400 ME. G. J. EOM"ANES ON PHYSIOLOGICAL SELECTION. 



statement of fact, namely, that all natural varieties which have 

 not been otherwise prevented from intercrossing, and which have 

 been allowed to survive long enough to develop any differences 

 worth mentioning, are now found to be protected from intercros- 

 sing by the bar of sterility — that is, by a previous change or 

 variation in the reproductive system of the kind which my theory 

 requires. In many cases, no doubt, this particular change, or 

 variation, has been caused by the season of flowering or of pair- 

 ing having been either advanced or retarded in a section of a 

 species, or to sundry other influences of an extrinsic kind ; but 

 probably in a still greater number of cases it has been due to wbat 

 I have called intrinsic causes, or to the " spontaneous " variability 

 of the reproductive system itself. In order to show how large 

 a part the principles thus explained have probably played in the 

 evolution of species, many arguments, which it would be tedious 

 again to enumerate, have been drawn from the inutility of so large 

 a proportion of secondary specific distinctions, from the swamping 

 effects of intercrossing in the absence of j)hysiological barriers, 

 from the multiplication of species, and from the leading or most 

 general facts of geographical distribution. Lastly, the relations 

 between natural and physiological selection have been shown to 

 be co-operative, the latter allowing the former to act by inter- 

 posing its laws of sterility, with the result that secondary specific 

 distinctions may be either adaptive or non-adaptive in character. 

 On the other hand, natural selection may assist physiological 

 selection by setting a premium both on the primary and on the 

 secondary distinctions — i e. encouraging the work both of ste- 

 rilizing species and of diversifying their cbaracters. 



In conclusion, therefore, it seems to me almost impossible to 

 doubt, when so many large and general facts combine in pointing 

 to the principles of physiological selection, that these principles 

 must be accredited with a highly important share intbe evolution 

 of species. Mr, Darwin has well said, " Erom the laws govern- 

 ing the various grades of sterility being so uniform throughout 

 the animal and vegetable kingdoms, we may infer tbat tbe cause, 

 whatever it may be, is tbe same, or nearly the same, in all cases." 

 This cause, as he candidly shows in the paragraphs from which 

 the quotation is made *, obviously cannot have been natural 

 selection. But to my mind it appears no less obvious that the 



* ' Origin of Species,' ed. 6. p, 248. 



