Evidences of Physiological Selection. 79 



a proportion of vegetable species should have origi- 

 nated in intimate association with their parent or 

 sister types, is clearly unintelligible on the theory of 

 natural selection alone ; there obviously must be some 

 other form of homogamy which, whether or not in 

 all places associated with natural selection, is the 

 primary condition to the differentiation. Such, 

 I hold with Nageli, is a logical necessity ; and this 

 whether or not I am right in believing the other 

 form of homogamy in question to be selective fertility. 

 But I go further and say, Surely there can be no 

 rational question that this other form of homogamy 

 must have been, at any rate as a highly general rule, 

 the one which I have assigned. For how is it that 

 in these ninety-five per cent, of cases, where vegetable 

 species are growing intimately associated with their 

 nearest allies, there is no hybridizing, or blending 

 and relapsing to the original undifferentiated types? 

 We know well the answer. These are fully differen- 

 tiated species, and, as such, are protected from mutual 

 intercrossing by the barrier of mutual sterility. But 

 now, if this bar is thus necessary for preserving the 

 specific distinctions when they have been fully 

 developed, much more must it have been so to admit 

 of their development ; or, otherwise stated, since we 

 know that this barrier is associated with " synoical " 

 species, and since we clearly perceive that were it 

 withdrawn these species would soon cease to exist, 

 can we reasonably doubt that their existence (or 

 origin) is due to the previous erection of this 

 barrier? If synoical species were comparatively 

 rare, the validity of such reasoning might be open 

 to question ; or, even if we should not doubt it in 



