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