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