Opinions on Isolation. 135 



should be accepted as one of the factors of organic 

 evolution *. 



My view from the very first has been that variations 

 in the way of cross-infertility are of frequent occur- 

 rence (how, indeed, can they be otherwise, looking 

 to the complex conditions that have to be satisfied 

 in every case of full fertility ?) ; and, therefore, 

 however many of such variations are destined to die 

 out, whenever one arises, " under suitable conditions," 

 " it must inevitably tend to be preserved as a new 

 natural variety, or incipient species." Among the 

 higher animals which are "comparatively few in 

 number " I think it probable that some slight change 

 of form, colour, habit, &c., must be usually needed 

 either to " superinduce," or, which is quite a dif- 

 ferent thing, to coincide with the physiological change. 

 But in the case of plants and the lower inverte- 

 brata, I see no reason for any frequent concomitance 

 of this kind ; and therefore believe the physiological 



1 Since the above was written, I have heard of some cases which seem 

 to present greater difficulties to our theory than those above quoted. 

 These refer to some of the numerous species of land mollusca which 

 inhabit the isolated rocks near Madeira (Dezertas). My informant is 

 Dr. Grabham, who has himself investigated the matter, and reports 

 as follows : 



" It is no uncommon thing to meet with examples of the same species, 

 sub-fossil, recent, and living upon one spot, and presenting no variation 

 in the long record of descent" Then, after naming these examples, he 

 adds, " All seem to vary immediately on attaining new ground, assuming 

 many aspects in different districts." 



Unquestionably these statements support, in a very absolute manner, 

 Mr. Wallace's opinion, while making directly against my own. It is 

 but fair, however, to add that the cases are not numerous v some half- 

 dozen at the most, and all within the limits of a single genus\ and that, 

 even in the opinion of my informant himself, the facts have not hitherto 

 been sufficiently investigated for any decisive judgement to be formed 

 upon them. 



