ME. G. J. ROMANES ON" PHYSIOLOGICAL SELECTION. 375 



which happened so to react on the reproductive system as them- 

 selves incidentally to erect a barrier, which might then serve — 

 as in the parallel case given in my illustration — to protect that 

 particular assemblage of secondary distinctions from extermina- 

 tion when they first arose, and afterwards to admit of their being 

 handed on in ever-increasing degrees of development ? And, in 

 point of fact, that this has been the case (supposing for illustra- 

 tion's sake the primary to have always been the result of secondary 

 distinctions) is proved by the very general association that is now 

 found to subsist between them — an association which can only 

 be accounted for by supposing that all other kinds of secondary 

 distinction failed in what may be termed their struggle for 

 existence, simply because they were not able to rear for them- 

 selves this barrier of sterility. 



Thus, we see, it really makes no essential difference to my 

 theory whether it be supposed, in any given case, that the primary 

 distinction was prior or subsequent to the secondary distinctions. 

 I have given my reasons for believing that in the great majority 

 of cases the primary distinction was, as I have said, the primor- 

 dial distinction ; and, if so, the causal influence of physiological 

 selection in the formation of species was in these cases absolute. 

 But I have also given my reasons for believing that in a minority 

 of cases the secondary distinctions determined the primary dis- 

 tinction ; and, if so, the causal influence of physiological selection 

 was in these cases relative, or conditional on other causes ex- 

 trinsic to the organism. But whether the ultimate causes of the 

 primary distinction be extrinsic or intrinsic, and whether this 

 primaiy distinction be historically prior or subsequent to the 

 secondary distinctions, in all eases (save where intercrossing is 

 otherwise prevented) it must be physiological selection that has 

 been the agency to which the preservation of the secondary dis- 

 tinctions has been due. Eor, as we have now so repeatedly 

 seen, any secondary distinctions, howsoever useful in themselves, 

 must be always liable to extinction almost at the moment of their 

 birth, unless they happen to be protected by the primary 

 distinction. Hence, whether the latter be given by independent 

 variation on the part of the reproductive system itself, or as an 

 indirect and concomitant result of variations taking place else- 

 where, it is equally true that the principles of physiological selec- 

 tion have been at work ; and, therefore, that it is to those 



