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large our insight into the hidden relations between the phenomena of nature.

Chapter XI. is devoted to an examination of the law of natural selection, as modified by the law of heredity, and I have here attempted to show that the acceptance of this secondary law will remove the most serious objections to the view that our present forms of life have been brought into existence through the survival of the fittest variations, and I have also called attention to the fact that the law of heredity is itself a result of the law of natural selection.

No one can deny that there are grave objections to the law of natural selection in its original form. Darwin admits this in many places, and able but dissenting critics have stated most of these objections with great ability. The evidence for the law of natural selection is so many sided, so extensive, and so satisfactory, that we may fairly conclude that the difficulties will disappear with greater knowledge, and as none of its hostile critics have proposed anything whatever to take its place, the difficulties which they have pointed out have hardly received from naturalists the attention which they deserve.

One of the most serious objections is that natural selection cannot effect any permanent modification of a race, unless great numbers of individuals vary in essentially the same way at nearly the same time, and that the chances against this are great beyond computation if variations are purely fortuitous in Darwin's sense of the word.

Darwin has acknowledged the weight of this objection, and there is no escape from the conclusion that natural selection fails to account for the origin of species, unless we can show that many individuals tend to vary at the same time. According to our view, the production of gemmules and the consequent variations are due to the direct action of changed conditions upon certain cells of the body, and any change which affects all the individuals of a species will cause the same part to vary in all of them at the same time. This objection to the law of natural selection is thus entirely removed.

The evolution of a complicated organism, or the modification of any part which includes a number of complicated structures, without destroying their harmonious adjustment to each other, demands a very great number of variations, and if these are fortuitous, we may well doubt whether there has been time enough for the evolution of life by natural selection. According to oar theory of heredity, a change in one part of the body is in itself a cause of variation in related parts; and as changes thus tend to occur where and when they are needed, the time which is demanded for the evolution of a complicated organ by natural selection is brought within reasonable limits, and one of the most fundamental objections is thus completely removed.

There are many reasons for believing that variations under nature may not be so minute as Darwin supposes, but that evolution may take place by jumps or saltations. According to our view a change in one part will disturb the harmony of related parts, and will cause their cells to throw off gemmules. A slight change in one generation may thus become in following generations a very considerable modification, and there is no reason why natural selection should not be occasionally presented with great and important saltations.

The law of heredity also enables us to understand the occasional occurrence of parallel or analogous variation, and the parallel evolution of polyphylletic-groups.

INDEX.

Adjustment between internal
and external relations, 40

Adler on parthenogenesis in
gall-wasps, 62

Adult organism, 6

Albrecht on parthenogenesis,
56, 58

Alcippe, 183

Alternation of generations, 115

American naturalist on spike-
horn deer, 298

Anchorella, 179

Ancon sheep, 299

Animalculist, 22

Annelids, 173

Anolis, 198

Antirrhinum, 132

Apple, 90

Apus, 58

Arbacia, 66

Argus pheasant, 201

Aristotle on parthenogenesis, 55

Aristotle on latent sexual char-
acters, 85, 105

Artemia, 58, 91

Arthropoda, sexual differences
in, 173

Asexual reproduction, 11, 17,

143, 249
Ass and horse, hybrids from,

127

Babyrusa, 205

Bachman on variation of tur-
key, 150

Balfour on polar globules, 71

Barnacle, sexual differences in,
179, 181

Barrington on colors of birds,
207

Basset on parthenogenesis, 62
Bechstein on spurred hens, 210
Bee, reproduction of, 9

"parthenogenesis in, 60

"variation of, 145
Beetle, sexual differences in,

191

Bell-bird, sexual differences in,
202

Birds, female modification in,
203

Birds, sexual differences in, 199
"weapons, etc., originally
acquired by male, 199

Bischoff on parthenogenesis,—

Blumenbach on Polish fowl,226

Bombyx, 58

Bonnet on evolution, 20, 85
Branchippus, 173
Buceros, 201
Budding in hybrids, 11
Bud'variation, 84, 144
Buffon on development, 26, 85

Buffon on effect of castration,
106

Butterflies, sexual differences
in, 196

Callionymus, 197
Cattle, hybrids from, 130
Cause of sex, 316
Ceratophora, 198
Chamelion, 199

Changed conditions cause sub-
sequent generations to vary,
149

Cherry, variation of, 149
Cladocera, sexual differences
in, 175

Claus on parthenogenesis, 58
Cohn on parthenogenesis, 66
Color changed by a change of

food, 90
Congenital characters not al-
ways hereditary, 93
Conn on cause of variation, 294
Copepoda, sexual differences
in, 175

Correlated variation, 84, 157
Crabs, sexual differences in, 186
Crayfish, dimorphism in, 188
Crossing as a cause of reversion,
132

Crossing as a cause of varia-
tion, 119

Cryptophyalus, 183

Cultivated plants, variation of,
146

Cyclops, sexual differences in,
175

Dall on saltatory evolution, 83
Daphnia, parthenogenesis in,
57

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variation of
of orange, 147
on causes of variation,
275

on compound person-
ality, 114

on complexity of the
germ, 114

on correllated varia-
tion, 84

on direct action of ex-
ternal conditions,90

on evolution of eye,
282

Spanned peacock,
299

on latent transmission
of sexual characters,
105

on modification of
male butterflies, 196

on pangenesis, 48

on parallel or analo-
gous variation, 302

on reciprocal hybrids,
129

on reversion, 10, 115,
132

on reversion in horse,
133

on selection, 12

on sexual characters
of birds, 199

on sexual selection,
169, 212

on transmission by
each sex, 100

on transmission with-
out fusion, 131

Darwin, on variation, 141, 146,
149, 153
"on variation from

crossing, 120, 122
"on variation of species
of large genera, 153
Development often indirect, 24
Dianthus, variability of hybrid,
124

Direct influence of external con-
ditions, 89

Dog, reversion in, 10
"hybrids from, 131
"variation of, 91, 150

Domesticated animals more va-
riable than wild ones, 142

Edmundston on sea-gull, 93
Education and culture, 272
Elephant, 205
Epigenesis, 20, 27

"and evolution, 24
Evadne, 58

Evolution and division of labor,
313

Bonnet on, 20
"of complicated or-
gans, 156
"definitions of, 20
"Huxley on, 20
"hypothesis logically

imperfect, 82
"morphological aspect
of, 813
saltatory, 157, 296

Falconer on variation of dog

and goat, 91
Female modification, 285
Fishes, sexual differences in,

196

Fish-lice, sexual differences in,
177

Fowls, hybrids from, 129, 131
"modification of female,
222

reversion in, 185

Gall-wasp, parthenogenesis in,
62

Gallus bankiva, reversion to,
135

Galton on pangenesis, 53
"on saltatory evolution,
296

Gartner on variability of hy-
brids, 120, 124

Gegenbaueron nature of ovum,
28

Gelassimus, 189
Gemmules, 82

Gerstaecker on parthenogene-
sis, 56

Godine on hybrid sheep, 129
Goose, inflexibility of, 142

Haeckel on perigenesis, 33, 45
on phylogeny of Me-
dusae, 304
on significance of onti-
geny, 80
Hagen on dimorphism of cray-
fish, 188
Haller on evolution, 20
Huxley on evolution, 20
Harvey, 22

Hemp, variation of, 90
Hen, parthenogenesis in, 67
Heredity, theory of, 16, 80

'' ontogenetic and phy-
logenetic, 811

"and memory, 37

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