i'^6 Darwin, and after Darwin. 



know, however, which in this connexion of things is the 

 cause and which the effect. A useful ancestral structure of 

 the flower may be conserved by an otherwise varying pro- 

 geny, on condition that the progress of diversity be not 

 disturbed by frequent intercrossings. [Therefore, if this 

 condition be satisfied, the structure of the flower in different 

 members of the group will continue constant : here the cause 

 of constancy in the flower (however much variability there 

 may be in the leaves, &c.) is its original inability to hybri- 

 dize.] On the other hand, in species or groups ready to 

 hybridize [or capable of hybridizing], the fixation of a new 

 specific type will require some change in the structure of the 

 flower, and a change considerable enough to alter the con- 

 ditions of fertilization. [Here the reason of the zVzconstancy 

 of the flower in different members of the group is the 

 original aptitude of their ancestral forms to hybridize.] 

 Perhaps there is something in this suggestion, but certainly 

 there are other efficient physiological relations, which are 

 at present unknown. Your theory of physiological selection 

 may serve to explain many difficult facts." 



The Importance of Prepotency. — A. Kerner shows by means 

 of his own observations on sundry species of plants which 

 hybridize in the wild state, that they do so very much more 

 frequently if both, or even if only one of the parent forms be 

 rare in the neighbourhood. This fact can only be explained 

 by supposing that, even in species most prone to hybridizing 

 under Nature, there is some degree of prepotency of pollen 

 of the same species over that of the other species ; so that 

 where both species are common, it is correspondingly rare 

 that the foreign pollen gets a chance. But if there were no 

 prepotency, the two species would blend ; and this Kerner 

 supposes must actually take place wherever two previously 

 separated species, thus physiologically circumstanced, happen 

 to be brought together. (Kerner's paper is published in 



