opinions on Isolation. 143 



me to ref^ard as almost certainly sound in themselves, 

 and no less certainly requisite as complements of the 

 Darwinian theory. I will now conclude by briefly 

 summarizing these matters of general principle in the 

 form of twelve sequent propositions. And, in doing 

 so, I may ask it to be noticed that the system which 

 these propositions serve to express may now claim, 

 at the least, to be a strictly logical system. For the 

 fact that, not merely in its main outlines, but likewise 

 in its details, it has been independently constructed 

 by Mr. Gulick, proves at any rate this much ; seeing 

 that, where matters of such intricacy are concerned, 

 nothing but accurate reasoning from a common 

 foundation of data could possibly have yielded so 

 exact an agreement. The only difference between us 

 is, that Mr. Gulick has gone into much further detail 

 than I have ever attempted in the way of classifying 

 the many and varied forms of isolation ; while I have 

 laid more special stress upon the physiological form, 

 and found in it what appears to me a satisfactory 

 solution of "the greatest of all the difficulties in the 

 way of accepting the theory of natural selection as 

 a complete explanation of the origin of species" — 

 namely, "the remarkable difference between varieties 

 and species when crossed." 



