250 APPENDIX III-—LETIERS PUBLISHED IN NATURE. 
the causes of divergence are all causes of segregation; while the causes 
of unification, whether of functions or of structures, are causes of 
intergeneration. If the environments which surround the isolated 
portions are the same, the use of the environment, and, therefore, 
the forms of selection, may become divergent; if the use continues 
unchanged, some useless divergence in the method of securing the 
use may appear; or, if all the relations to the environment, whether 
useful or useless, remain unchanged, ‘‘the adjustment of the male and 
female elements to each other” are liable to become slightly diver- 
gent, producing mutual infertility, or the preference of the sexes for 
certain shades or arrangements of color in their mates may become 
slightly different, or, through some slight difference in the hereditary 
elements distributed in each separated portion at the first, one, or all 
of these causes of accumulated divergence may be introduced. I 
think it is evident that we have here a general principle which is as 
applicable to a wide range of divergences as it is to the divergence 
that produces mutual infertility and sterility. 
The context shows that the prominent idea in Mr. Wallace’s mind 
was divergence in the adjustment of the male and female elements, 
through correlation with ‘‘some diversity of form or color,’’ resulting 
from divergent forms of natural selection, which had been induced by 
exposure to ‘‘somewhat different conditions of life.” But if the rea- 
soning is correct in the sentences I have quoted above it gives an 
explanation of similar divergences when the separated portions are 
exposed to the same environment and where there is no possible 
advantage to be gained by divergence. This is one of the principles 
I have used in the explanation of the divergences of Sandwich Island 
land mollusks; and I think that in the earlier stages of the develop- 
ment of infertility between allied forms it is often the only expiana- 
tion that is applicable. It should, however, be remembered that, for 
divergence of this kind, it is not always necessary that the isolation 
should be either complete or very long continued, and that, when the 
forms that are not fully fertile with each other meet and more or less 
commingle, there is, through the very laws of propagation, without 
any aid from natural selection, a constant increase in the ratio of the 
pure breeds to the mongrels, and an accumulating intensity in the 
segregative instincts and the physiological incompatibilities. As this 
point has been fully discussed in my paper on ‘‘Divergent Evolu- 
tion,’’ I do not need to enlarge on it here. 
There is, however, another phase of the subject which is indicated 
by Mr. Wallace’s suggestion that infertility depends on “‘such a 
delicate adjustment”’ that it is more easily affected by isolation than 
