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 eves on the righl 

 side and the other always on the left. As either arrangement would 

 be equally useful in the environment of cither 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. It is 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 inter sc, there is nothing t<> prevenl 

 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. 



