llomanes. 



THE DIFFERENTIATION OF SPECIES. 17 



over and over again, though he seems to have underestimated its importance 1 , and it forms 

 the keynote of Wallace's ' Island Life.' The more the geographical distribtion of birds is 

 studied, the more doubtful it seems to be that any species of bird has ever been differentiated 

 without the aid of geographical isolation. 



A theory of isolation has, however, recently been propounded by Mr. Romanes, in a 

 paper read before the Linnean Society on the 6th of May last year, entitled " Physiological 

 Selection ; an Additional Suggestion on the Origin of Species," which is deserving of special 

 attention. Mr. Romanes has done great service in calling attention to the " swamping 

 effects of free intercrossing ;" but the only conclusion at which I can arrive is that the Physiologi- 



. . . cal Isolation. 



theory of Physiological Selection is untenable, though the paper is a very valuable contribu- as- pro- 



tion to the literature of Evolution. It is seldom that the difficulties of Natural Selection ?, ou e y 



from Fortuitous Variations have been so clearly, so impartially, but so candidly set forth. 



If Mr. Romanes has failed to prescribe a remedy, he has accurately diagnosed the disease ; 



but, in my opinion, he has attacked the wrong organ. I agree with him that Natural 



Selection from Fortuitous Variations will only account to a very limited extent for the 



evolution of an existing species, and not at all for the differentiation of a new one. The 



point on which we differ is as to how the theory is to be amended so as to make it 



work. Mr. Romanes proposes to add Physiological Selection to Natural Selection to 



make it perfect. I fail to see that the position is thereby improved. I think that the 



weak point lies not in the Selection but in the Variation. If the variations be accidental 



I cannot see how they can be perpetuated, either by Natural or by Physiological Selection, 



except under a combination of coincidences so extraordinary that it can only be regarded 



as phenomenal. If the variations be not accidental then no kind of Selection is required 



to cause Evolution ; its only function is to increase the pace at which Evolution proceeds. 



It is impossible to avoid coming to the conclusion that Variation is not accidental, that 



there is no such thing as Fortuitous Variation, and that Spontaneous Variation, like 



Spontaneous Generation, is a myth. 



The theory of Physiological Isolation propounded by Mr. Romanes is an attempt to 

 account for the sudden creation of a species without going through the preliminary 

 stages of subspecies, incipient species, imperfectly segregated species, inosculating species, 

 morphological species, or by whatever name a species in the process of formation may be 

 called. It assumes that the number of species is so great that geographical isolation cannot 

 account for their existence, an argument very analogous to that of a naturalist who should 

 say that the number of men in this country is so great that they cannot all have been 

 children, and should thereupon invent a theory to account for the production of adults 



1 " Isolation also is an important element in the changes effected through Natural Selection. * * * Moritz 

 Wagner has lately published an interesting essay on this subject, and has shown that the service rendered by 

 isolation in preventing crosses between newly formed varieties is probably greater even than I have supposed." 

 (Darwin, ' Origin of Species,' 5th ed. p. 120.) 



D 



