Intracellular pangenesis, including a paper on fertilization and hybridization

ATTftl B 1910

LIBRARY

OF THE

UNIVERSITY OF CALIFORNIA.

BIOLOGY

LIBRARY

G

Intracellular Pangenesis

INCLUDING A PAPER ON

FERTILIZATION AND HYBRIDIZATION

BY

HUGO DE VRIES

PROFESSOR OF BOTANY IN THE UNIVERSITY OF AMSTERDAM

OF THE

UNIVERSITY

OF

TRANSLATED FROM THE GERMAN

*

BY

C. STUART GAGER

PROFESSOR OF BOTANY IN THE UNIVERSITY OF MISSOURI

CHICAGO

THE OPEN COURT PUBLISHING CO.
1910

C OLOGY

LIBRARY

6

GENERA

til

COPYRIGHT BY

THE OPEN COURT PUBLISHING Co.
1910

FOREWORD

The Intracellulare Pangenesis, of Hugo de Vries, was
such a source of stimulation to me at the time of its ap-
pearance that I feel greatly indebted to its author. By
creative imagination Hugo de Vries predicted much in
his book that gained a material basis only through the
histological research of the following decades. That is
what makes the study of his book to-day as interesting
as it is instructive.

In his paper, entitled Befruchtung und Bastardirung,
a translation of which is included in this volume, de Vries
has shown the same faculty of utilizing our present
knowledge from every point of view, and of looking
prophetically into the future. For in this paper also, on
the ground of theoretical considerations, he predicted
phenomena which were to furnish the basis for our con-
ceptions of fertilization and heredity, but which have be-
come actually known to us only through later works on
the most intimate processes of nuclear division.

Therefore I gladly comply with the wish of the trans-
lator to introduce his translation with a few words. I
say expressly "to introduce/' for works of Hugo de
Vries do not need a recommendation.

Bonn, . E. STRASBURGER.

June, 1908.

200228

my well-abused hypothesis of pangenesis"

Charles Darwin, Autobiography.

TRANSLATOR'S PREFACE

Every student of heredity is brought face to face with
the problem of some mechanism of inheritance. Pan-
genesis was Darwin's solution of this problem. But it
was not in the form in which Darwin left it that pan-
genesis became directly fruitful of results; and no one
felt the insufficiency of his hypothesis more keenly than
Darwin himself. Writing to Asa Gray in 1867 he said:
'The chapter on what I call Pangenesis will be called a

mad dream but at the bottom of my own mind

I think it contains a great truth."1 And to J. D. Hooker,
in 1868, he wrote : "I feel sure if Pangenesis is now still
born it will, thank God, at some future time reappear, be-
gotten by some other father, and christened by some other
name/'2

Many men discerned the weak features of the hypoth-
esis, but to Hugo de Vries belongs the credit of having
detected the "great truth" it contained. He became its
"other father," and rechristened it with another name
— a name more nearly like the original, no doubt, than
Darwin could have imagined.

The pangenesis of Darwin was hardly susceptible of
experimental verification, except to the extent that a more
intimate acquaintance with the facts showed that the
assumption of a transportation of "gemmules" was super-

iDarwin, C Life and Letters. 2: 256. New York, 1901.
2Loc. cit. p. 261.

vi Translator's Preface

fluous. But it contained the germ of de Vries's intra-
cellular pangenesis, the direct progenitor of the mutation-
theory. It was primarily because of this genetic rela-
tionship, together with the masterful way in which the
hypothesis is developed, and the accompanying wealth
of illustration, that the little German volume, here done
into English, was deemed worthy of translation at the
present time.

As those who have followed the more recent liter-
ature of theoretical biology well know, Delage has argued
against accepting any of the micromeric theories of the
structure of protoplasm. His argument is based upon
the idea that, by the law of probabilities, no one can ever,
by pure imagination, correctly conceive of the ultimate
structure of protoplasm in detail. Kellogg3 cites this
criticism of Delage as "a sufficient reason against accept-
ing any one of these highly developed theories of the
structural and functional capacity of invisible life units."
Possibly this is correct, but that depends upon what the
given hypothesis is to be accepted for. Of course no
unverified hypothesis should be accepted for truth. As
soon as the hypothesis can be so accepted it ceases to be
a hypothesis, or even a theory, and passes into the rank
of ascertained fact.

But that the argument of Delage can be advanced as
a reason for rejecting any hypothesis, not inherently im-
probable or absurd, as a working hypothesis, a point of
departure for further experiments, serving to orient a
whole body of investigators, seems to me entirely to miss
the point of the purpose of a hypothesis. Hypotheses
are not statements of truth, but instruments to be used
in the ascertainment of truth. Their value does not de-

3Kellogg, V. L., Darwinism To-day, p. 223. New York, 1907.

Translator's Preface vii

pend upon ultimate verification, but is to be measured
by their effect upon scientific research. All this is now
a truism.

What does it argue that no one, as Delage insists,
ever anticipated by imagination the striation of muscle
fibers, the existence of chromosomes and centrosomes, or
any other fact of minute structure revealed by the micro-
scope. May it not be asked in reply how long we should
have had to wait for the discovery of the inert gases of
the atmosphere, the accessory chromosome, and the ion,
had they not first been conceived in imagination and
formally embodied in working hypotheses? It is not
pleasant to contemplate what the effect on the develop-
ment of chemical science would have been had Dalton's
(micromeric) hypothesis of indivisible units been rejected
on the a priori grounds that the ultimate structure of
matter is beyond the power of the human intellect to
imagine in detail.

The hypothesis of intracellular pangenesis can never
be absolutely demonstrated as true — can never advance
beyond the rank of a theory — because the hypothetical
pangens are conceived to be invisible, ultra-microscopic
units, whose existence can never be more than inferred;
but the formulation of the hypothesis marks the beginning
of the greatest and most important forward step in the
study of the origin of species since 1859. The notion
of pangens became the parent-idea of unit-characters,
offered a simple mechanism for the disjunction of char-
acters in hybrids, and for continuous and discontinuous
variation, and thus lead up directly to the conception of
mutation as one method of the origin of species.4 And,
most important and significant of all, it resulted in per-

4Cf. footnote, p. 74 infra.

viii Translator's Preface

manently removing the entire question of organic evolu-
tion from the realm of ineffective speculation, and estab-
lishing it upon the firm basis of experimentation.

The term pangen is employed in its original sense by
Strasburger in his paper on "Typische und allotypische
Kertheilung."5

Recognizing the existence of some material entities
as the ultimate units of heredity, conceiving of them as
invisible, and accepting for them the name pangen, he
interprets the chromatin granules (chromomeres), which
can be directly seen, as larger or smaller pangen-com-
plexes, and suggests that we designate them "pangeno-
somes." The pangenosomes, owing to a "certain elective
affinity," he considers as combining into ids, (from the
idioplasm, of Nageli), and the ids, in turn, into chromo-
somes.6

Referring to de Vries's supposition, that the pangens
influence the cytoplasm by wandering out into it from the
nucleus and thus changing from an inactive to an active
state, Strasburger7 records his failure to detect any visi-
ble evidence that the bodies which he calls pangens thus
wander out from the nucleus into the cytoplasm, but refers
to the period in cell-division when the nuclear membrane
disappears and the spindle forms, as serving to bring the
chromosomes into direct contact with the cytoplasm, and
thus establishing a condition favorable for the ''forma-
tive influencing" of the cytoplasts by the nucleoplasts. A
similar influence might also result from extranuclear nu-
cleoli distributed in the cytoplasm. In the fertilization

5Jahrb. Wiss. Bot. 42: 1-71. 1905.

6Mottier's use of the word pangen to designate the visible chro-
momeres (Ann. Bot. 21; 307-347. 1907.), employs the term in a
sense entirely at variance with that for which it was originally pro-
posed (cf. p. 49.)

7hc. cit. p. 74.

Translator's Preface ix

of the egg he postulates a fusion of maternal with pater-
nal pangens.8 Thus, in the gametophytic generation, the
pangens must be considered as univalent (haploid), in
the sporophytic as bivalent (dlploid). This would lead
us to look for larger nuclei in the cells of the sporophyte
than in those of the gametophyte. This hypothesis was
verified in a number of plants, widely separated system-
atically. In Taxus baccata, for example, the nuclei of
the prothallus were noticeably smaller than those of the
sporophyte: and in nuclei with equally marked granula-
tion, Strasburger counted fifty granules in an optical sec-
tion' of the nuclei of the nucellus, and only one-half that
number in the nuclei of the adjacent prothallus.

But I cite this paper of Strasburger's chiefly to show
how the hypothesis of intracellular pangenesis, in other
hands that its author's, may assist in forming some com-
prehensible picture of the mechanism of matter in the
living state. The idea and the term pangen are also
adopted by Pfeffer in his Physiology of Plants.9

At the suggestion of Professor de Vries, a transla-
tion of his Haarlem Vortrag on "Befruchtung und Bas-
tardierung" is included in this volume, for the purpose of
showing the bearing of more recent research on the hy-
pothesis of intracellular pangenesis, and of thus bringing
the problem more nearly down to date. The translation
of this. Vortrag also appeared in "The Monist," for No-
vember, 1909.

It is a pleasure to record my profound gratitude to
Professor de Vries for his careful reading and annota-
tion of the manuscript of the translation, and for his inter-
est and encouragement throughout the undertaking.

*loc. cit. p. 61.

9Pfeffer W. The Physiology of Plants. Eng. Trans, by Alfred
J. Ewart. 1: 49. Oxford, 1900.

x Translator's Preface

I am deeply indebted to Professor Strasburger for his
kindness in preparing an introductory note, and wish, also
to express my sincere thanks to Miss Marie Onuf, whose
invaluable assistance rendered the completion of the work
possible.

C. S. G.

University of Missouri,

Department of Botany.

Nov. 13, 1909.

Verlag von Gustav Fischer in Jena

Im Jahre 1889 erschien in deutscher Sprache :

INTRACELLULARE PANGENESIS

von
HUGO DE VRIES

Prof, der Botanik a. d. Universitat Amsterdam

Preis: 4 Mark

XI

TABLE OF CONTENTS
INTRACELLULAR PANGENESIS

PAGE

AUTHOR'S INTRODUCTION 3

PART I

PANGENESIS

A. THE NATURE OF HEREDITARY CHARACTERS.

CHAPTER I. THE MUTUAL INDEPENDENCE OF HEREDITARY CHARACTERS.

§1. The Combination of Specific Characters Out of

Hereditary Characters 11

§2. The Similarity of the Differences Between Species

and Between Organs 15

§3. The Similarity Between Secondary Sexual Characters

and Specific Attributes 18

§4. The Variation of the Individual Hereditary Char-
acters Independently of One Another 19

§5. The Combination of Hereditary Characters 24

§6. Cross- and Self-Fertilization 29

§7. Conclusion 33

B. PREVAILING VIEWS ON THE BEARERS OF HEREDITARY

CHARACTERS.

CHAPTER II. THE SIGNIFICANCE OF THE CHEMICAL MOLECULES
OF THE PROTOPLASM WITH REFERENCE TO THE
THEORY OF HEREDITY.

§1. Introduction 37

§2. Protoplasm and Protein 41

§3. Elsberg's Plastidules 44

CHAPTER III. THE HYPOTHETICAL BEARERS OF SPECIFIC CHARACTERS.

§4. Introduction 50

§5. Spencer's Physiological Units 51

§6. Weismann's Ancestral Plasms 53

§7. Nageli's Idioplasm 57

§8. General Considerations 59

xii Contents

CHAPTER IV. THE HYPOTHETICAL BEARERS OF THE INDIVIDUAL
HEREDITARY CHARACTERS.

PAGE

§9. Introduction 62

§10. Darwin's Pangenesis 63

§11. Critical Considerations 66

§12. Conclusion 69

PART II

INTRACELLULAR PANGENESIS

A. CELLULAR PEDIGREES.

CHAPTER I. THE RESOLVING OF INDIVIDUALS INTO THE PEDIGREES
OF THEIR CELLS.

§1. Purpose and Method 79

§2. The Cellular Pedigrees of the Homoplastids 82

§3. The Cellular Pedigree of Equisetum 83

§4. The Main Lines in the Cell-Pedigrees 88

CHAPTER II. SPECIAL CONSIDERATION OF THE INDIVIDUAL TRACKS.

§5. The Primary Germ-Tracks 93

§6. The Secondary Germ-Tracks 95

. §7. The Somatic Tracks 100

§8. The Difference Between Somatic Tracks and Germ

Tracks 103

§9. Phyletic, Somatarchic, and Somatic Cell-Divisions.. 107

CHAPTER III. WEISMANN'S THEORY OF THE GERM-PLASM.

§10. The Significance of the Cell-Pedigree for the Doc-
trine of the Germ-Plasm 110

§11. The Views of Botanists 113

§12. A Decision Reached Through the Study of Galls. ... 118

B. PANMERISTIC CELL-DIVISION.

CHAPTER I. THE ORGANIZATION OF THE PROTOPLASTS.

§1. The Visible Organization 125

CHAPTER II. HISTORICAL AND CRITICAL CONSIDERATIONS.

§2. The Neogenetic and the Panmeristic Conception of

Cell-Division 128

§3. Cell-Division According to Mohl's Type 134

§4. The Regeneration of Protoplasts After Wounding. . 139

Contents xiii

PAGE

CHAPTER III. THE AUTONOMY OF THE INDIVIDUAL ORGANS OF
THE PROTOPLASTS.

§5. Nucleus and Trophoplast 144

§6. The Vacuoles ' ISO

§7. The Relation Between the Plasmatic Membranes and

the Granular Plasm 157

§8. The Question of the Autonomy of the Limiting

Membrane 160

C. THE FUNCTIONS OF THE NUCLEI.

CHAPTER I. FERTILIZATION.

§1. Historical Introduction 169

CHAPTER II. FERTILIZATION (continued).

§2. The Conjugation of the Zygosporeae 171

§3. Fertilization in Cryptogams 173

§4. Fertilization in Phanerogams 176

CHAPTER III. THE TRANSMISSION OF HEREDITARY CHARACTERS

FROM THE NUCLEI TO THE OTHER ORGANS OF THE

PROTOPLASTS.

§5. The Hypothesis of Transmission 179

§6. Observations on the Influence of the Nucleus in the

Cell - 183

D. THE HYPOTHESIS OF INTRACELLULAR PANGENESIS.
CHAPTER I. PANGENS IN THE NUCLEUS AND CYTOPLASM.

§1. Introduction 193

§2. AH Protoplasm Composed of Pangens 195

§3. Active and Inactive Pangens 199

§4. The Transportation of Pangens 201

§5. Comparison with Darwin's Transportation-Hypothesis 207

§6. The Multiplication of Pangens 212

CHAPTER II. SUMMARY

§7. Summary of the Hypothesis of Intracellular Pange-

nesis 215

FERTILIZATION AND HYBRIDIZATION
Fertilization and Hybridization 219

AUTHOR'S INTRODUCTION

AUTHOR'S INTRODUCTION

In the year 1868, in the second volume of his cele-
brated work, "The variation of animals and plants under
domestication," Darwin formulated the provisional hypo-
thesis of pangenesis. The discussion of this hypothesis
is preceded by a masterly survey of the phenomena to be
explained. Owing to this, as well as to his clear concep-
tion of the whole problem, this part of his book has at-
tracted universal attention. We find it mentioned in
almost all works which deal with general biological ques-
tions. While, however, the general part of the chapter
has until now remained the basis for all scientific consid-
erations of the nature of heredity, the hypothesis itself
has not enjoyed such general appreciation.

Darwin assumes (Variation 2: 369) that the cells,
as is generally accepted, multiply by division, and that in
so doing they preserve essentially the same nature. He
considers that this rule forms the basis of heredity. By
it, however, not all of the groups of phenomena brought
together by Darwin may be explained. Especially does
it not explain the effects of use and disuse, the direct ac-
tion of the male element on the female, and the nature of
graft-hybrids. In order to take into account these phe-
nomena, Darwin assumes that there exists, in addition to
cell division, yet another means of transfer of hereditary
qualities. Each unit of the body, according to his theory,

4 Author's Introduction

throws off minute granules1 which accumulate in the germ
cells and buds. These granules are the bearers of the
characters of the cells from which they are derived, and
thus transmit those characters to the germ cells and to the
buds.

Thus all the hereditary characters of the organism
are represented in the egg-cells, pollen-grains, sperm-
cells, and buds by minute particles. These they have re-
ceived, partly by descent from former germ cells, i. e.,
directly, but partly by later addition from the cells and
organs of the body. These minute granules are not the
chemical molecules; they are much larger than these and
are more correctly to be compared with the smallest
known organisms. Darwin calls them gemmules (small
germs). *

The hypothesis of these gemmules threw an unex-
pected light on a series of facts which had hitherto been
in absolute darkness. And if one reads attentively Dar-
win's discussion, he sees more and more clearly that the
transmission of gemmules by cell-division, from the
mother-cell to the daughter-cell, suffices to explain large
groups of phenomena. Only isolated groups of facts de-
mand in addition the hypothesis of transportation. The
doctrine of latent qualities and of atavism particularly
are drawn from their former darkness by Darwin's hy-
pothesis, and his discussion of this subject (p. 357)
clearly shows what great significance he imputes to this
circumstance. It demands, however, only the transmis-
sion of the gemmules in cell-division, not their transpor-
tation from the growing and full-grown organs to the
germ-cells.

1This is the term Darwin first uses. The Variation of Animals
and Plants. 2: 358. New York, 1900. Tr.

Author's Introduction 5

It has always seemed to me that most authors have not
sufficiently distinguished these two aspects of the hy-
pothesis, and that their objections against accepting the
theory of transportation have misled them into over-
looking the paramount significance of the doctrine of
gemmules. .

To my mind Darwin's provisional hypothesis of pan-
genesis consists of the following two propositions :

1. In every germ-cell (egg-cell, pollen-grain, bud,
etc.) the individual hereditary qualities of the whole or-
ganism are represented by definite material particles.
These multiply by division and are transmitted during
cell-division from the mother-cell to the daughter-cells.

2. In addition, all the cells of the body, at different
stages of their development, throw off such particles;
these flow into the germ-cells, and transmit to them the
qualities of the organism, which they are possibly lack-
ing. ( Transportation-hypothesis ) .

The second assumption possessed, for Darwin himself,
only limited importance, in the case of plants and corals,
as he considered a transportation of gemmules from one
branch to another impossible. It does not apply to the
workers of ants and bees, nor to the double stocks (gilli-
flower) mentioned several times by Darwin. These do
not possess any stamens and pistils themselves, and their
characteristics must therefore be transmitted from one
generation to the other through the fertile single specimens
of the race. The facts, for the explanation of which the
theory in question was brought forth, have gained neither
in number nor in trustworthiness during the twenty years
since the publication of Darwin's book.

Doubts of its necessity, therefore, are quite permis-
sible, and it is the chief service of Weismann to have

6 Author's Introduction

repeatedly emphasized these doubts, and to have shat-
tered the rather generally accepted doctrine of the hered-
ity of acquired characters.2

But even if, with this investigator, one rejects the
second proposition, that is no reason for likewise doubt-
ing the other part of the hypothesis of pangenesis. On
the contrary, it seems to me that by doing so its great
significance only becomes clearer. Besides, there have
been no convincing arguments brought forward against
this first dogma, and no other hypothesis concerning the
nature of heredity takes account of the facts in so simple
and clear a manner.

Yet most authors have considered that, by refuting
the transportation hypothesis, they have also refuted that
of the bearers of individual hereditary characters, and
they have hardly devoted any special discussion to it. In
consequence of this Darwin's view has unfortunately not
borne such fruit for the development of our knowledge
as its originator had a full right to expect.

My problem in the following pages will be to work
out the fundamental thought of pangenesis independently
of the transportation hypothesis, and to connect with it
the new facts which the doctrine of fertilization and the
anatomy of the cell have brought to light.

I shall be guided by the thought that the physiology
of heredity, and especially the facts of variation and of
atavism indicate the phenomena which are to be explained,
while microscopic investigation of cell-division and fer-
tilization will teach us the morphological substratum of
those processes. We shall not try to explain the mor-

2The designation "acquired" is not exactly well chosen. The
question is: Can characters which have originated in somatic cells
be communicated to the germ -cells. This possibility is rejected by
Weismann. Compare Part II, § 5. (p. 93).

Author's Introduction 7

phological details of those processes; our knowledge is
yet too limited for that. But, following the method of
Darwin, to find in the special cases the material substra-
tum of the physiological processes, that is our problem.

As the most important result of cell-investigation of
the preceding decades, I consider the theory that all the
hereditary predispositions (Anlagen) of the organism
must be represented in the nucleus of the cell. I shall try
to show that this theory leads us to assume a transporta-
tion of material particles which are bearers of the indi-
vidual hereditary characters. This does not mean, how-
ever, a transportation through the whole organism, nor
even from one cell to another, but one restricted to the
limits of the individual cells. From the nucleus the ma-
terial bearers of the hereditary characters are transported
to the other organs of the protoplast. In the nucleus they
are generally inactive, in the other organs of the protoplast
they may become active. In the nucleus all characters
are represented, in the protoplast of every cell only a
limited number.

The hypothesis, therefore, becomes one of intracellu-
lar pangenesis. To the smallest particles, of which each
represents one hereditary characteristic, I shall give a
new name and call them pangens, because with the desig-
nation "gemmule" (Keimchen) is associated the idea of a
transportation through the whole organism.

PARTI

PANGENESIS
A. THE NATURE OF HEREDITARY CHARACTERS

CHAPTER I

THE MUTUAL INDEPENDENCE OF HEREDITARY
CHARACTERS

§ /. The Combination of Specific Characters Out of
Hereditary Characters

Among the many advantages which have lent such
a prominent significance to the theory of descent in the
investigation of living nature, the shattering of the old
conception of species occupies an important place. For-
merly every species was regarded as a unit and the total-
ity of its specific attributes as an indivisible concept. Even
the latest theories on heredity accept this concept as one
that does not require any further analysis.

But if the specific characters are regarded in the light
of the theory of descent it soon becomes evident that they
are composed of single factors more or less independent <
of each other. One finds almost every one of these fac-
tors in numerous species, and their varying groupings^-
and combinations with less common factors causes the
extraordinary diversity in the organic world.

Even the most cursory comparison of the various or-
ganisms leads, in this light, to the conviction of the
composite nature of specific characters. The power to
produce chlorophyll and, by means of this, in light, to
decompose carbon dioxide, is evidently to be regarded as
a property which, in great measure lends to the botanical
world its peculiar stamp. This power, however, is lack-
ing in many groups throughout the system, and therefore

12 Mutual Independence of Hereditary Characters

is by no means inseparably connected with the other fac-
tors of plant nature.

Other factors are the predispositions (Anlagen)
which enable many species to produce definite chemical
compounds. First of all, the red and blue coloring mat-
ter of flowers, then the different tannic acids, the alka-
loids, etherial oils, and numerous other products. Only
a few of these are limited to a single species, many recur
in two or more species, which are often systematically far
apart. There is no reason for supposing that, in every
individual case there is a different mode of origin for
the same compound; rather it is obvious that essentially
the same chemical mechanism underlies the same process,
wherever we find it.

In a similar manner we must also accept as possible
the analysis of the morphological characteristics of the
species. It is true that morphology is not by any means
so far advanced that such an analysis could be carried out
in every individual case. But the same leaf-form, the
same leaf-edge, coarsely or delicately notched, recur in
numerous species, and even the customary terminology
teaches us that the configurations of all the various leaf-
forms are composed of a comparatively small number of
simple characters.

It would be superfluous to accumulate instances which
are easily accessible to every one, and it is only a question
of thoroughly familiarizing one's self with these ideas, so
that the synthesis of the whole out of its component parts
is clearly recognized. It will then be seen that the character
of each individual species is composed of numerous hered-
itary qualities, of which by far the most recur in almost
innumerable other species. And even if, in the building
up of any single species, such a large number of these

Specific Characters Composite 13

factors is necessary that we almost shrink from the con-
sequences of an analysis, it is clear, on the other hand,
that, for the building up of the sum total of all organisms,
there is required a rather small number of individual
hereditary characters in proportion to the number of
species. Regarded in this way, each species appears to us ^
as a very complex picture, whereas the whole organic
world is the result of innumerable different combinations
and permutations of relatively few factors.

These factors are the units which the science of hered-
ity has to investigate. Just as physics and chemistry go
back to molecules and atoms, the biological sciences have
to penetrate to these units in order to explain, by means
of their combinations, the phenomena of the living world.

Phylogenetic considerations lead to the same conclu-
sions. Species have gradually been evolved from simpler
forms, and this has taken place by the addition of more
and more new characteristics to those already existing. \
The factors which compose the character of a single spe-
cies are, therefore, in this sense, of unequal age ; the char-
acteristics of the larger groups being in general, older
than those of the smaller systematic divisions. But the
very consideration that the characteristics have been ac-
quired singly or in small groups, shows us again from
another side their mutual independence.

It is a striking, yet by far insufficiently appreciated
fact that frequently, in distant parts of the genealogical /
tree, the same character has been developed by wholly^
different species. Such "parallel adaptations" are ex-
tremely numerous, and almost every comparative treat-
ment of a biological peculiarity shows us examples
thereof. The insect-eating plants belong to the most
varied natural families, yet they all possess the power of

14 Mutual Independence of Hereditary Characters

producing from their leaves the necessary mixture of an
enzyme, and of an acid which is needed for dissolving
protein bodies.1 The agreement, emphasized by Darwin,
of this mixture with the gastric juice of the higher ani-
mals justifies even the supposition that those plants and
the animal kingdom have some hereditary qualities in
common.

The indigenous creeping and climbing plants, the trop-
ical lianas, the tuberous and bulbous plants, the fleshy,
leafless stems of the Cactaceae and Euphorbiaceae, the
pollinia of the Orchidaceae and Asclepiadaceae, and num-
berless other instances show us parallel adaptations. Very
beautiful pictures are furnished on the one hand by the
desert plants, which all have to protect themselves in some
way against the disadvantages of evaporation, and whose
anatomical relations have been so thoroughly described
by Volkens.2 On the other hand are the ant-plants, into
the adaptations of which to harmful and useful species of
ants Schimper has given us an insight.3

Everywhere we see how one and the same hereditary
character, or definite small groups of the latter, can com-
bine with other most diverse hereditary characters, and
how, through these exceedingly varied combinations, the
individual specific characters are produced.

1This statement is now known to hold true only in the case of
Nepenthes (Vines, Ann. Bot. 11: 563. 1897. 12: 545. 1898) and of
Drosera (see Fr. Darwin's articles). Schimper found no proteolytic
enzyme secreted by Sarracenias. (Bot. Zeit. 40: 225. 1882). His
results were confirmed by Miss Robinson, but she demonstrated the
secretion, by Sarracenia purpurea, of a starch-digesting enzyme.
(Torreya 8: 1908). Tr.

2Volkens, G. Die Flora der Aegyptisch-Arabischen Wuste.

3Schimper, A.F.W. Die Wechselbeziehungen zwischen Pflanzen
und Ameisen im tropischen Amerika. Bot. Mittheil. aus den Tropen.
Band I, Heft 1, 1888.

Organic Characters Composite 15

§ 2. The Similarity of the Differences Betzveen Species
and Between Organs

The comparison of species with the organs of a single
individual leads us to quite similar conclusions as does
the comparison of species with each other, for the dif-
ferences between the organs can be traced back, in the
same way, to various combinations of individual heredi-
tary qualities.

Even the simplest observation teaches us this. Just
as chlorophyll is lacking in some species it is also lacking
in single organs and tissues of higher plants. The red
coloring matter of flowers is limited to certain plant
species, and in these again to definite organs. Tannic
acid, etherial oils, and like substances, where present,
show a local distribution. Calcium oxalate is lacking in
most ferns and grasses, and on the other hand in the roots
of many species rich in calcium. The same is true, ap-
parently, of morphological attributes. I need not cite ex-
amples, for it will certainly be granted that a very close
agreement exists between the manner in which the or-
gans of a single plant differ from each other and the dis-
tinction between different species. Both depend upon
varying combinations and a varying selection from a
great range of given factors.

A series of phenomena, which we may summarize un-
der the name dichogeny, leads to similar conclusions. I
mean all those cases where the nature of an organ is not
yet decided during the early stages of its development,
but may yet be determined by external influences. Thus,
under normal conditions, the runners of the potato-plant
form at their tips the tubers, but on being exposed to
light, or when the main stem has been cut off, they de-

16 Mutual Independence of Hereditary Characters

velop into green shoots. By severing the stems, the
rhizomes of Mentha, Circaea, and many other plants, can
be made into ascending stems, and the transformations
which the thick almost resting rhizomes of Yucca undergo
after such treatment are remarkable. In a similar manner
Goebel has succeeded in causing the rudiments of bracts
to develop into green leaves,4 and Beyerinck5 observed
even the transformation of young buds of Rumex Aceto-
sella into roots.

In such cases it is clear that the possibility of develop-
ing in either of two different directions is dormant in the
young primordia. For this very reason I should like to ap-
ply the name dichogeny to this phenomenon. And it evi-
dently depends upon external influences what direction is
taken. Therefore a selection must take place from among
the available hereditary characters of the species, and this
selection may be influenced by artificial interference. For
the theory of hereditary characters such experiments are
therefore of the highest interest.

Here are naturally included the phenomena of bud-
variation. Many of these are cases of atavism. Let us
select an example. In plants with variegated leaves one
frequently observes single green branches. Since the
variegated plant is descended from green ancestors, this
case is regarded as a reversion. The variegated individual
evidently still possessed the characteristics of the green
ancestor, though in a latent condition. During the bud-
formation it split its entire character, but in such a way

4Goebel, K. Beitrage zur Morphologic und Physiologic des
Blattes. Bot. Zeit. 40: 353. 1882.

5Beyerinck, M. W. Beobachtungen und Betrachtungen iiber
Wurzelknospen und Nebenwurzeln. Veroffentl. Akad. Wiss. Am-
sterdam, pp. 41-41. 1886. Cf. also Tafel T, Fg. 9.

Bud- Variation 1 7

that in one branch the variegated combination predomi-
nated, in the other one the green color.

As a further illustration of bud-variation, I may men-
tion the nectarines. These are hairless peaches, which
originated in several varieties, and in some of them re-
peatedly through bud-variation. This fact can be ex-
plained only by saying that the possibility of producing
hairy fruit can become lost in single branches, easily and
independently from all other characters, or at least be-
come latent.

The characteristics which originate through bud-vari-
ation are usually preserved by propagation by means of
grafts, cuttings, et cetera, and, in isolated cases, are even
constant from seed. New varieties may therefore be pro-
duced in this manner. And, since we regard varieties as
incipient species, this consideration is further evidence of
an accordance in the differences between species and be-
tween organs.

Naturally included with bud-variations is the consid-
eration of monoecious plants, for the latter agree with the
former in the fact that different branches allow different
qualities to develop. In the young plant the sexes are not
yet separated, and frequently for a long time the possibil-
ity of producing both is retained. If this process, how-
ever, is started, it is accomplished by a kind of separation :
one bud develops into a staminate, the other into a pistil-
late flower. Or staminate and pistillate inflorescences are
produced, or whole branches are predominantly pistillate
and others staminate. The specific character was there-
fore present in the young plant as a whole, but in a latent
state, and, in order to manifest itself, it had to split into
its two chief parts.

The formation of organs, bud-variation, and the pro-

18 Mutual Independence of Hereditary Characters

duction of staminate and pistillate branches in monoecious
plants are therefore due to a kind of splitting. The po-
tentialities, united in the young plant, separate from each
other in order to be able to unfold. And the grouping
of the hereditary characters in the separate branches and
organs shows a very great agreement with the combina-
tion of such characters to form the various specific marks
of related organisms.

§ j. The Similarity Betzveen Secondary Sexual Characters
and Specific Attributes

Continuing in a similar manner as in the previous
paragraph we will now take into consideration the sec-
ondary sexual characters, for they lead to exactly the
same conception of a specific character.

This is most clearly seen in those cases where the two
sexes of one species, upon being first discovered have been
described as different species.6 But otherwise, too, the
secondary differences between the individuals of both
sexes are of the same order as the differences between the
various species in the same and in allied genera.

It is the same with those plants which bear flowers on
various individuals, the sex-organs of which exhibit con-
stant differences, the so-called cases of heterostyly. In
the Primulaceae we distinguish one form with long and
another with short style; in some species of flax there
occur three different forms of flowers in different indi-
viduals.

Although here the individuals belonging to two or
three different groups of the same species are different

6Catasetum tridentatum has three different forms of flowers,
which were formerly considered to belong to three different genera :
Catasetum, Monachanthus and Myanthus. de V., 1909.

Variation of Individual Hereditary Characters 19

neither according to sex nor to generation, nevertheless
they are distinguished by attributes which are as constant
and of the same order as the specific attributes taken from
the same organs in allied genera.

In the way of a supplement I will consider, in this
connection, the alternation of generations, because here
also the differences between the physiologically non-
equivalent individuals, belonging to different generations,
are of the same order as the specific characters. This we
are taught by the Uridineae and the Cynipideae, and all
those cases where the presence of an alternation of gen-
erations was discovered only after the single forms had
been described as species, and had been classified with dif-
ferent genera and families of the system. And even to-
day it is impossible to prove morphologically that two
forms belong together; experimental cultures alone can
decide this question. The successive alternating genera-
tions cannot be reduced to the same primary form, for
each of them compounds its characters by means of a dif-
ferent selection from the available hereditary endow-
ments of the species.

In summing up the result of this paragraph and the
two preceding ones, we find that every thorough consid-
eration of a specific character, and every comparison of
this with other characters, leads us to regard the former as
a mosaic, the component parts of which can be put to-
gether in various ways.

§ 4. The Variation of the Individual Hereditary Charac-
ters Independently of One Another

A comparative consideration of the organic world
convinced us that the hereditary characters of a species,
even if connected with each other in various ways, are

20 Mutual Independence of Hereditary Characters

yet essentially independent entities, from the union of
which the specific characters originate. Now let us see
whether or not this conclusion is supported by experi-
ment.

For this purpose let us turn to experiments on the
formation of varieties, especially to those which have been
made on a large scale by plant breeders. They teach us
that almost every character may vary independently from
the others. Numerous varieties differ from their ancestral
form, in only one attribute, as, for example, the white
sports of red-flowered species. The red color changes in
the corolla through all gradations, into white; it may be
lacking or it may be present not only in the blossoms, but
also in the stems and leaves, and can be developed to every
conceivable degree, without any other hereditary quality
being necessarily involved in the variation. In the same
way the hairine'ss, the arming with thorns and spines, the
green color of the leaves, may each vary by itself, and
even disappear completely while all other hereditary char-
acters remain quite unchanged. Frequently some charac-
ters that belong together vary in groups without exercising
any influence on the other groups. Thus an increase in
the number of petals is not rarely accompanied by a petal-
like development of the calyx or the bract-leaves, while
otherwise the plant remains normal. I have cultivated
a Dipsacus sylvestris, which offers all conceivable diver-
sities in the arrangement of the leaves, and which is other-
wise constant in thousands of specimens. The Papaver
sommferwn polycephalum deviates only in the transfor-
mation of numerous stamens into carpels. It is the same
for the cultivated Sempervirum tectorum. Such instances
are so numerous, in the plant kingdom as well as in the
animal kingdom, that the independent varying of single

Influence of Environment 21

characteristics forms the rule, while the combined varia-
tion of several of them is the exception. It is true that
in most cases it cannot be decided whether the given at-
tribute is determined by a single hereditary character or
by a small group of them.

On the other hand an accumulation of several varia-
tions in one race can easily be accomplished, and it occurs
quite commonly in cultures as well as in nature. But the
cases which were sufficiently well controlled and de-
scribed, usually show that the single variations have not
evolved simultaneously, but one after another, and this
is sufficient to prove their independence.

Such an hereditary character, isolated from the rest,
can now become the object of experimental treatment.
Through suitable selection it may be gradually strength-
ened or weakened, and at the will of the breeder it may
be brought into a certain relation to the other unchanged
characters. The red color of the copper-beech has been
so much intensified that even the cell-sap in the living
cells of the wood became intensely red. The doubling of
flowers frequently leads to a complete disappearance of
the sexual organs. And in numerous instances only those
organs change which are subjected to selection while the
others remain unaffected by it. The adaptation of the
cultivated plants of agriculture to the needs of man, and
of the horticultural ones to his aesthetic sense, demon-
strates this to us in the clearest manner.

Experimental treatment further leads to the study of
the influence of external circumstances on the unfolding
of hereditary characters. Here again these prove them-
selves to be factors of which each may vary independ-
ently from the others. Young varieties especially are
objects for study, and all those which have not as yet

22 Mutual Independence of Hereditary Characters

been sufficiently fixed, and in which, therefore, external
influences will still play a prominent part in answering
the question as to whether a given seed will produce a
true or an atavistic individual. Rimpau and others have
taught that with a given kind of seed disturbances and
interruptions of growth exercise a powerful influence on
the number of specimens that bear seed in the first year.7
And in horticultural and teratological literature one finds
scattered numerous data from which the importance of
external influences generally is clearly evident. To the
experimental investigator there is here opened a large and
almost untrodden field. Theoretically the chief task will
consist in isolating as much as possible the variations of
the hereditary characters in order to obtain, in this way,
a knowledge of the individual factors of the respective
character.

The variations which we observe in nature frequently
appear to us as if they had suddenly sprung into existence,
ancl the same is true of cultures on a small scale, or when
the single indivduals are not completely under control.

However, experience with cultivated plants, during
the first years after the beginning of their cultivation,
teaches us that the deviations often develop but slowly,
and that the modifying influences, as a rule, have to
^operate through several generations before they can ac-
cumulate their effect in such a manner that it becomes
evident.8 The facts with reference to this, collected by
Darwin, give the impression that the new characters at
first arise only in a latent state, and in this condition grad-

7Rimpati, A. W. Das Aufschiessen der Runkelruben Land-
wirtschaft. Jahrbilcher. 9: 191. 1880.

8On this point compare Darwin, The Variation of Animals and
Plants under Domestication. Ed. 2. 2: 39. 1875.

Atavism 23

ually gain in strength, until they finally reach the stage
necessary to make them visible. Here again it must there-
fore be assumed that every hereditary character is misci-
ble to any extent with the others.

The independence of the hereditary characters is most
beautifully shown in atavism. A character may remain
latent through a number of generations while all the
others unfold normally. From time to time it appears
again, mostly without exercising any kind of influence on
the other characters. We do not know what external
circumstances condition this reappearance; in all prob-
ability they do not act simply on the atavistic individuals, .
but we must conceive that the given potentiality is alway^
latent in the others, only it is very fluctuating in its
strength. To us only the crests of the highest waves are
visible.

To all appearance such qualities can be transmitted
through a long series of generations, from one generation
to another. Their existence can be reckoned by millen-
niums in those cases where they are at least as old as the
species itself. I mean the cases of reversion to the ances-
tors of the species, of which the zebra-like stripes of the
horse form such a well-known instance.9 We have a
similar illustration in the Primula acaulis var. caulescens,
which occurs from time to time in the field as a quite
isolated specimen among thousands of non-umbellate
plants, and then forms an inflorescence quite similar to
that of the most nearly allied umbellate species. Culti-
vation has taken possession of this more richly flowering
variety, and has put it on the market in many nuances of
color.

I should not close this section without having pointed

9Darwin, Joe. cit, 1: 59.

24 Mutual Independence of Hereditary Characters

out one phenomenon which greatly complicates the study
of hereditary characters. I refer to the circumstance, al-
ready repeatedly alluded to, of their being commonly
united in smaller and larger groups which behave like
units, the single members of the groups usually appear-
ing together. We see this in the staminate and pistillate
flowers and inflorescences of monoecious plants, in the
described cases of bud- variation and dichogeny. The
sexual characters of various individuals and the differ-
ence between the alternating generations of the same spe-
cies teach us the same thing.

This combination of the individual characters into
groups is therefore quite general, although it occurs in
all degrees, and although some hereditary characters, as
for instance the power of assuming a red color, do not
unite, as a rule, into a group with certain others. It is
recognized most clearly in those cases of the formation of
groups of green bracts instead of flowers, caused by
aphids, phytopters, and other parasites, where the stimu-
lus calls forth a whole series of characters that ordinarily
develop in other parts of the plant.

Every theory of heredity has to take into account this
combination of the hereditary characters into larger and
smaller groups, and different authors, like Darwin and
Nageli have strongly emphasized this point. But right
here lies a great difficulty which interferes with a working
out of the theory in detail, for in many cases it will ob-
viously be extremely difficult to decide whether one is
dealing with a single hereditary character or with a small
group of them. There is here a large field for morpho-
logical analysis which awaits working up.

§ 5. The Combination of Hereditary Characters

Hereditary characters can be combined to any extent

Combination of Hereditary Characters 25

and in any proportion. This is shown in variegated leaves
and striped flowers, where the result of this combination,
after corresponding splitting, is almost directly demon-
strated to us. Almost endless is the diversity of pattern
of variegated leaves, frequently on the same plant, or at
least on the different individuals of one and the same
crop. Striped flowers, according to Vilmorin, arise
through partial atavism from old white-flowering varie-
ties of red or blue species.10 Young varieties usually re-
vert by leaps to the ancestral form, while the older ones
do so by steps, through the appearance of isolated stripes
of the original color on the white back-ground. It is as
if the color potentialities were already too much weakened
to tint the whole corolla in one effort. The descendents
of the first striped flowers, however, soon form broader
stripes, and finally return, after a few generations, [at
least in some specimens,11] to the uniform color of the an-
cestral form.

Extremely peculiar are those cases where hereditary
potentialities, which in the active state necessarily ex-
clude each other, occur together in a latent state. Instead
of giving a long enumeration of many cases, I prefer to
describe a well-known case of variability, and select for
the purpose the arrangement of leaves in whorls.

Two-ranked whorls, the leaves of which stand cross-
wise over each other on the successive nodes, belong to
the best and most constant characteristics of entire nat-
ural families. Less frequent are the cases of three- and
more-ranked whorls. Quite frequently, however, one

10Vilmorin, L. Leveque de. Notices sur V ameliorations des
plantes par le semis, pp. 39-41. 1886. (According to modern views
the stripes are due to a separate character, de V. 1909.)

"Matter in the body of the text in brackets has been introduced
anew into the translation by the author of the original. Tr.

26 Mutual Independence of Hereditary Characters

species will change from its normal type into another form
of whorl, and in numerous plants with decussate leaves,
single branches with three- and more-ranked whorls
have been observed. The Fuchsias and the Weigelias of
our gardens, are common examples. The transitions from
one number in the whorls to the other usually take place
by leaps, in such a way that the whole shoot springing
from one bud is alike in this respect; however, branches
with another number in their whorls will frequently de-
velop from its terminal bud or its lateral buds. More
rarely a shoot will change, during its development, from
one number to another, as is the rule, for example, in
Lysimachia vulgaris. Intermediate forms between two-
or three- and four-ranked whorls are exceedingly rare,
although from our present knowledge, they may develop
quite readily, and have actually been observed from time
to time in most plants with whorled leaves.12 I mean those
whorls in which one leaf is more or less deeply split at its
apex, while the mid-vein forks. This splitting occurs in
all conceivable degrees and leads to a complete doubling
in those leaves which bear two blades on one cleft petiole.
Consideration of numerous examples gives the impression
that the single whorl-forms are antagonistic to each other,
and that each tries to exclude the other. It is rare that
they do not succeed in this effort, and then we get the
above mentioned leaves with the forked mid-vein, the
complete series of transition of which, from one leaf to
two leaves has been figured and described by Delpino.13

Therefore, even such qualities, which in the devel-
oped plant exclude each other, are miscible, apparently

12Cf. Delpino, F. Teoria generale della Fillotassi. Atti R.
Univ. Genova 4: 197. 1883.

™Loc. cit. p. 206, Taf. LX, Fig. 60.

Hereditary Characters Are Units 27

without difficulty, in the latent state. In truth, the prin-
ciple illustrated by this example holds good also in the
phenomena of monoecism and dioecism, of the di- and
trimorphism of flowers, and indeed, throughout the en-
tire range of organ-formation. Everywhere we find
characteristics which cannot exist simultaneously in the
same organ, and yet must be associated in a latent state
during its youth.

In summarizing briefly what has been said, we see
that experiments and observations on the origin and fix-
ing of variations teach us to recognize hereditary char-
acters as units with which we can experiment. They
teach us further that these units are miscible in almost
every proportion, most experiments really amounting
merely to a change in this proportion.

The above considerations are verified in a striking
manner by experiments in hybridization and crossing. In
no other connection does the concept of a species as a
unit made up of independent factors stand forth so
clearly. Everyone knows that the hereditary characters
of two parents may be mixed in a hybrid. And the ex-
cellent experiments of many investigators have taught us
how, in the descendents of hybrids, an almost endless
variation can usually be observed, which is essentially due
to a mixing of the characteristics of the parents in a most
varied manner.

The hybrids of the first generation have quite definite
characteristics for each pair of species. If one produces
a hybrid of two species, which previous investigators have
already succeeded in crossing, he can, as a rule, rely on
the description given of it tallying exactly with the newly
produced intermediate form. If the hybrid is fertile
without the help of its parents, and if its progeny are

28 Mutual Independence of Hereditary Characters

grown through a few generations in thousands of speci-
mens, one can almost always observe that hardly any two
are alike. Some revert to the form of the pollen-parent,
others to that of the pistil-parent ; a third group occupies
a central position. Between these are placed the others
in the most motley variety of staminate and pistillate
characteristics and in almost every gradation of mutual
inter-mixture.

Many and prominent authors have pointed out the
significance of hybrids for establishing the nature of fer-
tilization. With the same right we may use them in try-
ing to penetrate into the mystery of specific character.
And then they clearly prove to us that this character is
fundamentally not an indivisible entity. The character-
istics of a hybrid (of the first generation) are as sharply
defined and as constant, and on the whole of the same
order as those of the pure species, and the frequent spe-
cific name, hybridus,™ might go to prove that even the
best systematists felt this agreement.
•N Kolreuter, Gartner, and others have combined in one
hybrid two, three, and more species, and there is no rea-
son why any other than a purely practical limit should
be put to this number, and that, in fact, there should not
be combined in one hybrid characteristics which have
been taken from an unlimited series of allied species.
But this is of small importance, the chief point being the
proposition that the character of a pure species like that
of hybrids, is of a compound nature.

Crossings of varieties of the same species belong, es-
pecially in horticultural practice, to the most common
operations. Ordinarily the object pursued is simply that
of producing intermediate forms. Not infrequently,

14E. g. Papaver hybridum L., Trifolium hybridum L.

Cross- and Self -Fertilization 29

however, one desires to impart single definite qualities to
one variety and he derives these from another variety,
sometimes even from another species. Hardening against
winter-frost has frequently been transmitted in this man-
ner from one form to another. Carriere15 cites instances of
Begonias which, through crossing with a variety of an-
other species with variegated leaves, have been made
varigated without having their other qualities changed.
The conviction is really quite general in horticultural
practice that, by crossings, one may combine the charac-
ters of varieties at will, and thus improve his races ac-
cording to his needs in many as well as in individual
desirable points.

§ 6. Cross- and Self-fertilisation
In addition to the arguments dealt with in the pre-
ceding paragraph, which gives us the results of ex-
periments in crossing and hybridization, we will now
consider normal fertilization and see to what extent, in
this domain, the facts support our conception of the mu-
tual independence and miscibility of hereditary charac-
ters.

To fathom the meaning of fertilization is one of the
most difficult problems of biology. The numerous adapta-
tions of this process to the most varied conditions of life,
and the powerful influence which it has exercised on the
differentiation of species, especially through the develop-
ment of the secondary sexual characters, threaten always
to mislead us, and to make us mistake its essential nature
through its later acquired significance. Here, as in so
many cases, the conditions in the plant kingdom are clearer

15Carriere, E. A. Production et fixation des varietes, p. 22. 1865.
Other examples are given by Verlot, Sur la production et la fixation
des varietes. pp. 46 and 65. 1865. Cf. also Darwin, loc. cit. 2: 73.

30 Mutual Independence of Hereditary Characters

and simpler than in the animal kingdom, in which es-
pecially the exclusive limitation of propagation of the
higher animals to the sexual method makes us only too
easily over-estimate the significance of this process. To
this must be added the fact that, for the vegetable king-
dom, quite an unexpected light has been thrown on the
nature of this process through the exhaustive compara-
tive study of the significance of cross- and self-fertiliza-
tion, for which we are indebted to Darwin.

Darwin's experiments have taught us that the essence
of fertilization consists in the mixing of the hereditary
characters of two different individuals.16 Self-fertiliza-
tion, which takes place so readily in the vegetable king-
dom, and is so easily accomplished experimentally, has
not by any means the same significance. From seeds
obtained in the last named manner the individuals pro-
duced were always weaker in Darwin's experiments than
those obtained in a crop from crossed flowers. The
first ones were smaller, with less profuse branching, flow-
ering less abundantly and less constantly, and accordingly
they bore less seed. Crossing two flowers of the same
plant was more deterimental than the pollination of the
flowers with their own pollen.

Even the crossing of different individuals was not suf-
ficient to keep the species normal when it was cultivated
year after year in the same bed, and protected from being
fertilized by specimens of a different origin. The whole
colony deteriorated steadily and distinctly in the course
of a few years ; not only did the plants become smaller and
weaker, but their individual differences decreased so much
that they resembled each other almost completely. A

16Darwin, Origin of Species. 6 Ed., pp. 76-79, and Cross and
Self Fertilisation in the Vegetable Kingdom. 1876.

The Essence of Fertilisation 31

single cross, however, of such a colony with individuals
of another origin restored the original vigor.

The process of fertilization, in its essence, does not
consist, therefore, in the union of two sexes, but in the
mixing of the hereditary characters of two individuals of
different origin, or at least of such as have been subjected
to different external conditions. Therefore, a difference
in hereditary characters is obviously a condition for at-
taining the full advantage of fertilization ; this difference,
however, must have been acquired in the last instance
through a life under different influences.

Let us regard the individual hereditary factors as in-
dependent units, which can be combined with each other
in different proportions into the individual character of a
plant. Let us further assume that their relative increase
or decrease depends on external influences. Evidently
there is then a great probability that, under similar ex-
ternal conditions, the same factors will deteriorate in
different individuals, while under different conditions this
fate will befall other factors in every individual. Thus,
on crossing the plants of the same bed only, the individual
deviations of the same kind are strengthened; the weak-
ened factors are therefore made still weaker. But if we
cross individuals of the most different culture possible,
the differences in the individual factors are clearly bal-
anced, at least in part; and this the more so, the more
numerous the specimens which deviate from each other,
and which are used for the crossing.

It is well known to plant breeders that luxurious con-
ditions which are varied as much as possible lead to an
accumulation and increase of individual differences, while
simple and uniform circumstances make them disappear
gradually, and thus further the uniformity of all speci-

32 Mutual Independence of Hereditary Characters

mens. The first method is applied in improving races, the
latter in fixing newly acquired varieties.

To maintain a species with the required proportion of
all its hereditary factors, only an occasional crossing is
necessary. It need not precede every generation. Where
sexual generations alternate with asexual ones, as in the
gall-fly, and even where the latter occur in the majority,
as in many aphids, this is clearly seen.

With bees the fertilized eggs become females, the un-
fertilized ones males. But since every male descends
necessarily from a female that originated through fer-
tilization, it evidently profits sufficiently by the advant-
ages of an occasional crossing. The aphids, in which the
male as well as the female originate parthenogenetically,
teach us that here we have to do not with fundamental
relations, but with special adaptations.

The never-opening, so-called cleistogamous flowers,
the numerous devices for insuring self-fertilization in
flowers in case they are not visited by insects, and the
almost unlimited use of vegetative multiplication in plants,
all serve to teach us that an occasional fertilization is all
that is necessary for the normal preservation of the spe-
cies. That in higher animals every individual originates
in the sexual way, is therefore obviously only a special
adaptation.

In summarizing the result of these considerations, we
may say that the true essence of fertilization consists in
V mixing the hereditary characters of the different individ-
uals of a species. Hybrids have taught us how we are to
conceive this co-mingling. There is no doubt that the pro-
cess of mixing is, in principle, the same in both cases.
And just as Wichura17 succeeded in producing hybrids

17Wichura, Max. Bastardbefruchtung im Pflanzenreich er-
lautert an den Bastarden der Weiden. Breslau, 1865.

Conclusion 33

from six different kinds of willows, so should it be pos-
sible to combine, by crossing, the hereditary qualities of
several individuals into one.

In the preceding paragraphs we have seen how the
single hereditary characters occur as independent units
in the experiments of hybridization and crossing, and how
they can be attained in almost every degree. In the same
way, evidently, must we think of those units as inde-
pendent in the ordinary process of fertilization as well.

§ 7. Conclusion

Seemingly elementary, the specific character is ac-
tually an exceedingly complex whole. It is built of nu-
merous individual factors, the hereditary characters. The
more highly differentiated the species, the higher is the
number of the component units. By far the most of these
units recur in numerous, many of them in numberless or-
ganisms, and in allied species the common part of the
character is built up of the same units.

On trying to analyze species into these individual
factors, we are confused by their number, which, in the
higher plants and animals reaches probably into the
thousands. If, however, we regard the entire world of
organisms as the subject of our analysis, then the total ^
number of hereditary characters which is needed for the
building up of all living beings, is indeed large in itself,
but, in relation to the number of species it is small. In
that limited sphere our method of investigation leads ap-
parently only to complications, but, on the whole, it evi-
dently leads the way towards a very considerable simpli-
fication of the problems of heredity.

The hereditary factors, of which the hereditary charac-
ters are the visible signs, are independent units which may

34 Mutual Independence of Hereditary Characters

have originated separately as to time, and can also be lost
independently from one another. They can be combined
with each other in almost every proportion, every indi-
vidual character from complete absence through all
gradations being capable of attaining the highest devel-
opment. Frequently the conditions are so unfavorable
for some of them that they cannot manifest themselves
at all, and so remain latent. In this condition, they may
either persist for thousands of generations, or they may
appear in every generation during the development of the
individual from the fertilized egg, in which they are nearl)
all latent.

The hereditary factors compose the entire specific
character ; there is no separate basis to which they are at-
tached.

Although independent to the degree that each, of
itself, can become weaker and even disappear completely,
they are yet, as a rule, united into smaller and larger
groups. And the condition is such that, when external
influences, such as a stimulus to gall-formation, bring a
definite character into dominance, the entire group to
which it belongs is usually set into increased activity.

Independence and miscibility are therefore the most
essential attributes of the hereditary factors of all or-
ganisms.

To find a hypothesis which will make these charac-
teristics more comprehensible to us, is, according to my
opinion, the chief problem of every theory of heredity.

B. PREVAILING VIEWS ON THE BEARERS OF HEREDITARY
CHARACTERS

CHAPTER II

THE SIGNIFICANCE OF THE CHEMICAL MOLECULES OF

THE PROTOPLASM WITH REFERENCE TO THE

THEORY OF HEREDITY

§ i. Introduction

According to our present conception of all nature, the
wonderful phenomena of heredity must have a material
basis, and this basis can be no other than the living pro-
toplasm. Every cell originates through the division of
one that already exists; the living substance of the
mother-cell is distributed among the individual daughter-
cells and passes into them with all its hereditary qualities.
Microscopic investigation of the cell-body and the art of
the breeder, so far apart from each other until recently,
come nearer and nearer to working hand in hand. And it
is only through the co-operation of these two great lines
of human thought that we can succeed in establishing the
basis for a theory of heredity.

Chemistry teaches us that living protoplasm, like any
other^substance, must be built up of chemical molecules,
and that a final explanation of the phenomena of life can
be reached only when we shall succeed in deriving the
processes in protoplasm from the grouping of its mole-
cules, and from the composition of the latter out of their
atoms.

We are still, however, very far from this goal. The
chemists study chiefly pure bodies, that is, such as are
built up from like molecules; but protoplasm is evidently
a mixture of numerous, if not of almost countless differ-
ent chemical compounds. And by far the most of these

38 The Significance of Chemical Molecules

latter have been, even chemically, only very incompletely
investigated.

Of course, this consideration must not keep us from
utilizing the great truths of chemistry in the explanation
of life processes. Haeckel, and many other investigators
after him, have pointed out the great significance, for
such an explanation, of the power of carbon to combine
^in the most varied relations with other elements. "This,
in its way, unique property of carbon we must designate
as the basis of all pecularities of the so-called organic
compounds."1 The differences, which occur in the
growth of organic and inorganic individuals, are due to
the more complex chemical composition and the power
of imbibition of many carbon-compounds,2 et cetera.

In chemistry also this importance of carbon has been
\ emphasized. In his Vieivs on Organic Chemistry, van't
Hoff3 says : "From the chemical properties of carbon
it appears that this element is able, with the help of two
or three others, to form the numberless bodies which are
necessary for the manifold needs of a living being; from
their almost equal tendency to combine with hydrogen
and oxygen, follows the capacity of the carbon-com-
pounds to be adapted alternately for processes of r^duc-
tion and of oxydation as the simultaneous existence of a
vegetable and an animal kingdom requires." And, after
a discussion of the influence of temperature on the change
of the chemical property of carbon, he continues : "There-
fore, one does not go too far in assuming that the ex-
istence of the vegetable and animal world is the enor-

iHaeckel, E. Generelle Morpholgie, 1: 121. Berlin. 1886.
2Loc. cit. p. 166, and Haeckel, E. Die Perigenesis der Plastidule.
p. 34. 1876.

3 Van't Hoff. Ansichten iiber die organischc Chcmic. 1: 26. 1878.

Tivo Kinds of Life-Processes 39

mous expression of the chemical properties which the
carbon-atom has at the temperature of our earth."

Furthermore if we take into consideration the num-
berless isomers, which especially the more complicated
compounds of carbon, such as protein bodies, can form,
according to the present chemical theories, there can
hardly be any doubt that we shall some day succeed in re-
ducing the hereditary characters of all organisms to chem-
ical differences of their protoplasmic basis.4

But, much as such general considerations may help to
further our need for a uniform conception of all nature,
they are still far from serving us, especially at the present
time, as a basis for a theory of heredity.

Experimental physiology of plants and animals has
succeeded in reducing many of the processes of life to the
chemical effects of the involved compounds, to repeat
them in part outside of the organism, but in part also to
demonstrate the fact that their behavior in the living body
is ruled by the general laws of chemistry. Into an
understanding of the processes of breathing, nutrition,
and metabolism we have been initiated in a simply as-
tonishing manner by numerous investigators, and the
purely mechanical manifestations of energy which ac-
company growth and motion have also, in great part,
been analyzed and reduced to general laws. But the chief
discovery of these studies is that two kinds of processes
occur in the living body. In the first place, those that are
separable from living substance, and can therefore be ar-
tificially imitated, or even exactly duplicated. In the
second place, those that are inseparable from that sub-
stratum, and which indeed find their existence in the

4Cf. Haeckel, E. Generelle Morphohgie. 1: 277, and Sagiura,
Shigetake. Nature 27: 103. 1882.

40 The Significance of Chemical Molecules

processes of life of that very substratum. The former
processes are purely physical or chemical ; in a word, they
are aplasmatic processes ; the latter ones we must designate
as plasmatic; that is, as taking place in the molecules of
the living protoplasm itself. The former belong to phy-
siological chemistry and physics, the latter form the
proper subject of physiology. But toward an under-
standing of the latter we have taken only the first steps.

It is neither by general considerations, nor on an ex-
perimental basis, that we can penetrate, at the present
moment, into the relations between the qualities of the
chemical molecules of the protoplasm and the phenomena
of heredity. It can therefore be only a matter of try-
ing, by means of hypotheses, to get an insight into these
relations.

It is evident that we are justified in making such an
attempt. This right is very generally acknowledged, for
several prominent investigators have published their
views on this subject. Some have even made their hy-
potheses accessible to the critical consideration of others
by working out logically the consequences arising there-
from. And certainly, no one can doubt for a moment that
these hypotheses, much as they differ at present, have
aroused scientific interest in these questions.

The directions which these hypotheses take can, I be-
lieve, be summarized under three heads. Some authors
go directly back to the chemical composition of proto-
plasm and seek to derive the life-processes from it.
Others again assume that the chemical molecules are com-
bined into larger, but still invisibly small organic units,
and regard these units as the real bearers of heredity.
Some of them imagine that these units always represent
the whole specific character, and that therefore the in-

Protoplasm and Protein 41

dividual bearers of heredity in the same cell, with the
exception of insignificant differences, are alike. • Finally,
there is the directly opposite opinion of those investi-
gators who assume a special kind of material bearer for
every individual hereditary character; and according to
whom, therefore, protoplasm is built up of numberless
unlike hypothetical units.

It is these three different principles that we will sub-
ject to a thorough comparative examination in this and
the two following chapters. Before doing so, however,
we must first critically consider the relation between pro-
tein substances and protoplasm.

§ 2. Protoplasm and Protein

Lately the conceptions of protoplasm and protein have
been confused by many authors.5 This has led to the
hypothetical, and in no way justified assumption of a
living protein.6 This usage has exercised its influence,
even on the theory of heredity, and for this reason it
should not remain unmentioned here. Without this con-
fusion, the view which regards the chemical molecule of
protoplasm as the bearer of the hereditary characters
would probably never have met with any favor.

Protein is a chemical, protoplasm a morphological
concept. Chemistry is able to produce many pure pro-
teins, while the nature of protoplasm is conditioned by
its very heterogenous composition. Many protein bodies
can pass into solution, but nobody will ever think it pos-
sible to obtain a solution of protoplasm in a test-tube.

5Haeckel refers to protoplasm as a protein body: Generelle
Morphologic. 1: 278.

8A term proposed by Pfluger. Arch. Ges. Physiol 10: 251. 1875.
Tr.

42 The Significance of Chemical Molecules

Protein bodies are indeed products of life, but not the
bearers thereof; they do not offer us, in the chemical
laboratory, any essentially different quantities than the
other more complicated compounds. Protoplasm, how-
ever, is the bearer of life; it is distinguished from all
chemical substances by its power of assimilation and of
reproduction. The nature of these two processes will
undoubtedly be recognized some day, but up to the pres-
ent time they are still in complete darkness, and even
the boldest minds have not yet succeeded in lifting even
as much as a corner of the veil that covers them.

The designation of protoplasm as a protein body, or
as a mixture of such bodies, is based upon chemical analy-
ses and micro-chemical reactions. The latter undoubt-
edly betray the quite common presence of protein in pro-
toplasm. But the explanation of this fact is obvious.
Protein can very well be dissolved in the water of imbi-
bition of protoplasm, since it can be proven to occur fre-
quently in solution in the cell-sap. It is even not
improbable that, in killing the protoplasts, protein bodies
are frequently formed. But, in order to be able to assert
that protoplasm and protein are identical, it ought at least
to be demonstrated that protein-reactions are lacking
neither in any protoplasm nor in any individual organ
thereof. But such does not, by any means, appear to be
the case.7 Nucleus, trophoplast, and nucleo-plasm, have,
it is true, never been observed without protein, in well
nourished cells ; but, whether the wall of the vacuoles and
the plasma-membrane are structures that contain protein,
is still very questionable.8

Chemical analyses have, without doubt, brought to

7Cf. Zacharias, E. Bot. Ze'it. 4: 209. 1883.
8Cf. Jahrb. Wiss. Bot. 14: 512. 1883.

Morphological Units 43

light important conclusions concerning many compounds
developed from protoplasm. But whether those com-
pounds were present, as such, in the living protoplasm, or
have only developed after death, or through the influence
of reagents, as products of decomposition, is another
question.

The chief point for the theory of heredity is, however,
that protoplasm always offers us certain historical char-
acters besides physical and chemical properties. It is to
these that it owes its peculiarity. A synthetic composition
of protein bodies is no longer regarded by anybody as an
impossibility; but whether we shall ever succeed in ob-
taining living protoplasm in any other than the phyloge-
netic way, will probably remain for a long time a matter of
well-founded doubt.

The historical characters demand a molecular struc-
ture of such complicated nature that the chemistry of the
present time fails us entirely in our attempts at an ex-
planation. For the present, therefore, theory must be^
content to accept the idea that protoplasm is composed of
morphological units. These, of course, must themselves
be built up from chemical molecules, and among the latter
the protein bodies must play an important role. To con-
clude from this fact, however, that protoplasm itself is a
protein body, seems not at all justified.

Those invisible morphological units are of a hypothet-
ical nature and we will not follow up this subject any
further in this connection. I only wished to show how
this consideration also, leads us to that assumption of
pangens, with which we shall have to deal in the last two
chapters of this section.

44 The Significance of Chemical Molecules

§ j. Elsberg's Plastidules

The most thorough attempts to explain the phenomena
of heredity by the qualities of the molecules of living
matter were made by Louis Elsberg and Ernst Haeckel.
Elsberg, who called the cells plastids, chose for the com-
ponent particles the name of plastid-molecule or, abbre-
viated, plastidule.9 Haeckel considered this expression
a brief and suitable designation for the polysyllable pro-
toplasm-molecule,10 and secured general consideration for
the term in his "Perigenesis of the Plastidule."11

According to Elsberg, living matter consists entirely
of plastidules which multiply in such a manner, through
nutrition, assimilation, and growth, that new molecules
with the same characters as those present, are constantly
developed. At each cell-division these are transmitted to
the daughter-cells. The resemblance of children to
their parents, grand-parents, and ancestors is explained
in a simple manner by saying that they are essentially
built up of the same kind of plastidules, which they have
inherited from their ancestors. All individuals of one
species consist, on the whole, and apart from incidental
varieties, of the same plastidules; every species, how-
ever, contains the plastidules of its whole ancestry, and
consists therefore, of as many different plastidules as
there were different species in this ancestry. The dif-
ferences between individual species are conferred by their

9Elsberg, Louis. Regeneration, or The Preservation of Organic
Molecules : a Contribution to the Doctrine of Evolution. Proc.
Amer. Assoc. Adv. Sci. 23: 1874; and Elsberg, Louis. On the Plasti-
dule Hypothesis. Ibid. Buffalo Meeting, August, 1876. 25: 178. 1877.

10Haeckel, E. Jenaische Zeits. Med. Naturw. 7: 536. 1873.

"Haeckel, E. Die Peregenesis der Plastidule. p. 35. Berlin, 1876.

Elsberg's Plastidules 45

descent, and are, therefore, materially based on the dif-
ferences of the plastidules. Systematic affinity depends
upon the possession of the same plastidules, systematic
differences on the presence of different molecules in addi-
tion to the bulk of those that are alike.

Haeckel, who, in his "Generelle Morphologic," had
not yet considered the significance of the molecule for the
theory of heredity,12 has further carried out Elsberg's
train of thought13 in his above mentioned monograph.
"The sum total of physical and chemical processes, called
life, is evidently conditioned in the last instance by the
molecular structure of the plasson."1* In the non-nu-
cleated plasson (or protoplast) the plastidules are every-
where uniform; in the. nucleated ones they are differen-
tiated in such a manner that a distinction must be made
between plasmodules and coccodules (nucleo-molecules).
The differentiation of the organism into organs, and the
division of labor thereby achieved, Haeckel attributes to
a division of labor of the plastidules, for in this way they
are segregated more or less, and thus produce the various
kinds of protoplasm. Fertilization consists in the fusion
of two protoplasts which have developed in different
directions through a far-reaching differentiation of their
plastidules.15

We will limit ourselves to this part of the theory of

12Only in a general way does Haeckel point here to the signifi-
cance of "the numerous and minute differences in the atomic con-
stitution of the protein-compounds, which form the plasma of the
plastids." Gen. Morphol 1: 277.

13Elsberg later (Proc. Amer. Assoc. Adv. Sci. 25: 178. 1877.) in-
sisted that he had been misunderstood and misinterpreted by Haeckel
in the monograph above referred to. Tr.

l4Perigenesis. p. 34.

15Loc. cit. p. 52.

46 The Significance of Chemical Molecules

the plastidules, and not enter into the speculations on the
undulating motion of these particules. But, in critically
discussing that part, we can emphasize here the fact that
the theory is composed of two hypotheses :

1. Protoplasm is made up of numerous small units,
which are the bearers of the hereditary characters.

2. These units are to be regarded as identical with
molecules.

Trie first of these two hypotheses has obviously very
great advantages. It explains the fundamental phenom-
ena of heredity in a simple manner, and especially ac-
counts sufficiently for the independence and miscibility
of the individual hereditary characters. It is identical
with the first law of Darwin's pangenesis, as we shall see
more in detail in the third Chapter. We shall, therefore,
put off a more thorough discussion, especially as Elsberg
wrote a few years later than Darwin, and in not nearly
as clear a manner.

Let us now turn to a criticism of the second thesis.
Elsberg never expresses himself clearly about the identity
of his plastidule with chemical molecules. He defines
them as the smallest particles of a cell in which the hered-
itary characters lie hidden.16 These particles must be
larger than the molecules of the ordinary protein bodies ;
this follows from their much more complicated character.
Haeckel, however, devotes a detailed discussion to this
identity.17 "T*he plastidules possess, first of all, every
quality which physics ascribes generally to the hypotheti-
cal molecules, or combined atoms. Consequently each
plastidule cannot be analyzed any further into smaller
plastidules, but only into its component atoms. ..."

18Elsberg. loc. clt. p. 9.
17Perigenesis loc. cit. pp. 35-36.

Elsberg's Plastidules 47

As long as we are concerned only with the explana-
tions of the chemical processes in cell-life, this hypothesis
is certainly highly satisfactory. The production of vari-
ous compounds, as for example, the red coloring matter
of a flower, can be imagined as a function of definite
molecules of the protoplasm, more or less in the same
manner as the action of enzymes or chemical ferments.
Even the secretion of cellulose one might try to explain
thus by analogy. As soon, however, as we have to do
with morphological processes, this hypothesis fails us en-
tirely, because the frequently attempted comparison with
the formation of crystals furnishes only a remote simi-
larity. The hypothesis is quite useless when applied to'
that peculiar attribute of life, growth through assimila-
tion. It is obvious that any attempt to explain life-pro-
cesses from the properties of chemical molecules must
consider this phenomenon first of all. But in the great
realm of the lifeless there is no analogy for it. Chemical
molecules do not grow in such a way as to separate later
into two molecules which are like the original one. They
do not assimilate, and in this sense they are not capable
of independent multiplication. They do not possess any
qualities at all from which one could at present hypotheti-
cally explain a growth through assimilation.

Here lies the great difficulty of the plastidule hy-
pothesis. Indeed, Haeckel says, "Besides the general
physical properties, which modern physics and chemistry
ascribe to the molecules of matter in general, plastidules
possess some special attributes which are exclusively
their own, and these are, quite generally speaking, the
life-attributes which, according to the present concep-
tion, distinguish the living from the dead, the organic
from the inorganic." But it is easily understood that by

48 The Significance of Chemical Molecules

such an ancillary hypothesis the meaning of the hypothesis
as a whole is changed. For, with the same right, one
might say that the plastidules are not molecules at all, in
the sense of physics, but are distinguished from them
by their very life-properties.

It would be easy further to criticise the plastidule-
hypothesis in the same direction. It leads to pure specu-
lation. According to Haeckel, we must attribute sensa-
tion and will power to atoms.18 The plastidules possess
memory, according to his theory ; this faculty is lacking
in all other molecules.19 We shall not discuss, either, the
wave motion of the plastidule.

What is of interest to us, is to show that any attempt,
at the present time to reduce life-phenomena to the prop-
erties of the molecules of living matter, is, to say the
least, premature. We must either limit ourselves, with
Elsberg, to such deductions as can be derived from Dar-
win's gemmule-hypothesis, or be compelled to resort
everywhere to ancillary hypotheses, in place of explana-
tions. If we choose the first method, we arrive naturally
at the assumption of invisible units, of a higher order
than the molecules of chemistry, and of such a compli-
cated composition that every one of them must be made
up of a large number of chemical molecules. To these
units we must attribute growth and multiplication as
qualities which so far cannot be explained. In a like in-
explicable manner we must further assume that they are
the material substratum for hereditary characters. Leav-
ing this part unexplained, we can clear up many other
things. But in that case we cannot revert to the mole-
cules of protoplasm.

"Haeckel loc. cit. p. 38.
™Loc. cit. p. 40.

The Name Molecule Inappropriate 49

Therefore the material bearers of hereditary charac-
ters cannot be identical with the molecules of chemistry;
they must be conceived of as units, built up from the latter,
much larger than they, and yet invisibly small.

It does not seem to me correct to apply the name mole-
cule, or living molecule, to these units. This appellation
must lead to confusions and misunderstandings, and
I suppose it is employed only from lack of a simple desig-
nation. As such a term, the name "pangen," proposed in
the Introduction (p. 7), may be adopted.

CHAPTER III

THE HYPOTHETICAL BEARERS OF SPECIFIC
CHARACTERS

§ 4. Introduction

The majority of investigators assume that the ma-
terial bearers of hereditary characters are units, each of
which is built up of numerous chemical molecules, and is
altogether a structure of another order than the latter.

1 Growth through assimilation, and multiplication
by division are always assumed for them. For this
reason, as Darwin has already said, they are rather to be
placed in a class with the smallest known organisms, than
with the real molecules. An explanation of these prop-
erties is not attempted ; they are simply accepted as a fact.
Neither does the theory of heredity require such an ex-
planation; it can, for the time being, be reserved as a
problem for a later theory of life.

A second assumption in regard to the nature of those
hypothetical units is still .needed ; namely, one concerning
their relation to the hereditary characters. As to the man-
ner in which the latter are determined by the structure of
the bearers no suppositions are yet made, for the theory
of heredity does not, for the present, need this elabora-
tion. The only question is, whether the units are the
bearers of, all the specific attributes, or of the individual
hereditary characters only. Spencer and Weismann are
the chief representatives of the first view, Darwin's pan-
genesis assumes the latter.

Spencer's Physiological Units . ' 51

We have now critically to compare these various
opinions. In doing so the chief question is in how far
the hypotheses themselves, as they have just been de-
scribed, and without further ancillary hypotheses, can
lead to an explanation of the phenomena of heredity.

§ 5. Spencer's Physiological Units

In his famous system of Synthetic Philosophy, Her-
bert Spencer attempted, probably for the first time, to
formulate a material conception of heredity. His Prin-
ciples of Biology, which form the second and third volume
of that system, appeared in 1864 and 1867, therefore
before the publication of Darwin's pangenesis (1868).
His train of thought is essentially as follows :

Bud-formation from leaves, et cetera, teaches us that
the living particles of these organs possess the power of
reproduction, which is also shown in animals by the res-
toration of lost members. Now these particles cannot be
the cells themselves, because some cells can also replace lost
parts. Just as little can they be chemical molecules, be-
cause these are much too simply constructed for an ex-
planation of all the morphological differences. They
must, therefore, be units of intermediate size, invisibly
small, but composed of numerous molecules. Spencer20
calls them physiological units.

Every one of these units represents the entire specific
character; slight dissimilarities in their structure cause
the differences between allied species (p. 183).

Spencer finds it difficult to explain fertilization.
There is no sense in it unless there is some kind of dif-
ference between the two groups of physiological units.

20Spencer, H. Principles of Biology. Ed. 2. 1: 180-183.

52 Hypothetical Bearers of Specific Characters

This makes him assume that the units of different indi-
viduals are slightly dissimilar. From this it follows that
in the child the two kinds of units of both parents are
mixed, in the grandchild the four different units of the
grandparents, and so on. In this way one would arrive
at just the opposite of what was at first assumed, namely,
the similarity of all units in the same individual (pp. 253,
254, and 267).

To escape this difficulty Spencer points to hybrids. In
these the physiological units of two species are mixed.
The hybrids are liable to be inconstant in the following
generations, and to revert to the parental forms. There-
fore the unlike physiological units oppose a mixture, they
repulse each other, and try each, by excluding the dis-
similar kind, to form the whole individual (p. 268). In
the same manner the unlike physiological units exclude
each other in normal fertilization, and in this way uni-
formity within the individual is sufficiently assured.

The physiological units multiply at the expense of the
nutrient material (p. 254) and thus produce, as a rule,
new units that are quite alike. Under the influence of
external circumstances, however, they sometimes undergo
slight changes during the process of their multiplication,
and this is the cause' of their variability (p. 287).
Through fertilization, however, the balance thus disturbed
is regained (p. 289).

On this basis heredity is easily explained ; it is founded
on the fact that the child receives from father and mother
the material units that go to make up its characters.
Strong resemblance of the child to one of its two parents
is due to the predominance of the respective physiological
units; atavism depends upon the presence of units in-
herited from some given ancestor. Many other phenom-

Wcisni ami's Ancestral Plasms 53

ena are explained by Spencer in a similarly simple
manner.

Spencer's theory has, without doubt, the advantages
of a clear and concise system. But it does not take into
account the train of thought developed in our first section.
On the basis of those general considerations, therefore,
the theory is insufficient. Especially can it not explain
in a satisfactory manner the differentiation of organs, and
any attempt to bring it into accord with this process
would prove its fundamental inadequacy. Since the same
thing is likewise true of Weismann's theory of the ances-
tral plasms I refer the reader, in regard to it, to the con-
clusion of the next Section.

§ 6. Weismann's Ancestral Plasms

In a series of thoughtful writings during the last
decade, August Weismann has aroused the general in-
terest of the scientific public in the principles of heredity.
In doing so, he used, as a basis, the most recent achieve-
ments in the domain of cell-theory and the process of
fertilization.

Proceeding from the conviction that the development
of children from material particles of their parents is the
cause of heredity, and that the solution of the great
mystery is, in truth, to be looked for in the molecular
structure of the protoplasm, he tries to form a definite
conception of this structure. He begins by saying that,
in lower organisms, which do not possess a sexual dif-
ferentiation, the germ-plasm of each individual must
still be completely uniform. During fertilization, how-
ever, a mixing of the two parental germ-plasms must take
place, and thus in the child there are mixed two, in the

54 Hypothetical Bearers of Specific Characters

grand-child four kinds of germ-plasms.21 In the children
of the first sexually produced generation there will be only
one-half of the original amount of the two kinds of germ-
plasm, in the grand-children only one quarter. In every
succeeding generation the germ-plasm will consequently
consist of a larger number of unlike units, the so-called
ancestral plasms. But this can only continue until the
number of the ancestral plasms has reached that of the
smallest units of the entire hereditary substance. These
units, originally quite alike, are so no more, but each
possesses the tendency to transmit, under given condi-
tions, to the new organism, the totality of the character-
istics of the respective ancestors.

If now sexual propagation takes place in a species
with this kind of compound germ-plasm, (and all living,
sexually differentiated species must obviously have
reached this stage long ago), a further multiplication of
the ancestral plasms within the germ-plasm can no longer
continue. Therefore the number of the ancestral plasms
must be reduced from time to time. In the separation
of the polar bodies from the egg before fertilization, he
sees a process, the result of which is just this reduction.22

This reduction in the egg of the number of hered-
itary particles, as Weismann calls them, is obviously a
necessary consequence of the original assumption of the
uniformity of the germ-plasm. It is very instructive that
two such prominent thinkers as Spencer and Weismann,
starting from the same hypothesis, have arrived at an
ancillary hypothesis which is intrinsically the same. One
may well conclude from this that whoever does not wish

21Weismann, A. Ueber die Zahl dcr Richtungskorper, p. 30.
1887.

22Loc. cit. p. 32 ff.

Weismann' s Ancillary Hypothesis 55

to accept the ancillary hypothesis must also give up the
principle of the uniformity of the germ-plasm.

Weismann has connected his theory in a clear way
with the results of cell-study. He assumes that the nucleus
dominates and determines the nature of its cell, and also
that, for all functions of the cell, the material bearers of
the hereditary characters must be situated in. the nucleus.
He assumes further that these bearers are arranged in
rows on the chromatin-thread of the nucleus, and points
out how, with this assumption, all the hereditary char-
acters are divided through the longitudinal splitting of
the nuclear skein, and how they are distributed among the
two daughter-cells.

On the basis of these and similar conceptions, he also
treats the question concerning the cause of the differences
between the single organs of an individual. It is clear
that this question forms a great difficulty of the theory.
For the assumption of the ancestral plasms, every one of
which represents all the characters of the individual, can,
of itself, not serve as an answer, especially in connection
with the thesis just mentioned, that the nature of the
nucleus determines the character of its cell.

Let us see what ancillary hypothesis Weismann uses.
The theory of heredity demands that, on the germ-
tracks,23 the completeness of the germ-plasm be preserved,
for every egg-cell and every bud contain, on the whole,
the same hereditary elements as the germ-cells of the pre-
vious generation. In all the sequences of generations of
cells, which lead from one egg-cell to the germ-cells that
come next in order, (and these are the germ-tracks), the
germ-plasm must therefore remain the same. In all other
cells, however, which do not belong to the organs capable

23Cf. Part II, A. p. 79.

56 Hypothetical Bearers of Specific Characters

of reproduction, this, according to Weismann, need not be
the case. On the contrary, from the one-sided differen-
tiation of these cells, he believes that there is a corre-
sponding reduction of their germ-plasm. Every somatic
cell receives, at the time of its origination, only those
hereditary elements which will be needed by itself and its
descendents.

Against this assumption objections have been raised
from different sides, and some of them we shall describe
in detail in the Section on cellular pedigrees. Here, how-
ever, we must enter into the principal phase of the ques-
tion, namely, the relation of the ancillary hypotheses to
the main principle of the author.

That principle is the assumption of units, of which
every one is capable of reproducing all, or at least nearly
all, hereditary characters of the species. There is sup-
posed to be, for each individual, only one hereditary sub-
stance, only one material bearer of the hereditary tenden-
cies.24 To be sure, this is composed of ancestral plasms
which differ only slightly. A check must necessarily be
put to an excessive accumulation of various hereditary
tendencies by some kind of an arrangement. But, as we
have seen in our first section, the differentiation of the
organs demands the divisibility of the units of the germ-
plasm, and this in exactly the same high degree that the
differences of the individual organs and cells of an or-
ganism reach themselves. In the somatic cells the germ-
plasm must therefore gradually become divided into those
components, and hence, these are the bearers of the in-
dividual hereditary characters.

Let us continue to build a few moments longer on this
conclusion, without reference to the chief assumption. In

2*Ueber die Zahl der Richtungskorper, p. 29.

Nageli's Idioplasm 57

that case the germ-plasm must evidently consist, every-
where, of these same components, and, in the lowest
organisms, in which fertilization does not take place, as
well as in the germ-cells of the higher plants and animals,
we must assume, as the material basis of heredity, numer-
ous material bearers, which correspond to the individual
hereditary characters, and are not inseparably united. This
assumption, however, makes that of the ancestral plasms
completely superfluous. Thus it is easily seen that the
whole ancillary hypothesis regarding an occasional nu-
merical reduction of the ancestral plasms may fail.

In a word: In a consideration of the differentiation
of organs, Weismann's theory of itself leads to the quite
opposite assumption of individual material bearers for
the individual hereditary characters.

§ 7. Nageli's Idioplasm

In his mechanico-physiological theory of descent,
Nageli, a few years ago, advanced the concept of the
idioplasm25 In distinction to the other protoplasms, it
is the bearer of the hereditary qualities. A factor (an-
lage) representing every perceptible character, is present
in it; in every individual of the same species, even in
every organ of a plant, it has a slightly different compo-
sition. It is not limited to the nucleus, but runs through
the entire protoplast as a strand with many windings. All
cross-sections of this strand are alike, each one containing
every hereditary tendency. That is why, in cell-division,
the daughter-cells, with their part of the strand, are also
endowed with all the hereditary factors.

The nature of the idioplasm is determined by its mole-

25Nageli, C. von. Mechanisch-physiologische Theorie der Ab-
stammungslehre. pp. 21-31. 1884.

58 Hypothetical Bearers of Specific Characters

cular composition, and especially by the arrangement of
its smallest particles. These are combined in hosts, which
again are united into units of a higher order. The latter
represent the primordia of the cells, tissue-systems, and
organs. The idioplasm is a rather solid substance, in l/
which the smallest particles do not undergo any shifting
through the forces at work in the living organism, for it
is precisely the mutual arrangement of the molecules that
determines the nature of the hereditary factors.

The characteristics, organs, adaptations, and func-
tions, which are all perceptible to us only in a very com-
posite form, are, in the idioplasm, resolved into their real
elements. These elements are obviously the individual
hereditary factors, through the manifold changing com-
binations of which the visible characters originate. These
elements themselves are not strongly emphasized by
Nageli ; he lays greater stress on the fact that their prop-
erties are conditioned by their molecular structure, and
that they themselves, by their mutual association with
each other, again build up the entire idioplasm.

No definite conclusions can be drawn from the theory
in regard to the arrangement of the elements in the idio-
plasm, nor in regard to the question of how the idioplasm
develops its factors; here a wide field is still open to hy-
potheses.28 In general, however, the definite mutual ar-
rangement of the elements forms the chief points in which
Nageli differs from his predecessors. Neither Spencer
nor Weismann enter into this question, and Darwin's
pangenesis supposes a relatively loose combination of
those elements, which does not hinder a mutual penetrat-
ing and mixing. The question as to how the idioplasmic
strands of the two parents unite during fertilization is also

26Loc. dt. p. 68.

General Conclusions 59

only briefly mentioned by Nageli,27 and the whole pre-
sentation of this subject shows what great difficulties the
hypotheses of the solid composition of the idioplasm en-
counters.

Nageli 's theory tells us as little as any other theory
about growth through assimilation and the multiplication
of the material bearers of heredity. That the properties
of those elements are determined by their molecular
structure is just as little an advantage of his theory; it is
a conclusion derived from our most general conceptions,
which can be applied with the same right to the hypotheti-
cal units of every theory of heredity. But how that mole-
cular structure explains the hereditary factors, we, of
course, learn as little here as by any other thedfy. It is a
weak point of Nageli's work that these hitherto unex-
plained facts are not clearly designated as such, and that
the common basis of the various theories is not simply
mentioned as such.

§ 8. General Considerations

To my mind the above briefly sketched theories clearly
prove that the fundamental thought of pangenesis, that
is, of different material bearers for the individual hered-
itary characters cannot be avoided. Spencer, who wrote
before Darwin, did not have this thought, and it was im-
possible for him to give a satisfactory explanation of the
differentiation of organs. Weismann's theory, as we have
already seen, led its originator himself in that direction,
and forced him to admit, more or less clearly,. a divisibility
of the germ-plasm in this sense. And Nageli's idioplasm
is, on the whole, built up from those elements.

The more carefully we look into these theories in de-

**Loc. cit. pp. 215-220.

60 Hypothetical Bearers of Specific Characters

tail, the more we shall find that their efficiency lies in that
implicitly made assumption, while their difficulties arise
mostly through the other hypotheses. If, for the present,
we consider the material bearers of the individual charac-
ters, out of which we must imagine the physiological units,
the ancestral plasms, and the idioplasm to be composed, as
their elements, then the assumption of such elements is in
itself sufficient to explain the fact of heredity. The pre-
vailing resemblance of children to one of the parents, and
the phenomena of atavism become thereby comprehensi-
ble without any further assumptions.

The consequence which Spencer and Weismann em-
phasize as a necessity of their theory, namely the reduc-
tion of the number of units, (which, according to the
former, results through mutual repulsion, according to
the latter, through the polar bodies), is a difficulty which
arises from the union of the "elements," assumed by both
thinkers, and not from the assumption of the elements
themselves. If we discard the grouping of the elements
into units or ancestral plasms, such a reduction becomes
quite superfluous, because the individual elements can ar-
range themselves, after the fertilization in the egg, in a
similar manner as previously in the egg and in the sperm-
cell. And the phenomena of so-called specific atavism, in
which species preserve latent characteristics which they
have inherited from their ancestors, as, for example, the
Primula acaulis caulescens, show that latent characters
need not be thrown off, but may be preserved through
thousands of generations. In the idioplasm the firm union
of the "elements" is most strongly worked out, and it is
precisely in that point that every attempt fails to make the
theory harmonize with the phenomena of fertilization and
hybridization. For these processes teach us that hered-

Similarity of Various Theories 61

itary factors are miscible, but the idioplasmic strands are
not.

Variability teaches us that individual factors may con-
siderably increase, independently from others, and,
on the other hand, may almost completely disappear. And
in the formation of species this possibility has been util-
ized to the highest degree. In the solid union of the
idioplasm such a behavior of the individual "elements"
might be made extremely difficult, if not quite impossi-
ble.

We cannot, therefore, maintain the solid union of the
"elements" into physiological units, ancestral plasms, or
idioplasm. This leads, not only in the cases mentioned,
but almost everywhere, to contradictions with the facts,
or at least to superfluous assumptions. But it is just on
this union that the originators of these theories have laid
the greatest stress, while they have nowhere emphasized,
as an independent assumption, the conception of the "ele-
ments," and have not considered that as a thing apart
from their other hypotheses.

As soon as we do away with this union, the kernel of
all theories is the same as that of pangenesis, as has al-
ready been mentioned at the beginning of this Section.

CHAPTER IV

THE HYPOTHETICAL BEARERS OF THE INDIVIDUAL
HEREDITARY CHARACTERS

§ p. Introduction

The views on the nature of heredity expressed in the
first Section lead us to the conviction that hereditary
characters must be units, independent to a higher degree,
and combined in nature in the most varied groupings.

'On the other hand, a critical survey of the theories so
far discussed induced us to perceive in all of them a more
or less clearly defined kernel, which assumes material
bearers for the individual hereditary characters. To shell
this kernel was our task, and it had its justification in
those views. While the solution of the problem was
hitherto achieved with difficulty, this very nucleus is as
clear as day in Darwin's pangenesis.

The assumption of different material bearers for the
individual hereditary characters was worked out for the
first time by Darwin. The great phenomena of nature
which demand this assumption, and of which I could
make only a hasty sketch in the first Section, were clearly
comprehended and brought together in a masterful man-
ner by him. The entire work on "The Variation of Ani-
mals and Plants" amounts, so to speak, to establishing the
foundation of this fundamental idea, which he has then
worked out and tried to harmonize with contradictory
experiences.

It is remarkable that Darwin, with a modesty that puts
us to shame, presents this fundamental thought as a cur-

Darwin's Pangenesis 63

rent opinion, and not as his own discovery. He even
hoped to be able to identify his idea with Spencer's
theory.28 But so little did this view prevail that his critics
have separated it only in a few instances from the ancil-
lary hypotheses, and most of them have rejected the
fundamental thought, together with these secondary as-
sumptions. But let us proceed to analyze Darwin's
theory.

§ 10. Darwin's Pangenesis29

As already mentioned in the Introduction, the so-
called provisional hypothesis of pangenesis consists, in my
opinion, of the two following parts :

I. In the cells there are numberless particles which
differ from each other, and represent the individual cells,
organs, functions and qualities of the whole individual.

These particles are much larger than the chemical
molecules, and smaller than the smallest known organ-
isms;30 yet they are for the most part comparable to the
latter, because, like them, they can divide and multiply
through nutrition and growth.

They can remain latent through countless generations,
and then multiply only relatively slowly, and at some
later time they may again become active and develop ap-
parently lost characte/s (atavism).

They are transmitted, during cell-division, to the
daughter-cells : this is the ordinary process of heredity.

II. In addition to this, the cells of the organism, at
every stage of development, throw off such particles,

28Darwin, C. The Variation of Animals and Plants. 2: 371, note.

29I have already brought together the most important parts of
this paragraph in the Introduction (pp. 3-7) ; but a repetition cannot
be easily avoided.

30Darwin, C. loc. cit. 2: 372.

64 Hypothetical Bearers of Hereditary Characters

which are conducted to the germ-cells and transmit to
them those characters which the respective cells may have
acquired during their development.

These two parts must be considered separately. They
deserve this the more as their significance has been so far
generally misunderstood.

The hypothetical particles Darwin called "gemmules,"
on account of the analogy mentioned in the first proposi-
tion. This is a poorly chosen term, which has contributed
much toward the raising of insurmountable objections to
his theory. It has led many readers to imagine that they
were preformed germs (Keimchen) ; a conception which
does not in the least correspond to that of Darwin. On
the contrary, one would have to say, according to the
second proposition, that they originated only after the
acquisition of certain characters, or, at the most, simul-
taneously with them. But we will not enter any further
into this question.

The greatest number of investigators, in their criti-
cisms, have considered the second proposition only.
When pangenesis is mentioned, only this hypothesis is
usually meant. The whole theory is identified with this
second assumption, and the transportation of the gem-
mules is regarded as the chief point.31

I admit that, on a superficial 'reading, that chapter
might easily create such an impression. But when it is
read several times attentively, the transportation-hypothe-
sis is lost sight of, while the fundamental idea, which is
stated in the first proposition, becomes predominant.

This is partly due to the difficulty of familiarizing
one's self immediately with the great thoughts of the

^Darwin distinctly calls it "The chief assumption." The Varia-
tion of Animals and Plants. 2: 384. New York. 1900. Tr.

Darwin's Pangenesis 65

gifted investigator, partly also to the circumstance, al-
ready mentioned, that Darwin himself represents the first
proposition as a matter of course and generally known,
and presents only the second one as his own hypothesis.32
The assumption of the transportation of gemmules,
which was, especially for plants, very greatly limited by
Darwin himself, has been denied so frequently, and with
so much ingenuity that it would be superfluous to criticise
it any further here. Especially to Weismann is the credit
due of showing how little it is demanded by well known
facts and tested experience. The cases collected by Dar-
win, which seemed to require it,33 were exceptions, and
their trustworthiness has been strongly shaken by Weis-
mann.34 I believe I need only cite here the works of this
investigator.35

' Freed from the hypothesis of the transmission of
gemmules, pangenesis now appears to us in the purest
form. It is the assumption of special material bearers
for the various hereditary characters. It is true that
Darwin does not always express himself clearly as to
what he calls one hereditary. character, and occasionally

32In his letters also, he lays the greatest stress on this part. Cf.
Life and Letters of Charles Darwin. 3:72-120. (2:264. New York.
1901.)

33The well-known experiments of Brown-Sequard, which are so
frequently quoted as supporting the theory of the heredity of ac-
quired characters, were regarded by Darwin himself as opposing his
hypothesis of the transportation of gemmules. Cf. Darwin. The
Variation of Animals and Plants. 2: 392.

34Weismann, A. Ueber die Vererbung. 1883 ; also Die Bedeutung
der sexuellen Fortpflanzung fur die Selektionstheorie. p. 93, etc. 1886.

85The so-called graft-hybrids, and the remarks on the influence
of the male element on the parts surrounding the germ, give no proof,
to my mind, of the necessity of an assumption of transmission. Cf.
Part II, D, § 5, p. 207.

66 Hypothetical Bearers of Hereditary Characters

small groups of characteristics, or of certain morphologi-
cal units, are probably regarded as such. This, however,
lies in the incompleteness of our knowledge, which, in
certain cases, does not, even now, allow us to carry
through the principle, even though it is quite clear to our
author. Every character which can vary independently^
from others, must, according to him, be dependent on a
special material bearer.36

In what manner these hypothetical bearers are com-
bined in the cells, Darwin has not explained. He only
emphasizes that each of them can multiply independently
from the others, af though, as the phenomena of variabil-
ity teach us, this multiplication frequently takes place sim-
ultaneously in small groups of bearers.

In the Introduction I have mentioned the reasons
which induce me to reject the name "gemmule." It is,
in everybody's mind, too closely connected with the trans-
mission hypothesis. I may be allowed to christen the
hypothetical bearers of the individual hereditary predis-
positions by a new name, and call them pangens.37

§ //. Critical Considerations

Among the critics of Darwin, Hanstein deserves to
be named first, because no other has given as clear and
correct an appreciation of pangenesis as he, nor explained
in such a distinct manner the conclusions to which it
leads. Unfortunately, owing to his particular turn of
mind, Hanstein38 had to discard these conclusions, and
with them the whole theory.

Loc. cit. 2nd Ed. 2: 378. 1875.
37Cf. Introduction, p. 7.

38Hanstein, J. Beitrage zur allgemeinen Morphologic der Pflan-
zen. Bot. Abhandl 4: 1882.

Critical Considerations 67

Hanstein, with good reason, first rejects the name
gemmule, and calls the Darwinian units mikroplasts, or
archiplasts. And since he denies the transmission hy-
pothesis, he concludes from pangenesis :39 "One ought even
to make the hypothesis, that every cell of the entire plam>
body, at its very origin, is endowed by its mother-cells
with every kind of archiplast."40 The correctness of this
conclusion will probably now be admitted by all readers as
a necessary consequence of the assumption of archiplasts,
as these are indeed transmitted from one generation to the
other in the egg- and sperm-cells.41

Hanstein's objections I may here pass over. They
are based chiefly on his conviction that it is unavoidable
to assume a special power of nature for organisms.42

Weismann, in his work on heredity (1883. p. 16),
has expressed himself against the assumption of different
bearers of the individual hereditary characters. Accord-
ing to him, this conception does not show how these
"molecules" are to stay together in exactly those combi-
nations in which they exist in the germ-plasm of the
respective species. Without doubt this is the main diffi-
culty, and the fact that it has been the most important
cause of the establishment of the theories discussed in
the preceding chapter, shows what weight it carries.

But this difficulty is no objection. It is true that it
cannot be explained how the individual pangens may be
held together. But the more recent investigations on nu-
clear division have given us an insight into extremely
complicated processes, the object of which is evidently an

™Loc. cit. p. 219.
^Loc. cit. p. 223.
^Loc. cit. p. 219.
**Loc. cit. p. 225.

68 Hypothetical Bearers of Hereditary Characters

equitable distribution of hereditary characters among the
two daughter-cells. It is not to be thought that to-day we
already stand at the end of our investigations concerning
the nucleus. On the contrary, the great discoveries
which have been made up to the present time awaken
within us the hope that many more complex processes
within the nucleus, and of which we have not, as yet, the
slightest inkling, will some time be discovered. The fact
that we do not know how the hypothetical pangens are held
together is in harmony with this statement. But this
question does not need to be solved by auxiliary hypothe-
ses. It is simply to be reserved for further study of the
phenomena within the protoplasts and their nuclei.

An objection frequently urged is the necessity of as-
suming such a large number of different pangens.43 Ap-
parently the assumption of bearers of the whole specific
character is indeed much simpler. In that case only one
hypothetical unit is required for each species. However,
if we do not limit ourselves to the consideration of one
species, but extend our view over the whole world of or-
ganisms, this objection breaks down, as has already been
said in the first Section; for we then have to assume as
many units as there are and have been species, and their
number thus becomes increased without limits. But Dar-
win's units recur, most of them, in numerous plants or
animals, many in almost all of them, and a relatively
small number of such hypothetical pangens is sufficient
to explain, through the most varied possible groupings,
all the differences between species. On the whole, then,
the assumption of pangens is the simplest that can be
made, and this is obviously a great advantage.

43Cf. Weismann, Die Bedeutung der sexuellen Fortpflansung, p.
102 seq. 1886.

Conclusion 69

I think I can omit here a further comparison of the
doctrine of pangenesis with the theories established by
other investigators. Substantially it is contained in my
criticism of those views, and besides it will follow from
the working out of the fundamental thought in the suc-
ceeding paragraphs.

§ 12. Conclusion

The considerations of the first division of this Part,
and the critical explanations of the second division, have
led us to recognize, as unavoidable, a hypothesis of the
material basis of hereditary characters. It is, in a cer-
tain sense, a postulate at which everybody must more
or less surely arrive who thinks upon these questions,
and which we have always been able to trace as the kernel
of the best theories of inheritance.

Let us conclude now by presenting this hypothesis in
the most simple manner possible, and by indicating the
most important explanations which it is able to give us
without ancillary hypotheses.

In the first Division we arrived at the conclusion that
hereditary qualities are independent units, from the nu-
merous and various groupings of which specific charac-
ters originate. Each of these units can vary independ-
ently from the others ; each one can of itself become the
object of experimental treatment in our culture experi-
ments.

Hereditary characters are connected with living mat-
ter, and heredity depends on the fact that children origi-
nate from a material part of their parents. The visible
characteristics of organisms are determined by the invisi-
ble characters of the living matter. In this living substance
we assume special material bearers for the individual
hereditary characters. This is the fundamental thought

70 Hypothetical Bearers of Hereditary Characters

of Darwin's pangenfesis, at which almost all later investi-
gators arrived more or less clearly. At least, the critical
discussion of tfieir opinions leads, in the end, to this
postulate. Whether we speak of the molecules of the pro-
toplasm, or of the germ-plasm and idioplasm, as bearers
of the entire specific character; or whether we place in
the foreground the phenomena of hereditary; or, again,
whether, like Sachs and Godlewski, we use as a basis the
processes of growth and regeneration,44 we always finally
end by assuming different bearers of the inherited attri-
butes. But we reach this conclusion in the most certain
and clear manner if, following Darwin's example, we
regard the whole world of organisms from the most
general point of view possible.

According to the hypothesis concerning their nature,
these units have been given different names. For the one
adopted by me I have chosen the name, pangen.

These pangens do not each represent a morphological
member of the organism, a cell or a part of a cell, but
each a special hereditary character. These can be recog-
nized by each being able to vary independently from the
others. Their study opens a very promising field to ex-
perimental investigation.

The pangens are not chemical molecules, but morpho-
logical structures, each built up of numerous molecules.
They are the life-units, the characters of which can be
explained in an historical way only.

We must simply look for the chief life-attributes in
them, without being able to explain them. We must
therefore assume that they assimilate and take nourish-

44Sachs, J. Stoff und Form der Pflanzenorgane. Arbeit. Bot.
Instit. Wiirzburg. 2: 452. 1880. Godlewski, E. Bot. Centralb. 34:
82. 1888.

Conclusion 71

met and thereby grow, and then multiply by division,
two new pangens, like the original one, usually originat-
ing at each cleavage. Deviations from this rule form a
starting point for the origin of varieties and species.

At each cell-division every kind of pangen present is,
as a rule, transmitted to the two daughter-cells. What
combination of circumstances is the condition of this, and
what relation .is established by the practically uniform
multiplication of the various pangens of an individual,
we do not know.

The pangens, in smaller and larger groups must stand
in such a relation to each other that the members of one
group, as a rule, become active at the same time.45

All these conclusions follow naturally when we try
to connect the fundamental thought with the known
phenomena of heredity and variability.

The whole import of this fundamental idea will, I
believe, be made most clear by briefly grouping now the
most important advantages of the hypothesis in answering
some great biological questions. For entire large groups
of phenomena are made comprehensible to us in a simple
manner, and this without any ancillary hypothesis, by a
mere consideration of the ever changing relative quan-
tities in which the pangens must occur, according to the
nature and age of the cells. In the main these advan-
tages have already been pointed out by Darwin.

According to Darwin's idea, the phenomena of hered-
ity evidently depend on the fact that the living matter
of the child is built up of the same pangens as those
of its parents. If the pangens of the father predominate
in the germ, the child will resemble him more than the

45Darwin called these groups "compound gemmules.' Loc. tit.
2: 366. New York. 1900.

72 Hypothetical Bearers of Hereditary Characters

mother, if only certain pangens of the father prevail,
then this resemblance will be limited to single character-
istics. If certain pangens are fewer in number than
others, then the character represented by them is only
slightly developed; if they are very few, the character
becomes latent. If external conditions cause later a rela-
tively great increase of such pangens, the previously
latent character reappears, and we observe a case of
atavism. If certain pangens entirely cease multiplying,
the respective character is definitely lost, but this seems
to occur very rarely.

In the protoplasm, or at least in the nuclei, of the
egg- and sperm-cells, as well as in that of all buds, all
the pangens of the respective species are represented;
every kind of pangen in a definite number. Predominat-
ing characters correspond to numerous pangens, slightly
developed attributes to less numerous ones.

The differentiation of the organs must be due to the
fact that individual pangens or groups of them develop
more vigorously than others. The more a certain group
predominates, the more pronounced becomes the char-
acter of the respective cell. Connected with this is the
fact that external influences may frequently alter the
character of an organ in its earliest youth, but that this
becomes more difficult the more advanced it is in its
development, i. e., the more strongly definite pangens
are already predominating.

The regeneration of detached members, the restora-
tion of smaller lost parts of tissues, and the closing up
of wounds are evidently due to the fact that the pangens
of the lost parts are not limited to these parts, but that
all cells capable of reproduction contain all the pangens
necessary thereto.

Basis of Systematic Relationship 73

Some pangens represent characters which usually de-
velop only in quite definite organs. If these happen to-
predominate in the wrong place we get the phenomena of
metamorphosis.46 If, for example, the pangens which
determine the peculiarities of the petals develop in the
bracts the petalody of the bracts takes place.

Other pangens represent qualities which may appear
in many or in all members of the plant. And therein lies
doubtless the reason that such characters are so very
often equally strongly or feebly developed in all of
those members. Thus the red coloring matter of the
white-flowered varieties of red species is most frequently
also lacking in the stem and foliage, and plants with
variegated leaves not infrequently bear variegated fruit.

Phenomena of correlative variability, when not of
purely historical nature, i. e., if not originated by simul-
taneous accumulation of two independent qualities, find
their explanation in the union of the pangens into groups.

Systematic relationship is based on the possession of
like pangens. The number of identical pangens in two
species is the true measure of their relationship. The
work of the systematist should be to make the applica-
tion of this measure possible experimentally, by finding
the limits of the individual hereditary characters. Sys-
tematic difference is due to the possession of unlike pan-
gens.

According to pangenesis, there may be two kinds of
variability. These are differentiated in the following
manner by Darwin.47 In the first place the pangens
present may vary in their relative number, some may in-
crease, others may decrease or disappear almost entirely,

46Darwin, C Loc. cit. 2: 387.
"Loc. cit. p. 390.

74 Hypothetical Bearers of Hereditary Characters

some that have long been inactive may resume activity,
and finally the grouping of the individual pangens may
possibly change. All of these processes will amply ex-
plain a strongly fluctuating variability.
^> In the second place some pangens may change their
nature more or less in their successive divisions or, in
other words, new kinds of pangens may develop from
those already existing. And when the new pangens, per-
haps in the course of several generations, gradually in-
crease to such an extent that they can become active, new
characters must manifest themselves in the organism.
\ In a word : An altered numerical relation of the pan-
gens already present, and the formation of new kinds of
pangens must form the two main factors of variability.48
Unfortunately we have not yet succeeded in analyzing
the observed variations so far as to be able to determine
the share of each of those factors. But it is clear that
the former kind is more likely to determine the individual
differences and the numberless small, almost daily varia-
tions and monstrosities, while the second one has chiefly
to produce those variations on which depends the grad-
ually increasing differentiation of the entire animal and
vegetable world.

This conception of phylogenetic variability indicates
that the pangens, too, must have their pedigrees which
correspond to the pedigrees of the respective character-
istics. At every advance in the pedigree of the species
one or more new kinds of pangens must have developed
from those present. In the lowest organisms, therefore,
the pangens themselves become relatively simple, and not

48In a note to the translator, the author says: "That sentence
has since become the basis of the experiments described in my 'Mu-
tationstheorie.' " Tr.

Conclusion 75

very different from each other. With increasing dif-
ferentiation they must themselves have become more
complicated, and gradually more unlike each other.

But the farther we get away from the facts the more
likely we are to get lost in false speculations. My object
was only to place the fundamental idea of Darwin's pan-
genesis in the right light. I hope I have succeeded in
this.

OF THE

:UTY

OF /

PART II

INTRACELLULAR PANGENESIS
A. CELLULAR PEDIGREES

CHAPTER I

THE RESOLVING OF INDIVIDUALS INTO THE PEDI-
GREES OF THEIR CELLS

§ i. Purpose and Method

Since the founding of the cell-theory by Schleiden and
Schwann, cells have come more and more to the fore-
ground of anatomical and physiological consideration.
The theory of heredity, also, which about two decades
ago was hardly at all in touch with the cell-theory, has
given up this isolated position, and sees in the more re-
cent investigations on cell-division and the process of
fertilization an important furtherance of its problems.

Oninis cellula e cellula. Not only does this saying
dominate microscopic science, but it is steadily rising into
supreme command over all Biology. That every cell has
originated from a material part of its mother-cell, and
that it owes its specific characters to this origin, is now
accepted in the theory of heredity as the basis of all
thorough considerations. Whether or not this source is
sufficient for the explanation of all phenomena was the
question which induced Darwin to formulate his pan-
genesis. And this question remains the first to be an-
swered with reference to every new group of facts ap-
pearing within the domain of heredity.

The phenomena known at present, at least in so far
as they have been sufficiently thoroughly investigated,
demand an affirmative answer to that question. This
was conclusively demonstrated by Weismann, as has been

80 Cell-Pedigrees

already mentioned in the first Part. We need therefore
not deal with that question in this Section.

Not the organisms, but the cells, are therefore the
units of the theory of heredity. One has to go back to
these for a clear understanding. In the practical pedi-
grees of the animal- and plant-breeders of course only
the individuals figure, but for a scientific insight, these are
not sufficient, as is well known to the greatest authorities
among breeders.

Here the germ-cells (egg- and sperm-cells) come into
the foreground for consideration. They are the material
parts of the parents from which the children issue, and
hence form the material bond between the successive
generations. For every genii-cell we may trace a series
of ancestral cells back to the last preceding generations.
In this way we may proceed further, and follow up the
pedigree of the germ-cells through a series of generations.
The great scientific significance of these sequences of cells
has been strongly emphasized by Weismann; they form,
without doubt, the basis for the theory of cell-pedigrees.

But this kind of treatment Jeads to a one-sided con-
ception of the problem. We ought rather to trace the
ancestral line of all the cells of the entire body back
to the first cell from which the organism started. It is
true that thereby the task becomes much more extensive
and complicated, and it is a question whether a sufficient
anatomical and ontogenetic basis is at hand for its solu-
tion. Nevertheless it is only in this way that we can
approach a uniform treatment of the subject, and group
the available facts in such a way that they do not de-
ceive us, nor lead us to an overestimation of the signifi-
cance of isolated cell-sequences selected by us arbitrarily.

We should, therefore, trace out the pedigree of the/

Cell-Pedigrees 81

individual cells for the whole organism. Or, in other
words, we should resolve the individual into its cells and
and their lineage. To this end the history of develop-
ment must furnish us the requisite facts which, however,
must include all forms of reproduction.

The cellular pedigrees that are to be traced are of a
purely empirical nature. As Sachs has already empha-
sized, we have but to record the facts in as simple a group-
ing as possible,1 and see what conclusions can be drawn
from them without resorting to any hypothesis. The
harvest will, to my mind, be much richer than would be
imagined at first glance.

That the chief results of the consideration of cellular
pedigrees in both the plant and animal kingdoms will lead
to the same general conclusions, probably no one doubts at
present. But the conditions are quite different in the
plant world from those in the animal kingdom. The vari-
ous kinds of reproduction in the latter are not nearly
as numerous as in the former. A study of animals is
therefore much more exposed to the danger of one-sided
treatment than that of plants. Moreover, with the bot-
anist, the conviction that the anatomical and ontogenetic
investigation should always penetrate at least to the
individual cells has, under the influence of Mohl and
Nageli, for almost half a century, taken much deeper root.
Accordingly the ancestral sequence of by far the greatest
number of cells is, in innumerable cases, if not without
gaps, demonstrable with sufficient certainty at least in its
main lines.

Therefore I shall be able to limit myself in this sec-
tion, without danger, to the cellular pedigrees of plants.
And this the more so, as the most important lines of

1 Sachs, J. von. Vorlesungen uber Pflansenphysiologie. 1882.

82 Cell-Pedigrees

those pedigrees have lately been frequently emphasized
Afor the animal kingdom by Weismann and others, and a
comparison of both kingdoms with reference to this
point does not, therefore, offer any considerable diffi-
culties.

§ 2. The Cellular Pedigrees of the Homoplastids

In unicellular species the pedigrees of the individuals
coincide with the cellular pedigrees. But such is also
the case with those organisms of few cells, the cells of
which are as yet quite alike and not organized for various
functions. The Oscillariae are many-celled threads, but
all the cells are alike, every one of them is equally able
to propagate the species. Gotte has named such organ-
isms homoplastids, as compared with the.heteroplastids,
the cells of which are adapted for various functions.

It is clear that the ancestral trees of cellular descent of
the homoplastids are entirely composed of like branches.
It depends only upon external circumstances, and the
struggle for existence, which of the cells will become new
individuals, and which branches of the family tree, there-
fore, will continue the descent through the series of gen-
erations.

In the higher plants and animals, on the contrary,
only definite branches of the cellular pedigree lead, in the
normal course of development, to the cells that begin
the next generation, the other branches being already ex-
cluded, by their nature, from taking part in the normal
propagation of the species. The branches of the tree are
here, therefore, not only morphologically different, but
also intrinsically unlike in their relation to the pedigree
of the species.

The differentiation of the cellular pedigrees started

Cell-Pedigrees 83

with the development of the heteroplastids from the/
homoplastids. The undifferentiated cellular pedigrees of
the latter do not afford us any clue for judging the phe-
nomena of heredity. Hence we leave them aside, and
turn our attention entirely to the heteroplastids.

§ j. The Cellular Pedigree of Eqidsctum

Before we begin describing, at least in their main
lines, the extremely complex cellular pedigrees of the
higher plants, we will elucidate the whole method with a
rather simple example. I choose for the purpose the genus
of the horsetails (Equisetum). Their cellular pedigree
belongs, in spite of their alternation of generations, to the
simplest that are to be found among the leaf-forming
plants, or Cormophytes. There are two ways of arriving
at a conception of the main lines of the picture. One of
them is the progressive, the other the retrogressive. The
first one follows up the track of ontogeny, the second
one descends in the opposite direction. If one is inter-
ested in deciphering the combination for all the cells of
one plant, then the first method is obviously the simplest
and the safest. But, in choosing it, the relative value of
the two new twigs, into which the stem divides, can only
be judged when the ends of both twigs are constantly and
simultaneously kept in view. But, in tracing only the
main lines of the picture, it is, in most cases, much more
convenient to choose the opposite direction. For, in the
retrogressive direction, all paths evidently lead back to
the egg-cell, so that in this direction no erring is ever to
be feared.

I assume that through a combination of both methods
the picture of the cellular pedigree of an Equisetum-
species, e. g. of E. palustre has been developed and lies

84 Cell-Pedigrees

before us.2 The fertilized egg-cell in the archegonium
begins its growth by divisions, the first of which stands
nearly at right angles to the axis of the archegonium ;
this is followed by two walls at right angles to this and to
themselves. From the lower octants develop the root and
the foot of the young sporophyte, the latter by the for-
mation of a small-celled tissue body due to continued di-
visions. These branches of the pedigrees are thus ended.
From one of the upper octants of the embryo the apical
cell of the first shoot originates, the other octants partici-
pate in the formation of the annular thickening which
represents the first leaf-whorl, and thus soon end their
growth, after continued divisions.

The growth of the first, as well as of all successive
shoots is dominated by the apical cell. The latter occu-
pies the apex of the shoot, its upper cell-wall is spheri-
cally arched, while downward it is limited by three almost
plane walls. It has, therefore, the shape of an inverted
three-sided pyramid. It divides only by walls which run
parallel to the three sides of the pyramid ; every detached
piece is called a segment. By numerous divisions, the
three successive segments, parallel to the three sides
of the pyramid, always form an internode with a leaf-
whorl at its upper end. The whole shoot, therefore,
consists of sections each of which owes its origin to a
segment whorl of the apical cell.

The apical cell, therefore, evidently represents the
main stem of our pedigree; every segment corresponds
to a branch. During the development of the shoot, and
consequently, during the first year ®f vegetation of the

Illustrations of the required stages of development are found in
Goebel, K. Grundzuge der Systematik und Speziellen Pflanzenmor-
phohgie pp. 286-304. 1882.

Significance of the Apical Cell 85

individual, the main stem remains simple, and, since the
first shoot never bears a sporophore without modification
of its activity, it ends witl} the death of the shoot at the
end of the 'first summer.

Each segment that separates from the apical cell di-
vides first into an upper and a lower half; these, through
further walls, into a body of tissue, from which now all
the cells of the respective part of the internode and the
leaf-whorl arise. The sequence of division has been ex-
plained by Cramer and Rees and can be found in the
Lehrbuch der Botanik, of Sachs and Goebel. Further-
more, there should be emphasized, first of all, the fact
that, in the outer cell-layer of the body of tissue, and
alternating with the teeth of the leaf-blade, favored cells
are formed, each of which can grow into a lateral shoot.
The green shoots of older plants as a rule actually bear,
in every leaf-whorl, a circle of as many branches as the
whorl has members. But, in the first shoot, they usually
do not reach development. Every lateral bud, when de-
veloping into a shoot, possesses an apical cell, which starts
the development of the branch in the same manner as the
terminal cell of the main shoot.

Thus in every branch the apical cell again forms the
main line of the pedigree. It is true that this line does
not join the main stem in a simple manner but it can be
clearly traced back, through the first divisions of the
segment, to the stem. Now every segment, and within
it, during their first cleavages, those cells from the later
divisions of which the apical cells of the lateral branches
arise, we shall regard as the main stem of our pedigree.
All other cell-sequences will be considered as lateral
branches, for only in this manner can we get a clear
picture.

86 Cell-Pedigrees

Let us return now to the shoot during its first year of
vegetation. At the end of the summer it perishes. A
lateral bud in one of the basal leaf-whorls, however, con-
tinues to live, and develops during the next year into a
new shoot, which grows stronger and larger than the first
one, but does not yet bear any organs of fructification.
This course continues for several years, until the plant
has become quite vigorous. Sometimes the third or one
of the following shoots grows downward into the ground,
to form the rhizome, which, from now on, forms the
main-shoot of the plant, branching beneath the ground
and sending up into the air the leaf-bearing and spore-
bearing shoots. These are distinct in Equisetum arvense
and some other species. In the spring the pale, fertile
unbranching shoots arise, in the summer the extensively
spreading, green but sterile branches.

The cellular pedigree of the whole large plant would
very soon present an inextricable picture. To avoid this
danger, we must mark especially the main lines, perhaps
by indicating them by heavier marks. We must also
draw the lines as straight as possible. Supposing all of
this executed, we get a pedigree of the apical cells which
in the picture stands out clearly as a connected system,
and to which all the rest is laterally added. We shall
call the lines of the pedigree of the apical cells the
branches, the other ramifications the twigs. In order to
avoid misunderstandings, it must be remembered, that
the pedigree of apical cells does not consist exclusively
of apical cells, since these do not originate directly from
each other.

According to this definition the development of the
twigs of the pedigree is always limited, only in the
branches resides the ability of new ramifications, and

Cell-Pedigrees 87

thence of a continuation of the main-lines. But this is
not the case to the same extent for all branches as we shall
soon see.

In our picture two important parts are still lacking,
one of them being the roots, the other the organs of re-
production. The roots need only briefly be mentioned.
They grow by means of apical cells, the same as the
shoots, and are present in the lateral buds before the
latter arise from the leaf whorls. As a rule, every bud
at first forms only one root, which develops from an inner
cell, situated on its under side. This cell becomes the
apical cell of the young root. Therefore, in the genea-
logical tree every root, as well as every shoot, is repre-
sented by a branch with its numerous twigs. But since
the roots never bear leaf -buds, as in many ferns and pha-
nerogams, and therefore never produce any organs of
reproduction, they are always only sterile branches of the
pedigree.

In the case of Equisetum arvense this is the fate of
by far the greater portion of the branches of the cellular
pedigree. Because here only the pale, yellow shoots of
the later years, without chlorophyll, are selected for re-
production. Thus, here too, we distinguish sterile and
fertile branches.

At the apex of the fertile shoots stand the sporangia
in crowded spikes of four- to six-sided shields, which have
their stems in the center. Every one of these corres-
ponds in its origination to a tooth of a leaf-whorl. Hence,
the cell-pedigrees of the individual shields can be derived
in a similar manner from the apical cell of the shoot, as
in the vegetative part ; and in the same way the origin of
each single spore can be traced back to it. These lines
again we call branches, while all the lines leading to the

88 Cell-Pedigrees

other cells of the sporangial tissues must be regarded as
twigs. For here, too, the branches possess the power of
continuing the pedigree, but the twigs do not.

On germination the spores produce the male and the
female prothallia. The former bear only the male sexual
organs or antheridia, the latter only the female organs
or archegonia. In the cell-pedigrees we again imagine
heavy straight lines for those cell-sequences which lead to
the egg-cells and to the spermatozoids. These represent
for us the branches, all the others the twigs.

We have arrived at the completion of our sketch,3
since we have been through the much ramified path from
the fertilized egg-cell to the new germ-cells, and have
taken in its numerous side-paths. Let us glance once
more over the whole, and we shalK see that, by empha-
sizing the branches instead of the twigs we have, in spite
of the great complication a simple and clear picture. For
the branches again, we have to make a distinction be-
tween the fertile and the sterile. Only the former lead
finally to egg-cells, or to spermatozoids, i. e., to new in-
dividuals; the sterile branches do not do this. Funda-
mentally, then, they behave towards the fertile ones like
the twigs ; they take no part in the pedigree of the species.

§ 4. The Main Lines in the Cell-Pedigrees

For those cell-sequences, which in the cell-pedigree
lead from the fertilized egg-cell through the individual
to the next generation, I may, as a continuation of Weis-

3In order not to complicate the illustration I have not discussed
here the vegetative multiplication. I shall come back to it in the next
Section.

Germ-Tracks and Somatic Tracks 89

matin's clear statements employ the name germ-track.
This conception would then correspond exactly to the
fertile branches of the cell-pedigree in the illustration
selected above. We shall, in the future, keep this shorter
designation for it, and in contradistinction we shall call
all other sequences of generations of cells, the sterile
branches as well as the twigs of our illustration, the
somatic tracks.

A germ-track then, always leads in our cell-pedigree
from the fertilized egg-cell to the new egg- or sperm-cell ;
we imagine it drawn very straight and clear in our dia-
gram. Somatic tracks begin at all points of the germ-
tracks and lead, constantly branching, to all the vegeta-
tive cells of the body. The cells which are situated on
the germ-tracks, can be called germ-track-cells or, accord-
ing to Jager, phylogenetic, or perhaps still more distinct-
ively, phyletic cells. They are thus sufficiently distin-
guished from the ontogenetic or somatic cells.

It is a matter of course that the distinctions intro-
duced here, and therefore also the names and their defi-
nitions, are of a purely descriptive nature. There can be
no question as to their correctness since they are quite
arbitrary. The question is only, are they practical, i. e.,
can they lead us to a clear insight.

We must not wish to substitute a theoretical meaning
for the conception of the germ-tracks. Otherwise the
definition would not be sufficiently clear. Therefore
Weismann's germ-cells correspond only in their main
features, and not everywhere, with our germ-track cells.
This is especially shown by the circumstance that, ac-
cording to his theory, sexual cells are frequently produced
by somatic cells, and that he devotes a detailed discussion
to the fact that the splitting off occurs a little sooner in

90 Cell-Pedigrees

some groups of the animal kingdom and a little later
in others.4

In my picture, however, sexual cells are never pro-
duced by somatic ones, but the main lines are always
drawn through the ancestral rows of the germ-cells. Ac-
cordingly these produce all the somatic rows of cells.
We see that it is merely a matter of choosing the main
lines for the picture, and not of a comprehension of the
facts. But with my choice the picture becomes simple
and clear, and essentially the same for plants as for ani-
mals. To my mind the germ-cells of the hydroids and of
the phanerogams are not, as Weismann assumes,5 secreted
by the Metazoon itself, but are formed, as in the case of
all other sexually differentiated heteroplastids, on the
germ-tracks, only the number of cell-divisions which pre-
cede their origin on this track is here very great.

According to my definition, a germ-track never origi-
nates from a somatic track. A continuity of the germ-
cells does not occur as a very rare case,6 but everywhere,
and without exception, although sometimes at a great
distance, along the germ-track. The whole question of
whether somatic plasm can change into germplasm7 is,
on the basis of my conception, deprived of any founda-
tion in fact. But it certainly is not always easy to decide
whether a track is to be regarded as a somatic one or as
a germ-track, as will be seen from the next chapter.

For a clear comprehension of the phenomena of he-
redity the conception of the germ-tracks, as it has been

4Weismann, A. Zur Frage nach der Unsterblichkeit der Einzellig-
en. Biolog. Centr. 4: 683.
5Loc. cit. p. 685. ,

6 Weismann, A. Die Kontinuitdt des Keimplasmas. p. 11.
7Loc. cit. p. 52.

Germ-Tracks and Somatic Tracks 91

modified above, seems to me to be of prime importance.
While natural selection appears to act upon the qualities
of the finished organism, in reality it acts upon the bearers
of these characters hidden in the germ-cells.8 This im-
portant law has been raised above all doubt by the ex-
periences of animal and plant-breeders. Vilmorin, in his
breeding experiments, distinguished the individuals which
possessed in a higher degree the power of transmitting
their visible qualities to their descendants from those that
possessed it to a lesser degree.9 The former he called
bons etalons, and those he selected for breeding. But
whether a plant belonged to this privileged group the plant
itself did not show. This had to be decided by the de-
scendants and by these was the great breeder guided in
the selection of his breeding plants.

The body of the individual, therefore, gives only a
one-sided and very incomplete indication of the qualities
represented in its germ-tracks. But when one grows
from its seeds hundreds and thousands of specimens, these
furnish such a many-sided picture that the average may
be regarded as a criterion of those latent attributes.

By far the most of the hereditary character-units at-
tain their development only in the somatic paths ; it is only
here that the corresponding characters of the organism
become visible to us. But the transmission of a char-
acter and its development are, as Darwin says,10 distinct
powers which need not necessarily run parallel. The
transmission is accomplished invisibly, in the germ-tracks,

8Weismann, A. Ueber die Vererbung. p. 56.

9Vilmorin, L. L. de. Notices sur I' amelioration des plantes par
le semis. Nouvelle Edition, p. 44. 1886.

10Darwin, C. The Variation of Animals and Plants. 2: 38. New
York, 1900.

92 Cell-Pedigrees

the development mostly on the somatic tracks. It is only
with caution that we may utilize the latter in judging the
former.

In the following chapter I will discuss more in detail
the germ-tracks and the somatic tracks in the cell-pedi-
gree of the higher plants. In doing so I shall divide the
former into primary and secondary germ-tracks. Both
lead from the fertilized egg-cell to the new egg- or sperm-
cell. The former ones, however, do so by the shortest
route, that is usually within one individual, and, in the
case of alternation of generations, through the usually
small number of individuals involved. The latter, on the
contrary, reach their end indirectly, by means of vegeta-
tive multiplication, e. g., through adventitious buds. They
may frequently pass through an apparently unlimited
number of individuals before returning to an egg-cell.

CHAPTER II
SPECIAL CONSIDERATION OF THE INDIVIDUAL TRACKS

§ 5. The Primary Germ-Tracks
I designate as primary germ-tracks those sequences
of generations of cells which, in the normal course of
development of the organism, lead from the fertilized
egg-cell to the new germ-cells (egg-cells, spermatozoa,
pollen-grains). They will form the subject of the first
paragraphs. The secondary germ-tracks, leading through
adventitious buds, will be considered in the subsequent
paragraphs.

The primary germ-tracks, then, form the common,
or at least the shortest of the common, paths from one to
the next following generation of egg-cells. They are
never completely unbranched, because the normal multi-
plication of the species is incumbent on their ramification.
They probably always give off somatic twigs along their
entire length. But the manner and means of their ram-
ification, the number, position, and relative significance
of the individual somatic tracks, is subject to much modi-
fication.

Among extreme cases may be counted one one side
the well known instance of the Diptera, on the other hand
the Vertebrates, and, contrasted with both, the higher
plants and the corals. In the Diptera some of the first
cells that usually form from the egg develop into the sex-
ual glands of the body. Thus the initial cells for prac-
tically the entire body are directly separated from the

94 The Individual Tracks

germ-track at the first divisions, and this forms thereafter,
only the somatic tracks lying in the sexual glands. To
the Diptera must be added the Daphnoidae and Sagitta,
for the whole body of which, with the exception of the
organs of reproduction, the initial cells are also split off
very early from the germ-track, and by means of a rela-
tively small number of cell-divisions. In the vertebrates
the germ-track probably goes through hundreds of suc-
cessive cell-divisions, for the purpose of body-formation,
before it begins the development of the sexual organs.

Leaving the sexual organs out of our consideration, we
find that the somatic tracks composing the body arise
from the germ-track, in the Diptera as a single twig, in
the Daphnoidae and Sagitta as a small number of
them, in the vertebrates, however, as very numerous
twigs. But all the tracks for the body are always formed
before the germ-track begins to split into equivalent
branches in the region of the sexual organs.

Here lies the difference between the higher animals
and the plants. For in the latter the germ-track splits
at a very early period, and the majority of the somatic
tracks do not originate in the main-stem of the germ-
track, but chiefly in its branches. The picture of the pedi-
gree of the germ-cells coincides here with the picture of
the much ramified organism itself; it does not require a
detailed description. The colony-forming polyps present
a similar case.

The difference becomes clearest on introducing into
the picture only the germ-tracks, and leaving out the so-
matic tracks. The cell-pedigree of a higher animal stands,
then, as a straight tree, ramifying only a little at its top,
while that of the higher plants is so richly and repeatedly
branching from its very origin that the branches fre-

Primary and Secondary Germ-Tracks 95

quently overtop the main-stem which thus, not infre-
quently/is in the back-ground of the picture. Or, more
correctly speaking, there is no real main-stem, or at least
hardly any.

§ 6. The Secondary Germ-Tracks

In the higher animals the secondary germ-tracks are
lacking, in the vegetable world they are widely distrib-
uted. It is especially this circumstance which makes the
study of cell-pedigrees in the vegetable kingdom so much
more profitable than in the animal world, and the objec-
tions raised by Sachs, Strasburger, and other botanists
against Weismann's conception regard essentially the cir-
cumstance that the latter did not give due attention to
the secondary germ-tracks.

The secondary germ-tracks can by no means be re-
garded as exceptions. In no tree, in no shrub are they
lacking. Among perennial plants they are, if not of gen-
eral occurrence, at least very widely distributed, and only
the annual and biennial species are without this kind of
propagation. On the other hand the adventitious forma-
tions exhibit so many forms, such high differentiations,
and such beautiful adaptations, that they also are not
placed in the background, in this respect, as compared
with the primary germ-tracks.

For our purpose three cases are to be kept separate:

1. Nearly all cells of the body can develop into new
individuals.

2. Adventitious buds arise only from definite cell-
groups or cell-tracks preformed to this end, namely :

a. from meristematic tissues,

b. from mature cells.

The phenomena of regeneration of the Thallophyta

96 The Individual Tracks

and the Muscineae have in recent years repeatedly been
the subject of investigation, and the conviction has be-
come rooted in regard to them that, at least in some cases
of mutilation, every, or almost every cell that remains
unhurt can grow into a new individual. Pringsheim ex-
amined the mosses, Vochting the liverworts, Brefeld the
fungi.11 On continuing, under favorable conditions, the
cultivation of pieces cut off from these plants, one can
grow a new plant from every part that is not too small.
The stipe and the pileus of the fungi grow new pileuses
from the cut surfaces, the mosses form buds from any
given cell of the roots, leaves and shoot, even from the
sporangium and its stalk. At first the cells grow into the
thread-like protonema, on which the leaf -buds can then
develop in the usual manner. The Marchantiaceae, ac-
cording to Vochting, can be chopped up fine, and every
particle which has a sufficient number of uninjured cells
to keep it alive, will form a new plant. In the case of
Marchantia polymorpha I can confirm this observation
from my own experience.

In these cases, therefore, all, or nearly all the ramifi-
cations of the cell-pedigree form either primary, or at
least secondary germ-tracks. Somatic, that is, necessar-
ily sterile twigs are possibly present, although it has not
yet been proven. This case, which for Weismann forms
an exception, and demands a special assumption for its
explanation,12 is for us only an extreme one in the rich
abundance of examples.

11 Pringsheim, N. Ueber Sprossung der Moosfruchte. Jahrb.
Wiss. Bot. 11: 1. 1878.

Brefeld, O. Botanische Untersuchungen iiber Schimmelpilze,
Vol. I. Vochting, H. Ueber die Regeneration der Marchantiaceen.
Jahrb. Wiss. Bot. 16: 367. 1885.

12 Weismann, A. Die Kontinuitdt des Keimplasmas. p. 68.

Secondary Germ-Tracks 97

The second group of secondary germ-tracks, the ad-
ventitious buds from meristematic tissues, is by far the
most widely distributed in the vegetative world. Adven-
titious buds arise in part directly from the normal meri-
stematic tissues, in part throught the medium of the cal-
lus-tissue which leads to the closing up of wounds.
Those that originate from stems or branches, usually
become new twigs of the individual bearing them, the
leaf-born ones and the root-buds, however, develop for
the most part into new plantlets.

Bud-formation from callus is chiefly found in woody
plants, and almost every part of a branch or a root, if cut
for a slip or otherwise injured, can develop from the
youthful cells of the cambial zone, situated between the
wood and the bark, that undifferentiated tissue, oozing
out like drops of a semi-fluid substance, in which later
cork, bark, and wood, as well as the rudiments of numer-
ous buds develop. According to circumstances the buds
become roots or leafy twigs, and usually replace the lost
members of the individuals.

Since, as far as we know, every cell of the cambium
may contribute to the callus, and can produce therein the
mother-cell of a bud, we must designate the entire cam-
bium as a secondary germ-track which is as profusely
ramified as the cell-pedigree of the respective cambium
itself, and which bears the normal products of its activity,
wood and bark, as countless somatic twigs. It is to be re-
membered, however, that many cells of the wood and bark
retain, for a longer or shorter time, the power of con-
tributing to the formation of the callus, and even of pro-
ducing mother-cells of callus-buds.13 The line of de-
marcation between the secondary germ-tracks and the

13This point indeed still requires thorough investigation.

98 The Individual Tracks

somatic tracks is therefore to a great extent, obliterated
here, and perhaps even quite undemonstrable.

Callus-buds are also to be found in many herbaceous
plants. On leaves, too, they are not rare, but in such
cases they usually form new rooted plantlets.

Adventitious buds on leaves are very frequent phe-
nomena among the ferns. In the phanerogams they arise
at the base of detached leaves, especially in bulbous plants
and Crassulacese. Very well known instances are fur-
ther furnished by Bryophyllum calycinum, Cardamine pra-
tensis, and Nasturtium officinale.™ There can be no doubt
that in all of these cases there is present in every leaf
a germ-track, which is very frequently much ramified.

Root-buds are probably the most common and cer-
tainly the most completely and most thoroughly investi-
gated adventitious buds.15 And since many leaves, like
slips from stems and roots, can form roots after having
been detached from the plant and, by means of these
roots, give life to new plantlets, the importance of the root-
buds can hardly be exaggerated. Many plants, such as
Monotropa, multiply, except by seed, only in this manner,
others, like Rumex Acetosella and the thistles become the
most tenacious weeds by means of root-buds. Of all spe-
cies that possess this power, we can therefore say that
their root-system represents, in the cell-pedigree, a much
ramified germ-track with its somatic twigs.

14From the abundant literature on this subject I cite: Regel,
Vermehrung der Begonien aus ihren Blattern. Jenaische Zeits.
Naturw. p. 478. 1876. Beyerinck, Over het ontstaan van knoppen en
wortels uit bladeren. Ned. Kruidk. Archief. 3: 1. 1882. Wakker, J.
H. Ondersoekingen over adventieve knoppen. Amsterdam, 1885.

15This subject has been most exhaustively treated by Dr. M. W.
Beyerinck in his "Beobachtungen und Betrachtungen iiber Wurzel-
knospen und Nebenwurzeln." Verhandl. Kon. Akad. Wetenschappen.
Amsterdam, 1886.

Adventitious Buds 99

I should like to go further into this rich and tempting
field. But the reader who is familiar with the literature
will not need my guidance in forming a picture of the
secondary germ-tracks in the cell-pedigree, and in arriv-
ing at the conclusion that almost every larger branch of
this tree is to be regarded as a germ-track.

We still have to deal with the third case, that of the
adventitious buds from mature cells. Here the secondary
tracks run through formed cells, which frequently begin
only in an advanced age to rejuvenate, and to grow into
buds. This is illustrated by the begonias, which Darwin
has already used in his pangenesis for the explanation of
the almost universal distribution of the hereditary char-
acters throughout all the parts of the plant-body,16 and
which Sachs and Strasburger considered as opposing
Weismann's theory of the germ-plasm. This phenom-
enon has been thoroughly studied by Regel, Beyerinck,
and Wakker,17 and it seems sufficiently important to me to
be sketched here in its main lines.

The epidermal cells of the leaves and petioles, and also,
in some forms (e. g., Begonia phyllomaniacaj those of
the stem and its branches, possess the power of becoming
buds. This power is not limited to individual, privileged
cells, at least not in the leaves, but is inherent to the same
extent in all cells of the epidermis, especially in those of
the veins. If part of a leaf is laid on the ground in moist
air, after the veins have been previously cut through in
several places, there may be found, after some time, near
each wound, one or several new plantlets. The first pri-
mordium of these is a true rejuvenation. The epidermal

16Darwin, C. The Variation of Animals and Plants. 2: 362.
New York. 1900.

17See citations above (p. 98).

100 The Individual Tracks

cell, poor in contents, divides, without at first gaining in
size, into a small-celled body of tissue, in which rich pro-
toplasmic contents can now be observed. Gradually this
new formation grows and differentiates, by means of nu-
merous further cell-divisions into a bud.

Since these germ-tracks, which lead through a mature
but rejuvenating cell to a new generation, possess a high
theoretical value, and will be frequently mentioned in the
following pages, I shall give them a new name, and call
them fiseudosomatic.

§ 7. The Somatic Tracks

As Nussbaum has so strikingly put it, the germ tracks
are "the continuous foundation stock of the species, from
which the single individuals, after a short existence, fall
like withered leaves from a tree." With the difference
that every leaf is attached to the tree at some point,
whereas most individuals consist of the products of nu-
merous somatic tracks, which have originated successively
from the germ-track, and therefore cannot fall off without
a piece of the foundation stock.

The somatic tracks composing the individual usually
differ greatly from each other. Not only morphologi-
cally, in regard to the kind of cells, tissues, and organs to
which they lead, but also in their size and the extent of
their ramification. The whole aerial plant of Equisetum,
in the first year of its existence, represents a somatic
ramification. The leafy twigs of Tax odium, which fall
off in the autumn, and the leaves of all those plants which
are not capable of reproducing their species by means of
those organs, are further illustrations. There is an unin-
terrupted line of intermediate steps from these to the one-

The Somatic Tracks 101

celled somatic tracks which do not ramify any further, as
for example, the wood-fibres of some trees which are pro-
duced by the cambium.

The somatic tracks are, in general, the cell-pedigrees
of the single cells of the grown individual, with the excep-
tion of the germ-cells. In the case of every cell and every
cell-complex one can trace them back to the germ-track
from which they have evolved. In plants all the profusely
branching primary and secondary germ-tracks are prob-
ably closely set, along their entire length, with such bushy
lateral twigs. These give its characteristic appearance to
our picture. In the Diptera they originate chiefly from
one point of the germ-track, and thereby the picture is
entirely changed. In the higher animals, however, they
gradually branch off from the unramified part of the
germ-track, and very greatly surpass it in the richness of
their further ramifications.

The cells of the somatic tracks are usually composed
of the same protoplasmic organs as those of the germ-
tracks. Only here these organs are frequently adapted
to other functions, and therefore they bear other names.
Thus, in some somatic elements, the amyloplasts of the
germ-track cells become chloro'phyll-grains. Usually this
change is not only a more special adaptation, but also a
further differentiation. Especially do we meet again, al-
most without exception, in all somatic cells, such indi-
vidual parts of the germ-track cells as nucleus, tropho-
plast, vacuoles, nucleo-plasm, and lining layer.

Against this general rule some individual exceptions
must be mentioned. I do not take into account the nu-
merous cells, such as the many wood-fibres, and the stone-
cells and cork-cells, which die soon after their development
and lose their entire protoplast. They render their ser-

102 The Individual Tracks

vices to the organism in this lifeless condition, and form
the extreme instance of a reduction on the somatic tracks.

But there are also cases of a lesser reduction. Fre-
quently, in the Algae, as Schmitz describes, "In the in-
terior of the cells, the chromatophores, of which there is
no longer any need, and which, in the economy of the
whole plant, were equipped and adapted exclusively for a
definite single function, disappear."18 Especially is this
often the case in complexly organized and highly differ-
entiated algae. Sometimes, as it would seem, in the in-
most tissue-cells, but most commonly in the hairs and
rhizoids.

A further instructive instance is given by the spore-
sacs of the Ascomycetse. In these flask-like cells there
originate, through the division of the nucleus, the nuclei
for the individual spores, while the mother-cell, according
to the available data, does not retain any nucleus. When
the spores are formed the mother-cell has, therefore, be-
come a non-nucleated protoplast, although it has by no
means completed its life-task, since it has still to take a
very active part in the extruding of the spores, for which
purpose it must retain, in the interior of its numerous
vacuoles, the necessary osmotic pressure.

In our cell-pedigrees the ripe ascus forms the last
somatic twig of the germ-track which culminates in its
spores. This twig is simple, i. e., it does not necessarily
branch further. What lends importance to this illustra-
tion, however, is the present conception of the significance
of the nucleus. For, if it is the seat of the latent hered-
itary characters, we may assume that these are lacking
in the ripe ascus. And evidently the latter does not need

18Schmitz, Die Chromatophoren der Algen. p. 137. 1882.

Difference Between Somatic and Germ-Tracks 103

them for the fulfillment of the functions still devolving
upon it.

Therefore, we have here an instance of a somatic path
without latent hereditary qualities. At least, this is as
certain as observation can make it in the present state of
our knowledge. And it is evident that this instance com-
pels the assumption that on many other somatic tracks, as
well, a reduction of the hereditary characters, although
less extensive, may take place. But since our task is to
group facts, and not to make assumptions, we shall not
discuss this point any further.

§ 8. The Difference Between Somatic Tracks and Germ-
Tracks

We see now before us the rough lines of the picture
of the cell-pedigrees for the higher plants. And whoever
followed my description attentively, will have seen that
the picture is a purely empirical one, in which the promi-
nent lines are indeed arbitrarily chosen, but have been
drawn without any hypothesis. Especially is the differ-
ence between the somatic and the germ-tracks purely a
matter of fact, and in harmony with our present knowl-
edge. It claims nothing except to serve as an indication as
to whether any cell can, through its descendents, con-
tribute to the propagation of the species.

But, as a basis for theoretical considerations, the cell-
pedigrees will attain their full value only when we have
realized the significance of the difference between somatic
and germ-tracks. This is by no means a difference in
kind, but one of degree.19 This becomes clearest to us
when we try to define the limit exactly. We shall find,

19Weismann, A. Zur Annahme einer Kontinuitat des Keim-
plasmas. Ber. Naturf. Ges. Freiburg. 1: 7. 1886.

104 The Individual Tracks

then, that an apparently uninterrupted line of transitional
forms leads from the germ-tracks to the somatic tracks.

In the cell-pedigrees of one-celled organisms and of
homoplastids all the twigs are primary germ-tracks. In
the next higher plants primary and secondary germ-
tracks are to be distinguished and, the more highly the
organism is differentiated, the more are the latter pushed
into the background. They are lacking in the higher ani-
mals. But in such highly developed Thallophytes as the
fungi, and even in the mosses and liverworts, it is ap-
parent that all twigs in our picture have still the value of
germ-tracks. At least sterile side-twigs, that is, somatic
tracks, have not yet been demonstrated there. But, in the
case of the vascular plants, most of the tissue-cells, at
least when fully developed, can without doubt no longer
reproduce the species. Therefore the somatic tracks form
here an important part of the picture.

But let us now compare the somatic tracks of the vas-
cular plants with the secondary germ-tracks of the Mus-
cineae. Were not the significance of the latter known to
us through the investigations of Pringsheim and Voch-
ting, we would designate at least some of them as so-
matic tracks, for the question can be decided only by the
presence or absence of the power of reproduction. On
the other hand, it may possibly be shown, at some future
time, that some somatic cells of the vascular plants have
this power after all, and what we now call somatic tracks,
we will then have to regard as secondary germ-tracks.

The somatic tracks have obviously developed phyloge-
netically from the secondary germ-tracks. Not suddenly,
however, and at a leap, but quite gradually. The loss of
the power of reproduction makes them such. By this
means, however, only an adaptation, and no intrinsic dif-

Difference Between Somatic and Germ-Tracks 105

ference is conferred. It is true that, through further
adaptions, the differences may have become greater and
greater; the use of the power of reproduction, at first
limited to less and less frequent cases, may finally have
become quite impossible by the loss, not only of the adapt-
ive, but also of the inner conditions thereto. Doubtless
all transitions to the non-nucleated spore-sacs will have
been made.

But, in the plant world, by far the greatest number
of the somatic tracks are evidently still so much like the
secondary germ-tracks that we cannot assume an essential
difference between them. This is most clearly demon-
strated in those cases where homologous organs among
allied species consist, in one of them, of somatic tracks
only, while the other possesses secondary germ-tracks in
addition.

The most instructive illustration is given in the pseudo-
somatic germ-tracks of the begonias.20 Phylogenetically
these have obviously originated from tracks that we should
call somatic. But the very circumstance that, in the pro-
cess of the formation of species, this power of reproduc-
tion can make its appearance in cells in which it is lacking
in almost all the other phanerogams, teaches us that this
absence is only adaptive, I might almost say only apparent.
We are therefore compelled to attribute to the epidermal
cells of the leaves of the phanerogams in general a latent
power of reproduction. Yet they remain recorded as
somatic tracks in our empirical picture. Nevertheless it
seems perfectly clear to me that the difference is not quali-
tative.

Furthermore, the correctness of this conception is cor-
roborated by the not at all infrequent instances where

2°Cf. p. 100.

106 The Individual Tracks

parts of plants, which normally cannot form buds, produce
such in accidental variations or in varieties. Flower-bear-
ing twigs have been observed on a petal of a Clarkia and
of a Begonia, on the stem of the compound leaf of Lyco-
persicum, and on the leaves of Levisticum, Siegesbeckia,
Rheum, Urtica, and Chelidonium. Caspary saw more
than a hundred of them on a petiole of Cucumis. Every-
one is doubtless familiar with the flowers on the glumes
of the variety of barley cultivated as Hordeum trifurca-
tum.

Some leaves can take root when cut off and stuck into
moist ground. I saw those of Aucuba and of Hoya car-
nosa keep alive, in this way, for two years, without form-
ing buds; some are said to have existed for seven years
in this condition.21 Whether buds are ever developed from
the roots of such leaves, either normally or after wound-
ing, seems to be unknown. But this is not at all impossi-
ble, and in general the whole case deserves to be more
thoroughly investigated. Other leaves fail to take root
under like conditions, and simply perish. But those of
the Crassulaceae, and of bulbous plants, grow buds from
their base. Here, too, the line of demarcation between
somatic tracks and secondary germ-tracks is evidently not
a sharp one, at any rate not qualitative.

Finally, we have still to emphasize the fact that very
frequently the power of reproduction is restricted to
youth. This is most clearly shown by the callus-forma-
tion of woody plants, where the still living older cells of
the bark and the wood usually do not take any part in it.
In the petioles of plants that are rich in juice, as Peper-

21I have since succeeded in keeping a rooted leaf of Hoya car-
nosa alive for more than six years. It did not produce any bud. de V.
1909.

Phyletic, Somar tar chic and Somatic Cell-Divisions 107

omia, grown cells also take part in the callus- formation,
but, as it seems, only in a subordinate way. Perhaps by
far the greatest part of the somatic cells of plants have
this power in their youth, and the line of demarcation
between secondary germ-tracks and somatic tracks would
lose still more of its distinctness through this possibility.

§ p. Phyletic, Somatarchic, and Somatic Cell-Divisions

We will now look a little more closely into the cells
themselves, which are distributed along the individual
tracks. In the homoplastids all the cells and all the cell-
divisions have the same importance. The two daughter-
cells evolved from one mother-cell are of the same value.

But in the higher plants such processes are relatively
rare. They happen chiefly only where a germ-track di-
vides into two equivalent branches, or where a uniform
tissue is deposited on a somatic track. By far the greatest
number of divisions, however, furnish unlike products,
and to this fact is due the entire differentiation.

It seems more important to me to distinguish between
phyletic, somatarchic, and somatic cell-divisions. Those
divisions in which a germ-track-cell splits into two
daughter-cells, both of which, although in different ways,
continue the germ-track, are obviously phyletic. All the
somatic cell-divisions are divisions on the somatic tracks.
Where a track is laid down of such a nature that through
the division of a cell of the germ-track, there develops, on
the one hand, a cell which continues the germ-track, and
on the other hand, a somatic cell, the division is soma-
tarchic.

There can be no doubt that, in the phyletic divisions,
the hereditary factors are transmitted to the two daughter-

108 The Individual Tracks

cells. Such is the case, also, in the somatarchic divisions,
with reference to the daughter cells that continue the
germ-track. But as to whether or not this also holds true
of the other sister-cell, which forms the beginning of a
somatic track, opinions differ. As to whether or not, in
the somatic cell-divisions, a corresponding reduction of
the latent factors goes hand in hand with the advancing
adaptation and specialization of the cells will be discussed
in the next chapter.

I have still to emphasize that the successive genera-
tions of cells from the germ-tracks, which evolve from so-
matarchic cell-divisions, are not all alike. They have been
designated at times either as germ-cells or as embryonic
cells. But there is no necessary reason for this in the plant
kingdom. It is true that they are all alike in being the
bearers of all the hereditary characters of the species, but
they bear them only in a latent condition. They may be in-
trinsically very different in respect to their active heredi-
tary characters. And, even if the whole germ-track does
not pass through such a rich variety of forms and adapta-
tions as are furnished to us by the somatic cells, yet, com-
pared with a single somatic path, however profusely the
latter may branch, it may, by no means, be second to the
latter in regard to differentiation. On the contrary, the
very power of producing, one after another, the most
varied somatic tracks, indicates a continuous alteration in
its activity.

The cells of the germ-tracks are by no means always
such as remain in a juvenile condition during the whole
duration of their existence, or which, between quickly suc-
ceeding cell-divisions, have only a short individual life.
The prothallia of ferns and horse-tails consist of green,
vigorously assimilating cells, through the divisions of

Transmission vs. Development of Characters 109

which there is, at first, an increase in number, until, at
last, from some of them the sexual organs develop. There-
fore the cells on the main germ-tracks are here not dis-
tinguished by any visible characteristic from the purely
vegetative cells. The same is true of the already repeatedly
mentioned pseudo-somatic germ-tracks of the begonia.

Everywhere we are confronted with the statement of
Darwin, quoted above, that the transmission and the de-
velopment of hereditary characters are different powers.
In the cell-pedigree they run almost nowhere parallel.

CHAPTER III
WEISMANN'S THEORY OF THE GERM-PLASM

§ jo. The Significance of the Cell-Pedigree for the Doc-
trine of the Germ-Plasm

In the first two chapters of this section I have compre-
hensively described the cell-pedigrees for the plant world,
and, in order to draw a clear picture, I have been com-
pelled to introduce a number of new names. The fact that
all the cells of the whole plant-body are produced by
division, is now universally recognized, and herewith the
possibility of the establishment of cell-pedigrees is admit-
ted as a matter of course. Furthermore, the scientific
value of such consideration has been pointed out by dif-
ferent investigators in botany as well as in zoology.

The elaboration of the picture, however, as I men-
tioned in the beginning of this division of Part II, seemed
indispensable to me, because, up to the present time, the
higher animals have been put to the front in these consid-
erations, and for the further reason that this fact leads
only too readily to a one-sided conception. For here the
distinction between the germ-cells and the body-elements
is so great that it only too easily gives the impression of a
qualitative difference.

This contrast has been strongly emphasized by Weis-
mann in his interesting speculations on the "mortal" so-
matic cells and the "immortal" germ-cells,22 and forms, to
a large extent, the basis for his theory of the germ-plasm.

22Weismann, A. Ueber die Dauer des Lebens. 1882. Ueber Leben
und Tod. 1884.

Theory of the Germ-Plasm 111

This doctrine, and the hypothesis of the ancestral
plasms which is based on it, have already been critically
reviewed in the first Part. I have there (p. 56) also
pointed out the fact that, in the face of a detailed consid-
eration of cell-pedigrees, it cannot be maintained. Now
that we have become more familiar with these latter, it
must be our task to endeavor to establish this claim.

The true significance of the difference between the
germ-tracks and the somatic cells can be correctly judged
only when glancing over the whole richness of the ramifi-
cations of a highly differentiated cell-pedigree. And it is
only in plants that this differentiation reaches its highest
degree. Numerous intermediate forms lead here, with
almost imperceptible transitions, from the main germ-
track to the somatic tracks.

For this very reason I have laid particular stress on
the discussion of the secondary germ-tracks. They are
wanting in the higher animals. In the plant kingdom they
are present in all gradations. I have not attempted to
draw a sharp line of demarcation between them and the
main germ-tracks ; such an attempt would be thwarted by
the same difficulties which make impossible the exact lim-
itation of the concept "individual." We must be satisfied
here with an arbitrary limit, and choose the one that seems
most convenient.

The difficulties that confront us on the border-line be-
tween secondary germ-tracks and somatic tracks are of
a different nature. Here they are due to the incomplete-
ness of our knowledge. I call those tracks that do not
lead to a propagation of the species somatic. But many
cells, many a tissue-complex which, on this ground, we
now call somatic, will prove itself, on further experimen-
tation, to be provided with the power of reproduction.

112 Theory of the Germ-Plasm

The group of the pseudo-somatic tracks may be chosen as
an illustration,23 and I shall come back to further instances
in the last paragraph of this Section.

Therefore germ-cells and somatic cells do not present
any qualitative contrast in the plant kingdom. They are the
extremes of a long line of quantitative differences. This
law I regard as one of the most important results of the
consideration of vegetative cell-pedigrees. Sachs, Stras-
burger, and others, have pointed out the importance of
this law, and it seems to me that the foregoing compre-
hensive descriptions ought to contribute in causing the
conviction of its correctness to become general.

On the distinction between germ-cells and somatic
cells Weismann founded his theory of the germ-plasm.
The latter must, therefore, be present in all the germ-cells.
But according to Weismann, it is only in these that it needs
to be retained, while it must be lacking in the somatic
cells, because they cannot reproduce the species. They
are limited to the unfolding of a limited number of hered-
itary units, and thus need only that portion of the germ-
plasm requisite thereto. These considerations induced
Weismann to regard the germ-plasm as a special sub-
stance, which, in contrast to the remaining or somatic
plasm, is the vehicle of heredity.

In the first part we have seen how the theory of a germ-
plasm fails us in the explanation of the differentiation of
organs. There the assumption of one substance is not
sufficient ; special material bearers of the individual hered-
itary characters, the so-called pangens, were necessary for
the explanation. Their assumption, however, rendered
the assumption of the germ-plasm with its consequences,
superfluous.

23Cf. Section 6. p. 100.

The Views of Botanists 113

Now we have demonstrated that the empirical basis
for the assumption of the germ-plasm, which was to lie in
the qualitative difference between germ and somatic cells,
was only an apparent one and disappears when we con-
sider cell-pedigrees in detail, and from every point of
view.

Nor from this point of view can we recognize as justi-
fied the assumption of the germ-plasm. Because if we
were to attribute germ-plasm to all the cells of the en-
tire organism, the hypothesis would thereby become
superfluous, and the term practically synonymous with nu-
cleo-plasm.

I propose to follow out these general discussions more
in detail in the two following subdivisions of this chapter.

§ ii. The Views of Botanists

That all the cells of the germ-tracks must contain
the hereditary characters of their species, in either the
active or the latent state, can hardly be doubted. How the
somatic cells behave in this respect, cannot on the whole
be determined by experiment. Especially not negatively,
because the absence of latent hereditary characters can
never be experimentally proven. The quite isolated, non-
nucleated cells of nucleated .organisms form possibly an
exception. But positive experimental results would lead
us to recognize the investigated cells, which, up to that
time had been called somatic, as elements of secondary
germ-tracks. Therefore they only shift the limit without
deciding the question.

And yet, as we have seen in the preceding paragraph,
the question is one of high theoretical value. And as
long as this point has at all been an object for reflection,
botanists have been of the opinion that all, or at least by

114 Theory of the Germ-Plasm

far the most, of the cells of the plant-body have been
equally endowed in regard to latent characters. Turpin
and Schwann, later Miiller and Hanstein, but in recent
years, especially Vochting, have taken up the pen in the
support and development of this view.

This prevailing and so well substantiated doctrine was
opposed by Weismann in the year 1885. He advanced his
well known theory of the continuity of the germ-plasm,
and thus sought to create a basis for a theory of heredity.
The material bearer of the hereditary characters in
their totality, and including therefore the latent ones,
Weismann calls germ-plasm ; the bearers of the active
qualities in any given cell, somatic plasm. The somatic
plasm is, therefore, lacking in no cell, because they are all
active to a certain degree, even if only to the extent of
being capable of further division. The germ-plasm, how-
ever, is, according to him, restricted to those cells which
are charged with the transmission of the hereditary char-
acters to the following generations. In the true somatic
cells this power is said to be lacking.

Intimately connected with this conception, according
to Weismann, is the law that the character of every cell
is determined by its nucleus.24 The specific nature of a cell,
according to him, is dependent on the molecular structure
of its nucleus; every histologically differentiated kind of
cell possesses therefore its specific nucleo-plasm.25 Identi-
cal nucleo-plasm, ceteris paribus, means also identical cell-
body ; in every somatarchic cell-division, as well as in most
of the somatic divisions, the nucleo-plasm must therefore
split into two unequal parts, only that part of the hered-
itary characters being given to each daughter-cell, which

24E. g. Die Kontinuitat des Keimplasmas. p. 30.
25Loc. cit. p. 70.

Objections to the Theory 115

is necessary for the functions of its descendents.26 If the
progeny be unlimited, as in the germ-tracks, then the nu-
cleus receives the entire germ-plasm; but since the pro-
geny of a somatarchic cell is limited, and since it is
restricted in its morphological and physiological range of
development, it gets only the corresponding part of the
hereditary characters. Therefore they have no true germ-
plasm, but only somatic plasm.

On the hypothesis of the germ-plasm, Weismann
builds that of the ancestral plasm, which is directly op-
posed to pangenesis, and has been critically considered
in the last division of Part I. But the empirical justifica-
tion for the basis of that assumption, may here be con-
sidered from every possible point of view.

That Weismann has not succeeded in convincing bot-
anists is shown by the various objections to him, made
especially by Sachs and Strasburger. The essence of these
objections is that Weismann has not sufficiently consid-
ered the secondary germ-tracks, and has thus been in-
duced to assume a sharp contrast between germ-plasm and
somatic plasm. Now, not only the oft mentioned exam-
ple of the begonias, but the entire and very rich doctrine
of adventitious buds, teach that there is nowhere a sharp
line of demarcation between the secondary germ-tracks
and the somatic tracks of the plant. The latter have de-
veloped only quite gradually out of the former. And
even though they have in fact often lost the power of re-
production, everything speaks in favor of the fact that
they still very frequently possess it potentially. In other
words, the loss of germ-plasm need not necessarily go
hand in hand with the loss of the power of reproduction.

In his book, Ueber Organbildung im Pflanzenreich,

28Cf. also Part I, Chapter III, § 6, p. S3.

116 Theory of the Germ-Plasm

published about ten years ago,27 Vochting brought to-
gether the facts known at that time and the results of his
own rich experiments. At the end of the first volume he
discusses the pending question in detail. The experiments
teach directly (p. 251), that "in every fragment, be it
ever so small, of the organs of the plant-body, rest the
elements from which, by isolating the fragment, under
proper external conditions, the whole body can be built
up." Of course, this is true only if the fragment contains a
number of meristematic cells. On this basis the question is
discussed, "Whether there is a sufficient support for ex-
tending our proposition over any given complex of living
vegetative cells." This discussion again leads to the as-
sumption that every morphological form of tissue is po-
tentially in a condition to produce meristematic cells, and
therefore to reproduce the entire oVganism. But since
experiments involving the isolation of very small portions
of tissues encounter unsurmountable difficulties, and since,
on the other hand, the power of reproduction as an adap-
tation may very likely have been lost in many tissues,
there is, as a matter of course, no "strict proof attempted,
and it is simply claimed that this very plausible assump-
tion is probably correct."28

This assumption, however, in the now current lan-
guage, has no other meaning than that all, or at least the
greatest number of the cells of the plant-body contain all
the hereditary characters of the species in a latent condi-
tion. And this same assumption I have sought to estab-
lish, as far as possible, empirically, through a detailed
description of cell-pedigrees available through the most
recent investigations on the phenomena of regeneration.

"Vol.- 1,'Bonn, 1878; Vol. II, Bonn, 1884.
. cit. pp. 251-253.

Objections to the Theory 117

It is, indeed, not to be denied that Weismann's view
finds strong theoretical support in the usual economy of
nature. Why endow numberless cells and long genera-
tions of cells with characters which they will never need ?
But it must not be forgotten that such parsimony would
perhaps necessitate special adaptations, and that therefore
it might, in the end, be simpler not to make any differ-
ences at all between the individual cells in regard to their
latent characters.

However, I should not like to go quite so far as to at-
tribute to every somatic cell all the latent qualities. First
of all, as was pointed out at the beginning of this Part,
it would be impossible to support such a view experiment-
ally, and therefore it would remain permanently sterile.
Then I have pointed out the non-nucleated asci, which
doubtless represent somatic tracks without latent hered-
itary units, and therefore permit the assumption of a re-
duction of these qualities in other tracks. Here, too, a
very slowly advancing differentiation and specialization
is, on the whole, much more probable, according to our
present conception of living nature, than the sharp con-
trast between the chosen bearers of heredity and the so-
matic cells equipped only with the hereditary particles
required for their functions, as assumed by Weismann.

Weismann also expresses himself, on the ground of
botanical facts, to the effect "that he can see no theoreti-
cal obstacle to the germ-plasm, under certain conditions,
being admixed with cells of a pronounced histological dif-
ferentiation, or, indeed, even with all the cells of the en-
tire plant/' For the liverwort, serving as a,n illustration,
he admits this conclusion to be correct.29 And the more

29Zur Annahme einer Kontimiitat des Keimplasmas. Ber. Nat-
urforsch. Ges. Freiburg. 1: 10. 1886.

118 Theory of the Germ-Plasm

we study the cell-pedigrees of the plant kingdam, the more
we become convinced that there is no qualitative distinc-
tion in nature between the cells of the germ-track and the
somatic cells.

§ 12. A Decision Reached Through the Study of Galls

In the foregoing paragraphs we have repeatedly em-
phasized how, on the whole, it is impossible to decide the
pending question experimentally. The phenomena of re-
production by excised parts of plants make manifest the
existence of secondary germ-tracks hitherto unknown;
but they do not teach us anything about the nature of the
remaining somatic tracks.

An experiment which we cannot carry through is made
by the gall-forming parasites in such a great variety of
ways that a glance at their products may be made at this
point. The thorough and detailed examinations by Bey-
erinck have so far enriched our knowledge in this field,
that the whole history of development, as well as the an-
atomical structure in the grown condition, is clearly laid
before us in the case of all the more important forms of
galls.30 Two laws, especially important for our purpose,
have resulted from these studies. First of all, the galls,
even at their highest differentiation, are built up of only
such anatomical elements as are otherwise found in the
plant bearing them. Only the peculiar layer of stone cells
of some Cynipid-galls, which later change into a thin-
walled nutritive tissue, forms a hitherto unexplained, but

80Beyerinck, M. W. Beobachtungen iiber die ersten Entwick-
elungsphasen einiger Cynipidengallen. Veroffentlicht Kais. Akad.
Wiss. Amsterdam. 1882. The same, Die Galle von Cecidomia Poae.
Bot. Zeit. 43: 305, 321. 1885, and Ueber das Cecidium von Nematus
capreae. Bot. Zeit. 46: 1. 1888.

Importance of the Study of Galls 119

probably only apparent, exception from this rule. In the
second place plants have no special adaptations for the
purpose of gall-formation ; the adaptations lie completely
with the parasite which works only with the characters
that belong to its host.

But the galls are not at all restricted to the anatomical
elements of the organs on which they originate. Cells
which the plant otherwise forms in the bark of its stem
only, can frequently be found in the galls of leaf-inhabit-
ing Cynipids and Diptera. The same holds true for the
galls of the stem and the root. We may conclude from
this that the power of producing these elements belongs
not only to those organs which develop them normally,
but probably also to all the other parts of the plant.

Worthy of special notice here are the roots which, for
the purpose of covering the galls of Cecidomia Poae, de-
velop in a place, where, in the normal course of develop-
ment, neither the plant bearing them, Poa nemoralis, nor
any other kind of grass, is able to produce roots.31 Thus
the larvae here make use of a potentiality, the existence of
which we could never have conjectured, still less proven.
In Beyerinck's experiments, these gall-roots grew into nor-
mal, profusely ramifying roots; the cells of the internode,
stimulated to activity, must therefore have possessed, in
a latent condition, the qualities necessary thereto.

Through the experiments of this investigator, even a
direct transformation of apparently somatic tracks into
germ-tracks has been, if not entirely accomplished, at
least brought quite near completion.82 The galls which the
leaf -wasp Nematus viminalis, produces on the leaves of
Salix purpurca, possess an exceeding vitality. At the be-

^Bot. Zeit. 1888. 1. c.

32Bot. Zeit. 46: 1, 17. 1888.

120 Theory of the Germ-Plasm

ginning of autumn, when left by their inhabitants, they
are still quite turgescent. If they are now buried in hu-
mus, they will keep through the winter, and can even
enter upon a new life in the following summer. They will
then form new chlorophyll, by means of which they are
nourished, and the best among them will gradually begin
to put forth adventitious roots. These originate either on
the outer or on the inner surface of the wall surrounding
the cavity, and are always located on the vascular bundles
of the gall. Judging from their microscopic structure,
these rootlets, reaching a length of a few centimeters, are
identical with the normal young roots of the respective
species of willow. The required hereditary characters
must therefore be present in a latent state in the gall, in
which probably nobody would otherwise have looked
for a germ-track.

These important experiments will become still more
instructive for our purpose, when we shall succeed in mak-
ing the gall-roots develop so far that they are enabled to
form adventitious buds. But, since the roots of all woody
plants have this power, we may predict even now that this
experiment will succeed. Perhaps it will require special
measures, as for example, a graft on the roots of a willow.
But without doubt we may conclude from the complete
agreement in the anatomical structure, as proven by Bey-
erinck, that the physiological properties also, of the nor-
mal and of the gall-roots are the same.

And if anyone is ever successful in growing in this
way an entire willow from a gall, it will be clear, that, in
the latter, all the hereditary characters of the willow are
present in a latent state.

This would obviously be much more useless than their
presence on any given normal somatic track. The con-

Importance of the Study of Galls 121

elusion, however, that germ-plasm is by no means limited
to those cells which need it for their own development, nor
to their progeny, we may even now regard as perfectly
certain.

And this is probably the most important inference
which we may deduce from this entire section. With it
we have established one of those laws which can be ap-
plied as bases for our hypothesis. But we shall revert to
this in the last Section.

B. PANMERISTIC CELL-DIVISION

CHAPTER I
THE ORGANIZATION OF PROTOPLASTS

§ i. The Visible Organization

Protoplasm is the vehicle of the phenomena of life,
and therefore also of hereditary characters. Hence, any
theory of heredity must start from a definite view in re-
gard to the structure of this important substance. But
anatomical investigation, in spite of its astonishing prog-
ress during the last decade, has in this very field not yet
achieved a clear and generally accepted conception of this
structure.

This is essentially due to the circumstance that the
newer methods for the study of the nucleus and its division
have disclosed a field so important, and so rich in surpris-
ing results, that attention has been directed chiefly, and
frequently exclusively, to this organ. Often one even
meets with views which put the protoplasm (cytoplasm)
into the background with reference to the nucleus.

But the study of the nucleus is so much advanced at
present that one may hesitate at this one-sided treatment.
The researches of Flemming, Strasburger, and so many
other investigators, have disclosed the structure of the
nucleus and the changes of this structure during division,
and have, in the main, brought our knowledge to a definite
conclusion. Now, especially in botany, the investigation
of cell-division itself comes again to the front. And it is
not only a question of establishing the relation of the nu-
cleus to the cytoplasm ; it is just as essential a problem to

126 The Organisation of Protoplasts

find out the attitude of the individual organs of the latter
and especially of the vacuoles, the granular plasm, and the
plasmatic membrane. For the knowledge of cell-division
will be complete only when all the organs of the proto-
plast have been equally considered.

The described course of investigation makes it clear
why even a practical and simple designation of the living
cell-contents has not yet gained general recognition. Such
a designation was suggested by Hanstein, in his well-
known lectures, by the word "protoplast."1 The word
"protoplasm" was coined by Mohl for the semi-liquid
nitrogenous substance "which furnishes the material for
the formation of the nucleus and the primordial utricle,"
and from which originate the first solid structures of the
future cell.2 The formed body, built up from this sub-
stance, has frequently been called protoplasmic body,
plasm-body, sometimes even protoplasmic globule or drop,
expressions which are obviously inadequate to create a
clear conception in the minds of readers and hearers.

Compared with these designations, Hanstein's word
clearly and distinctly describes the individuality of the
living cell-contents. This individuality has long been re~
ognized by the best investigators. As early as 1862
Brikke said that protoplasm was an organic body; not "
drop of fluid, but an elementary organism.8 But the lack
of an appropriate name obscured the clearness of the con
ception, and it was Hanstein who supplied this want
Klebs and others have accepted his designation and

1Hanstein, J. von. Das Protoplasma als Tr'dger der pflanzlichen
und thierischen Lebensverrichtungen. 1 Theil. 1880.

2Mohl, H. von. Bot. Zeit. 4: 75. 1846.

3Briicke, E. Die Elementaroganismen. Sitsungsber. Kais.
Akad. Wiss. Wien. 442; 381. 1861.

Protoplasts Are Elementary Organisms 127

through their influence it will doubtless be more gener-
ally adopted.*

Protoplasts are elementary organisms in the true sense
of the word. They consist distinctly of individual organs,
which are more or less sharply distinguished from each
other and which possess a high degree of mutual indepen-
dence. In the greatest number of plants this structure is
clearly evident, but in the lowest organisms this differ-
entiation is entirely wanting, or at least it is limited to
a great extent. Sometimes one meets with the expres-
sion "unorganised plasm," even for organisms which by
no means lack differentiation. But doubtless this expres-
sion must be understood to mean that the methods so
far employed have not yet revealed any insight into the
organization, and not that the want of any kind of organs
has been thoroughly studied and definitely proven.

*As is well known, the term is now in common use. Tr.

CHAPTER II
HISTORICAL AND CRITICAL CONSIDERATIONS

§ 2. The Neo genetic and the Panmeristic Conceptions of
Cell-Division

Only a few decades back it was generally believed
that individual organs, such as the nucleus and the chlo-
rophyll grains, could always, or at least very frequently
originate from the undifferentiated protoplasm through
differentiation. However, in recent years, investigations
have not confirmed this neogenesis in a single instance.
Wherever the origin of an organ has been thoroughly
and comprehensively studied, with the present means of
investigation, the organ has been shown to originate
by a division of differentiated members already present.

The organization of the protoplasts is not periodical,
nor evident only in grown cells. It is permanent, inher-
ent in all cells, and in all stages of their development.
The assumption of formation de novo gives place every-
where to the recognition of divisions ; the neogenetic con-
ception gives way to the panmeristic.4

It is of interest to glance over the course of develop-
ment of our knowledge. In his "Lehre von der Pflan-
zenzelle," Hofmeister describes the nuclei according to
the knowledge of that time. They appear in the proto-
plasm as drops or masses of a transparent homogenous
substance, either in cells with few nuclei, of a definite

4The view that all the organs of protoplasts, as a rule, multiply
only by division I call panmeristic. This assumption was maintained
for plant-cells for the first time in my plasmolytic studies. Cf. Vries,
H. de. Jahrb. Wiss. Bot. 16: 489. 1885.

Neogenetic vs. Panmeristic Cell-Division 129

size from the beginning, or in cells with many nuclei, first
as small formations which increase through growth.
Sometimes they contain granules as soon as they become
visible, but frequently they occur at first without any inter-
nal solid structure, and attain this only later. Every cell-
division is usually preceded by a disappearance of the
nucleus, which is then followed by the appearance of two
or more new nuclei.5

The comprehensive investigations of Strasburger and
Schmitz have proven this assumption to be erroneous, at
first for isolated and then for an increasing number of
cases, and wherever a disappearance and subsequent re-
appearance of nuclei was assumed, the origination of the
new nuclei through division of the original ones could be
proven. Exceptions to this rule are no longer known.

The history is exactly the same for chlorophyll grains.
Even in the last edition of his text-book6 Sachs
said : "The chlorophyll bodies originate in young cells
through the separation of the protoplasm into clearly
distinct colorless portions that are becoming green.
The process can be conceived to mean that, in the
originally homogenous protoplasm, most minute particles
of a somewhat different nature are distributed or origi-
nate for the first time and then accumulate at various
points, appearing as differentiated bodies." That the
green bodies which had formed in this way could multi-
ply through division, and that the chlorophyll bodies of
many algae are usually cut through at every cell-division
by the forming wall, can easily be observed and was not
unknown at that time.

But it was Schmitz who first showed that, in the algae,

BHofmeister, Die Lehre von der Pflanzenzelle. p. 79. 1887.
*Lehrbuch der Botanik. 4. Auflage, p. 46. 1874.

130 Historical and Critical Considerations

division is the only way in which the chromatophores are
newly formed.7 Following up this idea with the phanero-
gams, Schimper discovered the colorless organs of the
youthful cells, which in these cells are exclusively charged
with the formation of starch and through whose assump-
tion of green color the real chlorophyll grains are formed.
In all cases that have been observed those amyloplasts
multiply only through division, and Schimper, as well as
Arthur Meyer, has accumulated such a number of obser-
vations on this manner of development that the former
view has been abandoned by all botanists. Some special
cases, it is true, still await explanation, but as long as
they have not been thoroughly investigated, there is no
reason for regarding the old conception more plausible
than the new one.

It is similar with reference to the vacuoles. Until
about four years ago they were generally regarded as a
new formation in the protoplasm, caused by the secre-
tion of superfluous water of imbibition. In my "Plas-
molytische Studien ilber die Wand der Vacuolen," I have
established the claim that, for them as well, the mode
of origin of nucleus and trophoplast8 must be the only
real one.9 I supported this claim by showing that all
vacuoles are surrounded by a living wall, which, accord-
ing to the method suggested by me, can always be easily
and convincingly demonstrated, and which I believe may
be regarded as an organ of the protoplast, with as much
right as the nuclei and the chromatophores.

This conclusion, drawn from my panmeristic concep-
tion of cell-division, has been completely confirmed by

7Schmitz, F. Die Chromatophoren der Algen. Bonn, 1882.

8By this name Arthur Meyer designates the amyloplasts and
their derivatives (chlorophyll grains, chromoplasts, etc.)

QJahrb. Wiss. Bot. 16: 489-505. 1885.

Neogenetic vs. Panmeristic Cell-Division 131

Went's investigations.10 Thereby, to my mind is proven
the correctness of this conception as opposed to that of
neogenesis. Now the situation is reversed. While up
to the present time the condition with reference to the
nucleus and the chromatophores could be regarded as
peculiar, there is now great probability that the different
members of a protoplast have the same mode of origin,
and therefore that they can claim the rank of independent
organs only in so far as they follow this rule.

Now that the mode of origin for nucleus, trophoplasts
and vacuoles has, in the main, been established, and that
the works of Wakker11 have taught us to recognize the
crystals, most of the crystalloids, and the aleurone grains
as contents of the vacuoles, the problem is chiefly con-
cerned with the plasmatic membrane and the granular
plasm.12 In regard to their behavior during cell-forma-
tion our knowledge is essentially the same as at the time
of Mohl and Hofmeister. Our insight into the pro-
cess of cell-division has indeed become deeper, chiefly
through Strasburger's work; but the very point in ques-
tion, the beginning of the dividing wall, which for some
time, seemed to be decided neogenetically, has again be-
come extremely uncertain through the discovery (to be
discussed later) of the cell-ring by Went13 as well as
through the objections of other investigators.

10Went, F. A. F. C. De jongste toestanden der vacuolen. Amster-
dam, 1886. Les premiers etats des vacuoles. Arch. Neerl 1887, and
Die Vermehrung der normalen Vacuolen durch Theilung. Jahrb.
Wiss. Bot. 19: 295. 1888.

"Wakker, J. H., Studien iiber die Inhaltskorper der Pflanzen-
zellen. Jahrb. Wiss. Bot. 19: 423. 1888. Preliminary contributions
are found in Maandblad v. Natuurwetensch. 1886, Nr. 7. 1887, Nr.
5 and 6, and in Bot. Cent. 33: 360, 361. 1888.

12Cf. § 6 below, p. 150.

13Cf. § 7 and 8, pp. 157 and 160.

132 Historical and Critical Considerations

For these reasons I believe that a critical review of our
knowledge in this field will be of substantial usefulness.
It will then be shown how, in almost all cases, the attitude
of the plasmatic membrane and of the granular plasm,
during cell-formation, is in fact unknown. At least in
all the cases which seem to contradict the panmeristic con-
ception.

It is not a question of whether this latter conception
is correct or not. This seems to me to have been proven
above any doubt by the researches of the investigators that
have been quoted. The question is whether, with this
conception, we are to regard the granular plasm and the
limiting membrane as two intrinsically different organs,
which pass over into one another as little as the nuclueus
and the chromatophores, or whether they stand in a sim-
ilar relation to each other as the amyloplasts and the chlo-
rophyll-grains. As long as it was thought that the gran-
ular plasm had the power of producing the other members
by a process of differentiation, it was natural to assume
a like mode of origin for the plasmatic membrance. It
is therefore not astonishing that, even at present, this
view is still regarded as the one that actually obtains.
The instance described by Mohl as a type of cell-di-
vision, and which involved the historically noteworthy
discussions of the question as to whether the ^protoplasmic
body played a passive or an active role during this process
is well known to all. Like Mohl's type of the filamentous
algae, Cladophora, Spirogyra is in more recent times pre-
ferred for this study. At the future plane of division
the limiting membrane and granular plasm fold into a
ring which, growing inwards, apparently simply cuts in
two the remaining part of the cell-contents. For the
daughter-cells the two new parts of the limiting membrane

Autonomy of the Plasmatic Membrane 133

originate as a continuation of the old membrane. Accord-
ing to Klebs's14 descriptions the Euglenidae also offer a
beautiful example of panmeristic cell-division.

It is very unlikely that in the case of such a funda-
mental process, the higher plants should behave differ-
ently from the lower ones. That there are differences in
minor points is self evident, and everybody knows that
there are important distinctions, especially in the relative
duration of the individual steps in the process. And the
same holds for the manner in which it is provided that
every daughter-cell gets its own nucleus. But, that the
completion of the plasmatic membrane should take place
through the insertion of a quite newly formed piece is so
much at variance with the rest of our knowledge, that
one cannot by any means accept it on the basis of the
older investigations. At any rate it must be held in doubt
until supported by direct observation.

Such, however, is not the case at present, as I shall
try to show in the last Chapter of this Section. On the
contrary many facts already speak in favor of the com-
plete autonomy of the membrane, although not with suf-
ficient certainty to serve as conclusive proof.

However that may be, whether the limiting membrane
can develop from the granular plasm, or whether both
are mutually autonomous, it is certain, at any rate, that
on the one hand these two, and on the other the nucleus,
the trophoplasts, and the vacuoles are independent organs,
which, in the normal course of things, multiply only by
division.

Hence, the organization of the protoplasts is hered-
itary, and this not in the sense that the organization of the
higher organisms is reproduced in each individual through

14Klebs, G. Arbelten Bot. Institut. Tubingen. 1: 282.

134 Historical and Critical Considerations

the development of invisible hereditary units, but through
the direct passage, from the mother-cell to the daughter-
cells, of all the organs which compose the organism.

The significance of this law for our hypothesis of
intracellular pangenesis will be discussed in the last divis-
ion of this Part. Here we will familiarize ourselves more
thoroughly with the actual basis on which it is founded.

§ j. Cell-Division According to Mohl's Type

The "Grundzuge der Anatomie und Physiologie der
Vegetabilischen Zelle," by Hugo von Mohl,15 was for a
long time the chief source from which beginning bota-
nists got their knowledge. It is only Hofmeister's Pflan-
zenzelle (1867) and Sachs's Lehrbuch (1868) which put
an end to its reign, but many illustrations and statements
from the "Grundzuge" are still vividly remembered by
older botanists.

The multiplication of cells through division is de-
scribed in the following manner in this book of Mohl's.16
It "is introduced by changes which the primordial utricle
of the dividing cell undergoes, in consequence of which
the dividing walls develop, growing gradually inward
from the periphery of the cell, and separating the cell-
cavity into two or more cavities." We have to dis-
tinguish those cases where the cell-division is preceded
by a doubling of the nucleus, from those in which this
is not the case (our present poly-nucleate cells). This
latter, less frequent, but simpler case occurs in Conferva
glomerata, and therefore Mohl begins his description
with this alga. But even where the formation of two
new nuclei precedes the formation of the dividing wall,

15Published in Wagner's Handworterbuch der Physiologie, 1851.
16Loc. cit. p. 211.

Cell-Division According to Mohl's Type 135

this latter process takes place in the same manner as in
the Conferva above mentioned. And this as well among
the algae as in the higher plants. According to Mohl,
then, the plasmatic membrane is always produced by new
parts growing out of old ones.

As to the historical aspect, it needs only to be em-
phasized that this law for the algae, which Mohl put
into the foreground, has been confirmed by all later in-
vestigators.17 Here its correctness is beyond any doubt,
and can be easily controlled by anybody. Who, therefore,
on theoretical grounds, is inclined, to assume that, in cell-
division, the same principles are valid for the entire plant-
world, must with Mohl, still regard the case in question
as a type.

In the uni-nucleate cells there are usually present very
peculiar structures, the function of which is to make the
new dividing wall pass exactly between the two new
nuclei. From our present conception of the significance
of the nucleus this cannot be wondered at, for what would
a cell be without its hereditary characters. In the higher
plants these structures are not cleared up in every respect,
though with the spirogyras this is, to a large extent, the
case, especially through the repeated publications of
Strasburger. We shall therefore describe the process
in this plant, making use of the last description of this
investigator as far as this serves for our purpose.

At the time18 when the nucleus approaches the end
of the prophase, the protoplasm collects around it and

17Cell-division through constrictions is widely distributed among
the lower algae. Cf., e. g., Klebs, Arbeiten Bot. Inst. Tubingen. 1:
336-343.

18The following is taken from Strasburger, Ueber Kern- und
Zclltheilung im Pflanzcnrdch. pp. 9-23. Jena, 1888.

136 Historical and Critical Considerations

assumes, in the region of the poles of the nucleus, a struc-
ture of parallel fibres. It soon becomes clear that we have
to do with the first signs of the spindle-fibres. These
develop quickly and continue through the interior of the
nuclear cavity, until they come into contact with each
other. There is no valid reason for the eventual assump-
tion that the spindle-fibres developing in the interior of
this cavity are of a different origin from the external
ones. On the aequator of the spindle the chromatic sub-
stance accumulates, touching the individual fibres at their
circumference.

Next occurs the formation and longitudinal splitting
of the nuclear skein, followed by the separation and
moving apart of the two halves of the segments. Dur-
ing this period one sees clearly that not all the spindle-
fibres have succeeded in uniting with the opposite ones.
Only those that were successful in this are retained as
connecting fibres between the two young nuclei which
move apart. The space forming between them is sur-
rounded by a protoplasmic mantle toward the outside,
and apparently there collects in it a substance with osmotic
action which enlarges this space and drives the young
nuclei apart. In the meantime the number of the con-
necting fibres on the mantle of this space is lessened more
and more, the mantle itself is made to bulge more and
more in a transverse direction, and becomes correspond-
ingly thinner. Yet it remains sharply and plainly visible.
The space has assumed now the well-known barrel-shape,
its wall is called the connecting cylinder, and remains for
some time as an extended vesicle, closed in on all sides.
Finally, by being strongly distended in an aequatorial
direction, this vesicle reaches the protoplasmic accumu-
lation at the margin of the protruding dividing wall. It

Cell-Division According to Mohl's Type 137

unites with the latter, and is now gradually flattened by
it, and finally constricted.

According to the principles of the theory of the vacu-
oles ascertained by Went and myself, it is probable that
the space containing osmotic substance and surrounded
by the connecting cylinder is a vacuole, which, contrary
to Strasburger's conception,19 must have penetrated from
the outside between the two younger nuclei. It is just as
evident that this vacuole must be surrounded by a wall
of its own, and that this also forms the inner layer of the
connecting cylinder. The latter is also separated from
the other vacuoles of the cell-space, by a wall, and between
the two walls there lies, at least in the beginning, granu-
lar plasm. The changes of that vacuole which forms the
interior of the barrel during the whole process require,
of course, special investigation, made on living material.20

But there can be no doubt about the correctness of
Strasburger's conception, where he places the whole pro-
cess of cell-division, with the one exception of the divi-
sion of the nucleus, in the protoplasm itself. The daugh-
ter-nuclei are passive in this, the cytoplasm alone is the
active element.

The chlorophyll-bands, the vacuole, and the granular
plasm are simply constricted by the plasmatic membrane
growing into the interior. The membrane itself finally
separates in the same manner, after having entirely closed
up the space remaining in the middle of the ring.

In those poly-nucleate algae, the nuclei of which are
evenly distributed over the entire lining layer of proto-

l9Loc. dt. p. 17.

20Zacharias, in his discussion of Strasburger's work (Bot. Zeit.
46: 449. 1888), emphasizes also "that, on the living object, things may
exist which can be better recognized and interpreted there than by
fixing and staining."

138 Historical and Critical Considerations

plasm, no particular devices have been observed for as-
suring the possession of one or more nuclei at the cell-
divisions of each daughter-cell. Moreover they do not
seem necessary, owing to the great number and regular
distribution of the nuclei. Nuclear spindle and nuclear
barrel have therefore lost their significance in this case,
and accordingly they are probably not present, at least
not as a rule. Cell-division is essentially performed by
the plasmatic membrane and the granular plasm only.

For the correct understanding of the processes of
normal cell-division, one law, which has been ascertained
by experiments on artificial division of living protoplasts
in former and more recent times, is of extreme import-
ance. I do not mean the adaptive processes of regener-
ation after wounding (these will be discussed in the
next paragraph), but the constriction of the uninjured
cell-contents in entire cells, and the division of the pro-
toplasts into two or more pieces during plasmolysis. The
respective cases I have put together in my "Plasmoly-
tische Studien ilber die Wand der Vacuolen."^ They
teach that, in artificial constrictions of a protoplast, the
limiting membrane, the wall of the vacuole, and the gran-
ular plasm close their edges, apparently without any dif-
ficulty, and round off to form a new unit. In plasmolytic
experiments this is easily verified. Here one sees also,
how upon the restoration of turgor, the parts flow to-
gether again, their members uniting with the correspond-
ing organs of the other parts of the same protoplast.

This power of combining with homologous parts
seems to be universally inherent in the three mentioned
organs of the plant-protoplast. The walls of the vacu-
oles show it wherever the numerous vesicles of cell-sap
. Wiss. Bot. 16: 501-505. 1885.

Regeneration of Protoplasts After Wounding 139

in young tissue-cells combine into one large vacuole dur-
ing the rapid growth in the transition to the adult con-
dition. When two or more like protoplasts unite to form
a so-called symplast, something similar takes place in
their walls, at least in some cases, as in the plasmatic
membrane and the granular plasm. The ontogeny of the
latex-vessels teaches this more clearly than anything else.
A fusion of like parts in the "feet" of many rhizopods has
also been repeatedly observed and described.

As far as we know, only simple contact is needed for
this fusion, besides the required degree of homogenity.
We may, therefore, regard it as a mechanical process and
use it as an element in the explanation of normal cell-
division. In Spirogyra it evidently accomplishes the fu-
sion of the spindle with the inward growing ring, and
later determines the final closing up of the opening that
was left in the ring.

§ 4. The Regeneration of Protoplasts after Wounding

Even though, in the normal course of development,
the individual organs of a cell multiply by division, this
does not necessarily imply that this rule must be without
exception, and that there cannot be cases where nature
tries to achieve its ends in another way. Especially where,
through outward interference, such as wounding and mu-
tilations, individual members of a protoplast are com-
pletely lost, it might be expected that a regeneration in
another way might be possible.

To be sure observations now available do not warrant
the assumption that such cases actually occur. But this
does not, by any means, exclude their possibility. And
on this possibility I want to lay great stress in this con-
nection, for the hypothesis of intracellular pangenesis

140 Historical and Critical Considerations

allows us to regard as possible an occasional neogenesis
of such organs out of pangens proceeding from the nu-
cleus.

Judging from the facts published up to the present
time, however, the phenomena of regeneration after
wounding are closely connected with the normal pro-
cesses. Nobody, at least recently, has maintained that in
such a case there is a new formation of nucleus and chro-
matophores. There have been only few investigations in
regard to a possible occurrence of new vacuoles. These
were made by Went for the very purpose of testing the
point in question, and teach at least one thing with cer-
tainty, that so far, wherever it had been thought necessary
to assume a formation de novo of normal vacuoles, such
does not really take place. For the vacuoles which have
been observed originate partly through constriction from
the large sap- vesicle of the cell, and partly through the
swelling of the smaller ones which are suspended in the
granular plasm. Especially in the case of the Vaucheria,
which was studied first by Hanstein, and later by so many
investigators, there surely can no longer be a well founded
doubt on this point.22

Since the time when, in my "Plasmolytische Studicn"
I expressed the opinion and sought to establish the fact
that the plasmatic membrane is a separate organ of the
protoplast23 no decisive facts on this subject have been
published. Klebs is opposed to my assumption on the
ground of an observation made on Vaucheria?* For the
study of these processes this investigator introduced a
new method, which makes it possible to demonstrate,
easily and with certainty, the beginnings of the formation

22Went, F. A. F. C. Jahrb. Wiss. Bot. 19: 330-341. 1888.

™jahrb. Wiss. Bot. 16: 493. 1885.

** Arbeit en Bot. fust., Tubingen. 2: 510.

Regeneration of Protoplasts After Wounding 141

of a cell-membrane around exuded masses of protoplasm.
He stains the water or the diluted solution in which the
threads are cut through, with Congo-red, which is stored
up with great avidity by these young cell-membranes.

Nevertheless this method does not yet decide the ques-
tion raised by me, because, as Klebs also says, there is no
means of deciding the presence or absence of a plasmatic
membrane on a portion of the mutilated protoplast that
forms a cell-membrane. "Among the free swimming balls
of protoplasm there are always a number of such that are
quite large and rich in contents which live several days
but without forming a cell-membrane." In the case of
most of them, however, the beginnings of the formation
of a cell-membrane are very soon evident.25 Wherein
the difference in the behavior of these two kinds of frac-
tional parts consists, was not further investigated by
Klebs. My assumption that the former lacked the limiting
membrane, while the latter got a part of this organ when
cut off, has not been at all disproved.

Nor does the great extensibility of the plasmatic mem-
branes during the enormous swelling of the vesicles which
later form the cell-membrane seem to me by any means
improbable or even surprising. Plasmolytic experiments
teach us at every step that the extensibility, not only of
the plasmatic membrane, but also of the wall of the vacu-,
oles and perhaps even of the granular plasm is very con-
siderable. And Went has comprehensively demonstrated
that the swollen spheres of Vaucheria contain only such
vacuoles as have originated by the enlargement, and
mostly also by division of the sap-vesicles present in the
uninjured plant. The assumption of an extensibility of
the plasmatic membrane which need not be much greater
than the proven elasticity of the wall of the vacuoles can-

*r*Loc. dt. p. 507.

142 Historical and Critical Considerations

not seem very surprising. The phenomena of regenera-
tion of Vaucheria demand renewed investigation in this
respect also. As long therefore, as there is no actual
proof of a neogenesis of this organ, independently of the
old one, we cannot recognize such great significance in
this instance as some authorities attribute to it.

Here also the observations by Haberlandt26 on the
same phenomenon are important. This investigator di-
rected his attention chiefly to the nuclei, and familiarized
himself with their behavior during regeneration. The
nuclei accumulate near the wound in the plasma deprived
of chlorophyll bodies, and are evidently more important
than the latter for the growth of the new cell-membrane.
In the exuded globules of protoplasm which remained
alive, Haberlandt succeeded almost always in demon-
strating the presence of one or more nuclei, but never the
absence of any. In spite of this, not all of them formed
a new cell-wall. "At times there occur cell-forms devoid
of a membrane and rich in plasm. If the sap-cavity is
lacking, the chlorophyll-bodies aggregate in the center,
and the nuclei lie in the peripheral, colorless plasma. In
case a cavity for cell-sap is present, the chlorophyll-grains
lie in the innermost layer of the plasma-body the nuclei
more toward the outside."27 The possession of nuclei is
therefore, in itself, not sufficient for the formation of a
cell-membrane. It would be important to find out whether
the parts of plasma referred to are perhaps the very ones
that did not get part of the old limiting membrane.

It seems to me to be of great interest to regard the
whole pending question from another point of view, and
one which has already been considered by Haberlandt.

26Haberlandt, G. Ueber die Beziehungen zwischen Funktion und
Lage des Zellkernes. pp. 83-97. Jena, 1887.
27Loc. cit. p. 92.

Regeneration of Protoplasts After Wounding 143

Regeneration is obviously an adaptation to guard against
the results of injuries which occur frequently in nature.
In such cases the higher plants usually give up the affected
cells; the large-celled algae and fungi, especially those
that have been designated by Sachs as non-cellular, evi-
dently cannot do that. Therefore one generally finds in
them the power of closing up wounds. That it would,
however, be of particular importance to keep escaped
globules of protoplasm alive is the less probable, as it is
only possible to do so in solutions which are quite a little
more concentrated than those in which the respective
plants naturally live. Therefore, the closing up of the
wound is primary, the processes in the escaped plasma
secondary. From the adaptive characters available for
the first, it ought to be possible to explain the latter. And
as long as the first can be explained without the hypothe-
sis of an independent neogenesis of the plasmatic mem-
brane, this assumption must be regarded as at least im-
probable for the latter.

This consideration leads us to include in the field of
these studies even the closing up of wounds in latex-tubes.
The investigations of Schmidt on the latex-vessels, and of
Schwendener on the latex-cells may serve as important
points of departure in this.28 For they teach that in parts
of latex-tubes which adjoin the wound of the cut, a closing
up of the tube can be accomplished in the same way as in
some Siphoneae (e. g., Bryopsis, Codium, Derbesia) and
in many pollen-tubes the injured part of the cell-cavity
is separated from the uninjured one.29

28 Schmidt, E. Ueber den Plasmakorper der Geliederten Milchroh-
ren. Bot. Zeit. 40: 462. 1882. -Schwendener, S. Einige Beobach-
tungen an Milchsaftgefassen. Sitsungsb. Kais. Akad. Wiss. Berlin.
20: 323. 1885.

29Schmidt, E. he. cit. p. 462.

CHAPTER III

THE AUTONOMY OF THE INDIVIDUAL ORGANS OF THE
PROTOPLASTS

§ 5. Nucleus and Trophoplast

A review of our knowledge concerning the anatomy
of the nucleus can be regarded as superfluous in this con-
nection. This knowledge is to be looked upon at present
as an established achievement of science, the significance
of which for the theory of heredity can hardly be doubted
any longer. Flemming in the zoological, Strasburger and
Schmitz in the botanical field have broken the way, and
their observations have been verified and extended in the
main by numerous other investigators.

It does not seem to be quite fully decided whether the
amitotic nuclei, which have originated through constriction
and scission, are of significance in questions of heredity,
or whether they occur in somatic cells only, and not on
the germ-tracks. In Chara the nuclei in the apical cells
divide, according to Johow's investigations, according to
the usual scheme of indirect nuclear division ; the smaller
cells of the grown plant, for example in the nodes, remain
forever uni-nucleate, while the larger ones become multi-
nucleate through constriction. This kind of nuclear form-
ation, however, is never followed by cell-division.30 Ac-
cording to Zimmermann direct nuclear division in the
plant-world "is limited to only those cases in which the
nuclear division is not accompanied by cell-division."31
' 30Johow, F. Die Zellkerne von Chara foetida, Bot. Zeit. 31:
729. 1881.

31Zimmermann A. Morphologic und Physiologic der Pflansen-
zelle. p. 34.

Nucleus and Trophoplast 145

In the multi-nucleat cells of Valonla Schmitz82 has fre-
quently observed division, and always observed it to take
place by constriction. It does not seem to be established
with certainty, for all cases, how the nuclei of the swarm-
spores originate here and in the case of the other Siphono-
cladiaceae, whether through direct or indirect division.

In this connection it should be mentioned that, accord-
ing to Van Beneden and Julin, direct and karyokinetic
nuclear divisions alternate in the spermatogenesis of As-
carls inegalocephala.^ Thus we see that this subject is
not yet ripe for theoretical use.

The amyloplasts, with all their derivatives, among
which the chlorophyll bodies are the most important, Ar-
thur Meyer calls trophoplasts. In the lowest plants they
are not yet differentiated, and, as far as these belong to
the Phycochromacese, the whole non-nucleated protoplasm
of the cell, according to Schmitz, is stained.34 But later
Hansgirg demonstrated nuclei and chromatophores in
some algae of this group.85 From the Chlorophycese up-
ward they are universal in the green plants. In the higher
plants, where they were discovered by Schimper,86 they
are usually colorless in young cells. As a rule they re-
main so in the underground parts, which are normally not
exposed to light.

Phylogenetically, therefore, plants with undifferen-
tiated colored protoplasm are probably older than those

32 Schmitz, F. Die vielkernigen Zellen der Siphonocladiaceen. p.
27. 1879.

83Van Beneden et Julin, La spermatogenese chez I'Ascaride me-
galocephale, Bruxelles, 1884.

84 Schmitz, F. Die Chromatophoren der Algen. p> 9. 1882.

8BHansgirg, A. Ber. Deut. Bot. Ges. 3: 14. 1885.

86Schimper, A. F. W. Ueber die Entwickelung der Chlorophyl-
korner und Farbkorper. Bot. Zeit. 41: 105, 121, 137, 153. 1883.

146 Autonomy of Cell-Organs

which possess special chromatophores. Hence we must
imagine them to have originated from the others through
differentiation. A further step in the differentiation is
then the development of colorless conditions of these
chromatophores. These are still lacking in the lower Al-
gae, occur first in the highest groups of this class, and at-
tain their full significance only in the higher plants. In
other words, we must regard the amyloplasts, although
they are generally the young condition from which chlor-
ophyll bodies develop, as the consequences of a higher
differentiation and assume that they have developed phylo-
genetically from the latter. This discussion is important
for the reason that it brings nearer to our understanding
the not infrequent changes of form of the trophoplasts on
the germ-tracks. On the whole, the cells of the germ-
tracks of the higher plants are, as many authors empha-
size, of an embryonic nature, and such cells probably
always possess colorless trophoplasts. But according to
our definition of the germ-tracks, there are many excep-
tions to this rule. Thus, to name only one instance, the
prothallia of ferns, in their youthful state, consist of
green, dividing cells, with well-formed chlorophyll-grains,
from which later the amyloplasts of the egg-cells will
originate. Also in the callus-formation of cut petioles of
Begonia, Peperomia, and other species, a reversion of
green trophoplasts into colorless ones may take place,
especially in the case of the production of adventitious
buds. And, since generally the amyloplasts occur in young
cells and their derivates in grown protoplasts, these and
similar cases would be illustrative of a pronounced reju-
venation.

' On the germ-tracks the amyloplasts usually take on a
simple roundish form, on the somatic tracks they change

Autonomy of Chromoplasts 147

their shape considerably, and with it the structure and size
of the starch-grains produced by them.

Among the most peculiar characters of the chromato-
phores in connection with the organization of the proto-
plasts, belong their autonomous movements. Since the
researches of Sachs on this subject, we know that the
chlorophyll grains of some plants are moved about by
streams of the granular plasm in such a way that, under
the influence of light, they take up positions which are
favorable for the assimilation of carbon dioxide. But in
this process they are passive. The beautiful researches of
Stahl, however, have disclosed independent movements
of these structures under the influence of the same stimu-
lation. They consist chiefly in changes of shape, through
which the organs in question either approach a more or
less globular shape, or that of a flat, circular disc. Thus
it is brought about that, in direct sunlight, they present
a smaller, in diffuse daylight, a larger surface for re-
ceiving the rays. And to us they afford an insight into
the high degree of their inner differentiation such as we
could never have attained by the simpler study of their
chemical activity.

According to Weiss, the yellow and orange chromo-
plasts at times also make autonomous movements, which,
according to the descriptions of this author, resemble the
changes of form of the amoeba and the white blood-cor-
puscles.37 These structures, therefore, may also be more
highly organized, and play a more important role, than
that of the simple task of giving their color to the respec-
tive plants.

I wish to lay quite particular stress here on these

87Weiss, A. Ueber spontane Bewegungen und Formanderungen
von Farbstoffkorpern. Sitzungsb. Kais. Akad. Wiss. Wien. 90: 1884.

148 Autonomy of Cell-Organs

phenomena, for up to the present time they have probably
not been utilized for the theory of heredity. But the more
plainly we see the independence of the individual organs
of the protoplasts, and the more clearly our conviction
grows that they require a high inner differentiation for
exercising their functions, the more will we be inclined
to give them, their due place in our theory, and especially
will we try to investigate the more thoroughly their rela-
tion to the hereditary factors accumulated in the nucleus.

Wherever, hitherto, we have succeeded in demonstrat-
ing with complete certainty the origination of trophoplasts,
we have found that they arise through a division of those
already present. That the chlorophyll grains, in the
higher plants as well as in the algae, can multiply through
constriction and scission has long been known. But it
was Schmitz who showed that this process is the only form
of their multiplication in the algae.38 In the Characeae he
discovered, in the apical cells, the colorless bodies from
which the green organs of these plants are derived in
the same way. These investigations are now so generally
known that it would be superfluous to describe them here
in detail. I shall only emphasize, as especially important,
the fact that the swarm-spores also possess only such
chromatophores as they have received from their mother-
cell, a fact that was especially mentioned in the case of
Cladophora and Halosphaera.39

The investigations by Schimper and others, who dis-
covered this same law for the phanerogams, have already
been discussed in one of the preceding Chapters.

Special consideration is still due to the rarer forms
derived from the more general chromatophores. In the

38Schmitz, Die Chromatophoren der Algen. 1882.
*»Loc. cit. pp. 135, 136.

Formation of Oil 149

first place we must mention the eye-spot40 observed in
many swarm-pores, and which, according to the opinion
of those investigators who have examined it more care-
fully, is probably a metamorphosed chromatophore, the
same as the chromatic bodies of the higher plants studied
by Arthur Meyer.41 In the Euglenae its origin has been
more carefully studied by Klebs. Here it always origi-
nates by division, the organs being always preserved in
the resting cells.42 It is not yet definitely decided whether
or not the pyrenoids in the chorophyll bodies of Spiro-
gyra and other algae are to be regarded as specially dif-
ferentiated parts of these organs. But it seems certain
that, at least in isolated cases, they multiply through di-
vision.43

On the origination of oil in plant-cells little i« known
with certainty. Pfeffer has demonstrated that the oil
does not form in the vacuoles, but lies imbedded in the
granular plasm. Special organs which accumulate it
within themselves have lately been described by Wakker
for Vanilla planifolia, and have been called elaioplasts.
Although it has not been possible to find out their mode
of origin, the most natural assumption is that they are
metamorphosed chromatophores.44 In some cases, as for
example in the diatomes, the oil-drops of the Algae evi-

40Cf. Zimmerman, Die Morphologic und Physiologic der Pflan-
zenzelle. p. 71. 1887.

41Meyer, Arthur, Das Clorophyllkorn. 1883.

42Klebs, Ueber die Organisation einiger Flagellatengruppen.
Unters. Bot. Inst. Tubingen. 1: 233.

43Schmitz, F. Die Chromotophoren der Algen. pp. 42 and 65.
1882. Schmitz, F. Beitrage zur Kentniss der Chromatophoren.
Jahrb. Wiss. Bot. 15: 142. 1884. Strasburger, E. Ueber Kern- und
Zelltheilung. p. 26. 1888.

'4tWakker, J. H. De Elaioplast. Maandbl. v. Natuurwetensch.
No. 8. 1887.

150 Autonomy of Cell-Organs

dently do not lie in the chromatophores, and this, accord-
ing to Schmitz, is a general rule.45 But in the higher
plants this seems at times to be the case.46

Last to be mentioned here are the microsomes. In
most cases it seems to be unknown what they are. Small
oil-droplets, starch-grains, inactive vacuoles, amyloplasts,
protein bodies formed by fixation47 through the coagula-
tion of the protein dissolved in the protoplasm, and per-
haps many other formations are frequently all classed
under this name. Very justly has Strasburger claimed
"that not the microsome but the hyaloplasm is to be con-
sidered the active substance."48 At any rate it ought
never to be forgotten that the word microsome stands
only for a question mark, and that we can talk of an in-
sight into the significance of these structures only after
the question concerning their nature in the cases con-
cerned shall have been answered.

§ 6. The Vacuoles

Vacuoles were formerly regarded as empty spaces in
the interior of the protoplasm. This accounts for their
name, and explains the small interest shown in them, until
recently, in the study of the anatomy of the cell. It is
only since Sachs discovered that the turgidity of growing
cells is not due to an imbibition of water in their walls,
as was previously assumed, but to an osmotic tension be-
tween the wall and the cell sap, that attention was directed
to the significance of the vacuoles.49

45 Schmitz. Loc. cit. p. 164.

46Cf. Meyer, Arthur. Das Chlorophyllkorn, pp. 14 and 31. 1883.

47 i. e. artifacts caused by the "fixing" fluid. Tr.

48 Strasburger, E. Neue Untersuchungen. p. 107. 1884.
49Sachs, J. von. Lehrbuch der Botanik, 3 Aufl. 1872; 4. Aufl.

1874, p. 757.

Autonomy of Vacuoles 151

%

This was still more the case through the demonstra-
tion furnished by the same author, that the tension to
which growing cell-membranes are subjected by the cell-
sap is one of the most essential mechanical causes of the
surface growth of these membranes. For with this dem-
onstration Sachs laid the foundation still valid, for the
whole mechanical theory of growth in length.

Building on this foundation, many investigators have
enlarged our knowledge of the mechanical causes of
growth in various directions. Some have especially
measured and analyzed the degree of extensibility of the
cell-membranes and the amount of force supplied by the
cell-sap. Others have studied the causes governing the
variations of extensibility of the wall in one and the same
cell, and which occur in different spots and in different
directions, and have explained them, as due, with great
probability, to local differentiations in the protoplast it-
self, which might regulate this elasticity through the
secretion of certain enzymes. Others again have at-
tacked the doctrine of intussusception, which was the
prevailing one at the time of the discoveries mentioned,
have proven it to be incorrect, and have tried to ressusci-
tate in its place, in a new form, the old "apposition
theory."

Although subject to misunderstandings from some
sides,50 Sach's theory has acquired a prominent position
in plant-physiology, and, since the two decades of its es-
tablishment, it has become, in ever increasing measure,

50In my "Untersuchungen iiber die Mechanischen Ursachen der
Zellstreckung" (p. 3, 1877.), I have distinctly emphasized the fact
that there are also phenomena of growth independent of turgor,
and that therefore this turgor is neither the only, nor even the first
reason for growth. Krabbe and Klebs arrived later at the same
conclusion. Cf. Arbeiten Bot. Inst. Tubingen. 2: 530. 1888.

152 Autonomy of Cell-Organs

the starting point of new investigations. It has been,
without doubt, one of the most fruitful thoughts for the
development of our science.

The further study of the cell-sap and the vacuoles,
suggested by this theory, has led in regard to the morpho-
logical aspect, which alone interests us here, to the proof
that the wall of the vacuoles is an essential, never wanting
part of the plant-protoplast.51 The method which made
it possible always to demonstrate the presence of this wall
consisted in the treatment of the living cells with a 10%
solution of potassium nitrate, which has been stained with
eosin. Directly, or after a shorter or longer period, the
outer protoplasm dies in the reagent, while the wall of the
vacuoles remains living for a while. It is then visible
as a distended bubble, more or less completely separated
from the dead parts, and entirely preventing the penetra-
tion of the eosin. In colorless cells, therefore, the bubble
carries contents as clear as water, while the remaining
protoplasm is stained red or brown by the eosin. Fre-
quently the original vacuole separates into several smaller
ones; and not infrequently one can follow this process
directly under the microscope.

The wall of the vacuoles is to be regarded as a special
organ of the protoplast, which regulates the secretion and
accumulation of the substances which are present in the
cell-sap in solution, and because of this function, it has
been given the name tonoplast. But frequently the sap-
spaces together with their walls are now designated as vac-
uoles.

In living cells the tonoplasts are, as a rule, not visible,
because they consist of translucent vesicles of an extreme

61Vries, H. de. Plasmolytische Studien iiber die Wand der Vac-
uolen. Jahrb. Wiss. Bot. 16: 465. 1885.

Autonomy of Vacuoles 153

thinness. But they are clearly and distinctly visible in the
tentacle-cells of some insectivorous plants, especially of
the Drosera rotundifolia and D. intermedia. The process
of aggregation, discovered by Darwin,52 taking place here
during the digestion of the prey, belongs to the most in-
teresting phenomena that the life of a cell presents for
our admiration. In the resting tentacle-cells there lies
usually a large vacuole containing red cell-sap. Under
the influence of stimulation it separates into several, and
soon into numerous smaller ones. These contract, while
secreting part of their contents, and are now carried
through the cells by the currents of the granular plasm,
with great rapidity, and in the most various directions.
Thus they lie as red vesicles in unstained sub-
stance, and can therefore be seen very distinctly. Dur-
ing these movements they undergo striking changes of
form ; sometimes they are drawn out into long tubes, and
thereupon split into numerous small globules, sometimes
two or more unite to form larger vesicles. Toward the
end of the phenomenon this last process has the pre-
cedence, and finally all the sap-bubbles have again united
into one, of the original volume.53

The above mentioned phenomena of aggregation, and
the division of the vacuoles, as it is so frequently ob-
served in plasmolysis placed the ability of these organs
to multiply by this process beyond any doubt. From the
analogy of these structures with the chromatophores I
then deduced the assumption, that "like the amyloplasts,
they can be produced in no other way than by division."54

52Darwin, C. Insectivorous Plants. Chap. III. 1875.

53Vries, H. de. Ueber die Aggregation im Protoplasma von
Drosera rotundifolia. Bot. Zeit. 44: 1, 17, 33, 57. 1886.

54Vries, H. de. Plasmolytische Studien iiber die Wand der Vac-
uolen. Jahrb. Wiss. Bot. 16: 505. 1885.

154 Autonomy of Cell-Organs

This supposition has since been completely confirmed
by Went.55 He showed first, that, contrary to the pre-
vailing opinion, vacuoles are present even in the youngest
cells of the meristerrk These multiply continuously
through division, and observation teaches that during
cell-division one-half of the vacuoles present goes to one
daughter-cell and the other half to the other. Some-
times it was possible to observe the constriction and after-
wards the transmission of the two sap-vesicles, formed
in this way, to the daughter-cells. From the vacuoles
of the meristem all the vacuoles of the entire plant can
thus be derived. Divisions of these structures are to be
found everywhere ; formations de novo nowhere. In the
same way, in the cryptogams that grow with an apical
cell, all the vacuoles originate from the original vesicles
present in these cells.

According to these investigations the vacuoles behave
exactly in the same way as the chromatophores, and are
just as independent cell-structures as the latter. And
through the demonstration of this independence, the pan-
meristic conception of cell-division has been definitely
proven as correct, in opposition to the former neogenetic
one.

According to later communication by the same author,
he succeeded also in observing the formation of vacuoles
in some special cases which had not been studied before.
Here should be emphasized the formation of these organs
in the swarm-spores which, according to a communication
by letter from Went, comes about by a division of the
sap-vesicle in the mother-cell in such a way that every

55Went, F. A. F. C. Die Vermehrung der normalen Vacuolen
durch Theilung. Jahrb. Wiss. Bot. 19: 295. 1888.

Autonomy of Vacuoles 155

swarmer receives into its body a portion divided off from
this bubble.

In the literature, an origination of sap-cavities in nu-
clei, chromatophores, or even in the granular plasm, out-
side the vacuoles already present, has repeatedly been
described. But, on investigating these cases, it was found
that here one had to deal, not with normal vacuoles, but
with pathological formations, which occur with the age-
ing or dying of the cell. Frequently they are also due
to the influence of the water in which the preparations are
examined.56

From the theory that the vacuoles originate only
through division, it may be concluded that the sap-vesicles
of germinating seeds are derived from those present in the
ripening ovules, and that, therefore, in the ripe condi-
tion, the vacuoles must indeed be dried out, but cannot
be entirely lacking. Following up this thought Wakker ar-
rived at the noteworthy discovery that the aleuron-grains
are the dry states of the vacuoles in the seed.57 During the
process of ripening, the amount of protein matter dissolved
in the cell-sap gradually increases until the fluid becomes of
a thick, slimy consistency. In drying, some of the protein
bodies crystallize and form the well known crystalloids,
while the remaining protein hardens into an amorphous
mass around them. When soaking the seed, these masses
soften gradually and are later utilized as nourishment.
By using a solution of one part nitric acid in four parts

56Went, F. A. F. C. De jongste toestanden der vacuolen, pp.
45-65.

57Wakkcr, J. H. Aleuronkorrels zyn vacuolen. Maandbl. r.
Naturw., Nr. 5. 1887. Over kristalloiden en andere lichamen die
in de cellen van zeuvieren voorkomes. Bot. Cent. 33: 138. 1888,
and Jalwb. Wiss. Bot. 19: 423. 1888. Since that time this result
has been confirmed by Werminski, Ber. Deut. Bot. Ges. 6: 199. 1888.

156 Autonomy of Cell-Organs

of water, one can bring about at will this hardening in the
still liquid cell-sap, and in this way artificially produce the
formation of aleuron-grains under his very eyes.

It is important that, in some seeds more, in others
less, the vacuoles divide during the process of ripening
into several smaller, frequently into very numerous ex-
tremely minute vesicles, which gradually fuse again into
one large vacuole at the beginning of germination.

The processes in the seed, therefore, fit beautifully
into the conception that the vacuoles originate only by
division.58

Just as the chromatophores can differentiate into the
most various organs, so also can the vacuoles, although
to a lesser extent. Went observed how, in different cells,
there lie vacuoles which remain separated throughout
their existence, and are distinguished by their different
contents.59 Frequently some of them are stained, others
are colorless, or some contain tannin, which is lacking in
others. In such cases the latter are called by that author
adventitious vacuoles.

The contractile or pulsating vacuoles form a special
system. In the swarm-spores of the algae they probably
originate from the other vacuoles60 through further dif-
ferentiation, but in the Euglense, according to the investi-

58In Miiller's bodies of the ant-plant, Cecropia adenopus, Schim-
per illustrates formations in the cell-contents which, at first glance,
look like vacuoles, and which, on account of their semi-fluid con-
tents, he compares with the aleuron-grains. Their origination from
vacuoles can hardly be doubted. Schimper, A. F. W. Die Wechsel-
beziehungen zivischen Pfianzen und Ameisen. 1888. Cf. especially
Taf. II, Fig. 11. Also Wakker, Jahrb. Wiss. Bot. 19: 467. 1888.

59Went. loc. cit. pp. 65-91.

60Or have the turgor-vacuoles possibly originated phylogenetically
from the pulsating ones?

Autonomy of Plasmatic Membranes 157

gations of Klebs, they multiply by division.61 They
possess here a wall of their own which resembles the
walls of ordinary vacuoles in its great power of resistance.
Klebs observed how the pulsation may continue for a
long time after the rest of the protoplast has been killed
by some mechanical interference. The view that, in
systole, the contents of these vacuoles are expelled into
the surrounding tissues, while, in diastole, fluid is taken
from the protoplast, is probably generally accepted for
rhizopods and flagellates. My own observation con-
vinced me of its correctness in Actinophrys Sol. The
same opinion may also apply to the pulsating vacuoles in
the plant-world.62

§ 7. The Relation Between the Plasmatic Membranes and
the Granular Plasm

While the investigations of the last two decades have
thrown a clear light on the organs of the protoplasts just
discussed, the relation between plasmatic membrane and
granular plasm is still completely in the dark. In our
knowledge of the mode of origin of the nuclei, tropho-
plasts, and vacuoles, the theory of heredity, as I have
tried to explain in this Section, finds its indispensable
basis. On the mutual relation of the two other men-
tioned parts of the protoplast, no facts have yet been
found, which might be utilized for the theory.

As already mentioned, what the nature of that relation
is, is certainly not of essential importance for the
hypothesis of intracellular pangenesis. Yet it remains an
important question whether granular plasm and plasmatic
membrane are mutually as independent as the granular

61Klebs, G. Arbeiten Bot. Inst. Tubingen, Bd. I. p. 250. ff.
62Pfeffer, Pflanzenphysiologie, pp. 399-401.

158 Autonomy of Cell-Organs

plasm and the wall of the vacuole, or whether they stand
in the same genetic relation as amyloplasts and chloro-
phyll-grains. As long as this question remains undecided,
the application of my hypothesis to the plasmatic mem-
brane and therewith to the surface growth of the cell-
membrane and all the formative processes of the cells,
is rendered very difficult. For this reason may I be
allowed to subject the respective phenomena to a critical
revision in order to encourage further research. I think
it will then be seen that the prevailing opinion that the
plasmatic membrane originates in every case from the
granular plasm is, for the present, not supported by cer-
tain and closely observed facts, but is adhered to only
from habit. This, however, it seems to me, ought not
to be allowed in view of the newer knowledge in regard
to the origin of the wall of the vacuole. For, as long as
no special wall was assumed for the vacuoles, it was nat-
ural not to regard the plasmatic membrane as a special
organ. Since the independence of the former has been
established, such is obviously most probably the case for
the latter also.63

Besides the incompleteness of the observations, which
is to be demonstrated in the next paragraph, the whole
course of the development of our knowledge in the field of
cell-anatomy on the one hand, and the already repeatedly
described differentiations of the plasmatic membrane and
the granular plasm on the other hand, controvert the
prevailing opinion. The latter does not form at all, as

63A method by which the plasmatic membrane could be arti-
ficially separated everywhere from the granular plasm, just as strong
plasmolytic reagents separate the wall of the vacuole, is particu-
larly desirable. Such a method could also render great service in
judging the hypothesis mentioned on page 160, Note 2, on the growth
in thickness of the cell-membranes.

Autonomy of Plasmatic Membranes 159

the old conception would have it, a ground-substance of
protoplasm, mixing constantly by its movements, and
therefore not organized in the ordinary sense. This is
most clearly seen in the Characeae. Here it consists, first
of all, of a moving portion and of a resting part that
contains the chlorophyll grains. When, sometimes the
green plastids are torn from their position, and carried
away by the current, one sees that they did not adhere
separately to the plasmatic membrane, for they are not
carried off singly, but in bands and groups, while within
these the grains retain their mutual position and distance.
Neither does the moving part form a whole, for the ra-
pidity of the current is not at all everywhere the same
on a cross-section. It is greater near the chlorophyll-
grains than next to the wall of the vacuole, and further-
more it increases from the two indifferent zones toward
the center of the green areas which are separated by them.
With declining vital energy the more torpid currents are
the first to suspend movement, while the more rapid ones
continue to move, and with decreasing rapidity the width
of the current diminishes at the same time.

Quite generally speaking, the granular plasm seems
to consist, in the plant-world, of moving and of resting
parts, the limits of which can be shifted by more or less
favorable life-conditions, or can also shift spontaneously
in the course of development, adapting themselves to
changing needs.

The latter condition is illustrated by the beautiful in-
vestigations by Dippel, Criiger, and Strasburger on the
relations between the plasma-currents and the internal
sculpture of the cell-wall.64 For along those places where

e*Dippel. Abhandl. Naturf. Gcs., Halle. 10: 55. 1864. Criiger,
H. Westindische Fragmente. Bot. Zeit. 13: 623. 1855. Stras-
burger, E. Jenaische Zeitschr. Naturwiss. 10: 417. 1876.

160 Autonomy of Cell-Organs

ledges, jutting into the interior, are in the process of for-
mation there generally run strong currents which evi-
dently bring and distribute the requisite food. But this
differentiation in the granular plasm is, to all appearances,
controlled by a corresponding differentiation in the plas-
matic membrane. For, according to Dippel, the bands
which form the layers of cellulose, consist of an outer
hyaline band, which is thicker than the rest of the plas-
matic membrane, and, like^ the latter, cannot be stained
with iodine, together with an inner, moving layer of the
granule-bearing plasm, which takes a deep yellow tint
when treated with iodine.65 The hyaline band is evi-
dently a differentiated part of the plasmatic membrane
which, on its inside is covered and nourished by the cur-
rent, and on its outside forms the ledges of the cell-
membrane.66

In naked protoplasts the cilia also bespeak an inner
organization of the plasmatic membrane. These are de-
scribed by Strasburger67 for the swarm-spores of Vau-
cheria. Here all the cilia adhere to a denser part of this
layer; they appear to be embedded in it by a thick root.

§ 8. The Question of the Autonomy of the Limiting
Membrane

While in cell-division, according to the type described
by Mohl, the multiplication of the limiting membrane by

65Loc. cit. pp. 57, 58.

66Strasburger's hypothesis that the growth of the cell-wall is
accompanied by a transformation layer by layer of the outermost
strata of the limiting membrane into cell-wall can, without difficulty
be combined with the assumption of the autonomy of this organ with
reference to the granular plasm, and therefore need not be discussed
in detail here.

67 Strasburger, Studien uber das protoplasma, p. 400. 1876.

Autonomy of the Limiting Membrane 161

division and growth is generally recognized, the insertion
of a new layer and its connection with the old membrane
is usually assumed for cell-formation in the higher plants.
In addition to this, there are some cases of cell-formation
which seem to argue quite directly in favor of a formation
of the limiting membrane de novo from the granular
plasm.

All these cases seem urgently to demand renewed in-
vestigation. It is only with the intention of encouraging
it that I shall briefly discuss them here.

In regard to the ordinary mode of cell-division the
situation has greatly changed during the past year through
a discovery by Went68 which has been confirmed by Stras-
burger.69 This discovery concerns the nature of the so-
called cell-plate, which, when nuclear division is
completed, forms at the equator of the now barrel-shaped
figure. As the name indicates, the cell-plate is regarded
as a layer which, cutting across the figure, later divides
into two layers, and between these secretes the new cel-
lulose lamella. These two halves of the layer are the two
complementary pieces of the plasmatic membrane ; as the
barrel becomes flattened and extends laterally toward the
cell-walls, they increase until they reach the old limiting
membrane of the mother-cell and blend with it.

Went succeeded in loosening this whole division fig-
ure from the cells after they had been fixed and stained,
and allowed it to float around in the fluid of the prepa-
ration. In this way it became possible, by turning the
cell-plate, to study a polar view of it, while hitherto only
the side-view had been studied and figured. As long as

68Went, F. A. F. C. Beobachtungen iiber Kern-und Zell-
theilung. Ber. Deut. Bot. Ges. 5: 247. 1887.

69Strasburger, Ueber Kern-und Zelltheilung. 1888.

162 Autonomy of Cell-Organs

the cell-plate is smaller than the daughter-nuclei, this
view, of course, does not teach anything, because it has
not been possible to remove the nuclei. But as soon as
the cell-plate protrudes sideways from betwen the nuclei,
it can be seen that it is not, by any means, a continuous
plate, but only a rather thin ring. This ring lies in the
connecting tube that separates the interior of the figure
from its surroundings and has probably the same signifi-
cance as in Spirogyra.™ This "cell-ring," as we must now
call the cell-plate, enlarges until it unites, first on one,
then gradually on all sides, with the peripheral protoplasm
of the mother-cell.

That the plane of the cell-ring is the place where the
dividing wall forms, is certain, and agrees essentially with
the previous conception of the cell-plate. But it has not
yet been possible to discover whether or not the secretion
of cellulose in the cell-ring begins before the latter has
joined the wall of the mother-cell at least on one side. As
soon as its presence can be proven by reagents, the new
membrane is already joining the wall of the mother-cell, at
least on one side.71 Likewise it has not been decided,
whether, in the plane of the ring there is extended a mem-
brane which crosses the vacuole situated there and sepa-
rates it into two separate sap-vesicles. But this is not
probable.

It is clear that, with the discovery of the cell-ring, the
old conception of cell-division that contradicts the auton-
omy of the plasmatic membrane, is weakened. For its
final refutation, however, further researches are neces-
sary, especially such as will include the wall of the
vacuoles in the figures of division.

70Cf. pp. 132-134.

72Strasburger, E. Bot. Praktikum, p. 597. 1884, and Ueber Kern-
und Zelltheihmg. p. 171 ff. 1888.

Autonomy of the Limiting Membrane 163

I agree here with Zacharias72 who, from observation on
Chara, is of the opinion that the cell-plate elements origi-
nate from the cytoplasm surrounding the nuclear figure.
I wish also to recall here an opinion of Flemming's, ac-
cording to which, cell-division in plants and animals
generally begins with a constriction of the protoplast.
This constriction has not been observed in many prepa-
rations for the only reason that it is frequently unilateral,
and therefore requires a special position of the cell under
the microscope in order to be seen.78

Platner's view that the spindle fibers are currents of
the granular plasm requires further investigation. For
this purpose direct observation on the living object is
necessary. Obviously the plasma-currents have, until
now, been sadly neglected in the study of cell-division.

There are still left for us to consider the instances of
so-called free cell-formation, which probably represent
the most striking exceptions to the rule of the autonomous
origination of the plasmatic membrane. By free cell-
formation is meant those cases in which not all of the
protoplast of the mother-cell is used in the formation of
the daughter-cells.74 The new cells were thought to have
originated in the interior of the mother-cell, and there-
fore without any contact with the limiting membrane.

72Zacharias, E. Ueber Strasburger's Schrift Kern-und Zell-
theilung im Pflanzenreiche. Jena. 1888. Bot. Zei't. 46: 456. 1888.

73Flemming. Zellsubstanz, Kern-und Zelltheilung. p. 243.
1882.

74In the most recent interview of the pertinent literature, Zim-
mermann suggests that the name free cell-formation be not used for
these phenomena, but for the formation of free cells, i. e., of such that
lose their connection with the mother-cell. If it should be discovered
that a free cell-formation in the old sense, does not exist in the plant-
world, this suggestion would certainly be acceptable. Cf. Die Morph-
ologie und Physiologie dcr Pfianzenzclle, p. 160. 1887.

164 Autonomy of Cell-Organs

Hence it was clear that their limiting membrane must have
been derived from the granular plasm.75

In the formation of the endosperm a new plasmatic
membrane seems to be formed only in contact with that
of the mother-cell. In small embryo-sacs, where each
nuclear division is followed by a cell-division, the condi-
tions are, evidently, not essentially different from those in
vegetative cell-division. And, for those embryo-sacs
which continue to grow after fructification, I am not able
to find, in the literature in question, any proof against the
correctness of this assumption.76

In a number of algae (e. g., Acetabularia, Hydrodict-
yon, Ulothriv) the swarm-spores arise from only a part
of the protoplasm of the mother-cell. In such a case this
part is always the peripheral layer, and every swarm-
spore receives, as far as the present literature allows us
to judge, not only a nucleus, chromatophores, and vac-
uoles,77 but also a part of the limiting membrane of the
mother-cell. Similar conditions seem to exist among the
fungi, e. g., in Protomyces macrosporus.78 In the case
of Hydrodictyon, Pringsheim states that the colorless,
ciliated, anterior end of the swarm-spores represents the
maternal membrane.79 In the Saprolegniaceae also, the

75At this point in the original occurs a discussion of the pro-
cesses of cell-division within the embryo-sac in their relation to the
question of the autonomy of the limiting membrane. Since the
points there considered are now definitely settled and agreed upon,
the two paragraphs are here omitted with the author's approval. Tr.

76See especially Hegelmaier, Zur Entwickelungsgeschichte endo-
spermatischer Gewebekorper. Bot. Zeit. 44: 529, 545, 561, 585. 1886.

"According to the communication by Went mentioned on p. 154.

78Cf. de Bary. Vergleichende Morphologic und Biologie der
Pilse, Mycetozoen und Bacterien, p. 86. 1884.

™Monatsbericht Kais. Akad. Berlin, p. 246. 1871.

Autonomy of the Limiting Membrane 165

oospores are formed in such a way that each takes up in
itself a part of the maternal membrane.80

We meet with a greater difficulty in the ascospores.
But their origin has not been carefully studied in late
years. Thus, though we know that divisions of the
mother-nucleus always precede their formation, the ques-
tion as to how they acquire their other organs has not
yet been studied. It is clear that every spore must get one
or more vacuoles through the division of the maternal
sap-vesticles, but how this comes about, nobody has yet
investigated. The consideration of the other question
also as to whence the spores obtain their plasmatic mem-
brane, must be most urgently recommended.

In the same way the origination of the egg-cell in the
oogonium of the Peronosporales awaits study by means
of modern methods. In this case, too, nothing definite
can be said for the present in regard to the origination
of the plasmatic membrane. Concerning the membrane
of the spermatozoids, consult the following Section (pp.
174-176).

As a final result of this review, we may therefore say
that, in all cases in which the arising of a new plasmatic
membrane is supposed to take place without contact with
the old one, this assumption is chiefly due to investigation
by the older and imperfect methods. Exceptions to the
rule are not at all known with certainty, although, accord-
ing to the hypothesis of intracellular pangenesis, they
must not be considered, a priori, as impossible.

8°De Bary. Abh. Senckenb. Naturf, Ges. 12: 261, 1881.

C. THE FUNCTIONS OF THE NUCLEI

CHAPTER I
FERTILIZATION

§ /. Historical Introduction

The first author who described the nucleus as the organ
of heredity was Ernst Haeckel. In the second volume of
his "Generelle Morphologic der Organismen/n he estab-
lished this conception, founding it especially on the be-
havior of the nucleus during cell-division. For him thfe
"inner nucleus has the work of transmitting the hered-
itary characters, the outer plasm has the part of adapta-
tion, accommodation or adjustment to the conditions of
the outer world." And just as the nucleus plays its princi-
pal role in propagation, so is nutrition the chief task of
the plasma. In the lowest, non-nucleated organisms the
two functions are not yet separated.

For almost ten years this prophetic utterance re-
mained without noticeable effect on the progress of cell-
anatomy and the theory of fertilization. It was only
when Oscar Hertwig discovered that in fertilization the
spermatozoids copulate with the nucleus of the egg-cells
that Haeckel's idea became the starting-point for a new
line of investigation.2 Hertwig first observed this fact
in the eggs of the Echinidae.

R. Hertwig, Fol, Selenka, Flemming, and others, have
lent their support to this opinion by further investigations,

!pp. 287-289. 1886.

2Hertwig, O. Beitrage zur Kenntnis der Bildung, Befruchtung
und Theilung de£ thierischen Eies, Morphol Jahrb. 1: 347. 1875.

1 70 Fertilisation

and in consequence of this it is quite generally recognized
at present in zoological science.

In the field of botany Strasburger has the merit, by
investigations of many years' duration, of having defi-
nitely proved the theory that fertilization consists essen-
tially in the union of the nuclei. His first studies on the
fertilization of the conifers, and later on the same process
in the angiosperms3 now form the foundation of this part
of our knowledge.

The other organs of the protoplasts take no part in
fertilization during copulation. And since, in spite of
this, the derivatives of the fertilized egg-cell possess later
the characteristics of both parents, it is clear that a trans-
mission to them of the hereditary characters from the
fertilized nucleus must take place. This transmission,
however, has, at least so far, eluded observation. But
many facts, even outside the scope of the theory of fer-
tilization, speak in favor of its existence.

It is my intention to put together in this Section, as
completely as possible, all the facts that might throw any
light on the nature of this transmission. The prevailing
conception regards this process as a dynamic one, while
my hypothesis of intracellular pangenesis assumes a
transport of material particles as bearers of the hereditary
characters. Therefore it is a question of ascertaining
which of these two conceptions is best supported by the
material available for observation.

3Strasburger, E. Uebcr Befruchtung und Zelltheilung, 1878.
Neue Untersuchungen uber den Befruchtungsvorgang bei den Phane-
rogamen, 1884.

CHAPTER II

FERTILIZATION (continued.)
§ 2. The Conjugation of the Zygosporeae

The behavior of the chlorophyll-band of Spirogyra
during conjugation is very instructive. De Bary4 has
already observed that in many species having one spiral
the two chlorophyll-bands of the conjugating cells join
their ends in such a way that they form a continuous
ribbon. For the one-spiraled species, 5. Weberi, how-
ever, Overton has quite recently described and figured
how the band of the maternal cell splits in the middle
during conjugation, and how the paternal band then in-
serts itself between the two halves and attaches itself to
their ends.5 Later, owing to the considerable swelling
of the pyrenoids, as well as to other processes, the
windings of the band gradually become more indistinct,
and finally, in the zygospore, quite indistinguishable, un-
til they reappear again during its germination.6

These data are quite sufficient to give us an idea of the
derivation of the chlorophyll-bands of the young germ-
plant. We assume, as a result of the above mentioned
investigations, that the chlorophyll-band of the germi-
nating zygospore consists of the bands of the two sexual
cells which are joined by their ends in one way or an-

4De Bary. Die Conjugaten. p. 3.

5Overton, C E. Ber. Deut. Bot. 5: 70. Taf. IV. 1888.

6See also on this subject Klebahn. Ber. Deut. Bot. Gcs. 6: 163.

172 Fertilization

other.7 What will happen to these first parts of the band
at the first divisions of the young plant? Evidently, in
the case described by de Bary, the first cell-division will,
by cutting the band through in the middle, give the ma-
ternal half to one daughter-cell and the paternal half to
the other. In 5. Weberi the two subsequent divisions will
do this ; the middle cells of the four-celled thread will then
bear the paternal, the two end-cells, the maternal band.

The result of this speculation is, that, for the individ-
ual cells of a one-spiraled Spirogyra-thread, it makes no
difference whether they get their chlorophyll-band from
the father or from the mother. However, there is no
doubt but that all the bands of the young plant possess,
later, the same hereditary characters, even though there
were individual differences between father and mother.
We must therefore assume that they necessarily got these
from the nucleus, after fertilization. If we attribute to
the process of conjugation any significance at all for the
active hereditary characters, and do not wish to restrict
its effect, through all generations, to the nuclei, we are
evidently compelled to accept this assumption.

But in this case the necessity of a transmission of the
hereditary characters from the fertilized nucleus to the
other organs of the protoplasts, lies before us in a simple
illustration.

We will generalize this theory, and say that in the
entire plant world it is indifferent for the new individual
whether, with the exception of the nucleus, it gets the
organs of its protoplasts from the father or the mother.

7In other cases the chlorophyll-band of the male cell is dis-
organized and resorbed. Cf. Chmielevsky. V. Eine Notiz iiber das
Verhalten der Chlorophyllbander in den Zygoten der Spirogyraar-
ten. Bot. Zeit. 48: 773. 1890.

Fertilization in Cryptogams 173

But the nucleus must be from both. The facts to be dis-
cussed in the two following Sections, teach us that, in
fertilization proper, the other organs come from the
mother only. But this is simply to be regarded as a spe-
cial adaptation.

The chromatophores of the other Zygosporeae, exam-
ined with this end in view, behave essentially similarly to
those of Spirogyra. They touch one another (Eplthe-
mia), or do not unite (Zygnema and many others), but
they never conjugate in the true sense of the word.8 At
the first divisions of the zygospore, the paternal and ma-
ternal chlorophyll grains must therefore always be dis-
tributed to the individual cells of the thread.

Schmitz, who was probably the first to observe the
conjugation of the nuclei in the Zygosporeae, and who
studied carefully the above mentioned behavior of the
chromatophores, demonstrated in a clear manner that, in
these cases also "the essential point is only the union of
the nucleus of the male cell with the nucleus of the female
cell/'9 And the facts which have been discovered later
have fully confirmed this statement.

§ j. Fertilization in Cryptogams

Schmitz, in his important monograph on the chro-
matophores of the algae, has comprehensively demon-
strated that these structures which, at each vegetative
cell-division, are transmitted from the mother-cell to its
daughter-cells, are usually entirely lacking in the sper-
matozoids.10 The egg-cells, however, always possess these

8Schmitz. Die Chromatophoren der Algen, p. 128. See also
Overton and Klebahn, loc. cit.
*Loc. cit. p. 128. note 2.
10Schmitz, loc. cit. p. 120 ff.

174 ' Fertilisation

organs. After fertilization they multiply by division, and
thus form the chromatophores of the new individual. In
regard to this point the organization of the protoplasts
is therefore inherited directly from the mother and not
from the father.

Let us now see, how the other members of the proto-
plast, with the exception of the nucleus, behave. To all
appearances the spermatozoids possess neither vacuoles
nor chromatic bodies, and hence the condition is the same
for the former as for the latter.

According to the best recent investigations, the sper-
matozoids do not originate, as some authors previously
assumed, from the nucleus only of the mother-cell, but
the rest of the plasma also takes part in their formation.
It is true that the nucleus forms the bulk of the body of
the male reproductive cell. Schacht has already voiced
the theory, on the basis of his observations and those of
others, "that the nucleus takes a very active part in the for-
mation of the spermatozoid and in a certain way blends
into it."11 He declares further that, in this process, the
granular contents of the mother-cell disappear. This trans-
formation of the nucleus, although denied by prominent
investigators12 at the beginning of the more recent re-
searches, is now generally recognized as the most im-
portant part of the whole process.

Outside the nucleus there lies, in the spermatozoids,
the limiting membrane, which protects this organ against
external influences, and, in a certain way, serves as the
little boat that carries it to its destination. The distinc-

"Schacht. Die Spermatozoiden p. 35. 1864.

12Comp. e. g. Sachs, Lehrbuch, 4. Auflage, p. 303; and Stras-
burger, Zellbildung und Zelltheilung, III Aufl. p. 94; also Bot. Zeit.
39: 847, 848. 1881.

Fertilisation in Cryptogams 175

tion of these two parts we owe chiefly to Zacharias, who
thoroughly investigated the micro-chemical reactions of
the male reproductive cells, and pointed out repeatedly
the different behavior of their external and internal
parts.13 The nuclein especially forms the chemical char-
acteristic for the substance of the nuclei. Fluids which
easily dissolve and extract this substance remove only the
inner part of the spermatozoids and leave the outer layer
and the cilia in general undissolved. In return the cilia
dissolve in pepsin, and do not, therefore, consist of nu-
clein.1* According to Campbell, also, the cilia of the sper-
matozoids are not developed from the nucleus, but from
the cytoplasm of the mother-cell.15

But, during fertilization evidently the nucleus alone
plays a part. The deep penetration of the entire sper-
matozoid into the egg-cells teaches that there is no prob-
ability of a conjugation of its outer layer with that of the
egg-cell. More likely do this organ and the cilia dis-
appear within the egg-cell, without playing any note-
worthy role therein.

Exceptionally the spermatozoids possess small chromat-
ophores which, perhaps, they may need on the way to the
egg-cell, either for taking the right direction, or for other
purposes. An example is found in Fucus, where Schmitz
proved that they arise by division from the chromato-
phores of the mother-cell.16 But no observation teaches
that they play any role in fertilization.

Phylogenetically, the spermatozoids of the algae have

isZacharias. Bot. Zeit. 1881-1888.

14Zacharias, E. Ueber die Spermatozoiden. Bot. Zeit. 39: 828,
836, 850. 1881.

15Campbell, D. H. Zur Entwickelungsgeschichte der Spermato-
zoiden. Ber. Deut. Bot. Ges. 5: 120. 1887.

16Schmitz, loc. cit. p. 122.

1 76 Fertilization

doubtless originated from conjugating swarm-spores. In
time they have gradually lost their chromatic bodies, and
probably also their vacuoles. For the disappearance of
the former Schmitz describes a number of intermediate
steps. May I be allowed to quote the following sentences
from his important treatment of this subject:17 "Some-
times, especially where the difference of the two kinds of
sexual cells is not yet very considerable, the spermato-
zoids act exactly like the isogametes, and like these
retain the chromatophores unchanged (e. g., in Scyto-
siphon lomentarium) . As that difference becomes greater,
however, the chromatophores of the male cells show a
distinct tendency to disappear, and especially does their
coloring become less intense (Bryopsis)"

This comparative study bridges the chasm lying be-
tween conjugation and fertilization, which is no doubt
chiefly due to the fact that, in the latter, the organization
of the protoplasts is inherited morphologically from the
mother only, while in the former, in some cells, the in-
heritance is from the mother, in others from the father.
But, on the other hand, the above mentioned phylogenetic
consideration leads to the conviction that the outer layer
of the spermatozoids has the same significance and the
same origin as that of the swarm-spores, and is just as in-
dispensable.

§ 4. Fertilization in Phanerogams

In the seed-bearing plants, also, the organization of
the protoplasts is directly inherited from the egg-cell
alone. From the pollen-tube only the nucleus penetrates
into the latter; other parts, even if they should be neces-
sary for the transportation of the nucleus and should ac-
cit. p. 121.

Fertilization in Phanerogams 177

company it, do not play any role in the true process of
fertilization.

Everybody is acquainted with the valuable investiga-
tions of Strasburger in this field which, since 1878, have
repeatedly treated this point and have completely proven
the above mentioned theories. It would be superfluous
to redescribe them here, or to enumerate their confirma-
tions by other investigators.

How the nuclei unite during fertilization is a question
which is very far from haying been satisfactorily an-
swered. Furthermore, differences predominate here
which are at least very striking. According to Stras-
burger, not only do the nuclear skeins fuse, but also the
nuclear vacuoles, and hence the nuclear sap.18 Accord-
ing to van Beneden, the nuclear skeins of the male
and the female cells in Ascaris megalocephala arrange
themselves side by side and form the segmentation nu-
cleus.19 They seem to unite at their ends, thus forming a
single nuclear thread, in which, therefore, only juxtapo-
sition takes place, and not a mutual penetration of their
elements. But while, in animals, according to the avail-
able data, fusion does take place during the state when
the chromosomes are arranged in the form of a star, it is
seen to occur in the plants in the state of rest. Whether
this difference really exists, and how the nuclear threads
generally unite, are questions which have to be more
thoroughly investigated.20

It is significant that the number of the chromosomes,
according to Strasburger's most recent investigations, has

l8Strasburger. Ueber Kern- und Zelltheilung, p. 230. Jena.

19Van Beneden, E. Recherches sur la maturation de I' oeuf.
1883.

29Strasburger. Ueber Kern- und Zelltheilung. p. 240. Jena. 1888.

178 Fertilisation

also been found to be constant in plants in the generative-
cells of every species, being the same for the male cells
as for the female. Sometimes it is the same for large
groups of plants as, e. g., for the Orchidacese 16; in the
Liliacese it varies21 between 8, 12, 16 and 24. For Ascaris
megalocephala it is 2, for A. lumbricoides 24. Obviously
this number does not have any systematic significance or
stand in any relation to the hereditary characters.

However, from a continued investigation in this field,
we may expect important disclosures on the question as
to which parts of the nucleus are the real bearers of the
latent hereditary characters. For the present the evi-
dence is in favor of the assumption that they are to be
looked for in the chromosomes.22 For the further work-
ing out of the theory of heredity this is, without doubt,
of the highest interest; for our hypothesis, however, a
decision is not absolutely necessary.

siStrasburger. Loc. tit. pp. 239, 242.

22Roux, Ueber die Bedeutung der Kernfiguren, 1883.

CHAPTER III

THE TRANSMISSION OF HEREDITARY CHARACTERS

FROM THE NUCLEI TO THE OTHER ORGANS

OF THE PROTOPLASTS

§ 5. The Hypothesis of Transmission

The question of a transmission of hereditary charac-
ters from the nuclei to the other organs of the protoplasts
has been repeatedly raised in the foregoing sections. But,
if we review all the facts combined in the preceding chap-
ter, and in this, the necessity of the assumption of such
transmission is forced upon us.

The protoplasts of the plant possess a visible organi-
zation, which, at every cell-division, is transmitted by
division of the individual organs, directly from the
mother-cell to its daughter-cells. The heredity is here
a visible and not a latent one. But the individual or-
gans are ontogenetically independent from each other;
they originate only through the division of such as are
already present. And even if, in the course of develop-
ment, they adapt themselves to various functions and, in
doing so, receive other names, and although their origin
in individual cases is not yet cleared up, so much is, on
the whole, certain, that the nucleus, the chromatophores,
the vacuoles and the granular plasm, and probably also
the limiting membrane, are primary organs which never
arise from each other, but only multiply side by side.

Each of these primary organs possesses a complement
of characters and potentialities which, together, form the

180 Intracellular Transmission of Characters

character of the species. These qualities can either be
seen directly under the microscope, or they betray their
presence by definite functions. That the hereditary char-
acters lie in the respective organs of the protoplasts can
hardly be doubted. But whether they also lie thus in cells
where they are present only in the latent condition is not
disclosed by the processes of vegetative propagation.

Here the process of fertilization serves as a clue. Hy-
brids teach, and daily observations on man confirm the
fact that children, on an average, receive their character-
istics, to the same extent, from both parents. But the
fertilized egg-cell receives its organs from the mother
only, while from the father only the sperm-nucleus
conjugates with the nucleus of the egg-cell. All the
hereditary characters of the father must therefore
be transmitted in the nucleus, as potentialities in a
latent state. And before they can become active in the
other organs of the protoplast, they must evidently be
transported to the latter ones from the nucleus. This
transmission is therefore a hypothesis, the assumption of
which may well be regarded as a necessity at the present
state of our knowledge.

May I be allowed to illustrate this transmission by a
few examples. I take them from hybrids, because here
the relations lie most clearly and convincingly before us,
and I chose the colors of the flowers bec?use they are
easily observed.

Let us first take the red color of flowers. Phase o-
lus multiflorous has red flowers, Phaseolus vulgaris nanus
white ones. By pollinating the latter with the pollen of
the former there came about several times, in 1886, in my
own cultures, -a hybrid seed. This does not deviate ex-
ternally from the normal seed of its mother-plant, but it

The Hypothesis of Transmission 181

develops into a plant which is similar to the twining P.
multiflorous, but remains smaller than the latter. The
flowers of the hybrid are of a pale red, being a tint midway
between the two parents, as I had the opportunity of
convincing myself personally. The red coloring matter
is found in solution in the vacuoles of the cells of the
petals.

The ability of the vacuoles to form the red erythro-
phyll comes from the father, in this instance. But the
vacuoles of the hybrid originate morphologically from
those of the mother. The power of producing erythro-
phyll must therefore have been transmitted in a latent
condition in the sperm-nucleus of the father to the nu-
cleus of the egg-cell, and must have been communicated
sooner or later to the vacuoles of the hybrid.

The same thing is taught by many other hybrids, as,
for example, Digitalis lutea 9 x purpurea $ , Linaria
vulgaris 9 x purpurea $ , Linaria genistaefolia ? x pur-
purea $ , et cetera.2*

The yellow color of the flowrers behaves in the same
way. Digitalis lutea-purpurea forms the best illustra-
tion. The two forms D. purpurea $ x lutea $ and D.
lutea $ x purpurea $ are quite alike, with the exception
of some slight variations in the color of the flowers.24
Naudin gives an illustration of the hybrid ; the flower has
a pure yellow color in one cluster, while in the other one,
yellow is mixed with pale red.25 Of the two mentioned
hybrids of the Linaria I do not find any record of the
reciprocal forms.

23Cf. Focke, Die Pflansenmischlinge, pp. 311, 315, and other
passages.

24Focke, loc. cit. p. 315.

25Naudin. Nouvelles recherches sur 1'hybridite. Nouvelles Ar-
chives du Museum d'histore naturelle de Paris, p. 95, PI. 2. 1869.

182 Intracellular Transmission of Characters

Like the qualities of the vacuoles, those of the chro-
matophores must be communicated to the hybrid during
hybridization, in a latent condition in the pollen-nucleus
of the father. As an instance I mention Raphanus sativus
9 X Brassica oleracea $ , Medicago sativa $ x falcata $ ,
Geum album 9 x urbamim $ , Verbascum phoeniceum 9
x blattaria $ .2Q

Similar instances can be found in great number in the
abundant literature on hybridization-experiments. But
science greatly needs a comprehensive miscroscopic study
of hybrids in relation to the anatomical structure of their
parents.27

Still more forcibly and more generally do we feel the
necessity for the assumption of a transmission, when we
observe the hybrids in the second and following genera-
tions. Almost always, when cultivated in a sufficiently
great number, some of them revert to the grand-mother,
others to the grand- father. The latter ones are so similar
that they could be easily confounded with the grand-
father. This teaches us that in hybridization, all the
characters of the father pass on to the hybrid, where they
are present in the latent state only, but that they become ac-
tive again in some of its children. All the organs of the
protoplasts must therefore be able to draw their active
characters from the nucleus.

In the hybrid, however, the characters of father and
mother are equally represented. Especially are both hy-

26These instances are from Focke, where more can easily be
found. I regret to say that I had no opportunity of controlling the
nature of the yellow coloring matter.

27The "Comparison of the Minute Structure of Plant Hybrids
with that of their Parents, and its Bearing on Biological Problems,"
by J. M. MacFarlane (Trans. Roy. Soc. Edinburgh, 37: 203. 1892) is
still practically the only investigation in this field. Tr.

The Influence of the Nucleus in the Cell 183

brids produced by two species, in which the one species
will function at one time as the father and at another
time as the mother, with few exceptions, essentially alike.
There is no ground for the assumption that the hereditary
characters, latent in the egg-cell and in the spermatozoid,
are inherited in a fundamentally different manner from
the father than from the mother. And thus we arrive at
the conclusion that the latter, too, must lie in the nu-
cleus, and are not distributed over the individual organs
of the egg-cell.

Hence the nuclei are the bearers of the latent hered-
itary characters. In order to become active, the greater
part of these characters,28 at least, must pass from the
nuclei into the other organs of the protoplasts

§ 6. Observations on the Influence of the Nucleus in the

Cell

Even the first investigators of this organ realized
that the nucleus plays a prominent role in the life of the
cell. They have given expression to this conviction in the
name itself. And, although later the supposed absence of
the nucleus in large groups among the Thallophytes gave
rise to a doubt as to the correctness of this opinion,29 it
has been entirely removed by more recent investigations.

At first it was impossible to form any idea as to the
nature of that role. The investigators mentioned in the
first chapter of this Section, Haeckel, Hertwig, Flem-
ming, Strasburger, and others, were the first to teach us
to regard the nucleus as the real organ of heredity.
And even in these later years there are some authors who

28The characters that regulate nuclear division, are probably
active in the nuclei themselves.

29Cf. Brucke, Sitzungsber. Akad. Wiss. Wien. 1861.

184 Intracellular Transmission of Characters

still, in opposition to Haeckel's positive assurance, re-
gard the nucleus as an organ of nutrition, ascribing to it
an influence on the formation of protein, starch, or other
products of assimilation.

Owing to the influence of the above named investi-
gators, attention has been directed, in recent years, more
and more to the nucleus. In consequence of this, a series
of observations have been made and published, which
speak in favor of the fact that the nucleus, although not
self -active, still exercises a very great influence on the
most important processes in cell-life. On the whole, the
conditions observed must, without doubt, be reduced to
this, that the hereditary characters, as long as they are
latent, are stored up in the nucleus, and become active
only in the other organs of the protoplasts. But it must
not be forgotten that, in individual cases, there may be a
special correlation between nucleus and protoplasm, which
must be attributed to specific adaptations, and not to
general laws. In the individual case it will usually be
very difficult to decide between these two possibilities.

First, I shall describe some of the conditions empha-
sized already by the older investigators. In young cells
the nucleus lies in the middle of the cell. With the in-
creasing size of the vacuoles, when the protoplasm reaches
the so-called foamy state, it remains in that position and
is connected with all the parts of the peripheral plasm by
bands and strands radiating from it by the shortest lines.
This familiar picture, and the considerable size of the nu-
cleus in young cells, may have been the first reasons for
attributing special importance to this organ. The nucleus
does not grow correspondingly with the increasing
growth of the cells. It becomes relatively smaller, and
the fusion of the vacuoles forces it out of its central posi-

The Influence of the Nucleus in the Cell 185

tion. Ordinarily, it does not take any definite position
after this, but is moved around in the cell by the cur-
rents of the granular plasm. As Hanstein describes it,
the nucleus traverses a long and very tortuous way within
a few hours, and sails in all directions throughout its
whole domain, "as if to inspect it everywhere."30 Every-
thing argues for the assumption that the activity of the
entire protoplast is under the regulating influence of the
nucleus.31

Besides the general behavior of the nuclei the in-
vestigations of Tangl, Haberlandt, Korschelt, and others,
have made us acquainted in recent years with a special
relation of the nuclei to individual processes in cell-life.

Tangl observed bulb-scales of A Ilium Cepa, which had
been recently wounded, for example, the day before.32 He
saw that near the wound-surface the nuclei are not, as
otherwise, irregularly distributed over the cells, but that
they had gone to that side of their cells which was nearest
to the wound. With them the granular plasm was also
accumulated on those walls. The shorter the distance
from the wound, the more pronounced was the phenom-
enon, but as far away as about 0.5 mm. it could still
be distinctly seen. These conditions probably indicate
that the process of regeneration which the wounds usually
cause proceed here, under the influence of the nuclei.

Haberlandt studied the position of the nucleus during
this process in a great number of cases in which the cells
of the higher plants show a more vigorous local growth

30Hanstein, Das Protoplasma. 1: 165. 1880.

31Cf. Strasburger. Neue Untersuchungen. p. 125. 1884.

32Tangl, E. Zur Lehre von der Continuitat des Protoplasmas
im Pflanzengewebe. Sitzb. Math.-Naturw. CL Akad. Wiss. Wien.
90: 10. 1884.

186 Intracellular Transmission of Characters

in some definite part of their circumferences.33 He did
so partly where, through localized surface growth, the
shape of the cells changes, partly where unilateral thick-
enings of the membranes, or a definite wall sculpture are
started. And although, owing to the abundance of in-
dividual phenomena, a rule without exceptions could not
be expected, he found, on the whole, that the nucleus most
frequently turns to where growth is strongest, and re-
mains longest where the latter continues longest.

According to Korschelt, the same rule is valid, in a
general way, for the animal cell.34 With chiefly unilateral
or local activity of the cells, this investigator succeeded,
in a number of cases, in observing for the nucleus a defi-
nite position which was as near as possible to the place
where this process was going on. Frequently, when the
distance is more considerable, the nucleus is connected
with such favored places by bands and accumulations of
protoplasm.

Where the nucleus does not betray its influence on
the processes in the protoplasm by a change of position,
it does so frequently by a definite arrangement of the
latter around the nucleus. The accumulation of the amy-
loplasts in the immediate vicinity of the nucleus, as is
frequently observed in young cells, has been ascribed by
various investigators to the influence of the nucleus on
their activity.35 Pringsheim has demonstrated that, in

33Haberlandt, G. Ueber die Beziehungen zwischen Funktion und
Lage des Zellkernes. 1887.

34Korschelt, E. G. Haberlandt, Ueber die Beziehungen zwischen
Funktion und Lage des Zellkerns bei Pflanzen, Jena, 1887, nebst
einigen Mitteilungen. Biol. Cent. 8: 110. 1888.

85Cf. e. g. Strasburger, Ueber Kern und Zelltheilung, p. 195.
1888. Schimper, A.F.W. Untersuchungen uber die Chlorophyll-
korper, und die ihnen homologen Gebilde. Jahrb. Wiss. Bot. 16:
1. 1885. Haberlandt, G. Die Chlorophyllkorper der Selaginellen.
Flora. 71:291. 1888.

The Influence of the Nucleus in the Cell 187

the cells of Spirogyra, the threads which radiate from
the nuclear cavity attach themselves especially to the
pyrenoids of the chlorophyll bands, and by ramifying,
frequently connect several of them directly with the nu-
cleus.36 In cell-formation in those embryo-sacs where
the new cells arise in a peripheral layer, after the forma-
tion of numerous nuclei, Strasburger has repeatedly de-
scribed radiated figures which unite the nuclei, and which
are present, not only between the two daughter-cells of
a mother-cell, but also are placed between the nuclei that
are not so closely related to each other. The repeated
studies of this investigator certainly remove all doubt
of the fact that along these rays some influence from the
nuclei makes itself felt during cell-division.37 _.

The multinuclear nature of the coeloblasts, discovered
and carefully studied especially by Schmitz,38 also argues
for the great importance of the nucleus. As a rule, here
the nuclei do not lie in the moving part of the granular
plasm, but in its resting layers. They are arranged
evenly at almost equal distances from each other, and
are mostly small and so numerous, that every detached
piece, if indeed not too small to remain alive, probably
always contains one or more nuclei. All parts of the
protoplasts can evidently be directly influenced by them.

Following the observations on uninjured cells, the
investigations on injured protoplasts must lastly be dis-
cussed. Schmitz has already drawn attention to the fact
that the extruded protoplasmic balls of Vaucheria and
other Siphonocladiaceae, are enabled to form a new cell-

86Pringsheim, N. Ueber Lichtwirkimg und Chlorophyll Function
in der Pflanze. Jahrb. Wiss. Bot. 12: 304. 1881.

37Cf. e. g. Strasburger, E. Bot. Praktikum, 1 Aufl. p. 610.

38 Schmitz. Die vielkernigen Zellen der Siphonocladiaceen. Fest-
schr. Naturf. Ges. Halle. 1879.

OF THE

UNIVERSITY )

OF

.;.^

188 Intracellular Transmission of Characters

membrane and to regenerate into new vital individuals
only when they possess one or several nuclei.39 This
must not be understood to mean that the nucleus is the
only condition. The chromatophores and the other or-
gans of the other protoplasts must also be present, but
the significance of these for growth and nutrition is of
such a nature that their indispensability may be regarded
as a matter of course. Nussbaum and Gruber have later
proven through extensive experiments in the division of
protozoa, that here too the fractional parts of the proto-
plasts can regenerate completely only when the nucleus,
at least, is not lacking.40

The experiments of Klebs on the culture of plas-
molysed cells are also important.41 I take from them
what follows : If cells of Zygnema and Oedogonium are
plasmolysed in a 10% solution of glucose, the contents
of the longer cells not infrequently divide into two or
more pieces, which, joined at first by thin threads, later
separate entirely from each other. If the threads are
now grown in light in this solution, the contracted pro-
toplasts surround themselves with a new cell-wall, which
gradually increases in thickness. Sooner or later they
begin to grow and divide, and in so doing, break through
the old cell-membrane. But in those cells where the
contents are split into two or more parts, of which, of
course, only one can get the nucleus, only this latter part
forms a new cell membrane; the non-nucleated pieces

S9Loc. dt. p. 34.

40Nussbaum, Ueber die Theilbarkeit der lebenden Materie,
Archiv Mikr. Anatomic. 1886. Gruber, A. Ueber Kunstliche Thei-
lung bei Infusorien. Biol Cent. 4: 717. 1885; Ber. Naturf. Ges.,
Freiburg i-B. 1886.

41Klebs, G. Ueber das Wachsthum Plasmolysirter Zellen. Bot.
Cent. 28: 156. 1886; Arbeiten Bot. Instituts. Tubingen. 2: 565. 1888.

The Influence of the Nucleus in the Cell 189

can, it is true, produce starch and nourish themselves,
but they are not able to grow.

In order to get more information on the role of the
nucleus a method would evidently be needed, which would
allow us to kill the nucleus without injuring the cell body.
Perhaps this end could be attained by making use of the
method suggested by Pringsheim, of partially killing the
cells in the focal point of a lens.42 By selecting a lens
that makes it possible to strike a single point of the cell,
it could be focused on the nucleus with a dim light, and
then a brief exposure to the direct rays of the sun might
produce the desired result in some of the cells. I there-
fore warmly recommend this method for further elabo-
ration in this direction.

In reviewing the results of the investigations that
have been discussed, we see that the nuclei have an in-
fluence on the activity of the other members of the proto-
plast. They exercise this influence only as long as the
respective members remain in the most intimate proto-
plasmic connection with them, preferably at the shortest
possible distance, or otherwise by direct plasma-bands.

42Pringsheim, N. Jahrb. Wiss. Bot. 12: 331. 1881.

D. THE HYPOTHESIS OF INTRACELLULAR

PANGENESIS

CHAPTER I

PANGENS IN THE NUCLEUS AND CYTOPLASM
§ i. Introduction

We shall now try to connect with each other the
conclusions to which the critical survey of previous the-
ories of heredity, in the first Part, and the review of the
present state of the cell theory, in the second Part, have
lead us.

The result of the first Part was that the comparative
consideration of the world of organisms, from the broad-
est standpoint, compels us to regard specific characters as
being composed of innumerable, more or less independ-
ent factors, of which by far the most recur in various,
and many in extremely numerous species. The almost
unbounded variety of living and extinct organisms is
thus reduced to the numerous different combinations
which a comparatively small number of factors makes
possible. These factors are the individual hereditary
characters, which, indeed, most frequently, are ex-
tremely difficult to recognize as such in the intricate sum
total of the phenomena, but which, however, since every
one of them can vary independently from the others,
may, in many cases, be subjected separately to experi-
mental treatment.

These hereditary characters must be groundedjnjiy-
ing matter; every vegetative germ-cell, every fertilized
egg-cell must potentially contain within itself all the fac-
tors that go to make up the characters of the respective

194 Pang ens in the Nucleus and Cytoplasm

species. The visible phenomena of heredity are hence
the expressions of the characters of minutest invisible
particles, concealed in that living matter. And we must,
indeed, in order to be able to account for all the phenom-
ena, assume special particles for every hereditary char-
acter. I designate these units, pangens.

These pangens, invisibly small, yet of quite another
order than the chemical molecules, and each of them com-
posed of innumerable such molecules, must grow and
multiply, and must be capable of distributing themselves
by means of ordinary cell-division, over all or at least
nearly all cells of the organism. They are either inac-
tive (latent), or active, but they can multiply in both
states. Predominantly inactive in the cells of the germ-
tracks, they usually develop their highest activity in the
somatic cells. And this in such a way, that, in higher
organisms, not all the pangens of any given cell probably
ever become active, but in every cell one or more of the
groups of pangens dominates and impresses its character
on the cell.

Fertilization consists in a fusion of nuclei. The
offspring receives from the father only that which was
contained in the nucleus of the sperm. All the hereditary
characters must therefore be represented in the nuclei
by their respective pangens. Nuclei, therefore, are to
be regarded as the reservoirs of hereditary characters.

In the nucleus, however, by far the most of the char-
acters remain latent all through life. They become active
only in the other organs of the protoplast. Haeckel has
already said "that the nucleus within had to take care of
the transmission of the hereditary characters, and the
surrounding plasm, of the adjustmment, accommodation,
or adaptation to environmental conditions." (Cf. p. 169).

All Protoplasm Composed of Pangens 195

Therefore, a transmission of the hereditary characters
from the nucleus to the cytoplasm1 must in some way
take place here, and the observations communicated in
the previous Section furnish important arguments for
the correctness of this deduction.

These are the conclusions that, to my mind, are fully
justified by the facts at hand. The assumption of pan-
gens is a hypothesis that seems to me indispensable at our
present state of knowledge. To my mind it is absolutely
necessary for the explanation of the allied relations of
organisms, provided that this explanation is attempted
on a material basis.

I shall leave now these general considerations, and
attempt to describe how I picture to myself the relation
of the pangens to the phenomena of cell-life. I am per-
fectly aware of the fact that the working out of a
hypothesis to its extreme consequences leads only too
easily to erroneous conclusions, and is of value for science
only when leading to definite problems that can be solved
experimentally. I shall therefore limit myself to only
one hypothesis, which, it seems to me, recommends it-
self by its simplicity. This hypothesis, with the deduc-
tions resulting directly from it, will form the subject
of this last section.

The hypothesis reads as follows : All living proto-
plasm consists of pangens; they form the only living
elements in it.

§ 2. All Protoplasm Composed of Pangens

From Hertwig's renowned discovery, some investi-
gators have inferred that only the nucleus is the bearer
of hereditary characters ; that they are entirely restricted

!By cytoplasm I mean all the protoplasm except the nucleus.

196 Pang ens in the Nucleus and Cytoplasm

to it. To my mind this is a much too far-reaching de-
duction, and without justification. The fusion of the
nuclei during fertilization is evidence only that all the
hereditary characters must be represented in the nucleus,
but this fact does not decide that they cannot be present,
in addition, in the cytoplasm.

The organs of the fertilized egg-cell are still the same
as those of the unfertilized ; the young plant has inherited
from the mother its chromatophores and vacuoles as such.
In the long succession of cell-divisions which are started
by the fertilized egg-cell, those organs, multiplying
steadily by division, are transmitted each time to the
daughter-cells. They have, so to speak, their independ-
ent pedigree in addition to that of the nucleus. There
is, therefore, an additional heredity outside the nucleus.

The smallest morphological particles, out of which
the chromatophores are built up, must evidently possess
the power of multiplying independently, otherwise neither
the growth nor the repeated divisions of these structures
could be explained. In this respect these particles are
obviously similar to the pangens of the nucleus. The
power of producing chlorophyll must be present in a
latent state in certain pangens of the nucleus; it is also
inactive in the smallest particles of the chromatophores,
in the higher plants, as long as the respective members
are in darkness, and becomes active only on exposure to
light.

We shall therefore either have to assume chlorophyll-
pangens in the nucleus, and special chlorophyll-forming
particles in the chromatophores, or identify the two, and
imagine that those hypothetical units are inactive in the
nucleus, and become active only when they pass on to
the chromatophores. The second assumption is obviously

All Protoplasm Composed of Pangens 197

the simpler one ; for the first requires, for every function,
two kinds of units, which multiply by growth and divi-
sion, and which must stand in such mutual relationship
that the units in the chromatophore can function only
in the manner prescribed by the respective pangens in
the nucleus.

Precisely the same argument can also be used for the
other characters of the chromatophores, and for the other
organs of the protoplasts, in a word, for all hereditary
characters.

Let us consider the question from the standpoint of
the theory of descent. In the first, as yet non-nucleated
organisms, we must also, as a matter of course, regard
the individual characters as being connected with pangens.
But here the latter must evidently lie in the protoplasm.
And. as soon as differentiation advanced so far that not all
qualities had to be active at the same time, active and
latent pangens must in these simple protoplasts, have
lain side by side and intermingled. According to age and
external circumstances, at one time some, at another
time other pangens would enter into activity. Here it
would be quite superfluous to assume, for each function,
two kinds of units, on the one hand latent pangens,
merely having charge of heredity, and on the other
hand, particles which might express the latent characters.
The assumption that the same pangens can be either ac-
tive or latent according to circumstances, is evidently
much simpler for these lower organisms.

It can hardly be doubted that protoplasm consists of
most minute particles which are able to multiply independ-
ently. This is indeed the real attribute of life. And it
also seems to me clear that we should regard only these
particles as life-units, and everything else, such as pro-

198 Pangens in the Nucleus and Cytoplasm

tein, glucose, and salts, present only in the water of im-
bibition, as secondary to them. How these particles are
constituted, whether they themselves contain water of
imbibition, or not, and how the visible characters are
conditioned by their structure, we do not know; much
less are we acquainted with their manner of dividing and
multiplying. Apart from these difficulties, which adhere
to any theory, the assumption that these particles are
identical with the bearers of the hereditary traits, is ob-
viously the simplest one that can be made with regard
to the structure of living matter.

From this point of view, the origination of the nucleus
in the phylogenetic differentiation of the lowest organ-
isms, appears to us as an extremely practical division of
labor. Hitherto, the active and the inactive pangens were
lying everywhere in the protoplasm, side by side and
intermingled. And the higher the differentiation that had
been reached, the greater would be the number of diverse
pangens, in the same protoplast; and the greater, also,
would have to be the number of the latent among the
active ones. The latter would thereby be distributed over
a relatively large space, and the efficiency of the whole
must therefore suffer. By the formation of the nucleus
this situation could be changed. In the latter the inactive
pangens would be accumulated and stored; the active
ones could come nearer each other.

Let us further elaborate the picture. As soon as the
moment arrived for certain pangens, which until then
had been inactive, to be set into activity, they would ob-
viously pass from the nucleus into the cytoplasm. But
in so doing they would retain their characters, and es-
pecially their power to grow and multiply. Only a few
like pangens would therefore have to leave the nucleus

Active and Inactive Pang ens 199

every time in order, by further multiplication, to impress
the characters of which they are the bearers, on a given
part of the cytoplasm. This process would repeat itself
at every change of function of a protoplast; every time
new pangens would leave the nucleus in order to become
active. In this way the whole cytoplasm would soon
consist of pangens drawn from the nucleus, and of their
descendants.

§ j. Active and Inactive Pangens

Darwin has already emphasized the fact that the
transmission of a character and its development, even
though they frequently occur conjointly, are yet distinct
powers.2 This point, derived from the phenomena of
atavism, has attained great significance in cell-theory
through the discovery of the function of the cell-nucleus.
i The function of the nucleus is transmission, that of the
cytoplasm, development.

Former theories assumed a complete contrast be-
tween nucleus and cytoplasm, imagining hereditary char-
acters to be limited to the former, and seeing in the rest
of the protoplasm only a passive substratum, by means
of which the nuclei do their work. Thus the nucleus
became the essential part of the cell ; not only did it dom-
inate, but also completely determine the functions. But
the experiments of Nussbaum, Gruber, Klebs, and others
have taught that non-nucleated fractional parts of lower
organisms are also able to exercise certain functions.
Especially do they seem to possess the power of contin-
uing later those functions in which they were already
engaged before being detached. Hence, the influence

2Darwin, The Variation of Animals and Plants. 2: 381. New
York. 1900.

200 Pang ens in the Nucleus and Cytoplasm

of the nucleus, for such functions at least, need not be
continuous; if the functions have once been exercised
they can continue later without the cooperation of the
nucleus.*

The simplest explanation of this lies obviously in our
assumption that nucleus and cytoplasm are both built up
from the same pangens, with this difference, only, that
in the nucleus every kind of pangen of the given species
is represented, while in the remainder of the protoplasm
of each cell essentially only those are present which shall
attain their power of activity in it. In the nucleus most
of them are inactive, that is, they only multiply. Nat-
urally there must be also some active pangens in the nu-
cleus, as, for example, those that carry out the intricate
process of nuclear division ; but this does not affect the
main point. In the organs of the protoplast the pangens
can continue their multiplication, and, to all appearances,
they probably always begin here with a relatively great
increase in number. With that they can here remain
active or inactive for a shorter or longer period ; or they
may be active and inactive by turns. Some become active
at their arrival, others later, some independently from
external conditions, others again only as a reaction to
definite stimuli that start their activity.

The most remarkable processes that take place in the
interior of the nucleus during nuclear division are quite
in harmony with the assumption of pangens. Most in-
vestigators regard the chromatic thread as the morpho-

*Godlewski's experiment, in which non-nucleated portions of sea-
urchin's eggs were fertilized by the spermatozoa of a crinoid, is now
well known. The resulting larvae manifested only maternal charac-
ters. In the fifth edition of his "Allgemcine Physiologic," Jena, 1909,
Verworn cites this experiment as establishing beyond doubt the fact
that hereditary substance is not entirely confined to the nucleus. Tr.

The Transportation of Pangens 201

logical place where the material bearers of the hereditary
qualities are stored.* This thread would, therefore, con-
sist of pangens united into smaller and larger groups,
and it shows, in its thickest portions a distinct structure
of special particles strung together. We can entirely
agree with the opinion of Roux, where he sees, in the
longitudinal splitting of the nuclear skein, the visible
part of the separation of the maternal factors into the two
halves destined for the two daughter cells.3 This concep-
tion is in most complete harmony with pangenesis.

§ 4. The Transportation of Pangens

Our hypothesis that all protoplasm consists of pan-
gens, led us to the conclusion that all kinds of pangens
are represented in the nucleus. Here, most of them are
inactive, while in the remainder of the protoplasm, they
can become active. From this it follows that, from time
to time, pangens are transported from the nucleus to the
other organs of the protoplast.

I am quite aware that, with most readers, this de-
duction will prove the chief difficulty against my view.
The pangens are invisible, therefore their transportation
eludes observation. It is true that the experiments of
Nussbaum, Gruber, and Klebs, discussed in the preceding
Sections, prove that, on cutting off the opportunity of
transportation, the functions of the protoplast are very
greatly restricted, but there is here a possibility of many
other influences being at work. Therefore I should here
like to emphasize the fact that, by rejecting my hypothe-

*Cf. the Translator's Preface, p. viii.

3Roux. Ueber die Bedeutung der Kerntheilnngsfiguren. Leipzig.
1883.

202 Pangens in the Nucleus and Cytoplasm

sis, one does not arrive at a satisfactory view of the re-
lation between nucleus and cytoplasm.

If my hypothesis is rejected and the prevailing con-
ception concerning the contrast between nucleus and cyto-
plasm is followed, we can imagine the effect of the
nucleus to be either dynamic or enzymatic.

Strasburger represents the first view. According to
him, the reciprocal action between the nucleus and the
cytoplasm is a dynamic one, meaning that it takes place
without transmission of substance.5 For this investigator
has never been able to discover, in his extensive studies,
a transmission of visible particles. "From the nucleus,
molecular excitations are transmitted to the surrounding
cytoplasm which dominate, on the one hand, the processes
of metabolism in the cell, and on the other hand, give a
definite character, peculiar to the species, to the growth
of the cytoplasm, which depends on nutrition." As long
as it is a question of general insight only, this assumption
is sufficient, but as soon as attention is directed to indi-
vidual processes, we meet with insurmountable difficulties.
Morphological phenomena are indeed far from having
been sufficiently analyzed to allow a true understanding,
but in the meantime we can turn to the much simpler
chemical processes.

Let us select an example. It is an hereditary charac-
ter of by far the greatest number of plants to produce
malic acid for the purpose of preserving their turgor, and
to store it in their cell-sap, most frequently in connection
with inorganic bases. We cannot imagine the secretion

5Strasburger, E. Neue Untersuchungen uber den Befruchtungs-
vorgang bei den Phanerogamen, p. 111. 1884. See also Weismann,
A., Die Kontinuitdt des Keimplasmas als Grundlage einer Theorie
der Vererbung, p. 28. 1885. Cf. Translator's Preface, p. viii.

The Transportation of Pangens 203

of this acid otherwise, than by means of definite particles,
which have this power, owing to their molecular consti-
tution, and which might best be likened to enzymes.

There is no difficulty in assuming that these particles
become active only when they are made so by molecular
excitations from the nucleus, and I do not doubt that such
co-relations frequently occur. But the difficulty lies in
the question as to whence the cytoplasm gets these par-
ticles. Because, obviously, the power of forming malic
acid cannot be communicated by those excitations to any
kind of substratum. Such excitations can only set free
a function, and only that can be set free which is already
present potentially. Whence then originate the malic acid
formers of the cytoplasm?

This question is not answered by the dynamic theory.
But, as previously stated, hybrids teach us that similar pa-
ternal characters can be inherited from the father, and
therefore be transmitted in a latent state in the sperm-nu-
cleus. Hence the producers of the malic acid must, them-
selves, be derived from the nuclei. They are simply the
active states of the malic acid pangens that are inactive in
the nucleus. And the same must evidently hold, in a
similar manner, of all the other hereditary factors.

In this way, we arrive at the assumption previously
made, that the pangens of the cytoplasm originate from
the nuclei.

Haberlandt has pointed out the possibility of an en-
zymatic influence of the nucleus on the cytoplasm. The
significance of peculiar positions of the nucleus, observed
by this investigator, in the vicinity of the place of most
vigorous cell-activity, remains, according to him, the same,
"if that influence should be not a dynamic, but a material
one, and if, consequently, a diffusion of certain chemical

204 Pangcns in the Nucleus and Cytoplasm

compounds, secreted by the nucleus, should take place
through the plasm to the place of growth. The effective-
ness of these substances would doubtless be dependent on
the degree of cencentration of their solution, and this in
such a way that the cytoplasm would react to them only
at a certain concentration."6

But in order to react in a definite manner on the sub-
stance secreted by the nucleus, the cytoplasm must already
possess the requisite characters. Starch will react to a
secretion of diastase, but not all kinds of substratum will
do so. Thus the assumption of enzymatic effects demands
the presence, in the cytoplasm, of hereditary characters,
which have been taken from the nucleus.

Therefore, no matter how strange the assumption of
a transmission of pangens from the nucleus to the cyto-
plasm may appear at first glance, we arrive by the most
various ways of reasoning at a recognition of its correct-
ness.

An important question is that of the time when this
transportation chiefly occurs. A comparative considera-
tion of the various forms of variability will in the end,
it is hoped, furnish the necessary material for its answer ;
in the mean time we may assume it as probable that im-
mediately after fertilization, as well as during or after
every cell-division, such a transportation takes place. Hy-
brids, and those variations that affect in a similar man-
ner all the members of a plant, argue in favor of the first
point, and for the other, the previously discussed phenom-
ena of dichogeny, where during the earliest youth of an
organ its later nature can be determined by external in-
fluences. When, for instance, the terminal bud of a
rhizome grows prematurely and turns into an upward

6Haberlandt, G. Ueber die Beziehungen zwischen Function und
Lage des Zellkcrncs, p. 14, note. 1887.

The Transportation of Pangens 205

shoot, or the primordium of a transformed leaf becomes
a normal leaf, we may assume that other pangens have
been given up by the nucleus, than would have been the
case without artificial interference. Therefore, in that
youthful state, the normal delivery cannot yet have come
to an end. When grown cells are stimulated to form
callus or wound-cork or, as in Begonia, to produce de
novo entire plantlets, it is to be supposed that the pangens
that thereby become active must first be aroused from
their latent state.

The transportation of pangens, and their conveyance
to the proper places, demands quite special arrangements,
the existence of which many a reader will hardly venture
to suspect. But who would have dared, ten years ago, to
assume the remarkably complicated structure of the nu-
cleus ? We must be as sparing as possible with our hypoth-
eses, but on the other hand we must not be blind to the
fact that since Mohl's time, the investigation of the
structure of the protoplast has disclosed more and more
differentiations, and that, most likely, we are still far
from the end.

To my mind the currents in the protoplasm form one
arrangement for the purpose of this transmission. Every-
body knows how they take place in youthful cells at paths
that radiate from the nucleus, and more recent investiga-
tions have taught how they frequently connect the places
of greatest activity directly with the nucleus.

A few years ago the conviction that these little cur-
rents are a quite common peculiarity of plant-cells, was
far from being prevalent. The phenomenon was imagined
to be limited to a number of instances. Hanstein has
already pointed out how little this view was justified,7 and
Velten has proven the presence of currents in all plants

7Hanstein, Das Protoplasma, p. 155. 1880.

206 Pangens in the Nucleus and Cytoplasm

examined with this point in view.8 In the Botanische
Zeitung for 1885, I have furnished proof that mechanical
contrivances are not sufficient for the transmission of the
assimilated nutrient matter in plants, and that, of the
processes known up to date, it can only be accomplished
by the currents of the protoplasm.9

In this connection I have carefully verified Velten's
statement, and have confirmed the quite common exist-
ence of currents in vigorously living plants.10

The mechanical possibility of a transmission of pan-
gens is, therefore, sufficiently assured for all plant-cells.
Only one difficulty has yet to be overcome. Following
the precedence of Hofmeister, it was generally assumed
that the currents in the cells begin only at the end of the
meristematic period, and that, until that time, the granu-
lar plasm is in a state of rest. Now the meristematic
period is not only that in which the cells originate, but
also that in which their later character is chiefly deter-
mined. Hence it is in this very period that we must place
the most important part of the transportation of the
pangens.

But Hofmeisters statement was based on insufficient
observations. A subsequent investigation by Went, with
the more modern methods, led to a quite different result.11
The movements are indeed slow, and one examination
will often not disclose them. But if the observation of

8Velten, W. Ueber die Verbreitung der Protoplasmabewegungen
im Pflanzenreiche. Bot. Zeit. 30: 645. 1872.

9Vries, H. de. Ueber die Bedeutung der Circulation und der
Rotation des Protoplasma fur den Stofftransport in der Pflanze. Bo.
Zeit, 43: 1. 1885.

10Over het algemeen voorkomen van circulatie en rotatie in
de weepelcellen der planten, Maandbl. v. Natuurw. No. 6. 1884.
Cf. ibid. No. 4, 1886, and Bot. Zeit. 43: 1, 17. 1885.

lxWent, F. A. F. C. Die Vermehrung der Normalen Vacuolen
durch Theilung. Jahrb. Wiss. Bot. 19: 329. 1888.

Comparison with Darwin's Hypothesis 207

the same object is continued for hours under favorable
life-conditions, there will be noticed all kinds of displace-
ments, which put the presence of slow currents beyond
a doubt.

From this side, therefore, no difficulty stands in the
way of the assumption that the transmission of the pan-
gens in plant-cells is accomplished by the currents of the
granular plasm. In the domain of animal physiology we
are far from possessing the necessary knowledge of the
currents of the protoplasm. But then the difficulties of
investigating are here considerably greater than in the
plant-world.

§ 5. Comparison with Darwin's Transportation-
Hypothesis

Possibly to some readers there will appear to be a
great similarity between the assumption of a transmission
of pangens from the nucleus to the other organs of the
protoplast, as described in the previous paragraphs, and
Darwin's hypothesis of the transportation of gemmules.
However, this agreement is only apparent and not real.
The two hypotheses are fundamentally different through-
out.

Darwin assumed a transportation of gemmules
through the entire body ; my view requires only a move-
ment within the narrow limits of an individual cell. But
this is not the chief difference. In the gemmule-theory,
the particles that are separated from a cell or a member
can again enter new cells, especially the germ-cells, and
thus endow them with new hereditary factors. Not only
can the latter then reach their development in the given
germ-cell, but they can also be transmitted to all its de-

208 Pangens in the Nucleus and Cytoplasm

scendents. To this end, however, they must, according
to the present state of cell-anatomy and of the study of
fertilization, be received into the nuclei. The hypothesis
of intracellular pangenesis obviously does not make such
an assumption; the pangens that have once left the nu-
cleus do not have to return to it, neither into the nucleus
of the same cell, nor into that of any other.

It is true that, with our present anatomical knowledge,
the possibility of a transmission of pangens from one cell
to another cannot be denied. The researches of Tangl,
Russow, and many other investigators on the direct con-
nections of the protoplasts of neighboring cells by means
of the delicate pore canals of the pits, even indicate the
path on which such a passage might eventually take place.
In the latex vessels the currents of protoplasm are un-
doubtedly not limited to the individual constituent cells,
the current continuing without regard to the former cell-
limits. This is especially the case with the mass-move-
ment after injuries, and probably also with the proper
movements of the granular plasm in the normal state. If
we assume that all living protoplasm consists of pangens,
their passage from one cell to another cannot be denied
here. But this phenomenon is obviously of no importance
for the theory of heredity. Similar considerations could
be made for other cases of cell-fusions, or symplasts.

The mode of origin of the secondary pores of the
Florideae, discovered by Kolderup-Rosenvinge,12 is also
worthy of note. The cortical cells, e. g., of Polysiphonia,
divide in the usual manner with preceding nuclear di-
vision. But one part contains almost the entire proto-
plast and the other but a small corner at its base. The

12Kolderup-Rosenvinge, L. Sur la formation des pores second--
aires chez les Polysiphonia. Botanisk Tidsskrift. 17: 10. 1888.

Comparison with Darwin's Hypothesis 209

wall arising between the two halves forms a primary pit.
At that place the wall between the separated corner and
the underlying cell is dissolved, and contact being thus
established between the two protoplasts, they fuse. The
old poreless cross-wall is thus replaced by a new one that
contains a pore. But the interesting point for our pur-
pose is the circumstance that the underlying cell has now
received a nucleus from its upper neighbor. It has two
nuclei, and later it becomes multi-nuclear by nuclear
divisions. For all those who regard the nucleus as the
bearer of the herditary endowment, a transmission of the
latter here takes place from one cell to another. But
obviously again without any significance for the theory
of heredity.

The possibility of a transmission of material bearers
of hereditary characters from one cell to another can
therefore not be denied. Further investigations will,
without doubt, bring to light other facts that can be util-
ized for the same purpose. And that here and there, in
plants, processes take place in a similar way, which stand
in direct relation to heredity can, of course, not be denied
a priori.

But it is quite another question whether such a trans-
mission occurs commonly, and plays an important role
in the transmission of hereditary characters in the whole
plant and animal world.

Anatomical facts alone are not sufficient to answer this
question. From them, only the possibility of a transmis-
sion can be deduced or, more correctly speaking, the con-
clusion that our present knowledge does not furnish any
reasons which would make that transmission appear im-
possible. It may be that such a thing will be discovered
later. But it is not likely that anybody will think it is

210 Pang ens in the Nucleus and Cytoplasm

therefore permissible to infer the actual occurrence of a
general intercellular transmission of the bearers of hered-
itary properties.

Hence, the answer to the question must be looked for
in a quite different field. The theory of heredity must
tell us whether there are facts for the explanation of
which the assumption of an intercellular transmission is
indispensable.

To my mind, this is not the case, as I have already
stated in the Introduction. I have there referred to Weis-
mann's writings, which contain copious demonstrations
that all observations which so far seemed to demand such
an assumption, could in reality have been explained as
well, and in most cases better, without them.

Especially should the so-called heredity of acquired
characters be mentioned here. I have previously, in an-
other place, drawn attention to the fact that in many cases
we have here to deal with malformations.13 If we limit
the meaning of that expression to the variations which
have arisen on the somatic tracks, and ask whether these
can be transmitted to the germ-tracks of the organism,
then the question has a clear meaning. In that case we
can join Weismann in quietly answering, no. But, if we
also call such characters as may have originated on the
germ-tracks acquired, the question is no longer of any
significance for the problem which occupies us here.14

In botany graft-hybrids and xenia are mentioned as

13"Over steriele Mais-planten," Jaarboek v. h. Vlaamsch kruidk.
Genootschap, Bd. 1. Gent. 1889.

14The conception of germ-tracks and somatic tracks in the sense
developed in the first Section of this second Part may contribute
much, in this connection, to help the mutual understanding. See also
e. g., in regard to Eimer's discussions, his work : Die Entstehung der
Art en auf Grund von Vererben erworbener Eigenschaften. Theil 1.

Xenia 211

arguments for an intercellular transmission of hereditary
qualities. But both groups of phenomena are much in
need of being critically investigated before they can be
reliably employed in this way. The transmission of the
hereditary characters of the crown-graft to its stock15 has,
to my mind, never been scientifically proven, and never
will be, as long as new experiments are not made, in
which the variations of the stock itself, are thoroughly
studied and have become well known. Because, until
then, the possibility is not excluded that this variability
of the stock itself forms the most important factor in the
phenomena that have been observed.

The cases where the pollen is supposed to have trans-
mitted hereditary characters outside the fertilized egg-
cell and the embryo issuing from it, to the tissues of the
maternal fruit, have been carefully arranged by Focke
under the name xenia.16 And his review shows
plainly that here one has to deal with exceptional cases
which have never yet been thoroughly studied and suffi-
ciently controlled.17 And I think that, without a control,
based on critical examination, these data cannot be given
that far-reaching significance that would make them the

15Cf. the critical summary of the material for observation bear-
ing on this point, by H. Lindemuth, Uber Vegetative Bastarderzeug-
ung durch Impfung. Landw. Jahrb. 7: 887. 1878.

16Focke, Die Pflanzenmischlinge, pp. 510-518. 1881. [See also,
Webber, H. J. Xenia, or the immediate effect of pollen on Maize.
U. S. Dept. Agr. Div. Veg. Physiol Pathol Bull. 22. Sept. 12, 1900;
Correns, C. Untersuchungen iiber die Xenien bei Zea Mays. Ber.
Deut. Bot. Ges. 17: 410. 1899. TV.]

17The best known instance of Xenia, that of corn, has since been
shown to be of a different nature, consisting in the hybridization of
the endosperm in the process of double fertilization. See de Vries,
Sur la fecondation hybride de 1'albumen. Compt. Rendus Acad. Sci.,
Paris, 129: 973. 1899, and Sur la fecondation hybride de 1' endo-
sperme chez le Mais. Revue generate de Botanique. 11: 129. 1900.

212 Pang ens in the Nucleus and Cytoplasm

bases for an assumption of an actual intercellular trans-
mission of hereditary qualities.

The facts of heredity so far known, do not, to my
mind, make the assumption of an intercellular transmis-
sion of pahgens necessary. When the pangens have once
left the nucleus they do not need the power of penetrating
back into that nor into any other nucleus. The pedigree of
the pangens lies in the nuclei, and its protoplasmic side-
branchings all end blindly, although often only after many
cell-divisions.

I believe that the passage of the pangens from the
nuclei is a necessary conclusion of our present knowledge
concerning the physiological significance of the nuclei.
I need not assume a penetration of the extruded pangens
or their descendents into other nuclei. And this hypothe-
sis would be inevitable if one were to connect Darwin's
transportation of gemmules with the results of more re-
cent cell-study. In this case one would have to resort to
a new ancillary hypothesis in order to explain facts,
which, according to the discussions mentioned above, do
not at all require such an explanation.

Let us summarize the difference between the two
transmission hypotheses. The pangens of the intracellu-
lar pangenesis, having once left the nucleus, need never
re-enter it. For the gemmules of Darwin's transporta-
tion hypothesis, however, this power is the essential con-
dition, because without it, the hereditary properties of
which they are the bearers, can never develop into visible
characters in the descendants of the respective germ-cells.

§ 6. The Multiplication of Pangens

The hypothesis, that the entire living substance of a
cell is built up of pangens, naturally implies that in every

The Multiplication of Pang ens 213

protoplast every kind of pangen must be represented in
great numbers. In addition, the relative number of the
bearers of the individual hereditary characters is of very
great importance. In the cytoplasm it determines the
function of the individual organs, in the nucleus the power
of inheritance. If a new character in the nucleus is rep-
resented by only a few like pangens, the likelihood of this
character becoming visible, is evidently very small. But
the greater the number of those pangens, in comparison
with the others, the more prominent will the character
appear. From seeds of a twisted specimen of Dipsacus
sylvestris I have grown over 1 ,600 plants, of which only
two showed torsion of the stem. The pangens which
caused this torsion must, therefore, have been in such
relatively small numbers that their chance of becoming
active amounted to 1 per 1,000 at the most. In other
young varieties this proportion is more favorable, and,
by making the right selection, that chance increases quite
considerably in the course of a few generations. The
simplest explanation for this is obviously, that by breed-
ing those specimens in which the characteristic is repre-
sented by the greatest number of like pangens, the relative
number of these is gradually increased.

I have repeatedly emphasized the fact that, according
to my hypothesis, the pangens can multiply in the nu-
cleus as well as in the cytoplasm. This multiplication is
of the same order as that of the cells and of the organ-
isms themselves. When a large tree bears, every year,
thousands of seeds, the pangens of the egg-cell from
which the tree has grown, must have multiplied in an in-
credible manner. And the same thing is taught by the
enormous number of eggs that a single tape-worm can
produce. In the face of such phenomena the multiplica-

214 Two Kinds of Variation

tion of the pangens in the cytoplasm of an individual cell
is only minimal.

The giving off of the pangens by the nucleus must, as
a matter of course, always be done in such a way that all
kinds of pangens remain represented in the nucleus. Al-
ways only a relatively small number of like pangens must
leave the nucleus. The division of the nuclei, however,
must take place in such a way that all the different kinds
of pangens are evenly distributed over the two daughter-
cells. Only in certain somatarchic cell-divisions18 is there
a deviation from this regularity.

The two kinds of variability which Darwin distin-
guishes on the ground of pangenesis, are naturally also to
be deduced from the description here given.19 Fluctuating
variability is simply based on the varying numerical rela-
tion of the individual kinds of pangens, which relation
can indeed be changed by their multiplication and under
the influence of external circumstances, but most quickly
by breeding selection. The "species- forming" variabil-
ity,20 that process by which the differentiation of living
forms has come about, in its main lines, must essentially
be reduced to the fact that the pangens, in their division,
produce, as a rule, two new pangens that are like the
original one, but that exceptionally these two new pangens
may be dissimilar. Both forms will then multiply, and
the new one will tend to exercise its influence on the visi-
ble characters of the organism.

In harmony with this is the idea that we must imagine
the higher organisms to be composed of a greater number
of unlike pangens than the lower ones.

18Cf. pp. 102 and 107.

19Cf. p. 74.

20Now commonly called mutability (de V. 1909).

CHAPTER II
SUMMARY

§ 7. Summary of the Hypothesis of Intracellular

Pangenesis

The view of Darwin (apart from the hypothesis of
the transportation of gemmules through the entire body),
that the individual hereditary qualities are dependent on
individual material bearers in the living substance of
cells, I call pangenesis. These bearers I call pangens.
Every hereditary character, no matter in how many spe-
cies it may be found, has its special kind of pangen. In
every organism many such kinds of pangens are assem-
bled, and, the higher the differentiation that has been
reached, the more there are.

The hypothesis that all living protoplasm is built
up of pangens, I call intracellular pangenesis. In the
nucleus every kind of pangen of the given individual
is represented; the remaining protoplasm in every cell
contains chiefly only those that are to become active in it.
This hypothesis leads to the following conclusions. With
the exception of those kinds of pangens that become di-
rectly active in the nucleus, as for example those that
dominate nuclear division, all the others have to leave the
nucleus in order to become active. But most of the pan-
gens of every sort remain in the nuclei, where they multi-
ply, partly for the purpose of nuclear division, partly in
order to pass on to the protoplasm. This delivery always
involves only the kinds of pangens that have to begin to

216 Summary of Intracellular Pangenesis

function. During this passage they can be transported
by the currents of the protoplasm and carried into the
various organs of the protoplasts. Here they unite with
the pan gens that are already present, multiply, and
begin their activity. All protoplasm consists of such
pangens, derived at different times from the nucleus, to-
gether with their descendants. There is in it no other
living basis.

The elaboration of this hypothesis, given in the pre-
ceeding chapters, is only an outline, the purpose of which
was to make the main idea comprehensible. It is, for the
present, the simplest form in which pangenesis can accom-
modate itself to our present knowledge of the structure
of the cell. In details I am well aware of not having been
able always to find the right explanation. But the only
object I had in mind was to demonstrate how easily the
greatly misjudged pangenesis covers all the facts discov-
ered since its establishment !

FERTILIZATION AND HYBRIDIZATION
A Paper

read at the 151st annual meeting to the Dutch Society of
Science in Haarlem, May 16, 1903

The essay on "Fertilization and Hybridization" was read in
Haarlem in the Dutch language, and appears here in an enlarged
form. My conception of the life-processes in the nuclei is chiefly
based on the renowned investigations of van Beneden and of Boveri,
as well as the most recent researches by Conklin (Contr. Zool. Lab.
Pennsylvania, XII, 192), Sutton (Biol. Bull. IV, Dec., 1902), Eisen,
(Jour. Morphol XVII, 1), Errera (Revue Scientif. Feb., 1903), and
of many others. For the literature I refer to E. B. Wilson, The
Cell in Development and Inheritance, and V. Hacker, Praxis und
Theorie der Zellen-und Befruchtungslehre.

My presentation of the processes of fertilization and hybridiza-
tion is an outcome of the experiments which I have described in the
second volume of my Mutationstheorie (Leipsic, Veit & Co., 1901-
1903. English translation by Open Court Publishing Co., 1909-1910.)

H de V.

FERTILIZATION AND HYBRIDIZATION

"Vom Vater hab' ich die Statur,
Des Lebens ernstes Ftihrcn,
Vom Miitterchen die Frohnatur
Und Lust zu fabuliren."1

In these lines lies the whole problem of heredity and
fertilization. What everybody can see, Goethe has voiced
clearly and concisely in beautiful, simple words. We have
one part from the father, the other from the mother. Or,
as it is now usually put, the hereditary characters of the
two parents are combined in the offspring.

It became the problem of scientific investigation to
seek out the cause of this phenomenon. It could not be
limited to man. The law mentioned by Goethe1 must be
general, it must be true of the entire plant and animal
world, wherever two beings unite for the production of
progeny. Furthermore it cannot concern ordinary fertil-
izations only, but also those abnormal cases in which unlike
individuals, belonging to different varieties or species,
fertilize each other. The products of such crosses we
call hybrids, and for science they possess the great im-
portance that, in them, the manner in which the charac-
tertistics of the parents are combined can be studied more
easily and clearly than in the children of a normal union.
For the more the parents differ from each other, with
the greater certainty must it be possible to determine the
share of each in the characteristics of the offspring.

1 Goethe, "Spriiche in Reimen," Gesammelte Werke, III, 83, 1871.

220 fertilization and Hybridization

Everywhere this law is confirmed, that the child in-
herits one part of its nature from the father, the other
from the mother. The child is, therefore, on the whole,
a double being, with twofold qualities, more or less dis-
tinctly separated, that may still be traced back to their ori-
gin. This principle of duality, as we might call it, domi-
nates the entire theory of heredity ; it forms the thread that
binds together apparently separated cases; it serves as a
guidance for the whole investigation.

This investigation occupies two different fields. On
the one hand we have experimental research, on the other
hand microscopical. Physiology ascertains the relations
of the offspring to their parents ; it analyzes their charac-
teristics into their individual units, and tries to demon-
strate their origin. The history of development discloses
to us the corresponding microscopic processes; it looks
for the smallest visible bearers of heredity in the cell, and
investigates how they are maintained during life, and how,
during fertilization, they pass on from father and mother
to the offspring.

Few investigators master both provinces ; their extent
is much too great for that. And especially has the study
of hybrids so greatly advanced in recent years, that even
here a division of labor will soon be necessary. Both lines
of work have therefore developed more or less indepen-
dently of each other. In both, the main features of the
problem begin gradually to arise out of the abundance of
individual phenomena. And thereby there is disclosed,
one might almost say, beyond all expectation, an agree-
ment in the results of both lines of investigation, which
is so great, that almost everywhere the physiological pro-
cesses are reflected in the microscopically visible changes.

It is true that the final analysis lies yet beyond the

The Double Nature of Organisms 221

limits of our present microscopical vision. Compared
with the enormous complexity of the herditary characters
of the organisms the anatomical structure of the cells and
their nuclei, as it is known to us, is much too simple. The
individual traits of father and mother can not yet be found
in the cells of the offspring, but the investigations of most
recent times indicate clearly that here also the limits of
knowledge are being constantly extended.

The double nature of all beings that have sprung into
existence through fertilization, is seen in their external
appearance, as well as in the finest structure of their nu-
clei. The principle of duality obtains everywhere, even if,
in individual cases, the demonstration of it is yet in its
beginnings. But as far as the visible marks can be an-
alyzed and the individual component parts of the nuclei
can be traced, so far can the validity of the principle be
proven even at present.

Let us consider first the external part, then the inter-
nal.

Goethe derived his stature from his father, and not
from his mother, and it was not a stature between the
two. The sum total of his qualities he had partly from
his father, partly from his mother. The illustration ex-
plains the rule in a clear manner. In the offspring the
characters of the parents are combined. Not always does
the child get an even half from each; on the contrary, as
everybody knows, it resembles the mother more in some
respects, and the father more in others.

It is exactly the same with hybrids. With them a
single character is generally derived either from the father
or from the mother. The hybrids of white and blue flow-
ers usually bloom blue, those of a hairy or a thorny
parent crossed by one without hairs or thorns are usually

222 Fertilisation and Hybridisation

hairy or thorny. The crossing of a common evening-
primrose with a large-flowered species results in a flower
of the size of the former. But, if there are two or more
points of difference they may be transmitted to the chil-
dren partly by the one parent and partly by the other, and
it is thereby possible in practice to combine the good char-
acters of two varieties into a single race. Thus has Rim-
pau created a series of hybrid-races of wheat, and Lemoine
has produced his large-blooming sword-lilies, able to with-
stand the winter, and thus have originated, in agriculture
and horticulture, the countless hybrids, in which the fa-
vorable characteristics of various varieties are combined
with more or less diversity. Combined, or as we usually
say, mixed ; though this is an expression which makes us
only too easily lose sight of the independence of the in-
dividual factors in the mixture.

This independence is frequently difficult to demon-
strate in the mixtures, that is, in the characteristics of the
hybrids. Our means of differentiation only too frequently
prove insufficient. In the clear cases, however, it appears
very distinctly, and the greater the number of hybrids that
are studied accurately and thoroughly, the more generally
is the validity of the principle established.

If, for example, we find combined in a wheat-hybrid,
the loose ear of the mother-plant, with the lack of awns
in the father, the share of each appears simple and clear.
In the mixture of the characteristics these two are so far
apart, that they are always easily recognized. How are
such characters united in the hybrid ? Are they fused into
one whole, or do they simply lie loosely side by side ?

The splittings, which occur regularly in many hybrids,
when propagated by seed, and also, in the case of a few, in
vegetative propagation, give us an answer to this question.

Cytisus Adami 223

Of the last kind the Cytisus Adami serves as the most
beautiful and striking instance. It is a hybrid between
C. Laburnum and C. purpureus. Unfortunately its great
significance for the main features of the whole problem
has been underrated for a long time owing to the fable
of its having originated as a graft. As a matter of fact,
no hybrids are obtained by grafting, no matter how great
the mutual influence of the wild stock and the crown
graft. As far as historical evidence goes, the Cytisus
Adami has always been propagated by grafts since its first
appearance, but it did not originally spring into existence
in this way.2

This tree teaches us how the qualities of the two pa-
rents are combined. Ordinarily they occur mixed, the
leaves as well as the flowers having some features of the
Laburnum and others of the purpureus. The totality of
the characters lies, therefore midway between the two pa-
rents. But splittings do occur, and not at all rarely, or
rather so commonly, that indeed every specimen of the
hybrid, if not too small, will show them. In these split-
tings the types of father and mother separate sharply and
completely. Some twigs will grow that are purely La-
burnum, while others are only purpureus. The former
are vigorous and long-lived, the latter remain weak and
often die after a few years, which is the reason for their
being seen less frequently. But even in this point they
resemble exactly the respective parents.

Within the hybrid, the bearers of the parental charac-
ters are therefore arranged in such a manner that, so to
speak, they can be completely separated, at any moment,

2Strasburger (Jahrb. Wiss. Bot. 42: 69-70. 1905.) finds entire
absence of any cytological evidence that C. Adami originated as a
graft-hybrid. Tr.

224 Fertilisation and Hybridisation

by a simple cut. And, if not by a simple cut, then at least
by a physiological splitting, which passes exactly between
the two parental groups and does not leave in one of them
any trace of the other.

In this manner we have to picture to ourselves, in a
general way, the internal, invisible structure of the hy-
brids. The bearers of the characters of both parents are
intimately connected, and together dominate the visible
characteristics. But they are not, by any means, fused
into a new indivisible entity. They form twins, but re-
main separable for life.

In all nature there is probably not another such beauti-
ful instance of splitting as the above-mentioned Cytisus.
But with lesser differences between the parents, splittings
of the parental types occur frequently in the vegetative life
of hybrids. Many horticultural plants, and especially the
bulbous plants, furnish instances thereof; peas, corn,
wood-sorrel, anagallis, oranges, and several others are
known instances. The fruits that are half lemon and half
orange, belong doubtless to this group. Among the hy-
brids of the common and the thornless thornapple (Datura
Stramonium'), individuals have been found, although very
rarely, that showed a similar splitting, and which even
bore on the same fruit armed, as well as thornless cells.
In my garden, I cultivated, for many years, a Veronica
longifolia which was a hybrid from the blue species and
the white variety, and correspondingly had blue flowers.
But from time to time splittings occurred ; either one single
spike bloomed white, or a few isolated white flowers ap-
peared on an otherwise blue spike.

During the entire life, up to the time of the formation
of the reproductive cells this internal dualism manifests
itself in this way. Sometimes proofs of it are even found

The Double Nature of Organisms 225

in the anatomical structure of the tissues, and of the indi-
vidual cells, where the parental characters are set free and
a mosaic-like structure results.

MacFarlane, who has made the most thorough study
of the anatomical structure of hybrids, recognizes every-
where the principle of duality, and goes so far as to regard
every individual vegetative cell of a hybrid as a herma-
phrodite formation. And the renowned French investi-
gator of hybrids, Naudin, also expressed himself about
forty years ago in a similar manner. "U hybrid* est une
mosaique vivante" said he; we do not recognize the in-
dividual parts as long as they remain intimately blended,
but occasionally they separate and then we are able to
distinguish them.

We therefore regard it as established that, in the chil-
dren, the inheritances from the fathers and mothers are
indeed combined, but not fused into a new entity. Acting
always conjointly under ordinary circumstances, they yet
do not lose the power of separating occasionally.

But now arises the question as to what is anatomically
visible of this union. Can the dualistic formation be ob-
served within the cell ? Do the parental inheritances, here
too, lie side by side as twins ?

The hereditary characters are contained in the nuclei,
as was first declared by Haeckel, and later demonstrated
by O. Hertwig, and, for plants, by Strasburger. This im-
portant law forms, for the present, the basis of the whole
anatomical theory of heredity, and is recognized as such
by all investigators. We may, therefore, expect to find in
the nuclei, as well, the dualism of the parental qualities.

Every cell, as a rule, possesses a nucleus. This nucleus
dominates the life-activity, and although the current func-
tions can run their course without it, no new ones can be

226 Fertilization and Hybridization

introduced. In certain filamentous algae (Spirogyra) Ge-
rassimow succeeded in producing cells without nuclei;
they retained life for several weeks, feeding vigorously,
but nevertheless they always perished without any repro-
duction. In some tissue-cells the nucleus is constantly in
motion, and according to Haberlandt's investigations, it
stops longest where the work of the cell is most pro-
nounced for the time being, as for instance in unilateral
growth, the formation of hair, local accumulation of
chlorophyll, etc.

This concentration of hereditary characters is most dis-
tinctly seen in the sexual cells. Here the other functions
are reduced to a minimum. The nucleus dominates com-
pletely. In the male sperms the activity of the proto-
plasm is limited to moving around and to seeking the fe-
male cells. The body is made up almost entirely of the
nucleus. In the higher plants the spermatozoids lack even
the organs of free motion; they are carried to the egg-
cell passively, in the pollen-tubes. The egg-cells are us-
ually immovable and heavy in comparison with the male
elements, since they contain the food substance necessary
for the incipient growth of the germ, and for the first
cell-divisions.

Now fertilization consists in the union of two cells,
the male spermatozoid and the female egg-cell. This
union is the means of combining the inheritance of the
two parents, and therefore the nuclei play the main roles.
The nucleus of the egg-cell lies usually in its center; the
male nucleus reaches it by passing straight through the
surrounding plasm. Sometimes one sees quite distinctly
that it no longer needs its own protoplasm since it strips
it off and leaves it at the border of the egg-cell. In the
Cycadaceae, in which the spermatozoa are just large

The Essence of Fertilization 227

enough to be discernible with the naked eye, the cyto-
plasm with all its cilia remains in the outer layers of the
egg-cell, while only the nucleus penetrates more deeply.
The beautiful investigations of Webber and Ikeno have
brought this process to light.

Finally the two nuclei come into contact and unite into
a single body. This is the most important moment of
fertilization, the whole physiological process is concluded
by this union.

Let us ask now what has been achieved by it. Appar-
ently very little, for the two parental nuclei are only
closely appressed to each other. A penetration or fusion
of their substance does not take place. They remain sep-
arate in spite of the union. With fertilization the life of
the new germ begins, and in most cases immediately.
Originally a single cell, the germ soon divides into two
and then into more cells. But this beginning of the vege-
tative life takes place everywhere before the two parental
nuclei have entered into closer union. Only after the
first division does the limit become unrecognizable, the
contact of the constituent parts of the male and female
halves being now so intimate that there is at least the
appearance of a fusion.

It was the Belgian investigator, van Beneden, who dis-
covered this all-controlling fact. He first observed the
independence of the paternal and the maternal nuclei
in the intestinal worm, Ascaris, then elsewhere in the ani-
mal kingdom, and immediately recognized its significance.
Since life could begin without fusion of the two nuclei,
he considered that such a thing was not necessary, and
assumed that all through life the two nuclei preserve their
independence more or less completely.

According to this view the nuclei are double beings,

228 Fertilisation and Hybridisation

and we thus find, in the material bearers of the hereditary
characters, the duality of which Goethe sang in his
"Spriiche in Reimen," and which the splittings of hybrids
put so clearly before our eyes. Van Beneden chose the
name pronuclei for the male and the female nuclei that are
thus united, and speaks of a pronucleus male and a pronu-
cleus femelle. This designation has been retained since
that time, and recommends itself especially for the reason
that the union of the two nuclei is usually simply called the
nucleus of the cell ; and this latter designation will prob-
ably not be changed, although the double nature of the
nucleus is recognized. Therefore the pronuclei are the
entities that concern us ; the nuclei are really double nuclei.

If the border line between the two pronuclei remained
as distinct through life as before the first cleavage and at
the time of it, van Beneden's view would hardly meet with
any difficulty. But this is not so. Gradually the line of
demarcation becomes blurred, and in most cases nothing
more is to be seen of it in later life. But the richness of
forms in nature is fortunately so great that the general
phenomena in different organisms appear to us with an
extremely varied distinctness. And thus it is also here.
In one species the border line of the pronunclei is lost
sooner, in others later. It is only a case of finding the
best illustrations, that is, of selecting a species in which
the paternal and the maternal inheritances remain longest
visibly separate.

The discovery of such instances is the great merit of
Riickert and Hacker. In the one-eyed water-flea of our
fresh waters, the well-known Cyclops vulgaris, and its
nearest allies, they found a group of animals in which the
pronuclei remained distinctly separate for a long time.
Sometimes during several consecutive cell-divisions, some-

Autonomy of the Pronuclei 229

times for a longer period, and, in the best cases, during
almost the entire vegetative life, the double nature of the
nuclei can here be directly seen. What van Beneden con-
cluded from the incipient stages was here irrefutably
proven.

The double nature of the nuclei was also demonstrated
more or less distinctly, and during a shorter or longer se-
ries of cell-divisions, in other cases, by other investiga-
tors. It was observed in Toxopneustes by Fol, in Sire-
don by Kolliker, in Artemia by Brauer, in Myzostoma by
Wheeler, in the Axolotl by Bellonci. These and numerous
other observations now place the law quite beyond doubt.
The independence or autonomy of the pronuclei corre-
sponds everywhere with the mode of union of the visible
parental characters in the offspring.

In the snail-genus Crepidula, Conklin recently discov-
ered a case in which the double nature of the nuclei can
be demonstrated perhaps even more clearly and easily
than in the Cyclops. If the nuclei remain side by side all
through life, the question arises as to how they dominate
together the development of the child, the unfolding of
its characteristics. Here, too, the results of physiology
and of anatomy work beautifully together, and here, too,
Goethe's lines serve as a guide. Certain peculiarities are
inherited from the father, others from the mother. One
individual inherits them in this, another in that mixture.
The inheritance therefore consists of separate qualities,
which may be united in various combinations in the off-
spring. We are taught the very same thing by hybrids,
especially in their progeny, and the rich floral splendor of
our horticultural plants shows us what an endless number
of combination-types have already been achieved with
comparatively few characteristics.

230 Fertilisation and Hybridisation

But we shall not yet leave the subject of the nuclei.
The independence of all the hidden potentialities, which
in the physiological field is most sharply defined in the
theory of pangenesis, we can of course not hope to see
reflected in the nuclei. We must, at least for the present,
be satisfied to find here any independent parts in the nu-
clei.

It was well known to the older investigators, and,
among botanists, especially to Hofmeister, that the nuclei
are not structureless formations, but that they exhibit
more or less distinctly certain internal organs. But only
about a quarter of a century ago by means of better
methods of investigation did Flemming in the zoological
field, and Strasburger in the botanical, succeed in getting
a deeper insight into this structure, and soon afterwards
Roux showed how these achievements are entirely in har-
mony with the requirements of the theory of heredity.
Since then, numerous investigations have confirmed and
extended these results, and especially has Boveri brought
out the main features in the wide range of phenomena.
To him we owe the principle of the independence of the
individual visible component parts of the nuclei, a princi-
ple, which, in spite of much opposition, is more and more
strongly supported, and which has found in the most re-
cent studies of Sutton a brilliant confirmation.

What Boveri's theory offers us is, in the main points,
as follows : All the bearers of hereditary characters lie in
the protoplasm of the nucleus, in the nuclear sap, as it is
usually called, as definite particles, which can be brought
out by various methods as distinctly recognizable parts,
and which are combined into threads. It is true that one
cannot see the individual bearers, because there are too
many of them and they are too small. Even a counting of

B oven's Theory 231

the smallest visible granules succeeds only rarely. In the
nuclei of an American salamander, Batrachoseps, the
members of the nuclear threads are most distinct ; at least
Gustav Eisen succeeded in making an approximate count
of the smallest visible granules. In every pronucleus they
form 12 chief parts, the so-called chromosomes. Every
chromosome showed as a rule a subdivision into six sec-
tions or chromomeres, and every chromomere, in turn,
appears again to be built up of six smallest granules, the
chromioles. All in all there are here then about 400 dis-
tinguishable particles in the individual pronucleus. The
number of hereditary characters must certainly be much
higher than 400 for such an organism; it would more
likely have to be estimated at ten times that value. We
must therefore be satisfied, for the present, with the ob-
servation of groups of units in the nuclei.3

In the end there will surely be found a way of seeing
the individual units also. But the resolving power of our
microscope will finally reach its limit, and we shall prob-
ably never be able to see much smaller granulations than
the smallest elements that are visible now. So far, even
the causes of many contagious diseases, in plants as well
as in animals, are still quite invisible. But the calculations
which Errera has lately made on the limits of the smallness
of organisms still allow us full play. In Micrococcus he
finds a structure composed of about 30,000 protein mole-,
cules, but many nuclei are much larger. It cannot yet be
estimated of how many molecules a whole nuclear thread
is composed, but it may be assumed with certainty that not
every one of its granules has such a complicated structure
that it could hold the factors for all peculiarities of the

3Cf. Translator's Preface, p. viii.

232 Fertilisation and Hybridisation

whole organism. Their smallness would rather lead us to
suppose that every one of them could, at the most, repre-
sent only a small group of such units.

To prove this, on the one hand microscopically, on the
other hand experimentally, is the task that Boveri set for
himself.

The filamentous framework in most nuclei, recogniz-
able by certain staining methods, is now admitted by all
investigators as the idioplasm, the bearer of the hereditary
qualities. This thread is very delicate, and seems to form
a skein. But when the nucleus prepares to divide, the
thread contracts, and thereby is seen, what had hitherto
been invisible, that it is composed of several separate
threads. In the nucleus there are several threads and not
one single one. When the contraction of the thread is ad-
vanced so far that the individual parts have become quite
short and thick, they are called chromosomes. In the
nuclei of the body-cells these always occur in an even
number, one-half belonging to the paternal, the other to
the maternal pronucleus.

In a series of classical investigations Boveri succeeded
in showing that the individual chromosomes, on elongat-
ing again, when the division is accomplished, retain their
independence. They remain the same during their whole
life, elongating and shortening alternately throughout
their entire development. The purpose of the shortening
is to make possible an even division of all parts during
cell-division; the threads then split lengthwise, in such a
way that every single bearer of heredity first doubles, and
then sends the two halves into the daughter-nuclei. This,
of course, could hardly be accomplished in a skein. On
the other hand elongation has for its object the freeing of
the bearers of heredity from that crowded accumulation,

Significance of the Nuclear Skein 233

their task being to control and to direct the life functions
of the cell, and to that end they must be able to enter into
as free a contact as possible with the granular plasm. An
arrangement in rows, at least of ,those bearers that are to
become active, is the necessary condition thereto, and it
is evidently reached by means of the elongation of the
threads and the formation of the skein.

In order to make possible an orderly retreat of the
individual threads out of the tangle of the skein, every
thread is firmly attached by one end to the nuclear wall.
It retreats to this point, which is at the same time the point
at which its two halves, during cell-division, are pulled
apart after the splitting. The whole regularity of the
process would be hard to explain without this firm im-
plantation of the individual nuclear threads, as demon-
strated by Boveri. Where the nuclei are sinuate and the
nuclear threads are attached in the individual curves, the
conditions are specially clear.

In the species of locust, Brachystola magnet, Sutton
found the same implantations of the nuclear threads on
the curves of the nucleus. But here every thread, of
which there are eleven in every pronucleus, forms a skein
after the cell-division. These skeins of one and the same
nucleus remain separated from each other for a long time,
and the independence of the chromosomes can hence be
directly demonstrated, even at the stage of the skein. This
locust has also proven very instructive in another point
of Button's studies.

In general, one finds the individual chromosomes to be
of unequal length in the most various nuclei. But, in the
species of locust mentioned, this length occurs in such a
characteristic manner that the chromosomes can be easily
recognized in the successive cell-divisions. The pictures

234 Fertilisation and Hybridisation

taken at the successive stages allow one to follow up, with-
out difficulty, the identity of the short and thick nuclear
threads. In doing so one sees that, in the double nuclei,
the nuclear threads lie in pairs, that is, that there are two
nuclear threads of each individual length. Evidently
these belong together in such a manner, that in every pair
one thread belongs to the paternal and one to the maternal
pronucleus. A border line between them is nowhere to be
seen, and yet their independence is very evident. And
this harmonizes with the conception, as detailed above,
that, according to the species examined, this limit can be
observed for a longer or shorter time.

Microscopic examinations teach us, then, to recognize
the independence of the two pronuclei, as well as the
autonomy of the individual nuclear threads or chromo-
somes during the development of the entire body. The
agreement of this observation with the phenomena of
heredity may be considered as fully established.

But it is another question whether the individual chro-
mosomes correspond also to special groups of hered-
itary characters, or, in other words, whether the bearers
of the latter are strictly localized in the nuclear threads.
Obviously, this question can be answered only physiologi-
cally. It amounts to a decision as to whether, if definite
chromosomes, or definite parts in them, as for example,
single chromomeres and chromioles, were wanting, defi-
nite external characters of the organism would also be
lacking. If it were possible to kill a nuclear granule with-
out otherwise injuring the germ, what would be the con-
sequences ?

Engelmann has taught us, in his revolutionizing in-
vestigation on the activity of the individual chlorophyll
grains, how the focal point of a lens can be moved over

The Role of the Chromosomes 235

the field of a microscopic preparation, thereby lighting
up quite small portions of a cell, and how these portions
can thereby also be heated, and in that way killed. If a
part of a nuclear thread could be killed in this way, the
externally visible consequences would certainly allow us
to draw conclusions on the relations of this part to the
hereditary characters. Perhaps an analaysis of heredity
can some day be made by this method, but the technique is
not yet sufficiently advanced for this purpose.

However, there is another means of removing individ-
ual chromosomes, and this again we owe to the classical
investigations of Boveri. He found it in abnormal pro-
cesses of fertilization as they occur at times in eggs of sea-
urchins and star-fish, and it can be quite easily produced
artificially. It would lead too far from the main question
to go into details here. The important point for our pur-
pose is that, by certain interferences, a fertilization of one
egg with two spermatozoa can be achieved. This process
of dispermia leads in the nucleus of the germ, not to a
double, but to a triple number of chromosomes. In the
successive divisions the conditions become correspondingly
intricate, and almost any imaginable abnormal number of
chromosomes occurs. Nevertheless, the germs develop in
some cases, and then show deviations from the normal
type which allow a recognition of their normal relations
to the structure of their nuclei. Without doubt the germs
can, in every case, develop only those qualities the repre-
sentatives of which happened to be preserved in their
nuclei.

We shall leave the nuclear threads, at present, and
return to the two pronuclei. We saw them intimately
combined during the entire development of the body.
Now the question arises as to how long this union persists.

236 Fertilisation and Hybridisation

And since the double nuclei of the body originated during
fertilization, it is evident that the conjugating cells must
have single nuclei, and therefore that the separation of the
pronuclei must take place at the origination of these cells.

This fact is now so generally established, for animals
as well as plants, that it may be regarded as one of the
strongest foundations of the whole theory of fertilization.
Wherever it is possible to count the chromosomes, we find
in the somatic cells twice as many as in the sexual cells.
The former contain double nuclei, the latter single nuclei,
or pronuclei.

The sexual cells in animals originate directly from the
somatic cells, but in plants there is more or less prepara-
tion. Correspondingly, the two pronuclei separate in ani-
mals at the formation of the egg- and sperm-cells, but in
the case of plants before that. In the seed-bearing plants
it is the period of the origination of the mother-cells of the
pollen and of the embryo-sacs. Therefore all cell-genera-
tions which appear after this moment, and up to the final
production of the egg-cells in the embryo-sac, and of the
sperm-cells in the pollen-grains and their tubes, possess
only pronuclei. Such cells are called sexual, and the
period of their formation the sexual generation. In ferns
the entire life-period of the prothallium lies between the
origination of the sexual cells and the appearance of the
egg- and sperm-cells. This small plantlet, though built up
of hundreds of cells possesses, therefore, as Strasburger
has demonstrated, only pronuclei. The alternation of the
sexual prothallia and the asexual fern-plant is called the
alternation of generations ; the two generations are hence
distinguished from each other fundamentally by their
nuclei, which in the leafy plants are always double nuclei,
and in the prothallia always pronuclei. This difference

Numerical Reduction of Chromosomes 237

is so constant that one feels almost inclined to call the pro-
nuclei prothallial nuclei.

At the moment when the two prounclei separate, single
nuclei appear in place of the double nuclei, and the double
number of nuclear threads is thereby reduced to a single
one. This process is usually called the numerical reduc-
tion of the chromosomes ; but this imposing name means
nothing but the separation of two nuclei which had so far
worked together for a period. It is like the parting of
two persons who have walked along together for a while,
and will be looking for other companionship presently.
And this they achieve by fertilization.

This parting has been minutely studied by numerous
investigators. It has the appearance of a nuclear division
of a very special nature, and is frequently called the reduc-
tion-division, or heterotypic nuclear division. It is neces-
sarily accompanied by a cell-division, since the two sepa-
rated pronuclei can only part in separate cells, but this
cell-division does not always follow immediately, but
only after a second essentially normal division of the
nuclei. There result, in that case, four sister-cells instead
of the usual two.

Shortly before their separation, the chromosomes lie
together in pairs, always one in the paternal pronucleus
united with the corresponding thread of the maternal
pronucleus. They are placed lengthwise side by side.
Hence the separation evidently occurs by a longitudinal
line, and, in by far the greatest number of cases, this so-
called longitudinal splitting of the chromosome-pairs has
been observed in the origination of the prouclei. It is
true that this does not always succeed at a first glance,
and it is right here that the differences of opinion between
different investigators have blurred the picture for a long

238 Fertilisation and Hybridization

time. But gradually it was discovered that there are a
number of secondary details which may obscure the main
features, and we owe it chiefly to Strasburger that the
latter stand out clearly in the plant-kingdom. In the ani-
mal kingdom, however, there is still a series of cases
which do not follow this rule, and where the chromo-
somes of the pronuclei 'are not placed lengthwise side by
side at the moment of separation, but are connected at
one end. Hence the separation here takes the form of a
transverse division. Some insects and fresh-water crabs,
some molluscs and worms offer the best known instances,
but according to the most recent studies of de Sinety, Can-
non, and others, the assumption gains ground that here too
the microscopic pictures, on closer observation, disclose
a better fitting into the otherwise general scheme. It is
also possible that, after the longitudinal splitting, the
nuclear threads still remain connected for a while by their
ends, before they finally separate.

The male and the female sexual cells usually originate
in separate organs, frequently on special individuals. This
goes to show that, at their origination from the body-cells,
the paternal pronuclei do not become sperms and the ma-
ternal ones egg-cells. On the contrary, the two pro-
nuclei of a mother-cell in the ovary can become egg-cells,
and the two pronuclei of a pollen mother-cell can both
give rise, by further splitting, to the formation of sper-
matozoids. Accordingly, one-half of the forming sperms
gets paternal or now grand-paternal pronuclei, and the
other half grand-maternal. The same is true of the
egg-cells, and this holds good in spite of the circum-
stance that, in consequence of the crowded condition of
the ovaries, the larger part of the female cells has regu-

Transmission of Grandparental Characters 239

larly to be sacrificed every time.2 Therefore fertiliza-
tion may result in offspring with pronuclei from the
grandfather or grandmother only, or from both. This
circumstance may not be without significance in consid-
ering the resemblance between grandparents and grand-
children among men.

But it is not by any means decisive; daily experience
teaches that not only in a part of the progeny, but doubt-
less in all the offspring, there may be an admixture of the
characters of the grand-parents also. This indicates that
the separation of the pronuclei is not of as simple a nature
as the microscopic pictures might lead one to believe.
Another process, which, until now, has defied detection,
must take place, probably in the smallest, but to us invisi-
ble granules of the nuclear threads. That this is the
case we learn especially from the processes in hybrids
and their propagation. Here, splittings and new combin-
ations of the characteristics of the grand-parents occur
in apparently incalculable numbers, and here it is dis-
tinctly seen that the pronuclei do not separate without
a lasting reciprocal influence.

We shall first try to get a conception of this influ-
ence, for the facts concerning hybridization are rather
involved; they can be most clearly explained by means
of such a hypothetical conception. We shall accordingly
assume a mutual influence as an established fact, and in-
quire how this can take place.

First of all it is clear that it must be finished before
the separation of the pronuclei. Once they are apart all
intimate relation between them ceases. They go their
separate ways, each living for itself. Only in the double

2The reference is to the resorption of the sister-cells (when such
occur) of the embryo-sac mother-cell. Tr.

240 Fertilisation and Hybridization

nuclei do the paternal and the maternal pronuclei lie so
close together that their individual parts can exercise an
influence on each other.

We have further seen that, during the life of a double
nucleus, throughout the successive cell-divisions, from
the origination of the germ to the complete formation of
the offspring, the contact of the pronuclei becomes grad-
ually more intimate. Before the first cell division they
are, as a rule, still visibly separated; soon afterwards the
border-line begins to 'look more indistinct, and, shortly
before the formation of the sexual cells, the double na-
ture is disclosed with certainty only in the rarest cases
by special structural relations. It is, therefore, clear
that their opportunity for mutual influence gradually in-
creases during somatic life. Perhaps it first occurs only
at the end, possibly even, only at the moment immediately
preceding their separation. A decision on this point has
not yet been reached.* But the above-mentioned vegetative
splittings of hybrids indicate that the process is deferred
as long as possible. It also seems simpler to assume that
it occurs only in those cells which actually lead to the
formation of sexual cells, because in the leaves, bark, and
other vegetative parts of the body, it would evidently be
without significance.

We therefore imagine the mutual influence to be exer-
cised towards the end, or even at the very last moment
before the separation of the pronuclei. In the first case

4More recent investigations indicate that the fusion of the male
and female chromatin elements is completed during the stage known
as "synapsis" which immediately precedes the reduction-division, or
heterotypic nuclear division, referred to above. During synapsis the
chromatin is aggregated into a compact mass within the nuclear
cavity. Tr.

A Special Unit for Each Character 241

it could extend over a long time ; in the latter it must take
place suddenly. In the first case the individual parts of
the nuclear threads could be mated one by one; in the
latter this would have to take place everywhere simultan-
eously.

How this process comes about is self-evident when we
assume special units, special granules in the nuclear
threads, for the visible characters of the organisms. There
must be as many units in the nucleus, as a plant or animal
possesses individual characters. And this, of course, is
the rule for both pronuclei. In the condition of the short
and thick chromosomes these units lie crowded together.
This is a definite stage in cell-division ; the units, at least
those of the interior of the group, remain in a condition
of enforced rest. But as soon as cell-division is com-
pleted, the nuclear threads stretch ; they become quite long
and thin, and indeed so long that a large part, perhaps
most of them, possibly all of them, come to the surface.
At least stretched out in a row in this way, the granules
must then be arranged one after another, perhaps in the
threads themselves, perhaps in their finest ramifications.
Now they become active, and if, at this time, nuclear
threads of the paternal and the maternal pronuclei lie
together in pairs, every granule can enter into communion
with its corresponding unit in the other pronucleus.

There is no reason to assume that the exceedingly fine
structure of the nuclei, which is so strikingly to the pur-
pose and yet so simple, should be limited to what is visible
to us at present. On the contrary everything points to
the probability that, in the internal structure also of the
nuclear threads this same serviceable rule must prevail.
The whole complicated process of nuclear division has
for its object the division of the two pronuclei in such a

242 Fertilisation and Hybridisation.

way, that their daughter-nuclei will share alike in the
hereditary characters that are present. The lengthen-
ing of the nuclear threads at the close of division, their
so frequent ramification, and the seemingly irregular in-
tertwining of their parts, evidently indicates the possi-
bility of a domination of the cell-life by the bearers of
the inheritable qualities. These must impress their
character on the surrounding protoplasm either dynami-
cally or, as I have assumed in my Intracellulare Pangen-
esis, through a giving out of material particles to the
surrounding protoplasm, and thus promote growth and
development, in the prescribed direction, into the specific
form of the species to which the organism belongs.

This secretion of material chromatin particles from
the nuclei was recently demonstrated by Conklin in Crep-
idula.5 In this way considerable quantities of chromatin,
and therefore probably of pangens also, are transferred
into the somatic protoplasm.

Thus we consider that the structure of the nuclear
threads is such that it not only makes possible, but regu-
lates and dominates the relations of the two pronuclei.
In an ordinary animal, or in a plant which is not a hybrid,
both pronuclei possess the same units, only with a some-
what unlike degree of development. We assume, there-
fore, that the cooperation comes about in such a way that
the individual units in the stretched threads lie in the
same numerical order. Then, when the threads are
closely appressed lengthwise, in pairs, we can imagine that
all the like units of the two pronuclei lie opposite each
other. And this is obviously the simplest assumption
for a mutual influence.

5Strasburger failed to find any direct evidence of such a transfer
of particles in plants. Cf. the Translator's Preface, p. viii. Tr.

An Exchange of Character-Units 243

If every unit, that is, every inner character or every
material bearer of an external peculiarity, forms an en-
tity in each pronucleus, and if the two like units lie oppo-
site each other at any given moment, we may assume a
simple exchange of them. Not of all (for that would
only make the paternal pronucleus into a maternal one),
but of a larger-, or even only a smaller part. How many
and which; may then simply be left to chance. In this
way all kinds of new combinations of paternal and mater-
nal units may occur in the two pronuclei, and when these
separate at the formation of the sexual cells, each of them
will harbor in part paternal, in part maternal units. These
combinations must be governed by the laws of proba-
bility, and from these, calculations may be derived, which
may lead to the explanation of the relations of affinity
between the children and their parents, the grandchildren
and their grand-parents. On the other hand a compari-
son of the results of this calculation and of direct obser-
vation will form the best, and for the time being, the only
possible means for a decision as to the correctness of our
supposition.

The mutual influence of the two pronuclei shortly be-
fore their separation is therefore brought about, accord-
ing to our view, by an exchange of units. Every unit
can be exchanged only for a like one, which means for
one which, in the other pronucleus, represents the same
hereditary character. This rule appears to me to be un-
avoidable and really self-evident. For the children must
inherit all specific characters from their parents, and they
must also transmit all of them to their own progeny.
This exchange must hence be accomplished in such a way
that every pronucleus retains the entire series of units
of all the specific characters, and this result can evidently

244 Fertilisation and Hybridisation

be obtained only when the interchange is limited to like
units.

We distinguish here specific characteristics from indi-
vidual features. The units in the hereditary substance
of the nuclear thread compose the former. Every species
has an often exceedingly large and yet definite and invari-
able number of them. The sum total of these units
forms that which distinguishes any given species from all
others, even from its nearest allies. A complete diagno-
sis of a species would have to embrace all of these char-
acteristics, and therewith all the material bearers under-
lying them.

The individual features, that is, the differences be-
tween the individuals within the species, and not only of
the systematic but of the so-called elementary species, are
of quite another nature. It is true that they are, in a way,
hereditary, but with that they are subject to constant
changes. The average stature of man remains the same
in the course of centuries, for the same race (elementary
species), but the individual stature changes constantly
from one individual to another. In the somatic cells of
man the bearers of the stature of the father lie opposite
those of the mother. At the moment of exchange these
are mutually transferred, and the sexual cells receive
partly one, partly the other stature, but this in the most
various combinations with the other characters. Thus
one might continue. Every visible quality, every trait
of. character is to be found in all individuals, only in some
they are strongly developed and prominent, in others
weak and recessive. Ordinary observation takes more
interest in differences than in similarities, and for this
reason the former are designated by contrasting expres-
sions, as large and small, strong and weak, forward and

Individual Variation 245

modest. But these are, in each instance, only degrees
of the same hereditary characteristic, or the same trait
of character. And such more or less differing stages
of development of the same inner units we represent to
ourselves as the entities which are exchanged by the nu-
clear threads.

Individual differences are thus not included in the
type of the species. They form deviations from this
type, and are conditioned by causes which were formerly
generally described as conditions of nutrition, but now
more frequently as environment. Under these influences
every character can develop more or less strongly than
the average type. And the environment, provided it re-
mains constant during the entire period of development,
must affect all the unfolding characters in the same way.
If it is favorable it furthers all parts of the body and all
mental gifts, if it is unfavorable it has the opposite effect
on all of them. Not, by any means, to the same degree
upon all of them : that does not depend upon the environ-
ment but upon the units themselves; this, however, can
not lead to essential differences between separate individ-
uals. But our supposition of such a uniform environ-
ment would probably be met with only in the rarest of
cases. And, as soon as it changed, it would influence
one individual differently from the others. Moreover
the characters do not unfold simultaneously, but success-
ively, the higher ones for the most part later than the lower
ones, mental characters later than those of the body, the
reason later than the memory. And all those wheels
work into each other so that small deviations will rather
tend to become greater than to be equalized. Though
children of the same parents but of different age might,
during their entire youth, live under the same circum-

246 Fertilisation and Hybridisation

stances, they will yet react differently to them. This also
holds true for plants where, in the same bed, a delay of
only one day in germinating will, according to the weather,
lead either to equal or to quite surprising differences in size
and qualities.

If favorable and unfavorable conditions of life alter-
nate during the individual development, and if they strike
a group of individuals sprung from like seeds at different
periods of their growth, quite a considerable degree of
individual differences must thereby result.

These differences play in nature the same role as in
human society. One is adapted for this kind of task, the
other for that. With men it is the duty of every one to
develop his own talents to the best of his ability, and to
render as favorable as possible the circumstances for the
most perfect development of his children. The highest
efficiency of society in general demands of each the
strongest effort in the direction of his most favorable
talents. To ascertain this direction ought to be one of
the chief aims of education and instruction. In animals
and plants this highest efficiency can obviously not be
achieved in the same way. And especially are the con-
ditions different for plants, which are tied for life to the
place where they germinated. Here, as is well known,
nature is assisted by the astonishingly great number of
seeds ; she sows so many in every individual spot that only
the best, that is, the individuals best adapted for the given
locality, need retain life. But, by sacrificing countless
seeds, she also accomplishes here that adaptation of the
individual specimens which is the condition for the com-
plete unfolding of their abilities and advantages.

Very great weight is therefore given to individual
differences in the life of the entire species. The greater

The Significance of Sexual Reproduction 247

they are, the greater the power of adaptation, the greater
the chance of victory.

And in this I see the significance of sexual reproduc-
tion. It mixes the potentialities that have developed in
the single individuals in the most complete mariner imag-
inable; it achieves, at one stroke, all possible combina-
tions. It cancels, as Johannsen expresses it, the previous
correlation's. Asexual propagation confers a certain
degree of variability, and this may be quite sufficient in
many cases, especially in the case of a low organization
or of quite special adaptation, as in many parasitic and
saprophytic organisms. Under such conditions the vari-
ability remains, in a certain sense limited, more or less
one-sided, because every individual is the result of the
varying, but, on the whole, one-sided environment in
which his progenitors existed. Only an exchange of qual-
ities can help to overcome this one-sidedness ; only this
can cause all the combinations to arise which are de-
manded by the varying environments. If we assume that
the bearers of the individual characters are, as a rule, in-
dependent of each other during their exchange, and also
that the latter is ruled by chance, two pairs of character-
istics would directly result in four, three in eight, four
in sixteen combinations. The sum total of the points of
difference of two parents must therefore give rise to such
an incredible number of possibilities that no struggle
for existence, no annual rejection of hundreds and thou-
sands of germs could demand a richer material.

Hence sexual reproduction brings individual variabil-
ity to its highest point. It produces a material that cor-
responds to almost any environment. It is the principal
condition for the greatest efficiency of cooperation, be it
by a selection as free as possible of the line of develop-

248 Fertilisation and Hybridization

ment for the single individuals, or by a sacrifice of all
the individuals that do not quite meet all the requirements.

This service of sexual reproduction is evidently not
limited to a single generation. It exercises its influence
throughout successive generations, and it is probably in-
different whether the effect follows directly, or whether it
manifests itself in the course of time. Even without that,
the complete utilization of all given possibilities requires,
as a rule, more individual beings than are born in a single
generation. And with this, the otherwise strange fact is
explained, that the exchange of the units does not imme-
diately follow fertilization, but only takes place a short
time before the succeeding period of fertilization. But
obviously an exchange, ruled by laws of chance, could not
benefit a given isolated individual or, more correctly speak-
ing, it would most likely, just as frequently be harmful
as useful. It can only be of use in connection with an
increase in the number of individuals, for it is its task to
bring about as great a variety as possible, and with that,
the highest possible prospect for the required quantity
of superior specimens. At the moment when the produc-
tion of the sexual cells begins, in such enormous numbers,
it also finds the best opportunity for fulfilling its task.

Thus, sexual reproduction has only a subordinate sig-
nificance for the children, while for the grandchildren it
is of the utmost importance, because only for them does
the urn mix up all its lots.

The same laws that govern normal fertilization, are,
of course, valid for hybrids also. There cannot be special
biological laws for them, because they are only derived
phenomena, deviations from the normal. Now the ques-
tion is, to which results, departing from the rule, will the
common laws lead in these special cases. And with this

Hybrids, Varieties, and Species 249

it is clear that the phenomena must keep nearer to the
normal the less the deviation is from the type.

This type is conditioned by the fact that the two or-
ganisms that fertilize each other belong to the same small
or elementary species. They have then, on the whole,
the same characters, even if these are, according to their
environment in various degrees of development. There
are no differences among them independent of this, at
least if we consider the cumulative effect of uniform in-
fluences in the course of several generations.

As soon as such independent differences occur, and as
soon therefore as there are present constant contrasts,
which are retained in the sequence of generations and
cannot be blended by environment, we call the sexual
union of two individuals a crossing or a hybridization.
If the contrasts are slight, we call the two races varieties,
if they are greater, they assume the rank of species. The
crossing of varieties keeps quite near to normal fertiliza-
tion ; that of the species deviates the more the slighter the
relationship between them. The crossing of varieties
forms a type complete in itself, that of the species forms
a series which descends from almost normal processes,
by gradual progress, to a complete reciprocal sterility.
The variety-hybrids are fertile like their parents, but in
the species-hybrids the diminished fertility indicates ab-
normal phenomena either in fertilization or in the ex-
change of the units.

We must therefore discuss these two groups sep-
arately, and we shall begin with the varieties.

In daily life and in horticulture, any thing that deviates
from the normal is called a variety. Even the new forms
obtained by crossing are quite commonly counted among
the varities. In science, therefore, the word would really

250 Fertilisation and Hybridisation

be useless. Nevertheless it has been retained and its
meaning has been gradually limited. Especially in de-
scribing horticultural plants the conception is sufficiently
restricted by excluding on the one hand the hybrids, on
the other hand the improved races obtained by selection,
and finally the so-called elementary species that, taken
together, form our ordinary species.

Upon reviewing the cases that are left, two types can
be plainly distinguished, the constant and the inconstant
varieties. The former are not inferior to true species in
point of constancy. Their characters vary, in the single
individuals, around a mean, but in the main not more so
than the corresponding characteristic of the species.
From this they are separated by a decided chasm. In
pure fertilization they never bridge this chasm, or at
least, extremely rarely, but in crossing they revert very
easily to the species. It is this very reversion that stamps
them varieties, and when the crossing is not artificial but
natural, brought about by insects, it escapes observation,
and only the fact of the reversion strikes the gardener.

These constant varieties are, as a rule, distinguished
from the species to which they belong, by lacking some
striking quality that adorns the latter. Most frequently
it is the coloring of the flower or, in the case of flowers
with combined colors, as in the yellow and red tulips, one
of the individual colors, that is wanting. Often they
lack hairs or thorns, very frequently the development of
the blade is arrested, and split leaves originate. In all of
these cases there is no ground for the opinion that the
failure of the visible character means also the loss of the
respective unit. Rather does everything point to the
fact that the unit has simply become inactive, that it is in
a state of rest, or as it is usually expressed, that it has be-

Inconstant Varieties 251

come latent. Especially the reversions, which in individ-
ual specimens of such varieties are, at times, quite com-
mon phenomena, betray this latent presence.

Inconstant varieties are distinguished by a strikingly
high variability, by an exceedingly great range of depart-
ure from the norm. But here we encounter the double
meaning of the designation inconstancy. On the one
hand the word means a certain relatively great richness
of individual forms, on the other hand it relates to differ-
ences between the parents and the progeny. In choosing
from an inconstant variety a single individual, and sowing
its seed, after pure fertilization, the whole play of forms
of the variety can be found again in the children, — hence
a palpable proof of the inconstancy. But, on choosing
several individuals, and on sowing their seeds separately,
each of them will produce almost the same series of forms.
The whole group is transmitted from year to year, and
does not change. The variety has a definite circle of
forms in which the descendants of every specimen choose
freely their place, but they do not go outside the circle.
The limits are constant, and remain so in the course of
generations ; within the limits, however, a motley variety
prevails.

Such is the concept of plants with variegated leaves,
of double and striped flowers, and many other most highly
variable garden-plants. The new character is not based
here on the loss or the latency of some characteristic of
the species. Indeed, on the contrary, it is usually a pecu-
liarity which is already present in the species itself, or at
least in one of its races, in a latent state. Especially do
variegated leaves occur, not so very infrequently, on
otherwise green plants, and the same is. true of stamens
with petal-like broadenings. The relation of the incon-

252 Fertilisation and Hybridisation

stant varieties to the species from which they are derived,
is therefore quite different from that of the constant
varieties.

Nevertheless, the two crossings behave in the same
manner in regard to their mother-species. From the lat-
ter they are distinguished, for the most part, only in one
point, though sometimes in several. But we have always
to deal with the distinction between active as contrasted
with latent, be it that the given character is active in the
variety and latent in the mother-species, or latent in the
former and active in the species itself.

If to this we apply the conception of the arrangement
of the units in rows on the nuclear threads, as explained
above, it is quite evident that everything will follow ex-
actly the same course as in normal fertilization. Every
unit in the paternal pronucleus corresponds to the repre-
sentative of the same peculiarity in the maternal one.
The nuclear threads fit as nicely into each other as in a
pure species, and all the units which do not directly bring
about the point of difference behave quite normally. Co-
operation in vegetative life, and exchange during the
formation of the sexual cells need not be disturbed. We
may confine our whole consideration to the point of dif-
ference, and we shall select, for the purpose, as simple
an illustration as possible, one in which there is only one
difference between the species and the variety, for exam-
ple, the color of the flower.

The material bearer of the color-characteristic is situ-
ated in the mother-species so that it can display its full
activity while in the variety it is unable to do so. If the
paternal and maternal nuclear threads of the hybrid come
into contact for the purpose of exchange, and with the
same sequence of units in both, the active unit of coloring

First vs. Second Hybrid-Generation 253

matter naturally gets the equivalent inactive unit as an
antagonist. With this it must therefore be exchanged.
We assume that in this the latent condition is without
significance, that hence the exchange comes about in the
same manner as in normal fertilization.

Over this, however, the crossings of varieties have the
great advantage that there the origin of the characteris-
tic 'in question can always be clearly and positively rec-
ognized. Both units of a pair of antagonists are other-
wise distinguished only by a more or less of development,
here by a sharp contrast. And for this reason it is experi-
mentally much easier to discover the laws with varieties
than with purely individual differences.

In doing this, two points have to be distinguished ; the
consequences of fertilization and the consequences of the
exchange of the units. The former we see in the hybrid
itself, the latter in its descendants.6 And since fertiliza-
tion and exchange are two such fundamentally different
things, we must not wonder that there exist such decided
differences between a hybrid and its descendants. These
differences show themselves essentially by the fact that
the hybrids of a mother-species with a variety of the same
are alike, even if they are obtained in great numbers,
while their descendants always display a certain variety.

Let us first consider the first generation of variety-
hybrids. How do the two pronuclei, notwithstanding

6In the fertilized egg, resulting from the crossing, the chromatin
from the male and female parents is not completely fused. As pointed
out in a preceding footnote (p. 240), this fusion, called synapsis,
occurs as almost the last step preceding the nuclear and cell-divisions
that give rise to the reproductive cells. The characters of the first
hybrid generation are a result of fertilization. Following synapsis,
the pure bred offspring of this generation differ from their parents
and also among themselves. Tr.

254 Fertilisation and Hybridisation

their inequality, cooperate in order to regulate the evolu-
tion ? This question amounts to the same as asking, what
is the sum of the influence of an active and a latent unit ?
At first glance one would expect that this influence would
correspond to half the value of a pair composed of two
active units. Previously this opinion was rather gener-
ally accepted, and there was an inclination to regard plants
with intermediate characters as hybrids. Especially many
plants with pale red or pale blue flowers were regarded
as such. But the experience of later years has decided
differently.

Variety-hybrids generally bear the characteristic of
the species, sometimes fully developed, sometimes more
or less weakened^ but this for the most part only so little
that superficial observation sees no difference. An active
and a latent unit are not essentially different in their co-
operation from two active ones ; a fact which may prob-
ably be best explained by the assumption that two cannot
accomplish more than one already does. This conception
finds a very strong support in the results of the most
recent investigations by Boveri on dispermia, which we
have already partly discussed. By fertilizing one egg
with two spermatozoa the composition of the structure
of the nuclear threads can be altered in different ways,
for instance, in such a manner that in one nucleus there
lie not two, but three pieces of any one of its chromo-
somes. It might then be expected that the given charac-
ters would be very strongly developed, to about one and
one-half of their intensity. But, as far as can be judged
from Boveri 's experiments, this is not the case, and the
influence of the three equivalent units is not noticeably
greater than that of two.

We come now to the progeny of hybrids, and we, of

Disjunction of Hybrids 255

course, presuppose self-fertilization. At the formation
of the sexual cells the two pronuclei separate ; this happens
at the origination of the egg-cells as well as of the sperms.
Through exchange, the active units of our differing pair
combine partly with new units of the other pairs, and
thereby new combinations originate as in ordinary fertili-
zation. But if we consider only the differing pair, exactly
one-half of the egg-cells must obviously have the pater-
nal, and the other half the maternal character. Or, in
other words, in one-half of the egg-cells the given charac-
ter occurs in the active, in the other in the latent state.
Exactly the same is true of the male sexual cells, the
sperms, in animals as well as in plants, and independently
from the circumstance that in the higher plants the sperm-
cells are conducted to the egg-cells in the pollen-tube.

The male sexual products of a hybrid are therefore
unlike each other, and the same holds true of the female.
In the simplest case selected both groups consist of two
types, in the more complicated cases this number will ob-
viously become greater. The paternal and maternal fac-
tors of the hybrid become, in its progeny, grandpaternal
and grandmaternal. Hence, in regard to the point of
difference, one-half of its egg-cells and one-half of its
sperm-cells have grandpaternal factors, while the other
halves possess grandmaternal ones.

By means of this principle the composition of the pro-
geny in the simple as well as in the complex cases, and for
constant as well as for inconstant varieties can be calcu-
lated. Thus we obtain the formulae which are now uni-
versally known as MendeTs law.

They indicate, foranygiver!* number of points of dif-
ference between two parents, how many children corres-
pond to every individual combination of the respective

256 Fertilisation and Hybridisation

character. And, on the whole, experience has so far
proven the reliabilty of these formulae for animals as well
as for plants.

It would be too great a digression to consider here the
formulae themselves. We shall therefore leave the field
of the variety-hybrids, and turn to the hybrids between
different species, especially between allied elementary spe-
cies.

In order to understand these we must get a clear idea
of the nature of the points of difference in this case, or in
other words, what is meant by relationship. Species orig-
inate from each other in a progressive way. The number
of the units in lower organisms is evidently only small,
and must gradually increase with progressing organiza-
tion. Every newly arising species contains at least one
more than the form from which it has arisen. Only in
this way can one imagine the progress of the entire plant
and animal world.7

It is indeed questionable whether the acquisition of a
single new unit, the increasing by one unit of the entire
stock, amounting to hundreds and thousands, would be
sufficient to make the impression of progress on us. The

7A quite different hypothesis is thinkable, as, for example, that
suggested by G. H. Shull, "The Significance of Latent Characters,"
Science N. S., 25 : 792. 1907.

"All the visible variations of the present plant and animal world
were once involved in some generalized form or forms, and the pro-
cess of differentiation pictures itself to us as a true process of evolu-
tion brought about by the change of individual character-determining
units from a dominant to a recessive state. This conception results
in an interesting paradox, namely the production of a new character
by the loss of an old unit."

This hypothesis, however, as de Vries has pointed out, seems too
much like a revival of the old evolution theory as opposed to epi-
genesis. Tr.

Avuncular y vs. Collateral Crossings 257

difference will in most cases be too slight. Only when
two or three or more units have been added successively
to those already present, will we recognize an increase in
the degree of organization.

The progress of every individual species can appar-
ently take different directions. In some genera there are
species so typical that they may be regarded as the com-
mon origin of the others. Where these are lacking it is
manifest that the systematic relations are still too incom-
pletely known to us, or that the given forms have died out.
Every species can therefore be compared with its own
ancestors or with other descendants of the same ancestors.

This consideration leads us to the recognition of two
different types of relationship, and therewith also of two
groups of crossings between allied species, which have to
be kept absolutely apart. One of them we shall call the
avunculary, the other the collateral. In the first case we
cross a form with an "avunculus" or ancestor in the direct
line, in the latter case with one of its lateral relatives.
Obviously the first relation is very simple while the latter
is more complicated.

Every character and every unit corresponding to it,
which in a crossing is present in one species and lacking
in the older one, forms a special point of difference.
Hence the simplest case is the one in which there is only
one such difference between the two parents of a cross.
But generally several of them exist.

Now in such a cross, the differing factors evidently
do not find any antagonists in the sexual cells of the other
parent. When, during fertilization, the pronuclei unite
into a double nucleus, all the other units are present in
pairs. Not so the differing ones ; they lie unpaired in the
hybrid.

258 Fertilisation and Hybridisation

If we apply this reasoning to our conception of the
arrangement of the units in rows on the nuclear threads,
the immediate result would be that their cooperation must
be disturbed. The threads no longer fit, neither during
fertilization and in vegetative life, nor later when the units
are exchanged before the formation of the sexual cells.

If we imagine two corresponding chromosomes of the
two pronuclei placed exactly side by side, and in such a
way that every unit of the one has the corresponding unit
of the other for a neighbor, this will occur in a species-
cross only as far as the point of difference. Here one nu-
clear thread has one unit more than the other. The latter
has, so to say, a gap.

The greater the number of points of difference, the
more numerous are these gaps, and the more will the co-
operation of the two nuclei be interferred with. And this
must diminish the vitality of the germ or at least the nor-
mal development of all characters.

If the differences between the two parents are too nu-
merous, a crossing, as is well known, remains quite with-
out effect. Crossings between species belonging to dif-
ferent genera succeed in very rare cases only, indeed
within by far the most genera even the ordinary system-
atic species are not fertile when united. Genera such as
Nicotiana, Dianthus, Salix, and others, which are rich in
hybrids, are, as a rule the very ones in which the species
are exceedingly closely related to each other.

Even if the agreement of two species is great enough
for mutual fertilization, the life of the hybrid is by no
means assured thereby. Some of them die as seeds with-
in the unripe fruit, as has been specially described by
Strasburger for the hybrid seeds of Orchis Morio after
fertilization with 0. fusca.

Sterility of Hybrids 259

Others become young plantlets, but are too weak to
develop any further, and perish during the first weeks
after germination, as I have frequently seen, for example
after crossings of Oenothera Lamarckiana and 0. muri-
cata. Or only the most vigorous individuals continue to
grow, while the weaker ones perish, and this, in diocious
plants, sometimes results in the male seedlings perishing
while some of the more vigorous female ones develop
flowers, as Wichura observed in several willows. Finally
there might originate hybrids that grow vigorously, but
do not flower at all or only incompletely, or begin too late
to do so. There is a whole series of cases between the
unsuccessful crossings and the development of hybrids
into adult plants. And on the whole this series runs
parallel with the increasing systematic relationship.

If the hybrid has succeeded in reaching the period of
flowering, that is, the period of the formation of the sex-
ual cells, a new difficulty arises at the moment of the
exchange of the units. Whereas, up to that time, the co-
operation of the two pronuclei was more or less disturbed,
now the gaps become very important. Hence the quite
common phenomenon that the production of egg- and
sperm-cells fails more or less completely, that the hybrids
either produce no ovules that are capable of being fer-
tilized, or no good pollen, or neither. They are more or
less or even completely sterile. They either form no seed
at all, or only an insufficient quantity. Only where the
differences between the parents are quite small, does one
succeed in harvesting any seed, and even here frequently
only a little.

How the unpaired characters behave during the ex-
change, when they are not numerous enough to make a
failure of the entire process, is at present unknown. Ex-

260 Fertilisation and Hybridization

perience teaches, however, that in these cases the descen-
dants of the hybrids do not display that multifariousness
of type, nor those splittings that are characteristic of
variety-hybrids. They usually all resemble each other
and their parents, the original hybrids, and this constancy
persists through the course of generations. Accordingly
there originate races of hybrids which, apart from their
possibly diminished fertility, can hardly be distin-
guished from true species. Sometimes they are found
wild, as for example a hybrid race between two Alpine
roses and other races of the kind in the genera Anemone,
Salvia, Nymphaea, etc. Sometimes they have been ob-
tained artificially or have accidentally originated in the
gardens. The genus Oenothera is exceptionally rich in
such hybrid races, especially in the sub-genus of the com-
mon evening-primroses, Onagra. Very frequently such
hybrids are simply described as species, on the one hand
because they can be reproduced, without deviation, from
seeds, and on the other hand because systematic works
frequently do not sufficiently consider the elementary
species. The distinguishing of the latter from hybrid
races is frequently by no means easy.

The purpose of my explanations compels me to restrict
myself to simple and clear cases. In nature these occur
relatively rarely, and the individual elements of the phe-
nomena are usually commingled in most motley variety.
By far the greater number of crossings take place between
parents whose mutual relations do not wholly fit either
the one or the other concept, but where the characteristics
of the different types of hybrids are intermingled. I
cannot consider these cases here; they are of too com-
plicated a nature for an address.

Only one point I wish to touch upon. In the preceding

Mutation-Periods 261

pages I have always taken for granted that the species
and varieties are in their ordinary and unchanging state.
But this is by no means always the case. The origination
of new species and varieties demands that their immutabil-
ity should not be absolute, or at least should be suspended
from time to time. Experience confirms this by showing
that there are periods in the life of species, during which
they are, so to speak, especially inclined to produce new
types. At that time they produce the new varieties and
species, not only once but repeatedly, and not only a single
one, but frequently a considerable number. Genera rich
in species, such as the pansies and the rock-roses,7 are the
remains of such periods of variability, and everywhere in
nature we meet with similar ones. In garden-plants we
see, from time to time, periods during which certain
varieties occur by preference, as the double dahlia of
about the middle of the last century, the forms of toma-
toes in recent decades, and numerous other instances
teach us. On its first appearance the gardeners call the
new form a conquest, the later appearances are only repe-
titions, and are therefore of only very secondary practical
value.

The power of reproducing one or more new species
indicates a condition of unstable equilibrium of the given
internal units. In the nuclei the new characteristic is al-
ready invisibly present, but inactive. Certain causes, un-
known to us, can transform this into a permanent condi-
tion. This state of unstable equilibrium may be main-
tained in the great majority of individuals, through a
series of generations, as is the case with my Oenotheras.
But from time to time, sometimes in individual cases
every year, there is a shock, and the equilibrium becomes

7Sonnenrdschen (Helianthemum). Tr.

262 Fertilisation and Hybridisation

stable. The given individuals overstep their bounds,
abandon the earlier type, and form a new species.

It is evident that in crossings such unstable units will
behave differently from normal, stable ones. Their
chance of becoming stable is evidently considerable, ow-
ing to the phenomena of fertilization and the exchange of
units. In this way constant races originate, at least in the
genus Ocnothera, and this, on the one hand, with the re-
spective characteristic in an unstable condition, or in other
words, in a state of mutability ; and on the other hand with
stable equilibrium corresponding to a new species. But
researches in this field are only in their beginning, and do
not yet permit of a detailed analysis. Besides they repre-
sent, for the present, a case in themselves.

In conclusion, on reviewing the course of our deduc-
tions, we see that hybrids follow normal fertilization quite
closely, the more completely the less numerous and the less
pronounced the points of difference between the parents
of the crossing. If these are of such a kind that the num-
ber of units in one parent is different from that in the
other, disturbances take place which, if of lesser influence,
diminish the fertility of the hybrids, and if of greater sig-
nificance, affect their own power of development, or even
make the crossing a failure. If these units are present
in equal numbers on both sides, and if the differences are
limited to latency in one parent and activity in the other,
the normal process is not at all disturbed, but striking
phenomena occur, which find their explanation in the pe-
culiar manner in which the parental inheritances co-oper-
ate in the hybrid and in the formation of its sexual cells.

This co-operation is reflected in the life of the nuclei.

Conclusion 263

In fertilization the nuclei of father and mother simply
touch each other. In the course of development the con-
tact becomes gradually closer, bringing their equivalent
elements as near to each other as possible, in such a way
that the latter finally all lie side by side in pairs. But the
pronuclei by no means lose their independence thereby,
and for the purpose of every nuclear division they sepa-
rate their component parts more or less distinctly. Shortly
before their separation, their leave-taking, they are still
the same as before. But now they exchange their indi-
vidual units, and thus cause the creation of those countless
combinations of characters, of which nature is in need in
order to make species as plastic as possible, and to em-
power them to adapt themselves in the highest degree to
their ever changing environment.

This increase of variability and of the power of indi-
vidual adaptation is the essential purpose of sexual repro-
duction. It can be attained only by a mutual combination
in all conceivable forms of the peculiarities developed in
different individuals in different directions and degrees.
To this end the pronuclei mutually exchange their units
from time to time, and by assuming, on the ground of ex-
periments with hybrids, that this takes place, on the whole,
according to the laws of chance, that is, according to the
theory of probability, we have gained a basis which al-
lows us to probe to its very bottom this most significant
and mysterious process.

INDEX

Acetabularia, 164.

Acids, tannic, 12, 15.

Actinophrys Sol, 157.

Adaptations, parallel, 13.

Aggregation, 153.

Aleurone grains, 131, 155

Algae, 102, 135, 148, 149.

Alkaloids, 12.

Allium Cepa, 185.

Alternation of generations, 19,
236.

Amyloplasts, 130, 146.

Ancestral plasms, 53.

Anemone, 260.

Ant-plants, 14, 156.

Aphids, 32.

Apical cell, 84.

Archiplasts, 67.

Artemia, 229.

Ascaris, 227; lumbricoidcs, 178;
megalocephala, 145, 177, 178.

Asclepiadaceae, 14.

Ascomycetae, 102.

Ascospores, 165.

Ascus, 102.

Atavism, 16, 23, 25; specific, 60.

Atoms, 13; memory in, 48; will-
power in, 48.

Aucuba, 106.

Avunculus, 257.

Axolotol, 229.

Batrachoseps, 231.

Bees, 32.

Begonia, 106, 146.
phyllomaniaca, 199.

Begonias, 29, 99, 105, 205.

BELLONCI, 229.

BEYERINCK, 16, 98, 99, 118, 119,

120.
BOVERI, 218, 230, 232, 233, 235,

254.

Brachystola magna, 233.
Bras sic a oleracea, 182.
BRAUER, 229.
BREFELD, 96.
BROWN- SEQUARD, 65.
BRUCKE, 126, 183
Bryophyllum calycinum, 98.
Bryopsis, 143, 176.
Buds, adventitious, 98; callus,

97, 98 ; root, 98.
Bud-formation, 51, 97.
Bud-variation, 16, 24.
Cactacese, 14.
Calcium oxalate, 15.
Callus, 97.
Callus-buds, 97, 98.
Cambium, 97.
Carbon, 38.

Cardamine pratensis, 98.
CARRIERS, 29.
CASPARY, 106.
Catasetum tridentatum, 18.
Cecidium, 118.
Cecidomia Poae, 118, 119.
Cecropia adenopus, 56.
Cell-division, neogenetic, 128 ;

panmeristic, 128 ; phyletic, 107 ;

somatarchic, 107; somatic, 107.
Cell-pedigrees, 80.
Cell-plate, 161.
Cell-ring, 162.
Cellulose, secretion of, 47.
Chara, 144, 163.

266

Index

Characeae, 148, 159.

Characters, composite nature of
specific, 11 ; hereditary, 11 ; mu-
tual independence of, 11 ;
transmission of hereditary,
179.

Chelidonium, 106.

Chlorophyceae, 145.

Chlorophyll, 15.

Chlorophyll-bodies, origin of,
129.

CHMIELEVSKY, 172.

Chromatin, secretion of from nu-
clei, 242.

Chromoplasts, 147.

Chromosomes, 177, 178.

Circaea, 16.

Cladophora, 132, 148.

Clarkia, 106.

Cleistogamy, 32.

CAMPBELL, 175.

CANNON, 238.

Coccodules, 45.

C odium, 143.

Coeloblasts, 187.

Compounds, chemical, 12.

Conferva glomerata, 134, 135.

CONKLIN, 218, 229, 242.

Copper-beech, 21. v^

Cormophytes, 83.

Corn, 211.

Correns, 211.

Crabs, fresh-water, 238.

CRAMER, 85.

Crassulaceae, 98, 106.

Crepidula, 229, 242.

Crinoid, 200.

Cross-fertilization, 29.

Crown-graft, 211.

CRUDER, 159.

Crystalloids, 131.

Cucumis, 106.

Cycadacese, 226.

Cyclops, 229; vulgaris, 228.

Cynipidese, 19, 119.

Cytissus, 224; Adami, 223; La-
burnum, 223 ; purpureus, 223.

Cytoplasm, 202; composed of
pangens, 200; defined, 195.

Dahlia, double, 261.

Daphnoidae, 94.

DARWIN, C., 3, 14, 22, 23, 24, 29,
30,46,50,51,58,59,62,63,64,
71, 73, 91, 99, 109, 153, 199, 207,
212, 214, 215.

DARWIN, FRANCIS, 14.

Datura Stramonium, 224.

DEBARY, 165, 171, 172.

DELAGE, \\.

DELPINO, 26.

Derbesia, 143.

Dianthus, 258.

Diatomes, 149.

Dichogeny, 15, 16, 24.

Digitalis lutea, 181 ; purpurea,
181.

Dimorphism, 27.

Dioecism, 27.

DIPPEL, 159, 160.

Dipsacus sylvestris, 20, 213.

Diptera, 93, 94, 101, 119.

Dispermia, 254.

Drosera, 14, 153; intermedia, 153;
. rotundifolia, 153.

Duality, principle of, 220, 221.

Echinidae, 169.

EIMER, 210.

EISEN, 218, 231.

Elaioplasts, 149.

ELSBERG, L., 44, 45, 46, 48.

Embryo-sacs, 164.

ENGELMANN, 234.

Epithemia, 173.

Equisetum, 83, 100 ; palustre, 83 ;
arvense, 86, 87.

ERRERA, 218, 231.

Index

267

Euglenae, 149, 156.
Euglenidse, 133.
Euphorbiaceae, 14.
Evening-primrose, 222.
Eye-spot, 149.

Ferns, IS; prothallia of, 108.
Fertilization, 169, 170, 171, 180;

essence of, 31, 32, 170, 194,

226; 263; in cryptogams, 173;

in phanerogams, 176; result

of, 253.
Flax, 18.
FLEMMING, 125, 163, 169, 183,

230.

Florideae, 208.
FOCKE, 181, 182, 211.
FOL, 169, 229.
Fuchsias, 26.
Fucus, 175.
Fungi, 96.
Gall-roots, 120.
Galls, 118; cynipid-, 118.
GARTNER, 28.
Gemmule, 4, 64, 71, 206.
Geum album, 182; urbanmn, 182.
Germ-plasm, 90, 110, 121.
Germ-tracks, 55, 89, 103; pri-
mary, 93, 104; secondary, 95,

105.

GODLEWSKI, 70, 200.
GOEBEL, 16, 84, 85.
GOETHE, 219, 221, 228, 229.
GOTTE, 82.

Graft-hybrids, 65, 210.
Granules, 4.
Grasses, 15.
GRUBER, 187, 199, 201.
HABERLANDT, 142, 185, 186, 203,

204, 226.

HACKER, 218, 228.
HAECKEL, E., 38, 39, 41, 44, 45,

46, 47, 48, 169, 183, 184, 194,

225.
Halosphaera, 148.

HANSGIRG, 145.

HANSTEIN, 66, 114, 126, 140, 185,
205.

Helianthemum, 261.

Hereditary characters, 24.

Hereditary factors, independent,
11, 34; miscible, 24, 34.

HERTWIG, O., 169, 183, 195, 225.

HERTWIG, R., 169.

Heteroplastids, 82.

Heterostyly, 18.

HOFF, VAN'T, 38.

HOFMEISTER, 128, 129, 131, 134,
206, 230.

Homoplastids, 82.

HOOKER, J. D., v.

Hordeum trifurcatum, 106.

Horse-tails, prothallia of, 108.

Horse, zebra-like stripes of, 23.

Hoya trifurcatum, 106; car-
no sa, 106.

Hyaloplasm, 150.

Hybrids, 221; disjunction of
characters of, 28; progeny of,
254; species, — 251; variety, —
251, 254; vegetative splittings
of, 240.

Hybridization, 27.

Hydrodictyon, 164.

Hydroids, 90.

Idioplasm, 57.

IKENO, 227.

Insects, 238.

Isogametes, 176.

JAGER, 89.

JOHANNSEN, 247.

JOHOW, 144.

JULIN, 145.

KELLOGG, V. L., vi.

KLEBS, 126, 133, 135, 140, 141, 149,
151, 157, 188, 199, 201.

KOLDERUP-ROSENVINGE, 208.
K6LLIKER, 229.
K6LREUTER, 28.

268

Index

KORSCHELT, 185, 186.

KRABBE, 151

Latex-vessels, 208.

Liegesbeckia, 106.

LEMOINE, 222.

Levisticum, 106.

Life-processes, two kinds of, 39.

Liliaceae, 178.

Linaria, genistae folia, 181 ; pur-
purea, 181 ; vulgaris, 181.

LINDEMUTH, 211.

Liverworts, 96.

Lycopersicum, 106.

Lysimachia vulgcuris, 26.

MAC FARLANE, 182, 225.

Maize, 211.

Marchantia polymorpha, 96.

Medicago, falcata, 182; sativa,
182.

Membranes, autonomy of limit-
ing, 160; limiting, 157; plas-
matic, 157, 158.

MENDEL'S law, 253.

Mentha, 16.

Metamorphosis, 73.

MEYER, A., 130, 145, 149.

Micrococcus, 231.

Microsomes, 150.

Mikroplasts, 67.

Mohl, 81, 126, 131, 132, 134, 160,
205.

Molecules, 13; chemical, 37; liv-
ing, 49.

Molluscs, 238.

Monachanthus, 18.

Monoecious plants, 17, 24.

Monoecism, 27.

Monotropa, 98.

Mosses, 96.

MOTTIER, viii.

MULLER, 114.

Muller's bodies, 156.

Muscineae, 96, 104

Mutability, 214.

Mutation-periods, 261.

Myanthus, 18.

Mysostoma, 229.

NAGELI, viii, 24, 57, 58, 59, 81.

Nasturtium officinale, 98.

NAUDIN, 181, 225.

Nectarines, 17.

Nematus capreae, 118; viminalis,
119.

Nepenthes, 14.

Nicotiana, 258.

Nucleo-molecules, 45.

Nucleus, 194, 202; composed of
pangens, 200, 215; double na-
ture of, 227 ; influence in cell,
183; origin of, 198.

NUSSBAUM, 100, 188, 199, 201.

Nymphaea, 260.

Oedogonium, 188.

Oenothera, 260, 261, 262; La-
mar ckiana, 259; muricata, 259.

Oil, formation of, 149; etherial,
12, 15.

Onagra, 260.

Orchidaceae, 14, 178.

Orchis fusca, 258; Mario, 258.

Organism, elementary, 126.

Oscillariae, 82.

OVERTON, 171.

Pangenesis, 63, 73; intracellular,
defined, 215.

Pangenosomes, viii.

Pangens, viii, 7, 49, 70, 74, 193,
195, 215 ; active and latent, 197,
199, 254; transportation of,
201, 202, 204, 215; multiplica-
tion of, 212, 213.

Pansies, 216.

Papaver hybridum L., 28 ; somni-
ferum polycephalum, 20.

Peperomia, 106, 146.

Peregenesis, 44.

Peronosporales, 165.

Petalody of bracts, 73.

Index

269

Petals, increase of, 20.

PFEFFER, 149, 157.

PFLUGER, 41.

Phaseolus multiflorus, 180, 181 ;
vulgaris nanus, 180.

Physiological units, 51.

Plasma-membrane, 42.

Plasson, 45.

Plastidules, 44, 46.

PLATNER, 163.

Poa nemoralis, 119.

Polyps, colony- forming, 94.

Polysiphonia, 208.

Potato, 15.

Primula acaulis var. caulescent,
23, 60.

Primulaceae, 18.

Principles of Biology, 51.

PRINGSHEIM, 96, 104, 164, 186,
187, 189.

Pronuclei, 228.

Propagation, asexual, 247.

Protein and protoplasm, 41 ; ar-
tificial synthesis of, 43; living,
41.

Prothallium, 236.

Protomyces macrosporus, 164.

Protoplasm, 41, 125, 126; arti-
ficial synthesis of, 43 ; com-
posed of pangens, 37, 43, 195,
197, 216; currents in, 205, 216.

Protoplasts, 125; connection of,
208; regeneration of, 139.

Pseudosomatic tracks, 100.

Pyrenoids, 149.

Races, how improved, 31, 32.

Reduction of chromosomes, 237.

Reduction-division, 240.

REES, 85.

REGEL, 98, 99.

Regeneration, 95, 139, 143.

Rejuvenation, 99.

Relationship, systematic, 73.

Reproduction, significance of

sexual, 247, 248, 263.
Rheum, 106.
RIMPARA, 22, 222.
ROBINSON, Miss, 14.
Rock-roses, 261.
Raphanus sativus, 182.
Roses, Alpine, 260.
Roux, 178, 201, 230.
RUCKERT, 228.
. Rumex Acetosella, 16, 98.
Russow, 209.
SACHS, 70, 81, 85, 99, 115, 129,

134, 143, 147, 150, 151, 174.
Sagitta, 94.

SAGIURA, SHIGETAKE, 39.
Salamander, 231.
Salix, 258; purpurea, 119.
Salvia, 260.
Saprolegniaceae, 164.
Sarracenia purpurea, 14.
SCHACHT, 174.

SCHIMPER, 14, 130, 145, 156, 186.
SCHLEIDEN, 79.
SCHMIDT, 143.
SCHWANN, 79, 114.
SCHMIDTZ, 102, 129, 145, 148, 150,

173, 175, 176, 187.

SCHWENDENER, 143.

Scytosiphon lomentarium, 176.
Sea-urchins, 200, 235.
SELENKA, 169.
Self-fertilization, 29, 30.
Sempervirum tectorum, 20.
Sexual characters, secondary, 18.
SHULL, 256.
SINETY, 238.
Siphoneae, 143.
Siphonocladiaceae, 145, 187.
Slredon, 229.

Somatic tracks, 89, 100, 103, 105.
Species, how originate, 256.

270

Index

Species-hybrids, 249.

Specific characters, composition
of, 34.

SPENCER, SO, 51, et seq., 58, 59, 60.

Spermatozoids, origin of, 174.

Spitogyra, 132, 139, 149, 169, 171,
173, 187, 226, 227; Weberi, 171,
172; Zygospore of, 171.

STAHL, 147.

Star-fish, 235.

STRASBURGER, viii, 99, 115, 125,
129, 131, 135, 137, 150, 159, 160,
161, 162, 170, 174, 177, 178, 183,
186, 187, 202, 223, 225, 230, 236,
238, 242, 258.

SUTTON, 218, 230.

Swarm-spores, 149, 164.

Sword-lilies, 222.

Symplasts, 209.

Synapsis, 240, 253.

TANGL, 185, 209.

Tape-worm, 213.

Tax odium, 100.

Thallophyta, 95, 104, 183.

Thistles, 98.

Tonoplast, 152.

Toxopneustes, 229.

Transportation-hypothesis, 207.

Trifolium hybbridum L., 28.

Trimorphism, 27.

Trophoplast, 42, 130, 144.

Turgidity, cause of, 150.

TURPIN, 114.

Ulothrix, 164.

Uridineae, 19.

Urtica, 106.

Vacuoles, 150; contractile, 156;
pulsating, 156; wall of, 152.

Valonia, 145.

VAN BENEDEN, 145, 177, 218, 227,
228, 229.

Vaucheria, 140, 141, 142, 160, 187.

Vanilla planifolia, 149.

Variability, correlative, 73; fac-
tors of, 74; fluctuating, 214;
phylogenetic, 74; species-form-
ing, 214; two kinds of, 214.

Variations, sudden origin of, 22.

Variety-hybrids, 249, 254.

Varieties, how fixed, 31, 32; in-
constant, 251; result of cross-
ing, 249.

VELTEN, 205, 206.

Verbascum blattaria, 182; phoe-
niceum, 182.

VERLOT, 29.

'Veronica longifolia, 224.

Vertebrates, 93.

VILMORIN, 25, 91.

VINES, 14.

VOCHTING, 96, 104, 114, 116.

VOLKENS, G., 14.

WAKKER, 98, 99, 131, 149, 155.

WEBBER, 211, 227.

Weigelias, 26.

WEISMANN, 50, 53, et seq., 58, 59,
60, 65, 68, 79, 80, 90, 91, 103,
110, 202, 210.

WEISS, 147.

WENT, 131, 137, 140, 154, 155, 156,
161, 206.

Wheat, 222.

Wheat-hybrid, 222.

WHEELER, 229.

Whorls, 25.

WICHURA, 32, 259.

Willows, 33, 259.

WILSON, 218.

Worms, 238.

Xenia, 210, 211.

Yucca, 16.

ZACHARIAS, 42, 137, 163, 175.

Zea Mays, 211.

Zimmermann, 144.

Zygnema, 173, 188.

Zygosporeae, 173.

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