CHAP. VI. ON HETEROSTYLED PLANTS. 267 



the same average number of seeds when left to fertilise 

 themselves spontaneously under a net. 



We must therefore look to the appearance of inner 

 or hidden constitutional differences between the indi- 

 viduals of a varying species, of such a nature that the 

 male element of one set is enabled to act efficiently 

 only on the female element of another set. We need 

 not doubt about the possibility of variations in the 

 constitution of the reproductive system of a plant, for 

 we know that some species vary so as to be completely 

 self-sterile or completely self-fertile, either in an appa- 

 rently 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 

 their 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 



* Gartner, ' Bastarderzeuguug Plants under Domestication,' 2nd 



im Pflunzenreich,' 1849, p. 105. edit. vol. ii. p. 109 ; The Effects of 



t 'Origin of Species,' Oth edit. Cross and Self-fertilisation,' p. 463. 



p. 217; ' Variation of Animals and It may bo well here to remark 



