ORIGIN OF ADAPTATIONS AND OF SPECIES 251 



lated forms varied around a mean condition of their own, and 

 no longer around the mean of the species as a whole. 



Besides this, the influences of new food and new climatal con- 

 ditions as means of modification must be taken into account. 

 Furthermore, though a new species might conceivably arise on 

 an island without the aid of natural selection, it is very likely 

 that selection has often played a part in the formation of such 

 a species, as in the apterous or subapterous forms that pre- 

 dominate on oceanic islands. While it is possible that the 

 earliest arrivals were already apterous, and arrived safely be- 

 cause on that account they clung to driftwood instead of flying 

 away, it is probable, on the other hand, that on wind-swept 

 islands the full-winged and more venturesome individuals 

 would be carried out to sea and drowned, leaving the poorly 

 winged and less venturesome ones to remain and transmit 

 their own life-saving peculiarities ; which would become inten- 

 sified by continual selection of the same kind. Romanes, in- 

 deed, regards natural' selection itself as but one form of iso- 

 lation. 



Physiological isolation, which though important will not be 

 discussed here, " arises in consequence of mutual infertility 

 between the members of any group of organisms and those of 

 all other similarly isolated groups occupying simultaneously 

 the same area." (Romanes.) 



