MR. G. J. ROMANES ON PHYSIOLOGICAL SELECTION. 365 



this would be tlie case that is required by physiological relation — 

 namely, a variety relatively sterile towards its parent form, while 

 relatively fertile within itself. 



These anomalous cases, however, have only been given to show 

 the highly variable and capricious character' of the reproductive 

 system both in plants and animals; and hence to show that the 

 much less remarkable kind of variation which is required by my 

 theory is not antecedently improbable. But, as a matter of 

 argument, I do not require these anomalous cases ; for enough 

 has been previously said to prove that the particular kind of 

 variation required actually does occur as between individuals, 

 between races, and between species. Nevertheless, for the sake 

 of adducing yet one further argument of an a priori kind, I may 

 notice the very general fact that different varietal characters in 

 parents belonging to the same species persistently refuse to 

 blend in the offspring. This, indeed, may be said to be the rule 

 both in plants and animals *. But the varietal character with 

 which we are concerned belongs to the reproductive system 

 itself, independently of any other part of the organism. There- 

 fore, if this variation follows the rule of variations in general, 

 there must be more difficulty in its blending v/ith the parent (or 

 unchanged) form than there is in its blending with other simi- 

 larly changed forms. But, in this particular case, failure to 

 blend means failure to propagate — i. e. sterility, whether partial 

 or absolute. The varietal. form will thus be more fertile within 

 itself than it is towards its parent stock. 



Aegtjments a postekioei. 



Hitherto the evidence which I have adduced in favour of 

 physiological selection as an agency in the evolution of species 

 is only prima facie. That is to say, although we have evidence to 

 show the occurrence of this particular kind of variation, and 

 although we can see that whenever it does occur it must be pre- 

 served, as yet we have had no evidence to indicate to what extent 

 this particular kind of variation has been at \a ork in the forma- 

 tion of species. Thus far all I have been endeavouring to show 

 is that we have many and weighty considerations of an a priori 

 kind whereby to render the theory of physiological selection 



* See, for example, ' Variation of Plants and Animals under Domestication,' 

 vol. ii. p. 72. 



