268 CONCLUDING REMARKS Chap. VL 



ently spontaneous manner or from slightly changed 

 conditions of life. Gartner also has shown * that the in- 

 dividual plants of the same species vary in their sexual 

 powers in such a manner that one will unite with a 

 distinct species much more readily than another. But 

 what the nature of the inner constitutional differences 

 may be between the sets or forms of the same varying 

 species, or between distinct species, is quite unknown. 

 It seems therefore probable that the species which 

 have become heterostyled at first varied so that two 

 or three sets of individuals were formed differing in 

 the length of their pistils and stamens and in other 

 co-adapted characters, and that almost simultaneously 

 the ^^reproductive powers became modified in such a 

 manner that the sexual elements in one set were 

 adapted to act on the sexual elements of another set; 

 and consequently that these elements in the same set 

 or form incidentally became ill-adapted for mutual 

 interaction, as in the case of distinct species. I have 

 elsewhere shown f that the sterility of species when 

 first crossed and of their hybrid offspring must also be 

 looked at as merely an incidental result, following from 

 the special co-adaptation of the sexual elements of the 

 same species. We can thus understand the striking 

 parallelism, which has been shown to exist between the 

 effects of illegitimately uniting heterostyled plants and 

 of crossing distinct species. The great difference in the 

 degree of sterility between the various heterostyled spe- 



* Gartner, ' Bastarderzeiigung act on the reproductive system of 



im Fflanzenreich,' 1849, p. 165. most organisms, it is probable that 



t' Origin of Species,' 6th edit, the close adaptation of the male to 



p. 247 ; ' Variation of Animals and the female elements in the two 



Plants under Domestication.' 2nd forms of the same heterostyled 



edit. vol. ii. p. 169 ; ' The Effects species, or in all the individuals 



of Cross- and Self-fertilisation,' p. of the same ordinary species, could 



463. It raajr be well here to re- be acquired only under long-con^ 



mark that, judging from the re- tinned nearly uniform conditions 



markable power with which ab- of life, 

 mptly changed conditions of life 



