ME. G. J. EOMANES ON PHYSIOLOGICAL SELECTION. 377 



render the prevention of intercrossing wholly unnecessary, and 

 thus not in any way to require the protecting influence of phy- 

 siological selection. I will consider these points separately. 



!Pirst, other conditions may co-operate with physiological selec- 

 tion to prevent intercrossing with parent forms, and therefore, in 

 whatever degree such co-operation is furnished, a correspondingly 

 less degree of sterility will be required in order to secure a dif- 

 ferentiation of specific type. Of these other conditions migra- 

 tions and geographical barriers are probably the most important ; 

 and as such barriers may occur in all degrees of efficiency, from 

 wholly secluding small sections of species in oceanic islands to 

 imposing but slight difficulties in crossing streams &c., it is 

 evident that in many cases physiological selection may be thus 

 assisted in a great variety of degrees. Again, even where there 

 are no geographical barriers of any kind, varieties will occasionally 

 be segregated by their different degrees of adaptation to differ- 

 ences of climate — the adaptation having no special reference to the 

 reproductive system, and yet, by determining that the variety 

 shall live under a different climate from the parent form, more or 

 less effectually preventing intercrossing with that form. The 

 same thing applies to varieties occupying stations of their own*, 

 and also, in the case of higher Vertebrata, to all the members of 

 the same variety preferring to pair together, rather than with 

 their parent form, or with other varieties t. In all these cases 

 where the principles of physiological selection have been in any 

 degree accidentally assisted by other conditions, a correspond- 

 ingly less degree of variation in the reproductive system 

 would have been needed to differentiate the species. That is 

 to say, if the variation has been sufficient in amount, or in re- 

 lation to all the other conditions of the time, to prevent inter- 

 crossing with the parent form in any extinguishing degree, the 

 resulting sterility need not always be absolute, even as between 

 compatriots, but may occur in any corresponding degree ; while, 

 as between species which have been independently evolved on 

 different geographical areas, fertility may remain unimpaired, or 

 even be accidentally increased. 



Secondly, in other cases species may have become differentiated 



without the variations requiring to be protected from intercrossing, 



either by physiological, geographical, or any other barriers. In 



these cases, therefore, physiological selection has had no part in 



* See ' Origin of Species,' ed, 6, p. 8. t Ibid. 



