Download The Law of Heredity - a Study of the Cause of Variation, and the Origin of Living Organisms, Second Edition, Revised PDF

TitleThe Law of Heredity - a Study of the Cause of Variation, and the Origin of Living Organisms, Second Edition, Revised
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LanguageEnglish
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Total Pages365
Table of Contents
                            Brooks-Ch00.pdf
Brooks-Ch01.pdf
Brooks-Ch02.pdf
Brooks-Ch03.pdf
Brooks-Ch04.pdf
Brooks-Ch05.pdf
Brooks-Ch05.pdf-2.pdf
Brooks-Ch06.pdf
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Brooks-Ch08.pdf
Brooks-Ch09.pdf
Brooks-Ch10.pdf
Brooks-Ch11.pdf
Brooks-Ch12.pdf
Brooks-Index.pdf
                        
Document Text Contents
Page 1

MALE AND FEMALE WOOD DUCK, TO SHOW SECONDARY
SEXUAL CHARACTERS.

LFronL photographs of slufled specimens in the Collection of the Mary-
land Academy of Sciriices 1

Page 2

THE

LAW O F HEREDITY.
A STUDY OF TEE

CAUSE. OF VARIATION,

AND THE

O R I G I N OF LIVING ORGANISMS,

BY

W. I(. BROOKS,
ASSOCIATE IN BIOLOBY, Joms HOPKINS UXIVEBSITY.

BALTIMORE AND NEW YORE:
JOHN XURPHY & CO., PUBLISHERS.

Page 182

The Evidence from Variation. 155

2CTrdurnl #election caniiof produce Race Motlijicaiion
tinless the #ante Part tends t o vary i l l . a iVumber of
Iiidiuiduals at the Sirvne Tints.
Tliis argument, which seems to me to be the most im-

portnnt one wliicli has ever beeii adduced against the
theory of natural selection, was first adraiiced by a writer
in the North B1itish Review in June, 1876.

The anthor points out tliat since the cliance of sur-
viral of m y lx~rticular individual wliicli is borii is very
slight indeed, the birth of an individual with any par-
ticular slight advant:ige, and its conseqnen t superiority
over its fellows, would not be sufficient to over-balance
the cliance of its destruction. The objection, wliicli is
purely logical, and not experimental, will be stated at
length in amtlier place. At present the fact that those
who are best qualified to judge, Darwin among them,
hare acknowledged its great weight, will suffice to shorn
that i t is a real and' valid objection, and that the foot-
hold of the theory of natural selection wonlcl be greatly
strengthened if we could show that the causes which
produce variation act in such a way as to cause the mme
part to wry at tho same time in great numbers of in-
dividuals.

According to our theory of heredity, this will gener-
ally be the case. We suppose that an unfavorable change
in the environment of a particular cell causes this cell to
throw 08 gemmnles. It is plain that a change in the
external world, which unfavorably affects any partic-
ular cell or group of cells in one individual, will usually
affect the corresponding cells of other individuals of the
species at the same time. JVlien any particular cell is
prolific of gemmnles in one individual of a species, the
same thing will usually be true of the same cell in other

Page 183

156 Heredity.

individuals, and tlic corrcsponding cell will tliereforc bc
a hybrid, and will tend to vary in many dcscendants.

I n each of tliese descendants this hybrjcl will be com-
posed of almost identical clements, aiid they wil l all tcncl
to vary in the same or nearly the same manner; and as
each variation causes other cells to throw otT gemmnles,
the number of individuals which arc similarly modified
will tend to increase from generation to generation,
and natural selection will therefore act, not on a single
exceptional individual, but upon a grcat number, all of
wliicli are modified in essentially the samc way.

If Fariatioft is Purely Fortuitous, the Evolzction of a
Corriplicated Organ composed of H a i q Pnrts by h-at-
ural Selection demands a Period o j Time is
almost Injinite.
This obvious objection .to the law of natural selec-

tion lias been so frequently discussed that it is un-
necessary to dwell upon it at present, cspecially as I shall
examine i t in detail in another place. At prcscnt I will
onlycallattention to thefact tliatavariation inany part of
a complicated organ will, in itself, disturb the harmonious
adjustment of other parts, and mill thus cause them to
throw off gemmules, and thus to induce variability in
the next generation.

The fact that change is needed in any part will be the
cause of variation in this part, and the time which is
needed to restore all parts of an organ to a position of
equilibrium will thus be almost in6nitely redirced. The
argument of those who hold that life has not existed
upon the earth long enough for the evolution of :dl the
adaptations of nature by the sclection of fortuitous
variations will thus lose all its weight.

Page 364

Index. 826

Sexual differences in lucifer,
184
notodelphys, 177
pit is , 193
orcliestia, 1S7
ostracoda, 175
papilio turnus, 237
paradise birds, 237
pliasmidae, 236
pollicipes, 181
rotifera, 171
snphirrina, 176
scalpellum, 181
shrike, 237
social insects, 237
tanais, 188

Sexual dimorphism, 187
" reproduction, 11,17,143,

" selection, 169, 212

" modification of male,

'' variation in, 297

289

Sheep, hybrid, 129, 131

222

Siebold on parthenogenesis, 55
Simpson on hermaphroditism,

" on latent transmis-
109

sion, 106
Sitana, 198
Smerinthus, 59
Solenobia, 61
Sow, parthenogenesis in, 67
Spurred hen, 210
Spathogaster,
Species of large genera, 153
Speculations on heredity, 18
Spermatozoa, discovery of, 21
Spencer, definition of life, 38

'' on Irish elk, 287

Spike-horn deer, 298
Staphylinid3c, 194
Strongylocentrotus, 66
Sweet pea, hybrid, 132

Tanais, 188
Theory of heredity, 319, 16
Tiger, liybrid,.l30
Transmission without fusion,

Turkey, variation of, 150
131

Uhler on polymorphism, 238

Variability, how caused, 82
" of.offspring of hy-

brids, 122
" of sexual charac-

ters, 154
Variations, 13, 17

cau'jes of, 140, 275, 293
caused by climate, 142.

" change of food,142
" crossing, 119
" excessof food, 143

correllated, 157
of exceptional parts, 153
of homologous parts, 158
of hybrids, 18
law of equable, 154
of male butterflies, 195
of organisms produced sex-

parallel or analogous, 302
and sexual reproduction, 249
of species of large genera, 153
summary of chapter on, 161

ually, 143, 249

Viragines, 105
Von Baer, 22

Page 365

336

birds, 208
" on polymorphism, 238

Walrus, 205

Index.

brid willow, 124
Wilson on parthenogenesis, 66
Wollff on heredity, 22

Wart hog, 205
Waterton on hen with male

Weismann on parthenogenesis,
characters, 106

Walsh, law of equable varia- 1 Xiphophorus, 198 tion, 154
Parrell on removal of oviduct,

on variation of dog, 150
105

''

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