Chapter VIII: 345

The Factors of Evolution

Variation along definite lines and Natural Selection are undoubtedly
important factors of evolution—Whether or not sexual selection is a
factor we are not yet in a position to decide—_Modus operandi_ of
Natural Selection—Correlation an important factor—Examples of
correlation—Correlation is a subject that requires close
study—Isolation a factor in evolution—Discriminate
isolation—Indiscriminate isolation—Is the latter a factor?—Romanes’
views—Criticism of these—Indiscriminate isolation shown to be a
factor—Summary of the methods in which new species arise—Natural
Selection does not make species—It merely decides which of certain
ready-made forms shall survive—Natural Selection compared to a
competitive examination and to a medical board—We are yet in
darkness as to the fundamental causes of the Origin of Species—In
experiment and observation rather than speculation lies the hope of
discovering the nature of these causes.

Footnotes 389

Index 389

LIST OF ILLUSTRATIONS

FACING PAGE

Heck’s Curassow feeding Young Bird, which has the Plumage of the Hens of the Globose Curassow, its Father’s Species _Frontispiece_

_By permission of the Avicultural Society._

A Turbit belonging to Mr H. P. Scatliff 92

_From “The Modern Turbit,” published by “The Feathered World,” London._

Yellow-Rumped and Chestnut-Breasted Finches, with Specimens in Transitional State 98

On the left, the yellow-rumped finch; on the right, the
chestnut-breasted; birds in state of change in the middle.
_By permission of the Avicultural Society._

Male Amherst Pheasant 122

The chief colours of this species (_Chrysolophus amherstiæ_) are
white and metallic green, so that it is very different in
appearance from its near ally the gold pheasant.

Harlequin Quail (_Coturnix delegorguei_) 124

_By permission of the Avicultural Society._

Rain Quail (_Coturnix coromandelica_) 124

The markings on the throats of these quails are of the type usually
put down as “recognition marks,” but as the Harlequin Quail is
African and the Rain Quail Indian, the two species cannot possibly
interbreed. The pattern, then, can have no “recognition”
significance.
_By permission of the Avicultural Society._

Bouru Friar-Bird 222

Like most of the group to which it belongs, this honey-eater
(_Tropidorhynchus bouruensis_) is a soberly coloured bird, but is
noisy, active, and aggressive.
_By permission of Messrs Hutchinson & Co._

Bouru Oriole 222

This “mimicking” oriole (_Oriolus bouruensis_) is of the same tone
of colour as its supposed model the Friar-bird of the same island.
_By permission of Messrs Hutchinson & Co._

King-Crow or Drongo 232

This very conspicuous black bird (_Dicrurus ater_), ranging from
Africa to China, is a striking feature of the landscape wherever it
occurs.
_By permission of Messrs Hutchinson & Co._

Drongo-Cuckoo 232

The fork of the tail in this bird is unique among cuckoos, but is
nevertheless much less developed than in the supposed model, and
may be an adaptation for evolutions in flight, as such tails
usually appear to be.
_By permission of Messrs Hutchinson & Co._

Shikra Hawk 236

The upper surface of the tail, not shown in this drawing, exactly
corresponds with that of the cuckoo “mimic.”
_By permission of Messrs Hutchinson & Co._

Hawk-Cuckoo 236

This species (_Hierococcyx varius_) is commonly known in India as
the “Brain-fever bird.”
_By permission of Messrs Hutchinson & Co._

Brazilian Troupial 284

This species (_Icterus vulgaris_) is that most frequently seen in
captivity; the pattern of colour is found in several other allied
forms.
_By permission of Messrs Hutchinson & Co._

Indian Black-Headed Oriole 284

Several other orioles besides this (_O. melanocephalus_) have the
black head.
_By permission of Messrs Hutchinson & Co._

Queen Whydah 298

This species (_Tetraenura regia_) is a typical example of seasonal
sexual dimorphism, the male being long-tailed and conspicuously
coloured only during the breeding season, and at other times
resembling the sparrow-like female.
_By permission of the Foreign Bird Club._

Courtship of Skylark 314

Illustrating display by a species with no decorative colouring or
sex difference.

THE MAKING OF SPECIES