358 ME. G. J. ROMANES ON PHYSIOLOGICAL SELECTION. 



sterility towards the varietal form, with unimpaired fertility 

 towards the parent form. But a little thought will show that 

 this criticism would be shallow. The important fact is that among 

 a number of individuals of the same species, all exposed to 

 apparently the same conditions of life, some of the number so far 

 deviate from the specific type in respect of their reproductive 

 systems as to be absolutely sterile with certain members of their 

 own species, while remaining perfectly fertile with other members. 

 In terms of the above criticism, therefore, this fact might be 

 translated into saying that if the reproductive system can be 

 proved to undergo so remarkable a variation as that of individual 

 incompatibility, much more is it likely to undergo the " opposite " 

 variation, wherein a similar incompatibility would extend to a 

 larger number of individuals. Eor certainly the most remarkable 

 feature about this individual incompatibility is the fact of its being 

 only individual. It would not be nearly so remarkable, or 

 physiologically improbable, that such incompatibility shoidd run 

 through a whole race or strain. Therefore, the fact of individual 

 incompatibility appears to me to furnish most important evidence 

 of my theory ; for it proves that even the most apparently capri- 

 cious and wholly unaccountable variations may spontaneously 

 arise within the limits of the reproductive system — variations 

 which, physiologically considered, are much more remarkable, or 

 antecedently improbable, than anything that my theory requires. 

 2. Baces. — But of even more importance to us is the direct 

 evidence of such a state of matters in the case of varieties, 

 breeds, or strains. Incompatibility between individuals is, indeed, 

 of very great importance to my theory, because it constitutes 

 the first link in a chain of direct evidence as to the actual 

 occurrence of the particular kind of variation on which the theory 

 depends ; here we have, as it were, the first beginning in an 

 individual organism of a change which, under suitable condi- 

 tions, may give rise to a new strain, and so eventually to a new 

 species. But, seeing that the individual is so small a constituent 

 part of his species, unless his peculiar incompatibility has reference 

 to the majority of other individuals, so that it becomes only the 

 minority of the opposite sex with whom he can pair, the proba- 

 bility is that the peculiar condition of his reproductive system 

 would not be perpetuated by heredity, but would become ex- 

 tinguished by intercrossing. As I have already said, it is, 

 physiologically considered, even more remarkable that such 



