circumstances to which
he mainly points in support of his view is that the mutation bred
true, but this is only evidence of its genetic distinctness, which
may, of course, be admitted by those who remain unconvinced
as to the original cause of its appearance. He adds that he is
making similar experiments with some twenty genera; but what
is more urgently needed is repeated confirmation of the original
observation. When it has been shown that this mutation can
be produced with any regularity from a plant which does not
otherwise produce it on normal self -fertilisation, the enquiry
may be profitably extended to other plants.

A curious and novel experiment, which however, led ulti-
mately to a negative result, was made by F. Payne. Many dis-
cussions have been held respecting the blindness of cave animals.
The phenomenon is one of the well-known difficulties, and most
of us would admit that the theory of evolution by the natural
selection of small differences does not offer a really satisfying ac-
count of it. Those who believe in the causation of such modifica-
tions by environmental influences and in their hereditary trans-
mission make, of course, the simple suggestion that the darkness
is the cause of the loss of sight, and that disuse has led to the
reduction of the visual organs. Payne bred Drosophila ampelo-
phila, the pomace-fly (which is easy to keep in confinement, fed
on fermenting bananas) , for sixty-nine generations in darkness.
At the end of that period there was no perceptible change in the
structure of the eyes, or in any other respect. The number of
generations may possibly be regarded as insufficient to prove
anything, but comparing them, as he does, with the generations
of mankind, we see that they correspond with a period of about
two thousand years, an interval far longer than those which
many writers in particular cases have deemed sufficient.

In his first paper Payne states that, though no structural
difference could be perceived, the flies which had been bred in
the dark reacted less readily to light than those which had been
reared under normal conditions, and he inclined to think that
the treatment had thus produced a definite effect. After more

230 PROBLEMS OF GENETICS

careful tests, however, he withdrew this opinion. It proved
that both individual flies and individual groups of flies, both of
those bred in the light and of those bred in the dark, differed
greatly in their reactions, which were measured by counting the
time that it took for a fly to travel to the light end of a covered
tube, various sources of error being eliminated. He found further
that these differences of behaviour were not inherited in any
simple way, but he is disposed to attribute them to accidental
differences in the nature of the food, an account which seems
probable enough.17

In several recent publications Blaringhem18 has described
the origin of many abnormal forms of plants, especially of maize,
which he attributes to various mutilations practised upon the
parents. Respecting these the same difficulty which has been
expressed in other cases reappears, that before drawing any
conclusion as to the value of such evidence we require to know
that the plants treated belong to a really pure line, which if
left to nature in the ordinary circumstances of its life in that
locality would have had normal offspring. Abnormalities abound
in the experience of everyone who examines pans of seedlings
of almost any species of plant, and in maize they are well known
to be exceptionally common. Some of those which we meet
with when we attempt to ripen maize in this country are very
similar to those which Blaringhem describes, consisting in ir-
regularities in the distribution of the sexes, in the shapes of the
panicles, etc. Many of these are doubtless imperfections of
development, due to the dullness of our climate, but others are
presumably genetic and would recur in the offspring however
treated. If some one working in a climate where maize could
be raised in perfection would repeat these experiments, and show
that a strain which was thoroughly reliable and normal in its
genetic behaviour did, after mutilation, throw the miscellaneous
types observed by Blaringhem, that would be evidence at least
that the development of the seed could be so influenced by
injury to the parental tissues that its properties were changed.

"Payne, Fernandus, Biol. Bull., XVIII, 1910, p. 188, and ibid., XXI, 1911,
p. 297.

18 See especially, Mutation et Traumatismes, Paris, Felix Alcan, 1908.

CAUSES OF GENETIC VARIATION 231

Such evidence could be used for what it is worth ; but pending an
inquiry of this kind I am disposed to regard these observations of
variation following on parental injury as suggestive rather than
convincing.

Some evidence of a remarkably interesting kind has been
collected by J. H. Powers19 respecting the structure and habits
of Amblystoma tigrinum, which led him to the conclusion that
striking differences in the form, anatomy, and developmental
processes could be effected directly by change in the conditions
of life. It is well known that a profusion of forms, distinct in
various degrees, is grouped round Amblystoma tigrinum. Some
of these are believed to be geographically isolated, others occur
together in the same waters, and, as usual, authorities have dif-
fered greatly as to the number of names to be given. These forms
were studied in detail by Cope who described them in the Ba-
trachia of North America. The view which he inclined to take
was that the individual variations of Amblystoma tigrinum re-
sulted from variations in the time and completeness of the
metamorphosis, and these were regarded as due to external
causes, such as differences in season, temperature, and geo-
graphical conditions. Powers, however, states that collecting
within a radius of six or eight miles he found almost if not quite
the whole "gamut of recorded variation in this species." Some,
however, as he states, occurred rarely except under experimental
conditions, but considerable differences in temperature were not
found necessary in producing them. Every year, he says, he
has been able to add to the number of peculiar types found in
the same small area in nature, until the amount of natural
variation at least equals that seen by Cope in the collections of
the National Museum and those of the Philadelphia Academy.

Powers states that his observations by no means confirm
Cope's view that these differences are in the main referable to
variation in the completeness of metamorphosis, and on the
contrary, he regards metamorphosis as on the whole a levelling
process, tending to obliterate diversity. The enormous dif-

19 J. H. Powers, "Morphological Variation and its Causes in Amblystoma
tigrinum." Studies from the Zoological Laboratory. The University of Nebraska,
No. 71, 1907.

232 PROBLEMS OF GENETICS

ferences in size and proportions which he describes can only be
appreciated by reference to his figures. They affect almost all
features of bodily organisation. These striking differences he
looks upon as brought about by differences in nutrition, "diversi-
ties in habitual locomotion," and diversity in the age at which
metamorphosis occurs, and to sexual difference. Apart from
sexual difference he regards the chief distinctions, in brief, as
"acquired variations of the larva."

As an example he gives the great elongation of some of the
forms as "due first to slow growth, second to the free-swimming
habit, third to the prolongation of larval life, and finally to the
assumption of sexual maturity as males," either in the branchiate
or non-branchiate condition. He describes the rapid growth of
some and the slow growth of others. A larva of intermediate
type may grow about a centimeter a month, but a rapidly growing
specimen may grow more than four times as much. The slower
rate of growth may, he says, be induced by winter feeding, and
other treatment.20

When, however, he goes on to describe the influences which
he regards as exerted by the habit of freely swimming, I am led
to wonder whether after all in most of these illustrations, the
primary distinctions are not in reality genetic. "Specimens
raised in the same aquarium or in similar aquaria, side by side
with all conditions as uniform as it is possible to make them,
seldom fail to furnish striking examples of broad-headed, short-
bodied, and short- tailed types which are habitually found at the
bottom, while others, slender and elongated, are free swimmers,
and maintain themselves in almost as continual suspension and
motion as does a gold fish." Later, again, he writes, "Yet despite
the uniformity of these favourable conditions, the larvae soon
began to split up into two noticeably distinct groups, the one of

20 In connexion with this case I would refer the reader to some remarkable
observations of Dr. T. A. Chapman on various types of larvae which he reared
from the moth Arctia caja (Ent. Rec., IV, 1893, p. 265, and following parts). From
a single mother he raised a great diversity of forms, some which fed up rapidly
and passed through their development without assuming certain stages, and others
which were, as he called them, "laggards," moulting more times than their brethren
and developing at a much slower rate. It is greatly to be hoped that such a case
may be critically investigated by analytical breeding.

CAUSES OF GENETIC VARIATION 233

unusually compact proportions, the other of uniform intermediate
build, such is most commonly met with." It is to my mind
scarcely possible to resist the inference that, though there may be
definite responses to certain conditions, yet the chief distinctions
are genetic, and that it is these distinctions which confer the
power to respond. The parts respectively played by cause and
effect are always difficult to assign; but when it is stated that
"a weak-limbed, long-bodied and long- tailed animal becomes
well nigh perforce an undulatory swimmer, while the strong-
limbed, short-tailed, heavy-bodied specimen, when these charac-
teristics are rapidly forced upon it, is, under certain circum-
stances, just as forcibly induced to become a crawler," we feel
how erroneous any estimates of causation are likely to be.

One of the most remarkable and interesting sections of
Powers' paper is that in which he describes the differences in
bodily structure and habits which he attributes to cannibalism,
and the whole account of the phenomena should be read in the
original. It appears that there are two extremely distinct
types of larvae, those with narrow heads and slender bodies
which live for the most part on small Crustacea such as Daphnias,
and those with huge mouths and very wide heads, which dis-
regard such small animals altogether and live on amphibian
larvae, whether of their own or other species. As the illustra-
tions show, the differences between these two types are very
great, and the differences in instinct and behaviour are no less.
The cannibals take no heed of the pelagic Crustacea, lying slug-
gishly at the bottom, rousing themselves immediately to a
violent attack on the larger living things which approach them.
Nothing but the most incontrovertible evidence based on abun-
dant control experiments should convince us that such differences
are not primarily genetic, and in the present state of knowledge
I incline to think that the families really consist of individuals
which are ready to assume the cannibal habit if opportunity
offers, and others which are congenitally incapable of it. It may
readily be that if all chance of cannibal diet be excluded, the
full development of the wide head and mouth, or the other
peculiarities, would never become pronounced, but I doubt whether
such change could be induced in any individual taken at random.

CHAPTER XI.

STERILITY OF HYBRIDS. CONCLUDING REMARKS.

WHEN we consider the bearing of recent discoveries on those
comprehensive schemes of evolution with which we were formerly
satisfied, we find that certain details of the process are more easy
to imagine. We readily now understand how varieties once
formed, can persist, but at the same time difficulties hitherto
faced with complacency become formidable in the light of the
new knowledge. So generally is this admitted by those familiar
with modern genetic research that most are rightly inclined to
postpone the discussion. The premisses, indeed, on which such
a discussion must be based are almost wholly wanting.

The difficulties to which I chiefly refer are not those created
by the phenomena of adaptation, though they are serious enough.
In treating of that subject I* have felt obliged to express scepti-
cism as to the validity of nearly all the new evidence for the
transmission of acquired characters. At the present time the
utmost we are bound to accept is the proof that (i) in some
parthenogenetic forms variations, or perhaps we may say mal-
formations, produced in response to special conditions, recur in
one or perhaps two generations asexually produced after removal
to other conditions. (2) That violent maltreatment may in rare
instances so affect the germ-cells contained in the parents as to
cause the individuals resulting from the fertilisation of those
cells to exhibit an arrest of development similar to that which
their parents underwent.

I do not doubt that evidence of this type will be greatly
extended. As a contribution to genetic physiology these facts
are very important and interesting, but I cannot think that any
one, on reflexion, will feel encouraged by such indications to
revive old beliefs in the direct origin of adaptations.

In these respects we are simply left where we were. The
force of objections based upon the existence of adaptative

234

STERILITY OF HYBRIDS 235

mechanisms is no greater than it has always been. On the con-
trary the fact that variations can now so generally be recognized
as definite is some alleviation of the difficulty. We can moreover
disabuse ourselves of the notion that for all characters which are
definite or fixed, some utilitarian rationale may be presumed.
Upon that point the study of variation has provided a perfectly
clear answer.

In frankly recognizing that the fixity of characters in general
need not connote usefulness to their possessors we deliver our-
selves of a distracting pre-occupation and prepare our minds for
an investigation of the properties of living organisms in the
same spirit as that in which the chemist and the physicist
examine the properties of unorganized materials. The creature
persists not merely by virtue of its characteristics but in spite
of them, and the fact of its persistence proves no more than
that on the whole the balance of its properties leaves something
in its favour.

It may be noted by the way that the fact that the structures
of living things are on the whole adaptative was not always
obvious. Though to naturalists of this generation it is a truism,
we have only to turn to Buffon to find that in his philosophy of
nature it played no essential part. The passage in which Buffon
describes what he regards as the forlorn and degraded condition
of the Woodpecker is well known. We have come to think of
the Woodpecker as a capital example of adaptation to the mode
of life; but Buffon after enumerating the hard features of the
bird's existence, forced to earn its living by piercing the bark of
trees in an attitude of perpetual constraint, remarks1 " Tel est
1'instinct etroit et grossier d'un oiseau born£ a une vie triste et
chetive. II a regu de la Nature des organes et des instrumens
appropries a cette destinee ou plutot il tient cette destinee meme
des organes avec lesquels il est ne" (my italics). His reflexions
on the Stilt (Himantopus) read even more strangely to us,
accustomed as we are to see in the prodigious length and thinness
of the shanks and in the other features of its organisation pal-
pable adaptations to a wading life. For Buffon, however, this

1 Buffon, Hist. Nat., Oiseaux, 1780, VII, p. 3.

236 PROBLEMS OF GENETICS

curious bird seemed a poor, neglected production, extravagant
in its disproportions, one of the misfits of creation, left as a
shadow in the picture composed of nature's more successful
efforts.2 This theme he develops at some length, being evidently
well pleased with the idea.

Our way of regarding these things is doubtless sounder and
more fruitful than Buffon's, but it is well to remember that what
seems so obvious to us looked quite differently to other excellent
observers; and stupid as it may have been to have overlooked
plain examples of adaptation, it is a far worse mistake to see
adaptation everywhere. I do not seek to minimise the real
and permanent difficulty which the existence of adaptations
creates, but by the suggestion that all normal specific differences
are adaptational that difficulty was quite gratuitously increased.

In these respects it may be claimed that progress has been
made, even if that progress seem outwardly of small account.

But all constructive theories of evolution have been built on
the understanding that what we know of the relation of varieties
to species justifies the assumption that the one phenomenon is a
phase of the other, and that each species arises or has arisen
from another species either by one or several genetic steps. In
the varieties we have accustomed ourselves to think that we see
those steps. We still know little enough of the mode of occur-
rence of variation, but we do begin to know something, and if we
ask ourselves whether our knowledge, such as it is, conforms at
all readily with our former expectations, we cannot with any
confidence assert that it does. Among the plants and animals
genetically investigated are many illustrations of very striking
and distinct varieties. Many of these might readily enough be
accepted as species by even the most exacting systematists, and
not a few have been so treated in classification; but when we
have examined their relationship to each other we feel not merely
that they are not species in any strict sense but that the dis-
tinctions they present cannot be regarded as stages in the direc-
tion of specific difference. Complete fertility of the results of
inter-crossing is and I think must rightly be regarded as incon-

2 ibid., viii, p. 115.

STERILITY OF HYBRIDS 237

sistent with actual specific difference; and of variations leading
to that consequence no clear indication has yet been found.
As an example of possible exceptions mention should perhaps be
made of the case of a giant form of Primula sinensis investigated
by Keeble.3 It arose from a " Star " Primula of normal size,
and though fertile with its own pollen all attempts to fertilise
it with the pollen of other forms failed. Miss Pellew, who did
these fertilisations, tells me that very extensive trials were made,
and repeated in several seasons. Ultimately two plants were
raised from it fertilised with a plant of the strain from which it
sprang, and these proved sterile. In the light of modern expe-
rience the significance of such isolated instances is doubtful.

All the strains known as " Giants " are, as Messrs. Sutton
have always found, more or less sterile, and their sterility is
presumably due to some negative defect.

In regard to the fertility of Primula species there are several
paradoxes. For example the long-styled varieties, apart from
giants, are fertile with their own pollen, and for many years
short-styled plants have not been used in most strains. Auriculas
and Polyanthuses, on the contrary, are generally if not always
bred from short-styled plants, as the florists have decided that
the long-styled are inadmissible. Mr. R. P. Gregory tells me
that, though most strains of P. sinensis give seed enough
when only long-styled plants are used, he finds nevertheless
that when a " legitimate " union is made the amount of seed
usually increases much as Darwin observed. Darwin's state-
ment that plants of " illegitimate " origin are less fertile than the
" legitimately " raised plants is also in general confirmed by his
experience. To this rule there were some marked exceptions in
strains derived from long-styled plants, which though illegitimate
showed a high degree of fertility, but illegitimate unions between
short-styled plants always produced comparatively sterile off-
spring. I have no records of the behavior of Auriculas and
Polyanthuses. It would be interesting to know whether among
them pure strains of short-styled plants (dominants) have
appeared, and, if so, how their fertility is affected. Without

•Keeble, Jour. Gen., 1912, II, p. 173.

238 PROBLEMS OF GENETICS

much more critical data I suppose no one would nowadays be
inclined to follow Darwin in instituting a comparison between
the sterility of hybrids and that of illegitimately raised plants of
heterostyle species.4 It is even difficult to imagine any essential
resemblance between these two phenomena, nor has evidence
ever been produced to show that illegitimately raised plants
have bad pollen grains, which is the usual symptom of sterility
in hybrid plants and the consequence, as we believe, of failure
of some essential division in the process of maturation.

The difficulty that we have no knowledge of the contemporary
origin of forms, from a common stock, which when crossed together
give a sterile product, is one of the objections constantly and
prominently adduced from the time of the first promulgation of
evolutionary ideas. In the light of recent work the objection
has gathered strength. Why, if we are able to produce instances
of variation colourably simulating specific difference in almost
all other respects, do we never find an original appearance of this
most widely spread of all specific characteristics? No doubt all
breeders know that sterile animals and plants occasionally appear
in their cultures, but it is more in accordance with probability
that the sterility in these sporadic instances should be regarded
as due to defect than that it should be thought comparable
with that of the sterile hybrids. For their sterility must, by all
analogy with results elsewhere seen, be attributed not to the
absence of something, but to the presence and operation of
complementary factors leading to the production of inhibition
of division; and consistently with that interpretation, we find
that when from a partially sterile hybrid comparatively fertile
offspring can be raised, their comparative fertility continues in
the posterity generally if not always without diminution. The
distinction between these several kinds of sterility was of course
not understood in Darwin's time. The comparison, for example,
which he instituted5 between the sterility of " contabescent "
anthers and that of hybrids no longer holds, for at least in those
cases in which the nature of contabescent anthers have been
genetically investigated (Sweet Pea, Tropaeolum) they proved

4 Animals and Plants, ed. i, 1868, II, pp. 180-5.
B Animals and Plants, ed. i, 1868, II, p. 165.

STERILITY OF HYBRIDS 239

to be a simple recessive character. Nor can we now easily
suppose that the attempt there made by Darwin to suggest
resemblance between the sterility produced by unnatural condi-
tions and that of hybrids has any physiological justification.

In regarding the power to produce a sterile or partially
sterile hybrid as a distinction in kind, of a nature other than
those which we perceive among our varieties, I am aware that
I am laying stress on an impression which may hereafter prove
false. The distinction nevertheless is so striking and so con-
tinually before the eyes of a practical breeder that he can
scarcely avoid the inference that when he meets a considerable
degree of sterility in a cross-bred he is dealing with something
belonging to a distinct category, and not merely a varietal feature
of an exceptional kind.

Besides the sterility of hybrids appeal has often been made to
the phenomenon of incompatibility, in its several stages of
completeness, as distinguishing species. No one doubts that in-
compatibility may arise from a variety of causes of most diverse
degrees of importance, but though sometimes referred to as an
extreme case of interspecific sterility, it is really a very different
matter. In regard to one phase of this incompatibility, that
associated with self -sterility, some progress has been made, and
we are not wholly without experimental evidence of its being
within the range of contemporary variation.

Given the outline of Mendelian teaching as to gametic dif-
ferentiation and the classification of individuals in a mixed
population, it seemed highly probable that what we call self-
sterility must mean that the species really consisted of classes,
some of which are capable of interbreeding with others while
others are not. According to the received account every indi-
vidual, though incapable of fertilising itself, was supposed to be
able both to fertilise and to be fertilised by any other individual.
This notion has always seemed to me a self-evident absurdity,
for it would imply that there can be as many categories as
individuals. Such experiments, however, as I made did cer-
tainly give results consistent with that belief. I first tried
Cinerarias, which are usually self-sterile, but I found no in-

c4o PROBLEMS OF GENETICS

compatible pairs of plants. Whether I was deceived by the
consequences of apogamy, or whether the pollen of certain plants
may belong to more than one class I do not know. The results
were confused in various ways. Usually the self-fertilised plants
set little or nothing, and cross-fertilised they set fully with such
uniformity that the few failures could plausibly be attributed to
mistakes in manipulation or to other extraneous causes. Later
de Vries announced6 (without giving particulars) that he had
proved the existence of such classes in Linaria vulgaris; but on
making experiments with that species I again got no positive
results, and I came to the conclusion that in spite of inherent
improbability the conventional belief must be substantially true.
At last, however, the work of Correns, lately published,7 does
definitely show that in one species, Cardamine pratensis, classes of
individuals exist such that individuals of the same class are
incapable of fertilising themselves or each other, but fertilisation
made between the classes is usually completely effective. Many
complications were encountered and some contradictory evidence
is recorded, but the general bearing of the results was positive
and indubitable.

We know far too little of this phenomenon as yet to be able
to understand its significance, but I suppose we may anticipate
with some confidence that it will be found to be a manifestation
of dissimilarity between the male and female gametes of the
same individual, comparable with that first seen in the Stocks
(Matthiola) which throw doubles — a state of things in all likeli-
hood to be found widely spread among hermaphrodite organisms.
Whether the incompatibility between species is to be associated
with that of the self-steriles also cannot be positively asserted,
though it seems not unreasonable to expect that such an associa-
tion will be discovered.

The case of the apple and the pear is an impressive illustration
of this possibility. The two species are of course exceedingly
alike in all outward respects, but nevertheless the pollen of each
is entirely without effect on the other. Presumably we should

6 Species and Varieties, 1905, p. 471.

7 Correns, Festschr. med.-nat. Ges. zur 84 VersammL Deutsch. Naturf. u. Aertze.
Munster i. W., 1912.

STERILITY OF HYBRIDS 241

interpret this fact as meaning not so much that the apple and
the pear are in reality very wide apart, but rather that either,
each is lacking in one of two complementary elements, or that
each possesses a factor with an inhibitory effect. Their incom-
patibility may well be of the same nature as that of the classes
in Cardamine pratensis.

Returning now to the problem of inter-specific sterility; we
note, as I have said, the absence of contemporary evidence that
variation can confer on a variety the power to form a sterile
hybrid with the parent species. The considerations based on
this want of evidence have for a long while been familiar to all
who have discussed evolutionary theories, and it is worth observ-
ing the exact reason why the difficulty strikes us now with a new
and special force. In pre-Mendelian times all that was known
was that some forms could freely interbreed without diminution
of fertility in the product, while others could not. But now we
find that, by virtue of segregation, from one and the same pair
of parents, or even, in the case of hermaphrodites, from one and
the same individual, offspring commonly arises howing among
themselves exactly such differences as distinguish species — and
very good species too. This we see happening again and again.
But to forms capable of arising as brethren in one family the
title species has never been meant to apply, and if we are going
to use the term in application to fraternal groups we must
definitely recognise that by " specific " difference is to be under-
stood simply difference, without any immediate or even ulterior
physiological limitation whatever. Naturally, therefore, we begin
to think of the appearance of sterility in crosses as something
apart, and as a manifestation which distinguishes certain kinds
of unions in a very special way.

I am perfectly aware that there are gradations in the sterility
of hybrids as in every other characteristic upon which it has been
proposed to base specific definitions; but, as also so often happens
in the matter of defining intergrading categories, the difficulty
in practice is not often such as to lead to actual ambiguity. I
am speaking of course of those examples which are amenable to
genetic experiment. As to the rest there is complete and perma-

17

242 PROBLEMS OF GENETICS

nent uncertainty. But the experience of the practical breeder
does, I think, on the whole, support the contention to which
systematists have so steadily clung under all the assaults of
evolutionary philosophers, that, though we cannot strictly define
species, they yet have properties which varieties have not, and
that the distinction is not merely a matter of degree.

The first step is to discover the nature of the factors which by
their complementary action inhibit the critical divisions and so
cause the sterility of the hybrid. Thus expressed, we see the
problem of inter-specific sterility in its right place; and the
question why we do not now find contemporary instances of
varieties lately arisen in domestication, which when crossed back
with their parents, or with their coderivatives, can produce
sterile products, is perceived to be only a special case of a problem
which in its more general form is that of the origin of new and
additional factors.

For the requisite evidence no comprehensive search has been
made, but perhaps it will yet be found. All that we can say at
the present time is that the incidence both of hybrid sterility,
and of incompatibility also, is most capricious; and provided
that two forms have such features in common that a cross between
them seems not altogether out of the question, no one can predict
without experiment whether such a cross is feasible, and if
feasible whether the product will be fertile, or sterile more or
less completely. For instance, though probably all the British
and some Foreign Finches (Fringillidae) have been crossed
together, and some of these crosses, as for instance, the various
Canary-mules have been made in thousands, I believe no quite
clear example of a fertile hybrid can be produced. Many species
of Anatidae cross readily and produce fertile hybrids : others give
results uniformly sterile. Though most of the Equidae can be
crossed and some of the hybrids are among the commonest of
domesticated animals there is no certain record of a fertile mule.
Among the Canidae the dogs, wolves and jackals all give fertile
hybrids, but there is no clearly authenticated instance of a cross
between any of these forms and the European fox. In spite of
their close anatomical resemblance it is doubtful if the rabbit

STERILITY OF HYBRIDS 243

and the hare have ever interbred. Many of the wild species of
Bos have been crossed and recrossed both with each other and
with many domesticated races, but I understand that no cross
with the Indian buffalo (Bos bubalus) has yet been successful
even in producing a live calf.8 In the genus Primula many
hybrids are known and several of them occur in nature, but
hitherto no certain hybrid between P. sinensis and any other
species has been made, in spite of repeated attempts.

In Nicotiana many — doubtless all — the various forms of N.
tabacum can be crossed together without diminution of fertility,
though some are very distinct in appearance, but crosses between
tabacum and sylvestris are highly sterile (in my experience totally
sterile9) , though the distinctions between them are not to outward
observation nearly so great as those which can be found between
the various races of Primula sinensis.

Recently some remarkable experiments bearing closely on
these questions have been published by F. Rosen.10 They con-
cern the forms of Erophila (Draba) verna, celebrated in the history
of evolutionary theory as the plants especially chosen by Alexis
Jordan for the exposition of his views on these subjects.

The " species " contains a profusion of forms dissimilar in
many structural characters, such as the size and shape of leaves,
flowers, fruits, etc. Of these forms many grow in association.
Jordan found, on experiment, that each, to the number of some
two hundred, bred true, and that therefore, the conventional
assumption that polymorphism of this kind must mean great
contemporary variability had no foundation in fact. So far

8 This is a case of a somewhat different order and I mention it partly for that
reason as an illustration of the complexity which such negative instances may
present. The difficulty is that though the buffalo and the zebu can breed together,
the foetus is too large to be born alive. (See Ackermann Ber. d. Ver. f. Naturk.,
Kassel, 1898, p. 69. Prof. S. Nathusius, of Halle, who has great experience in
crossing Bovidae, tells me that he has always failed to cross the buffalo with
other species.)

9 In a paper to be published in the Report of the Genetic Conference, Paris,
1911, Bellair states that he obtained some partially fertile hybrids in the cross
N. sylvestris X tabacum. As to the various degrees of sterility in hybrids between
Nicotiana species see Lock, R. H., Ann. Roy. Bot. Gardens. Peradeniya, IV, 1909,

P- 195-

10 Beitrage zur Biol. der Pflanzen., X, 1911, p. 379.

244 PROBLEMS OF GENETICS

indeed is the evidence from favouring the belief that such forms
are in any way transitional or indeterminate, that, as is well
known, Jordan used it with every plausibility to support the
doctrine of the fixity of species. To certain aspects of Jordan's
work we will return later in this chapter, but the matter is in
the present connection of especial interest for the reason that
Rosen has lately found by experiment that some of these pre-
sumably very closely allied forms, crossed together, gave hybrids
more or less sterile. In the case of the offspring of one pair of
forms only (E. cochleata and stricta) was the fertility undi-
minished, and the various degrees of sterility found in the
other crosses ranged up to the extreme infertility of the hybrids
between E. stricta X data. From this cross ten plants were
bred. Of these the four strongest were chosen to breed from,
but two of the four proved totally sterile ; one had only bad seeds ;
and from the fourth a single seedling was raised which in its
turn proved to be sterile. From the less sterile hybrids F2
families were raised, with the usual experience that in this and
subsequent generations the sterility diminished among extracted
forms, new and true-breeding types with complete fertility being
thus derived from the original cross.11

The production of sterility as a consequence of crossing
plants so nearly approaching each other as these Erophila
" species " do is not a little interesting, and the fact well ex-
emplifies the futility of the various attempts to frame general
expressions as to specific properties or behaviour. Commenting
on his results Rosen argues that the polymorphic group commonly
called by systematists Erophila (Drabd) verna may now be
regarded as having arisen by crossing, as did his own types men-
tioned above. The question, however, what species were the
original progenitors of the group cannot be answered. Rosen
considers that no form which he knows satisfies the requirements,

11 One very peculiar feature was observed, namely, that all the new forms in Fa
which were bred from came true. As I understand, this statement applied to five
such new types, and they were represented by 76 individuals in F3, but further
details on this point are desirable. Another curious fact was observed, namely
that one of the Fi forms (cochleata X radiata) when fertilised by cochleata gave a
highly polymorphic family, but fertilised by radiata the resulting offspring were
almost uniform.

STERILITY OF HYBRIDS 245

and that it or they must be supposed to be lost. This conclusion
will recall the similar problem raised by the Oenothera mutants
(Chap. V); and unsatisfactory as it may be to have recourse
to such hypotheses we must remember the possibility that as a
consequence of hybridisation, subsequent segregation and re-
combination of factors, species may have thus actually, as we
may say, exploded, and left nothing but a polymorphic group of
miscellaneous types to represent them in posterity. If this way
of regarding the phenomena be a true one, the sterility now seen
when some of the group are re-crossed, becomes analogous to
that " reversion or crossing " which we now so well understand
to be a consequence of the recombination of characters separated
at some previous point in the history of descent. In the partial
sterility of the contemporary hybrid we see this character re-
appearing, formed now as it was on the occasion of the original
cross, by the meeting of complementary factors.

Another case that may be mentioned in this connection is
that of the crosses between various culinary peas (Pisum sativum)
and a peculiar form found by Mr. Arthur Sutton growing os-
tensibly in a wild state in Palestine. This Palestine Pea is low
growing, rarely reaching 18 inches. It is in general appearance
like a small and poorly grown field pea. The stems are thin and
rather hard. The most obvious differences which distinguish
this from other field peas are the marked serration of the stipules,
and the development of pith in the pods. Such pith is often
present in the pods of peas more or less, but in the Palestines it
is so strongly developed as almost to form a lomentum. Curi-
ously enough, though the flowers are purple much as those of
ordinary field peas, there is no coloured spot in the axils. On
the other hand, the stems have coloured stripes running up
from the axils. Though this plant differs so little from domes-
ticated peas, all crosses with them either failed, or produced
hybrids quite or almost quite sterile. This was Mr. Sutton's
experience, and on repeating the experiments with material
kindly given by him I found the same result.12

In a large series of crosses some seeds died or gave rise to

u I also had a few Fi seeds given me by Mr. R. H. Lock.

246 PROBLEMS OF GENETICS

feeble plants. Of the plants which lived, few gave any seed.
The seed, however, that was obtained from FI plants grew well
enough, and the Fz plants proved, as often in such cases, fertile.
In these, indeed, no sign of sterility was noticeable. The experi-
ment is being repeated in various ways, for, as the genetic
behaviour of peas is comparatively well known, the subject is an
exceptionally favourable one for these investigations.

Such an example shows the confusion produced the moment
we attempt to harmonize conceptions of specific difference with
results attained by experimental methods. It has been usual
to regard the field pea (P. arvense) as a species distinct from the
edible pea (P. sativum). De Candolle and others regard the
field pea as derived from a form wild in Italy, but the origin of
the edible pea is considered to be unknown. From breeding
experiments we find no sterility whatever in the crosses between
the various arvense and sativum types, nor in the crosses made
between them and several other peculiar types from various
countries; whereas this Palestine Pea, which only differs from a
small arvense in what might have been thought trivial characters,13
either fails to cross altogether or gives a sterile product, whatever
type be chosen as the other parent.

Examples of this kind have at least the merit that they lead
to more precise delimitations of the problem. We are confronted
with two distinct alternatives.

I. We may apply the term Species promiscuously to all
distinct forms. If we do so it must be clearly understood that
we cannot even rule out the several combinations of " presences
and absences " represented by the various types whether wild or
domesticated. For we may feel perfectly assured that at least
all the arvense and all the sativum types yet subjected to experi-
mental tests are on precisely the same level in this respect. There
is no distinction, logical or physiological, to be drawn between
them. Some contain more factors, and others contain fewer.
In some the re-combinations have been brought about by natural
variation or crossing, while the same consequences in the others
have resulted from man's interference.

u In a paper about to appear in Jour Linn. Soc. Mr. A. W. Sutton identifies
this Palestine pea as Pisum humile of Boissier and Noe.

STERILITY OF HYBRIDS 247

2. We may follow the conventions of systematists and dis-
tinguish the outstanding or conspicuous forms such as arvense,
guadratum, sativum and perhaps a few more as species, and leave
the rest unheeded. If this course is followed it must be clearly
understood and permitted as a piece of pure pragmatism, deliber-
ately adopted for the convenience of cataloguers and collectors,
without regard to any natural fact or system whatsoever.

But while following either the one plan or the other we shall
be still awaiting the answer, which only genetic experiment
can provide, to the question whether among the various types
there are some which differ from the rest in a peculiar way:
whether by having groups of characters linked together in
especially durable combinations, or by possessing ingredients
which cause greater or less disturbance in the processes of cell-
division, and especially in the processes of gametic maturation,
when they are united by fertilisation with complementary
ingredients.

Before any but the vaguest ideas regarding the nature and
significance of inter-specific sterility can be formed, a vast
amount of detailed work must be done. Sterility as a result of
crossing, as well as that which is alleged sometimes to arise in
consequence of changed conditions, is at best a negative charac-
teristic, and there are endless opportunities for mistake and mis-
interpretation in studying features of this kind. No one, I
suppose, would now feel any great confidence in most of the data
which from time to time are resuscitated for the purpose of such
discussions. Even the best collections of evidence, such as those
given by Darwin in Forms of Flowers, cannot be regarded as
critical when judged by present-day standards. Nothing short
of the most familiar acquaintance with the habitual behaviour
of individuals, and of strains kept under constant scrutiny for
several years would enable the experimenter to form reliable
judgments as to the value to be attached to observations of
this class.

The admission must, however, be faced that nothing in recent
work materially tends to diminish the surprise which has always
been felt at the absence of sterility in the crosses between co-

248 PROBLEMS OF GENETICS

derivatives. We should expect such groups of forms to behave
like the Erophila types, and frequently to produce sterile products
on crossing. Whatever be the explanation, the fact remains
that such evidence is wanting almost completely. In spite of
all that we know of variability nothing readily comparable with
the power to produce a sterile hybrid on crossing with a near
ally, has yet been observed spontaneously arising, though that
characteristic of specificity is one of the most widely distributed
in nature. It may be that the lacuna in our evidence is due
merely to want of attention to this special aspect of genetic
inquiry, and on the whole that is the most acceptable view which
can be proposed. But seeing that naturalists are more and
more driven to believe the domesticated animals and plants to be
poly-phyletic in origin — the descendants, that is to say, of several
wild forms — the difficulty is proportionately greater than it was
formerly, when variation spontaneously occurring was regarded
as a sufficient account of their diversity.

CONCLUDING REMARKS.

The many converging lines of evidence point so clearly to
the central fact of the origin of the forms of life by an evolutionary
process that we are compelled to accept this deduction, but as
to almost all the essential features, whether of cause or mode,
by which specific diversity has become what we perceive it to
be, we have to confess an ignorance nearly total. The trans-
formation of masses of population by imperceptible steps guided
by selection, is, as most of us now see, so inapplicable to the
facts, whether of variation or of specificity, that we can only
marvel both at the want of penetration displayed by the advo-
cates of such a proposition, and at the forensic skill by which it
was made to appear acceptable even for a time.

In place of this doctrine we have little teaching of a positive
kind to offer. We have direct perception that new forms of life
may arise sporadically, and that they differ from their progenitors
quite sufficiently to pass for species. By the success and main-
tenance of such sporadically arising forms, moreover, there is
no reasonable doubt that innumerable strains, whether in isola-

CONCLUDING REMARKS 249

tion or in community with their co-derivatives, have as a fact
arisen, which now pass in the lists of systematists as species. For
an excellent account of typical illustrations I would refer the
reader to the book lately published by R. E. Lloyd14 on the rat-
population of India. The observations there recorded are typical
of the state of things disclosed whenever the variations of large
numbers of individuals are closely investigated, whether in
domestication or in natural conditions.

Guided by such clues we may get a good way into the prob-
lem. We see the origin of colourable species in abundance.
Then, however, doubt arises whether though these new forms
are as good species as many which are accepted as such by even
cautious systematists, there may not be a stricter physiological
sense in which the term species can be consistently used, which
would exclude the whole mass of these petites esp&ces.

If further we find that we have, with certain somewhat
doubtful exceptions, never seen the contemporary origin of a
dominant factor, or of inter-racial sterility between indubitable
co-derivatives, it needs no elaboration of argument to show that
the root of the matter has not been reached.

Examination of the inter-relations of unquestionably distinct
species nearly allied, such as the two common species of Lychnis,
leads to the same disquieting conclusion, and the best suggestion
we can make as to their origin is that conceivably they may have
arisen as two re-combinations of factors brought together by the
crossing of parent species, one or both of which must be supposed
to be lost.

All this is, as need hardly be said, an unsatisfying conclusion.
To those permanently engaged in systematics it may well bring
despair. The best course for them is once for all to recognise
that whether or no specific distinction may prove hereafter to
have any actual physiological meaning, it is impossible for the
systematist with the means at his disposal to form a judgment of
value in any given case. Their business is purely that of the
cataloguer, and beyond that they cannot go. They will serve
science best by giving names freely and by describing everything

"Lloyd, R. E., The Growth of Groups in the Animal Kingdom, London, 1912.

250 PROBLEMS OF GENETICS

to which their successors may possibly want to refer, and gen-
erally by subdividing their material into as many species as
they can induce any responsible society or journal to publish.
Between Jordan with his 200 odd species for Erophila, and
Grenier and Godron with one, there is no hesitation possible.
Jordan's view, as he again and again declares with vehemence, is
at least a view of natural facts, whereas the collective species is a
mere abstraction, convenient indeed for librarians and beginners,
but an insidious misrepresentation of natural truth, perhaps
more than any other the source of the plausible fallacies regarding
evolution that have so long obstructed progress.

Nevertheless though we have been compelled to retreat from
the speculative position to which scientific opinion had rashly
advanced, the prospect of permanent progress is greatly better
than it was. With the development of genetic research clear
conceptions have at length been formed of the kind of knowledge
required and of the methods by which it is to be attained. If
we no longer see how varieties give rise to species, we may feel
confident that a minute study of genetic physiology of varieties
and species is the necessary beginning of any critical perception
of their inter-relations. It is little more than a century since
no valid distinction between a mechanical mixture and a chemical
combination could be perceived, and in regard to the forms of
life we may well be in a somewhat similar confusion.

As yet the genetic behaviour of animals and plants has only
been sampled. When the work has been done on a scale so
large as to provide generalisations, we may be in a position to
declare whether specific difference is or is not a physiological
reality.

INDEX OF SUBJECTS

PAGE

Abraxa grossulariata 105, 193

Aceras hircina, local variability. . . 123

Achatinellidae, local forms of 133

Acquired characters, inheritance of,

1 88, et seq., 217, 233

Acronycta psi, melanic 138

Adaptation, problem of 187, 234

Agelaius, local forms 120

Agrotis, fixed and variable species. 25

Alkaptonuria 83

Alpine Plants, growing larger, if

protected 183

Alpine Varieties 165

Alytes obstetricans, Kammerer's

experiments on 199. 210

Amblystoma, races of 230

Amphidasys betularia.melanic form,

136, 138

dimorphic larvae 141

Anodonta, polymorphism of 130

Antirrhinum, striped 57

species-hybrids 99

albinos no

Apple, will not cross with pear 239

Arctia caja, effects of temperature. 192

larval variation in 231

Arctic varieties 165

Argynnis paphia and valesina in

Italy 121

Armadillo, polyembryony 42

Artistic faculty 89

Arum, rights and lefts 57

Auriculas, short-styled selected . . . 236

Axis of symmetry in hand and foot 48

Axolotl, alleged effect of conditions 230

Azalea, bud-sports 55

Bacillus anthracis, unsegmented

form 71

Bacillus prodigiosus, variation in. . 213

Bacteria, variation in 212

Bacterium coli, variation in 214

Baeolophus, geographical races of . . 159

Barley, right and left-handed 58

Basilarchia, geographical races of. . 161

Begonia phyllomaniaca 50

hybrids 51

Bizarre Carnation, genetics of 54

Black, as a variation from red .... 148

PAGE

Blackbird, varying 150

Black Cock, fixity of 28

Boarmia repandata, melanic form . 136

rhomboidaria 137, 139

Botrytis susceptibility to 108

Bovidae, hybrid 242

Brachydactyly 89, 95

Bradypus, vertebral variation 68

Bud-sports geometrically irregular 54-57
Buffalo, attempts to hybridize. . . . 242

Bullfinch, gynandromorph 45

Bulimus detritus, local variation of 126

Canary, asymmetrical markings in 154

Canidae, hybrid 241

Capsella 100

Cardamine pratensis 239

Cat, Polydactylism 53

Carnation, Picotees and bizarres

compared 54, 58

Cataract, hereditary 89

Certhiola, melanic 142

Chladni figures 60

Choloepus, vertebral variation in . . 68

local variation in 119

Cinerarias, self -sterility in 238

Cistudo, local variation in 119

Climatic varieties 164

Coccaceae, variation in 213

Coenonympha arcania, climatic

forms of 179

satyrion 180

Coereba, melanic 142

Colaptes, geographical races, 147, et seq.

chrysoides 154

Colloids, growth in 65

Colorado beetles, experiments on. . 218

Colour blindness in twins 44

Continuous variation, possible ex-
ample of 173

Coracias, geographical races of. ... 160

Cotton, genetics of 98, 100

Coupling no

Crab, extra claws 74

Crustacean appendages and Serial

Homology 63

Crystals, analogy with 78

Cyclopian monsters, artificial 50

Daphnia, changed by environment 216

251

252

INDEX OF SUBJECTS

Dasypus, polyembryony 42

Dianthoecia, fixed and variable

species 25

Disease-resistance 87

Division, power of, a fundamental

attribute of living things 38

Genetics of 46, 50

Dogger Bank, large varieties on. . . 125

Dogs, hybrid 241

Dominance, nature of 95

Dominants, origin of new. . .88, 90, 95

Double monsters 42

Draba, experiments with 242

Drosophila 91

Payne's experiments on 228

Earthworm, regeneration 77

Elephant, tusk segmented 38

Entelechy 80

Environmental treatment, effects of,

188, et seq.

Enzymes and genetic factors 86

Epilepsy, inheritance of traumatic. 197

Equidae, sterility of hybrid 241

Erophila, experiments with 242

species 249

Exacum, right and left 57

Euphonia elegantissima, local forms 120
Eupithecia rectangulata, melanic

form 137

Factors, new 88

loss of 96

Factorial representation of varieties

158, 165

Falcons, geographical races 147

Fasciation 49

Ferments, Boyle on 54

Finger-prints of twins 44

Fixity and Variability in species. . 25

Flax, climatic experiments 197

Fowl, Silky 84

Leghorn 85, 90

Dominant white 94

Wyandotte 97

Rumpless 46

Foxes, incompatibility with dogs. . 241

Free-martin 44

Fringillidae, sterility of hybrid .... 241
Fundulus, cyclopian 50

Callus, invariability of wild species 13
and origin of poultry 90, 97

Genitalia, a basis for classification
in insects 13

Gentians, climatic experiments. ... 197

Geometrical structure and differen-
tiation 54, 56

Geometrical distinction between

germ-cells and somatic cells 58

Gladiolus, right and left 57

Gnophus obscurata, protective col-
ouring 141

Goldfinch, geographical races 147

Gonioctena variabilis, variation in

sexes of 121

Gouldian Finch, polymorphism, 148, 149
Gracilaria stigmatella, experiments

on 193

Grantia, large varieties of 125

Ground-Squirrels, local forms of. . 132

Grouse, red, variation 29

Guillemot, Ringed 150

Guinea-pig, Brown-Sequard's ex-
periments on 198

Gynandromorphs 45

Heliconius erato, forms of 122, 164

Helix lapicida, local variation of . . 126

striata 127

Heripensis 127

Caespitum 127

trochoides 127

nemoralis and hortensis 128

Helminthophila, geographical races

of 157

Hemerophila abruptaria, melanic. . 142

Hepialus humuli, in Shetland 119

Heterostyle plants 236

Hieracium 9

Himantopus 234

Homoeosis 68

Hybernia progemmaria 139

Hybrids, sterility of 233 et seq.

Incompatibility between certain

allied species 239

Individual, geometrical indepen-
dence of 58

Inhibiting Factors 95

Intermediates, nature of 131, 135

Isolation, consequences of 118

Lacerta muralis, Kammerer's ex-
periments on 209

fiumana 210

Leptinotarsa, Power's experiments

on 218

Limbs, extra, in pairs 72

Limnaea, sinistral 134

Linaria vulgaris, self-sterility 239

Loasa fruits, right and left 57

Lobster, extra claws 76

Locality, variation connected with

14, 118, 146, et seq., 208

Lumbricus, regeneration 77

INDEX OF SUBJECTS

253

Lychnis dioica and vespertina, in-
ter-relations of 18

macrocarpa, possibly a common
parent of 19

Machetes pugnax, polymorphism of

male 28

Maize, Blaringhem's experiments

on 229

Maize, cumulative factors in 116

Malformations, dominants, arising

de novo 89

Manx Cat, heredity 46

Matthiola 84, 104, 113

Melanic varieties 135, et seq.

Memory, analogy with heredity. . . 190

Meristic variation 69, 83, 86

Mirabilis, striped 57

Models of segmentation 59, 60

"Modes," Coutagne's conception of 126
Modling, peculiar race of Pieris

napi at 178

Mole, albino 27, 28

Mule, Linnaeus on 8

Mutation, Matthioli on 4

in Mercurialis 5

in Kales 5

alleged in bulbs 5

Theory 97

periods of 114

in Bacteria 214

Mutilation, consequences of 71

alleged effect of, on offspring. . . 229
Myxococcus, variation in 213

Narwhal, asymmetry of tusks. ... 44

Nemesia strumosa 91

Neuration, a basis for classification 13

Nicotiana, sterility of hybrid 242

Nightjars, varying 150

Noctuidae, fixity and variability. . 25
Noctua, polymorphic and fixed

species 25

Noctua castanea, local forms of . . . 122

Nomenclature, future of 94, 245

Notonecta, variations of 130

Odontoptera bidentata, melanic

form 137

Oedipodidae, protectively coloured 140

Oenothera, new dominant in 92

nibricalyx and rubrinervis 92, 95

Lamarckiana 92, 101

origin of 102, 244

has bad pollen-grains 102

factorial analysis of 103

pollen and egg-cells genetically
dissimilar 104

Oenothera, "twin hybrids" 105

laeta and velutina 105

reciprocal crosses in 105, et seq.

possible coupling in in

dwarfs 112, 114

"Triple hybrids" 114

alleged variation due to treat-
ment 227

Ophrys, local variability 125

Orange, polyembryony 45

Osmotic growth 65

Overlapping forms 146, 174

Papilio, geographical races of 162

Papilio turnus, variation of 144

Pararge egeria, geographical forms

1 66 et seq.

Parthenogenesis 50

Partula, local forms of 133

Passer domesticus and montanus,

distinctions 22

Pea, round and wrinkled 95

Pear, will not cross with apple 239

Pelargonium, variegated 55

bud-sports 56

Periodic phenomena in structure. . 63
Peronea, fixed and variable species 26

" Petites especes " 248

Petunia, double 104

Phalanger maculatus, local varia-
tion 119

Pheasant, fixity of 29

Phigalia pilosaria, melanic. . . .139, 140
Phratora vitellinae, experiments on 193

Phyllotaxis 69

Pied varieties common in Passer
domesticus unknown in Mon-
tanus 23

Pieris napi and bryoniae. ... 174, et seq.

Pig, mule-footed 46

Pigeon, web-footed 46, 49

Indian Rock, a recessive form ... 98

Pigments, nature of 83

Pisum humile, hybrids with culi-
nary peas 244

species 246

Planar ian, regeneration of 71, 77

Plotheia frontalis, polymorphic. .26, 29

Plusia, fixity and variation in 26

Poephila gouldiae, variation of . 148, 149

Polarity of individual 44

Polia chi, melanic 138

Polyanthus, short-styled selected . . 236

Polydactylism in Cat 52, 53

Polyembryony 45

Potato, variation in 91

Poultry, evolution of 90

Primula obconica 91

254

INDEX OF SUBJECTS

Primula sinensis, flaked 57

Leaf -shapes 70

new dominant in 92

sterility in 236

"Giants" 236

Primula, species-hybrids 242

Protective coloration 140

Pyrrhulagra, local forms 120

Python, twin- vertebrae 60

Quiscalus, geographical races of . . . 156

Rabbit, Angora 46

colours of 93

Incompatibility with hare 242

Raimannia odorata, Macdougal's

experiments on 226

Rats, Variation in 248

Recessives, origin of 90

Reciprocal crosses, giving distinct

results 105, et seq.

Regeneration 70

Repulsion no

Reversal on Regeneration 77

Rhamphocoelus, geographical forms,

159, 184
Rhinoptera, variation in jaws of . . 38

Rhythm in repetition 69

Ribs, variation of 68

Rights and Lefts 57~58

Ripples, analogous to segments 60, 66, 67

regeneration of 79

Rollers, geographical races of 160

Ruff, polymorphism of male 28

Salamandra, maculosa and atra

182, 199, 203

spotted and striped 207

geographical variation of 208

Segmentation, nature of 63

simulated mechanically 64

compared with rippling 65

analogies with 68

Segmentation of normally unseg-

mented structures 38

Selection, Natural, an insufficient
cause of definiteness of types,

17, 134, 142

Sempervivum 250

Serial Homology, the true nature of,

62, 66

Setina, Alpine varieties 181

Sex of Twins 44

Sex-factors, possible coupling of . . . in
Sexual characters, variation in 1 19 et seq.

Siamese twins 44

Silky Fowl 84, 85

Simocephalus, changed by environ-
ment 218

Sinistral forms I33~4

Situs transversus 43

Skate's jaws, variation in 38

Sloths, vertebral variation 68

Species, conceptions of, 3, 94, 99, 240, 245

allied, distribution of 185

alternative uses of the term 245

Specific difference, universality of . . 12
of organisms compared with

those of inorganic materials, 15
failure of theory of Selection to

explain 18, 134, 247

Sphyropicus varius. 149, 156

Spilosoma lubricipeda, varieties of 181

Zatima, Heligoland form 181

Spinal nerves, segmentation of. ... 67

Sporadic variation 131, 134, 248

Squashes, polymorphism of 100

Staphylococcus pyogenes, variation

in 213

Sterility of hybrids, in general .... 233

in Lychnis hybrids 20 et seq.

in crossing forms of Draba 243

Significance of 244

Self 238

Stilt 234

Stocks 84, 104, 113

Striped varieties 57

Substantive variation 84

Subtraction-stages 93

Supernumerary limbs 72-76

Sweet pea, variation of 91

sterile anthers in 237

Symmetry compared with heredity 41
Symmetry of body approximate.. 78

Syndactyly 47

in foot 48

Syntheticf ormulae, in nomenclature 94

Taeniocampa, fixed and variable

species 25

Tamias, local forms of 132

Tanagers, geographical races of . . . 159

Teeth, variation in 67, 39

Tephrosia consortaria and con-

sonaria 137, 139, 140

Tephrosia species, separated by

season 119

Terminal members, variation of . . . 68
Thais rumina, local variation in. . . 27
Tolerance, persistence of diversity

due to 17, 134

Tomato, number of cells in fruit. . 46
Transitional populations, rarity of, 165

an example 178

Tropaeolum, sterile anthers in. ... 237

INDEX OF SUBJECTS

255

Trypanosomes, variation in 215

Tusk, of Elephant, segmented .... 38

of Narwhal 44

Twinning 4*. 44. 7*

heredity of 45

in organs 46

Uria troile, variety of 150

Vanessa urticae, effects of tempera-
ture 191

Variation, a medley of phenomena, 14, 15

sporadic 131, 134

and locality 118

Causes of genetic ... 86, 87, 131, 212

Substantive and meristic 83

Veronica, specific difference in. 16

intermediates between species.. 17

Vertebrae, division in 60, 61

homologies of 66

Vespa, specific difference in 23

Vortex, living organism compared

with 40

Wave-motion compared with repeti-
tion of parts 62, 67, 79

Wheat, cumulative factors in 116

climatic experiments on 195

Woodpecker 234

Zebra, pattern of stripes compared

with ripples 38

INDEX OF PERSONS

PAGE

Ackermann 242

Agar 218

Allen, J. A 132, 147. *59

Annandale 47

Arrigoni degli Oddi 167

Backhouse SO

Baker, G. T 166

Bangs, Outram 120, 142, 155

Barrett 26, 136, 167, 173, 178, 193

Baur, E 55. 99

Baur, G up

Beneden, van 75

Bentham, on species of Veronica .. 16

Lychnis 21

Primula 22

Bernadin 42

Bishop, L. B 153. 157

Blaringhem 229

Bobart 5

Boisduval 182

Boissier 19

Borradaile 74. 75

Boulenger, E. G 208

Boulenger, G. A 182, 207, 209

Boyle 5. 54

Brewster, W 149. 150

Britton 227

Brown, T. Graham 198

Brown-Sequard 197 et seq.

Bruant, P 51

Buffon 234

Butler, S 189, 190

Buysson, R. du 24

Candolle, de 245

Carpenter, J. H 172

Chapman, F. M 148, 156, 157, 158

Chapman, T. A 13, 167, 182, 231

Church, A. H 69

Cieslar 197

Clark, Austin 142, 144

Cockayne, E. A 43

Cockerell, T. D. A 224

Compton, R. H 50, 58, 227

Cope 230

Cory 142

Correns 239

Coutagne 125, et seq.

Darwin, on Variation ; i, 2

Systematics 10

Selection 134, 139

Heterostyle plants 236, 237

Darwin, F 190

Darwin, Sir G 41

Davenport 46

Davis, H. M 102

Delcourt 130

Deschange 181

Dobell 215

Doncaster 105, 121, 136

Driesch 80, 81

Duchartre 51

East 91, 116

Edwards, W. H 162

Ehrlich 215

Fellmer 215

Field, W. L. W 161

Fischer, E 192

Fleck 171, 174

Fletcher, W. H. B 26, 181

Foster, Sir N 39

Galle 123

Garrod 83

Gates 92, 95, 102

Gayner, F 177

Godron 249

Gold, E 196

Goldschmidt 116

Goodwin, E 137

Gortner 226

Greene, E. L 8

Gregory, R. P 92, 100, 236

Grenier 249

Grover 173

Gruber 48

Gulick 119, 133

Hamling 142

Hampson, Sir G 26

Harris 142

Hartlaub 182

Herbst 42

Heribert-Nilsson 1 16

Hewett 182

Honing 105

256

INDEX OF PERSONS

257

Hunter, John 44

Jakowatz 197

Janet 24

Jeans 41

Jenkinson 4<>

Jentink 120

Johannsen 195

Jordan 185, 242, 249

Kammerer 199. et seq.

Keeble 236

Klebs 250

Krancher 182

Kiichenmeister 44

Kudicke 215

Lamarck 9

Lang, A 128

Lawrence, W. N 142, 145

Leake, H. Martin 98, 100

Leavitt 185

Lecoq 99

Lederer 167

Leduc 64, 65, 80

Leydig 182

Linden, M. von 192

Linnaeus 6, 7, 8

Lloyd, R. E 248

Locard 130

Lock, R. H 242, 244

Loeb 42, 45. So, 71, 77

Lotsy 99

Lowe, P. R 143

Macdougal, W. T 102, 226

Marchant 7

Mathew 171

Matthioli 4

Mayer, A. G 133

Mendel, Rediscovery of 2

On Fasciation 49

Merrifield 169, 172

Miller, W. D 120, 149

Morgan 71, 77, 91, 198

Moggridge 125

Nathusius, S 242

Nettleship 44

Newman, H. H 42

Newsholme 48

Nilsson-Ehle 116, 169

Norman, A. M 125, 156

Ober 142

Oberthiir 168, 170, 193

Oliver, J 45

Page, H. E 167, 180

Patterson, J. T 42

Payne, F 278

Pellew 236

Poll 45

Porritt. 136

Poulton 141

Powers, J. H 230

Pringsheim, H 213

Przibram. . .72, 78, 178, 194, 197, 199

Punnett no

Ray 4, 5

Raynor 105

Ridgway 10, 120

Roedelius 195

Rolfe 20

Rosen, F 242

Rosner 42

Rowland-Brown, H 167, 180

Sargent 185

Saunders, E. R 84, 104, 112

Schima 177

Schroder 193, 194

Schiibeler 195

Semon, R 190 et seq.

Sharrock 5

Shull 100

Speyer, A 166, 170, 181

Spillman 47

Standfuss 135, 181, 191

Staples- Browne 49, 98

Staudinger 170, 179

Stockard 50, 71

Sutton 236, 244

Tornier 72

Tower, W. L 218-226

Trechmann 133

Tugwell 181

Tutt, J. W. On Definiteness of

Species 13

On Plusia interrogationis 26

On Tephrosia 119

On N. castanea 122

On Pararge egeria 167 et seq.

Verity, R 171, 177

Vries, H. de 101-115, 222, 239

Walker, G 49

Weir, Jenner 119

Weismann 176, 188

Wendelstadt 215

Werbitzki 215

Werner 209

Wettstein 197

258

INDEX OF PERSONS

Wheeler, G 168, 171

Wheldale 83

Wilder 44

Wille 197

Williams, H 167, 172

Windle, B. C. A : . . 43

Winslow 213

Wolf, F 213

Woodforde 123

Woltereck 215

Zeijlstra 114

CALIFORNIA