vii ON THE INFERTILITY OF CROSSES 181 



so, though we have at present no evidence whatever in 

 support of it, it remains to be considered whether such physio- 

 logical varieties could maintain themselves, or whether, as in 

 the cases of sporadic infertility already discussed, they would 

 necessarily die out unless correlated with useful characters. 

 Mr. Romanes thinks that they would persist, and urges that 

 " whenever this one kind of variation occurs it cannot escape 

 the preserving agency of physiological selection. Hence, even 

 if it be granted that the variation which affects the re- 

 productive system in this particular way is a variation of 

 comparatively rare occurrence, still, as it must always he 

 preserved whenever it does occur, its influence in the manu- 

 facture of specific types must be cumulative." The very positive 

 statements which I have italicised would lead most readers to 

 believe that the alleged fact had been demonstrated by a 

 careful working out of the process in some definite supposed 

 cases. This, however, has nowhere been done in Mr. Romanes' 

 paper ; and as it is the vital theoretical point on which any 

 possible value of the new theory rests, and as it appears so 

 opposed to the self-destructive effects of simple infertility, 

 which we have already demonstrated when it occurs between 

 the intermingled portion of two varieties, it must be carefully 

 examined. In doing so, I will suppose that the required 

 variation is not of "rare occurrence," but of considerable 

 amount, and that it appears afresh each year to about the 

 same extent, thus giving the theory every possible advantage. 

 Let us then suppose that a given species consists of 100,000 

 individuals of each sex, with only the usual amount of 

 fluctuating external variability. Let a physiological variation 

 arise, so that 10 per cent of the whole number — 10,000 

 individuals of each sex — while remaining fertile inter se 

 become quite sterile with the remaining 90,000. This 

 peculiarity is not correlated with any external differences of 



themselves infertile or quite sterile ; and it is this infertility or sterility of the 

 hybrids that is the characteristic — and was once thought to be the criterion 

 — of species, not the sterility of their first crosses. Hence we should not 

 expect to find any constant infertility in the first crosses between the distinct 

 strains or varieties that formed the starting-point of new species, but only a 

 slight amount of infertility in their mongrel offspring. It follows, that Mr. 

 Romanes' theory of Physiological Selection — which assumes sterility or in- 

 fertility between first crosses as the fundamental fact in the origin of species 

 — does not accord with the general phenomena of hybridism in nature. 



