UNSTABLE ADJUSTMENTS AND ISOLATION. 249 
derived from the difference in the methods. So, easy recognition of 
other members of the species is of the highest importance for every 
species; but difference in ‘‘recognition marks”’ in sections of the same 
variety separated in different districts of the same environment is no 
advantage. Under the same conditions, habits of feeding may 
become divergent; but, since any new habit that may be found ad- 
vantageous in one district would be of equal advantage in the other 
district, the divergence must be attributed to some difference in the 
activities of the two portions of the species. 
I have recently observed that, of two closely allied species of flat- 
fish found on the coasts of Japan, one always has its eyes on the right 
side and the other always on the left. As either arrangement would 
be equally useful in the environment of either species, the divergence 
can not be considered advantageous. 
II. UNSTABLE ADJUSTMENTS AS AFFECTED BY ISOLATION.* 
In a brief passage in his volume on ‘‘Darwinism,’’ Mr. Wallace 
refers to a principle which seems to me to be worthy of much wider 
application than he has given to it. Itis a key which requires only 
a little filing to prepare it for unlocking some difficult problems in 
divergent evolution. Speaking of the infertility of crosses, he says 
(p. 184): 
It appears as if fertility depended on such a delicate adjustment of the male and 
female elements to each other that unless constantly kept up by the preservation 
of the most fertile individuals, sterility is always ready to arise. * * * So 
long as a species remains undivided, and in occupation of a continuous area, its 
fertility is kept up by natural selection; but the moment it becomes separated, 
either by geographical or selective isolation or by diversity of station or of habits, 
then, while each portion must be kept fertile cuter se, there is nothing to prevent 
infertility arising between the two separated portions. 
Here is an application of the principle of segregation (or of like to 
like in groups that do not cross) in which indiscriminate separation 
is followed by increasing divergence in the different portions, not 
because they are exposed to different environments, not because there 
is any advantage in such divergence, not because there is any need 
that the function should be performed more perfectly in one portion 
than in the other, but because intergeneration, which is the principle 
by which correspondence of function is secured, has been suspended 
for some generations; and, in the absence of intergeneration, neither 
natural selection, nor any other principle, is capable of preserving 
complete correspondence. In organisms that reproduce sexually 

* Published in Nature, May 8, 1890. 
