Opinions on Isolation. 131 



which Mr. Wallace is specially considering in the 

 part of his book where the sentence above quoted 

 occurs. How, then, does he meet this case? He 

 meets it by assuming that in all the numerous 

 adjacent valleys of a small island there must be 

 as many differences of environment, each of which 

 is competent to induce slight varietal changes on 

 the part of its occupants by way of natural selection, 

 although in no one case can the utility of these 

 slight changes be surmised. Now, against this ex- 

 planation there are three overwhelming considerations. 

 In the first place, it is purely gratuitous, or offered 

 merely in order to save the hypothesis that there 

 can be no other cause of even the most trivial change 

 in species than that which is furnished by natural 

 selection. In the second place, as Mr. Gulick writes 

 to me in a private letter, " if the divergence of 

 Sandwich Island land molluscs is wholly due to 

 exposure to different environments, as Mr. Wallace 

 argues on pages 147-150, then there must be com- 

 pletely occult influences in the environment that 

 vary progressively with each successive mile. This 

 is so violent an assumption that it throws doubt 

 on any theory that requires such support." In the 

 third place, the assumption that the changes in 

 question must have been due to natural selection, 

 is wholly incompatible with the facts of isolation 

 elsewhere namely, in those cases where (as in that 

 of Ireland) a large section of species, instead of 

 a small section, has been indiscriminately isolated. 

 Mr. Wallace, as we have seen, inadvertently alludes 

 to these " many other cases of isolation " as evidence 

 against apogamy being per se a cause of specific 

 K 2 



