Meaning of the term—Fatal to Wallaceism—Sexual Selection—The law of battle—Female preference—Mutual Selection—Finn’s experiments—Objections to the theory of Sexual Selection—Wallace’s explanation of sexual dimorphism stated and shown to be unsatisfactory—The explanation of Thomson and Geddes shown to be inadequate—Stolzmann’s theory stated and criticised—Neo-Lamarckian explanation of sexual dimorphism stated and criticised—Some features of sexual dimorphism—Dissimilarity of the sexes probably arises as a sudden mutation—The four kinds of mutations—Sexual dimorphism having shown itself, Natural Selection determines whether or not the organisms which display it shall survive.
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.
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._
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.