ISOLATION 357 



In the chapter on geographical distribution many instances 

 of Romanes'" indiscriminate" isolation will be found, as for 

 example, by the formation of islands, when, by the encroach- 

 ment of the sea a section of a species becomes cut off (rom the 

 parent body on the mainland. The section so isolated, as has 

 been shown in the chapter referred to, invariably comes at last 

 to differ, more or less markedly, from the parent species. And 

 this because any inherent tendencies to vary in this or that 

 direction will have a better chance of surviving, or rather per- 

 haps because the changed conditions have introduced a new 

 standard of selection. Here, in short, natural selection is really 

 at work, for, from whatever cause these variations arise, only 

 such variations as tend to bring the isolated more into harmony 

 with their environment will survive. Or, to put it another 

 way, any variations, whether of size, or colour, or of habit, or 

 instinct, which tend to produce disharmony with the environ- 

 ment, will bring about extinction. Indiscriminate isolation 

 merely removes the checks to the development of new characters 

 that were enforced under the earlier conditions. And the same 

 holds good of discriminate isolation, as when certain individuals 

 governed by the same motives seek new quarters, or adopt 

 new feeding habits. Isolation, then, whether discriminate or 

 indiscriminate, certainly does play a not unimportant part in 

 the origin of new species, but it is a secondary part. It may be 

 likened to a new turn of the kaleidoscope. If the repressive 

 forces of the old order are removed, new conditions are imposed. 

 The fact that, among birds, as among other animals, island 

 forms differ from those of the parent stock on the mainland is 

 probably due chiefly to the influence of new conditions, and in 

 the case of oceanic islands, to discriminate selection, or what 

 amounts to this. 



As touching physiological selection it seems clear that the 

 potency of this factor in evolution has been overestimated. The 

 evidence afforded by birds, at any rate, seems to afford this 

 hypothesis but little support : and it is probable that had 

 Romanes, in elaborating this hypothesis, sought for his evidence 

 among birds, he would have been less insistent on the magnitude 

 of the part it is supposed to have played in the evolution of 

 species. 



Briefly, Romanes held that new species might arise in the 



