132 
Rev. J. T. Gulick on the 
Unstable Adjustments disturbed by Isolation . 
There is a sentence in another chapter of Mr. Wallace’s 
hook w hieh attributes to isolation (though without recognizing 
the important results that must follow) just that kind of 
influence in introducing a certain class of physiological diver- 
gences, which I claim for it in introducing not only physio- 
logical, but also psychological and morphological divergences* 
I claim that there is in many species more or less variation 
with unstable adjustment in the habits which determine what 
forms of food it shall appropriate, and that, when a few indi- 
viduals of such a species (the offspring perhaps of a single 
female) are isolated, this adjustment is often so disturbed by 
the failure of the few individuals to completely represent the 
average character of the species and by their being freed from 
competition and wide interbreeding with those of their own 
kind that divergent habits of feeding are formed. I further 
claim that for the production of this result it is not at all 
necessary that the environments presented in the isolated 
districts should differ in any respect. Indeed, if all but one 
pair of a variable species should be destroyed, the descendants 
of that pair, remaining in the same area and under the same 
environment, would probably differ more or less from the 
original stock. Those that breed together must have habits 
that enable them to do so ; and the offspring of those that 
interbreed widely will for the most part inherit the powers 
and habits that enabled their ancestors to interbreed widely ; 
but if the offspring of a single family are carried to an isolated 
area presenting the same environment, there will be nothing 
to ensure the perpetuation of exactly the original powers and 
habits, unless the power of heredity is such that each pair is 
sure to transmit the complete average character of the whole 
species ; and this is not the condition of all species that pair, 
if of any. Within the limits of each freely interbreeding 
portion of a species a mutual harmony and adjustment of 
habits is preserved, because it is the condition of propagation 
within those limits ; but between portions that are prevented 
from interbreeding there is nothing but heredity to prevent 
divergence in the kinds of adjustment ; and in variable species 
the probability is that divergence will in time show itself more 
or less distinctly. Though Mr. Wallace considers this reason- 
ing fallacious when applied to divergence in habits, he uses 
an exactly parallel reasoning in the portion of the following 
passage which I designate by italics : — u It appears as if fer- 
tility depended on such a delicate adjustment of the male and 
female elements to each other that } unless constantly kept up by 
