Chap. VI. ON HETEEOSTYLBD PLANTS. 261 



been shown that heterostyled plants occur in fourteen 

 natural families, dispersed throughout the whole vege- 

 table kingdom, and that even within the family of the 

 Eubiacea3 they are dispersed in eight of the tribes. We 

 may therefore conclude that this structure has been 

 acquired by various plants independently of inheritance 

 from a common progenitor, and that it can be acquired 

 without any great difficulty — that is, without any very 

 unusual combination of circumstances. 



It is probable that the first step towards a species 

 becoming heterostyled is great variability in the length 

 of the pistil and stamens, or of the pistil alone. Such 

 variations are not very rare : with AmsincJcia spectahiHs 

 and Nolana prostrata these organs differ so much in 

 length in different individuals that, until experiment- 

 ing on them, I thought both species heterostyled. 

 The stigma of Gesneria pen&ulina sometimes protrudes 

 far beyond, and is sometimes seated beneath the 

 anthers ; so it is with Oxalis acetosella and various 

 other plants. I have also noticed an extraordinary 

 amount of difference in the length of the pistil in cul- 

 tivated varieties of Primula veris and vulgaris. 



As most plants are at least occasionally cross-fer- 

 tilised by the aid of insects, we may assume that this 

 was the case with our supposed varying plant ; but 

 that it would have been beneficial to it to have been 

 more regularly cross-fertilised. We should bear in 

 mind how important an advantage it has been 

 proved to be to many plants, though in different 

 degrees and ways, to be cross-fertilised. It migh. 

 well happen that our supposed species did not vary 

 in function in the right manner, so as to become 

 either dichogamous or completely self-sterile, or in 

 structure so as to ensure cross-fertilisation. If it had 



