396 ME. G. J. ROMANES ON PHYSIOLOGICAL SELECTION. 



theory of the origin of genera, families, orders, and classes, than 

 it is a theory of the origin of species. Physiological selection? 

 on the other hand, is almost exclusively a theory of the origin of 

 species, seeing that it can but very rarely have had anything to 

 do with the formation of genera, and can never have had anything 

 at all to do with the formation of families, orders, or classes. 

 Hence, the evidence which we have of the evolutionary influence 

 of physiological selection, unlike that which we have of the 

 evolutionary influence of natural selection, is confined within the 

 limits of specific distinctions. 



Again, physiological selection differs from natural selection in 

 that the variations on the occurrence of which it depends are 

 variations of an unuseful kind. But, if the princi pie acts atll, 

 it must resemble natural selection in being quite as vigilant in 

 the selection, and quite as potent in the formation of organic 

 types ; seeing that any variation in the reproductive system of 

 the kind in question must be preserved by the principle in ques- 

 tion, and this with even more certainty than are the useful 

 variations which furnish material to the working of natural selec- 

 tion. Eor while these useful variations — especially in their 

 incipient stages, when few in number and unjDronounced in 

 character — are obviously exposed to the most serious risk of 

 extinction from intercrossing, there is no such risk in the case of 

 this non-useful variation. Here the obliterating effects of inter- 

 crossing on the new variety are from the first excluded by the 

 very fact of its being a variety, or in virtue of the very peculiarity 

 which distinguishes it as a variety. Physiological selection 

 therefore, has this great advantange over natural selection, — 

 although it is confined to selecting only one kind of variation, 

 and this only in the reproductive system, whenever this one kind 

 of variation occurs it cannot escape the presei^ing agency of 

 physiological selection. Hence, even if it be granted that the 

 variation which aff"ects the reproductive system in this particular 

 way is a variation of comparatively rare occurrence, still, as it 

 must always be preserved whenever it does occur, its influence in 

 the manufacture of specific types must be cumulative, and, there- 

 fore, in the course of geological time, probably immense. 



So much, then, for the resemblances and the differences between 

 the two theories. It only remains to add that the two are com- 

 plementary. I have already shown some of the respects in which 

 the newer theory comes to the assistance of the older, and this in 



