i26 Darwin, and after Darwin. 



showing the hypothetical divergence of character undergone by 

 the descendants of seven species. Thus, he attributes divergence 

 of character exclusively to the influence of natural selection. 



Now, this argument appears to me unassailable in all save 

 one particular; but this is a most important particular: the 

 argument wholly ignores the fact of intercrossing with parent 

 forms. Granting to the argument that intercrossing with parent 

 forms is prohibited, and nothing can be more satisfactory. The 

 argument, however, sets out with showing that it is in limited 

 areas, or in areas already overstocked with the specific form in 

 question, that the advantages to be derived from diversification 

 will be most pronounced. It is where they "jostle each other 

 most closely" that natural selection will set a premium upon 

 any members of the species which may depart from the common 

 type. Now, inasmuch as this jostling or overcrowding of 

 individuals is a needful condition to the agency of natural 

 selection in the way of diversifying character, must we not feel 

 that the general difficulty from intercrossing previously con- 

 sidered is here presented in a special and aggravated form ? 

 At all events, I know that, after having duly and impartially 

 considered the matter, to me it does appear that unless the 

 swamping effects of intercrossing with the parent form on an 

 overcrowded area is in some way prevented to begin with, 

 natural selection could never have any material supplied by 

 which to go on with. Let it be observed that I regard Mr. 

 Darwin's argument as perfectly sound where it treats of the 

 divergence of species, and of their further divergence into genera ; 

 for in these cases the physiological barrier is known to be 

 already present. But in applying the argument to explain 

 the divergence of individuals into varieties, it seems to me that 

 here, more than anywhere else, Mr. Darwin has strangely lost 

 sight of the formidable difficulty in question ; for in this 

 particular case so formidable does the difficulty seem to me, 

 that I cannot believe that natural selection alone could produce 

 any divergence of specific character, so long as all the in- 

 dividuals on an overcrowded area occupy that area together. 

 Yet, if any of them quit that area, and so escape from the 

 unifying influence of free intercrossing, these individuals also 

 escape from the conditions which Mr. Darwin names as those 



