132 Rev. J. T. Gulick on the 



Unstable Adjustments disturbed by Isolation. 



There is a sentence in another chapter of Mr, Wallace's 

 book which attributes to isolation (though without recognizinf^ 

 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 s}>ecies 

 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: — ^^ It appears as if fer- 

 tility depended on such a delicate adjustment of the male and 

 female elements to each other thaty unless constantly kept up) by 



