356 A HISTORY OF BIRDS 



is by no means proved, or generally admitted, a great deal 

 of evidence may be produced to show^ that spatial, geographical 

 isolation is commonly associated w^ith the evolution of new 

 species. 



But we shall probably be near the truth in regarding 

 natural selection as the dominant factor in evolution — as the 

 mill of evolution ; isolation is the " hopper " thereof, which is 

 fed by spontaneous variation. By geographical isolation, for 

 instance, more or fewer individuals temporarily escape the 

 restrictions of natural selection imposed on the individuals 

 occupying the area from which any given species has escaped : 

 thereby these individuals become free to develop incipient 

 characters hitherto held in check. Sooner or later, however, 

 any such fresh development is again checked by natural 

 selection, which inevitably discovers the refuge of the escaped, 

 and imposes thereon the terms under which the newly occu- 

 pied area may be held. 



Romanes' hypothesis, though untenable in its entirety, is. 

 however, most suggestive, and demands a careful examination. 

 He contended, we may repeat, that new species arise only when 

 more or fewer individuals of any given species become isolated, 

 either by physical barriers or by internal, physiological changes 

 affecting reproduction. That is to say, spatial and physiological 

 segregation are the two prime factors in the origin of species. 

 Spatial, geographical isolation he perceived to be of two 

 kinds, (i) Indiscriminate, and (2) Discriminate. When, by 

 geological subsidences, a number of individuals of a species 

 become cut off from the main body of the species and conse- 

 quently no selection of individuals possessing certain peculi- 

 arities in common is made the isolation is Indiscriminate. 

 This form of isolation he likens to that of the shepherd who 

 divides a ilock of sheep without regard to their special charac- 

 teristics. When, however, he places all the white sheep in one 

 field and all the black sheep in another, "he is isolating 

 discriminately". And similarly, when a section of a species 

 develops " a change of instinct determining migration to another 

 area," or when the members of such an imaginary section all 

 develop in common a change of habitat, or change in the choice 

 of food, on the same area as that occupied by the main body 

 of the species, then the segregation is discriminate, 



