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