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...... 16

CHAPTER III.'

HISTORY OF 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

CHAPTER IV.

A NEW THEORY OF 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 stands midway between Darwin's theory of

natural selection and Lamarckianism.

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-

Summary of chapter..

CHAPTER VI.

THE EVIDENCE FROM 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...

CHAPTER VII.

THE EVIDENCE FROM 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 vary-Tendency 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..

CHAPTER VIII.

118

140

THE EVIDENCE FROM SECONDARY SEXUAL CHARACTERS..... 166

CHAPTER IX.

THE EVIDENCE FROM SECONDARY SEXUAL CHARACTERS CON-

TINUED.THE CAUSE OF THE EXCESSIVE MODIFICATION OF

MALE CHARACTERS.

The explanation of Daines Barrington and Wallace-Reasons

for considering it inadequate-Darwin's explanation—

History of domesticated races shows that this does not go

to the root of the matter-The view that the male is more

exposed than the female to the action of selection-A

more fundamental explanation is needed-This is fur-

nished by our theory of heredity-Special difficulties-

Summary...

207

CHAPTER X.

THE EVIDENCE FROM THE INTELLECTUAL DIFFERENCES BE-

TWEEN MEN AND WOMEN..

242

CHAPTER XI.

THE THEORY OF HEREDITY CONSIDERED AS SUPPLEMENTARY TO

THE THEORY OF NATURAL SELECTION.

Darwin believes that variations are purely fortuitous-Natural

selection cannot give rise to permanent race modifications

unless many individuals vary in nearly the same way, at

about the same time-The chances against this are very

great if variations are fortuitous-Argument from North

British Review-Darwin acknowledges the great weight

of this objection-It is removed by the theory of heredity

-The co-ordinated modification of complicated organs-

The time demanded by Darwin practically infinite-

Murphy's argument from the complexity of the eye-

Herbert Spencer's illustration-Our theory removes this

difficulty-Mr. Conn's objection-Saltatory evolution—

Evidence that it occurs-Spike horn buck-Ancon and

Mauchamp sheep-Black-shouldered peacock-The the-

ory of heredity accounts for saltatory evolution-Parallel

variation-Evidence of its occurrence-Evolution of the

medusæ-General and special Homologies..

CHAPTER XII.

RECAPITULATION AND CONCLUSION.

275

312

NOV 18 1916

13RARY HEREDITY.

*

CHAPTER I.

WHAT IS HEREDITY?

The development of an animal, with the complex and beautiful structural adjustments, the instincts, habits, and individual 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 heredityCharacteristics 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.

To the ordinary unscientific reader the word heredity may perhaps suggest nothing more than a few curious cases where an odd peculiarity of the parent has been transmitted to the children, or it may recall the hereditary transmission of a tendency to certain diseases, or the mental or moral idiosyncrasies of the parents.

To the breeder of domestic animals or plants it has a somewhat wider significance, and recalls the transmission by choice or fancy breeds of the features which give them their value. To him heredity is the law which enables him to modify his animals and to build up and perpetuate new varieties.