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PKEFACE.

The subject which is treated in this book has occupied my thoughts for ten years or more, but I have refrained from publishing my views, as I hope that I may some time be able to submit them to the test of experiment.

Many experiments have suggested themselves to me, but as most of them involve the cultivation and hybridization, for many generations, of such animals and plants as will thrive and multiply in confinement, they can only be carried out by some one who has the means for experimental researches, and who has also a permanent home in the country, where organisms of many kinds may be kept under observation for years, and where many specimens of hybrids between various wild and domesticated species can be reared to maturity.

My own studies have been in a different province of natural science, and it has therefore seemed best to publish this volume in order to call renewed attention to this most fascinating subject.

I have little hope that my views will be permanently accepted in the form in which they are here presented, but I do hope that they may serve to bind together and to vitalize the mass of facts which we already possess, and that they may thus incite and direct new experiments.

If this book should serve to turn the attention of others into this channel, and should thus ultimately help us to a clearer insight into the nature of the forces which have acted, and still act, to guide the evolution of life, this result will far outweigh the acceptance or rejection of the speculations which are here advanced.

CONTENTS.

CHAPTER I.

WHAT IS HEREDITY?

The development of an animal, with the complex and beauti-

ful structural adjustments, the instincts, habits, and in-

dividual traits of its parents is one of the most wonderful

phenomena of the material universe—Heredity is not due

to the external conditions which act upon the ovum, but

to something within the ovum itself—The phenomena of

reversion—Asexual and sexual heredity—Possibility of an

explanation of heredity—Characteristics which are now

hereditary were at one time new variations—Heredity and

variation are opposite aspects of the same problem—We

may hope that a more perfect acquaintance with the laws

of heredity will remove many objections to the theory of

natural selection .' 5

CHAPTER II.

HISTORY OF THE THEORY OF HEREDITY.

Requisites of a theory of heredity—Historical sketch of specu-

lation on heredity—Evolution hypothesis of Bonnet and

Haller—Ovists and spermists—Modern embryological re-

search has shown that it is impossible to accept the evolu-

tion hypothesis in its original form—Buffon's speculations

upon heredity fails to account for variation—Hypothesis

of epigenesis—This hypothesis is logically incomplete—

The analogy between phylogeny and ontogeny gives no

.real explanation of the properties of the ovum—Haeckel's

plastidule hypothesis—This hypothesis is not logically

complete unless it involves the idea of evolution—Jager's

hypothesis—Ultimate analysis shows that this is at bottom

an evolution hypothesis—No hypothesis of epigenesis is

satisfactory—No escape from some form of the evolution

hypothesis—This conclusion is accepted by Huxley 10
1 CHAPTER III.

HISTORY OP THE THEORY OF HEREDITY—(Continued).

Some form of the evolution hypothesis a logical necessity—

Darwin's pangenesis hypothesis—This is an evolution

hypothesis, since all the characteristics of the adult are

supposed to be latent in the germ—Miscellaneous objec-

tions to it—These objections do not show that it conflicts

with fact—Difficulty in imagining detailed working is no

reason for rejecting it—Galton's experimental disproof—

There are many reasons for believing that the sexual ele-

ments have different functions—The evidence from par-

thenogenesis—Polar-cell hypothesis—The evidence from

hybrids, from variation, and from structures confined to

one sex—The pangenesis hypothesis recognizes no such

difference in the functions of the reproductive elements—

We must therefore distrust its absolute correctness—Sum-

mary of last two chapters 47

CHAPTER IV.

A NEW THEORY OP HEREDITY.

The objection to the hypothesis of pangenesis would be

almost entirely removed if it could be simplified—State-

ment of a new theory—Heredity is due to the properties

of the egg—Each new character has been impressed upon

the egg by the transmission of gemmules—Tendency to

form gemmules is due to the direct action of external con-

ditions—The ovum is the conservative element—The male

cell is the progressive element—This theory has features

of resemblance to most of the hypotheses which have been

noticed—It fills most of Mivart's conditions also—It is not

necessary to assume that the ovum is as complicated as

the adult—There are many race characters which are not

congenital—There are many congenital characters which

are not hereditary—Direct action of external conditions—

Our theory 6tands midway between Darwin's theory of

natural selection and Lamarckianism 80

CHAPTER V.

ON THE OPINION THAT EACH SEX MAY TRANSMIT ANY

CHARACTERISTIC WHATEVER.

The argument from hybrids—This argument is inconclusive

—The argument from the homology between the ovum

and the male cell—Homology does not involve functional

similarity—The argument from the dual personality of

each individual; from reversion; and from polymorphism

—These phenomena admit of a simpler explanation—

Summitry of chapter 99
CHAPTER VI.

THE EVIDENCE PROM HYBRIDS.

Importance of the subject—It furnishes a means of analyzing

or isolating the influence of each sexual element—Hybrids

very variable—Hybrids from domesticated races more

variable than those from wild races—The descendants of

hybrids more variable than the hybrids themselves—The

offspring of a male hybrid and the female of a pure species

are much more variable than those of a female hybrid and

the male of a pure species—These facts inexplicable on

any view, except the one here presented—Reciprocal

crosses—They differ in fertility and in structure—The

. difference is exactly what our theory requires—Difficulty

in explaining transmission of characters without fusion—

Reversion caused by crossing—Two kinds of reversion—

Summary 118

CHAPTER VII.

THE EVIDENCE PROM VARIATION.

Causes of variation—Changed conditions of life induce varia-

bility—No particular kind of change is necessary—Varia-

bility is almost exclusively confined to organisms produced

from fertilized ova—Bud variation very rare—History of

the Italian orange—The frequency of variation in organ-

isms produced from sexual union, as compared with its

infrequency in those produced asexually, receives a direct

explanation by our theory of heredity*—Bud variation

more frequent in cultivated than in wild plants—Our

theory would lead us to expect this—Changed conditions

do not act directly, but they cause subsequent generations

to varyTendency to vary is hereditary—These facts

perfectly explicable by our theory—Specific characters

more variable than generic—Species of large genera more

variable than those of small genera—A part developed in

an unusual way highly variable—Law of equable variation

—Secondary sexual characters variable—Natural selection

cannot act to produce permanent modification unless

many individuals vary together—Our theory is the only

explanation of the simultaneous variation of many in-

dividuals—This theory also simplifies the evolution of

complex structures—Saltatory evolution—This is ex-

plained by our theory of heredity—Correlated variation

of homologous parts—Parts confined to males more vari-

able than parts confined to females—Males more variable

than females—Summary of last two chapters 140

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