Physiological Selection. 55 



quired way. But in such cases free intercrossing will 

 have prevented the perpetuation of any morphological 

 changes, save those which have the power of so re- 

 acting on the reproductive system as to produce the 

 physiological change, and thus to protect themselves 

 against the full and adverse power of free intercrossing. 

 We know that slight or initial changes of structure, 

 colour, &c., frequently occur as varieties, and yet that 

 on common areas very few of these varieties become 

 distinct species : free intercrossing prevents any such 

 further divergence of character. But if in the course 

 of many such abortive attempts, as it were, to produce 

 a new species, nature happens to hit upon a structural 

 or a colour variation which is capable of reacting on 

 the sexual system in the particular way required, then 

 this variation will be enabled to protect itself against 

 free intercrossing in proportion to its own development 

 Or, in other words, the more it develops as a morpho- 

 logical change, the more will it increase the physio- 

 logical change ; while the more the physiological 

 change is thus increased, the more will it in turn 

 promote the morphological. By such action and 

 reaction the development of each furthers the develop- 

 ment of the other, till from an almost imperceptible 

 variety, apparently quite fertile with its parent form, 

 there arises a distinct species absolutely sterile with 

 its parent form. In such cases, therefore, it is still 

 the physiological conditions which have selected the 

 particular morphological changes capable of so react- 

 ing on the reproductive system as to produce cross- 

 infertility, and thus to protect themselves against the 

 destructive power of free intercrossing. So to speak, 

 free intercrossing is ah\ays on the watch to level 



