Evidences of Physiological Selection. 85 



for the origin and permanence of these synoical 

 varieties, in all degrees of differentiation up to well- 

 defined synoical species. Least of all. as we have 

 seen, can natural selection alone have had anything 

 to do with such a state of matters ; while, as we have 

 likewise seen, in all its details it is exactly the state 

 of matters which the theory of physiological selection 

 requires. 



Nevertheless, although this inference is so strongly 

 grounded, we ought to remember that it is only an 

 inference. In order fully to verify the theory of 

 physiological selection, we ought to prove by experi- 

 ment the fact of cross -infertility between these synoical 

 varieties, as we learn that it afterwards obtains between 

 synoical species. It is to be regretted that the theory 

 of physiological selection did not occur to the mind 

 of Nageli, because he would then, no doubt, have 

 ascertained this by actual experiment. As it is, the 

 great value of his observations goes no further than 

 establishing a strong presumption, that it must be 

 selective fertility which causes the progressive dif 

 ferentiation of synoical varieties; and also that, if 

 so, this must be the principal factor in the differentia- 

 tion of vegetable species, seeing that some ninety-five 

 per cent, are of synoical origin. 



Evidence from Experimental Research. 



My paper on Physiological Selection pointed out that 

 the whole theory would have to stand or fall with the 

 experimental proof of the presence or the absence of 

 cross-infertility between varieties of the same species 

 growing on common areas. From the facts and 

 considerations which we have hitherto been dealing 



