166 GBOEGE JOHN EOMANES issi- 



common area is prevented, and specific difEerentiation 

 rendered possible. Innumerable varieties are known 

 to occur which do not pass into distinct species, 

 the reason being that this initial variation, that is, 

 incipient infertility whereby the swamping efiects of 

 intercrossing might be obviated, was lacking, and the 

 variations became re-absorbed. That is, given any 

 degree of sterility towards the parental form which 

 does not extend to the varietal form, then a new 

 species must take its origin. Without the bar of 

 sterihty, in Mr. Eomanes' opinion, free intercrossing 

 must render the formation of species impossible. 

 Mutual sterihty is thus the cause, not the result, of 

 specific differentiation. As regards the occurrence of 

 this initial variation, the reproductive system is known 

 to be highly variable, its variability taking the form 

 either of increased fertihty, or of sterihty in all degrees, 

 and depending on either extrinsic causes (changes of 

 food, climate, &c.), or on an intrinsic cause arising in 

 the system itself. 



From the nature of this additional factor at work 

 in the formation of species, Mr. Eomanes called his 

 theory 'physiological selection.' 



Physiological selection is conceived of as co- 

 operating with natural selection, the former allowing 

 the latter to act by interposing its law of sterihty, 

 with the result that the secondary specific characters 

 may be either adaptive or non-adaptive in character. 



To Miss C. E. Bomanes. 



Aix-les-Bains : May 1886. 



The Linnean Society paper went off admirably. 

 There was a larger attendance than ever I saw there 

 before. But this may have been partly due to the 

 president (Lubbock) having had a paper down for the 

 He was considerate enough to mth- 



