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 lhkely 
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 1s 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.) 
