

Chap. VI. ON HETEROSTYLED PLANTS. 257 



common progenitor. But an immense lapse of time 

 will have been necessary in all such cases for the 

 modified descendants of a common progenitor to have 

 spread from a single centre to such widely remote and 

 separated areas. The family of the Kubiaceaa contains 

 not far short of as many heterostyled genera as all the 

 other thirteen families together ; and hereafter no doubt 

 other Eubiaceous genera will be found to be hetero- 

 styled, although a large majority are homostyled. Sev- 

 eral closely allied genera in this family probably owe 

 their heterostyled structure to descent in common; but 

 as the genera thus characterised are distributed in no 

 less than eight of the tribes into which this family has- 

 been divided by Bentham and Hooker, it is almost 

 certain that several of them must have become het- 

 erostyled independently of one another. What there 

 is in the constitution or structure of the members of 

 this family which favours their becoming heterostyled, 

 I cannot conjecture. Some families of considerable 

 size, such as the Boragineas and Verbenacese, include, 

 as far as is at present known, only a single heterostyled 

 genus. Polygonum also is the sole heterostyled genus 

 in its family; and though it is a very large genus, 

 no other species except P. fagopyrum is thus charac- 

 terised. We may suspect that it has become hetero- 

 styled within a comparatively recent period, as it 

 seems to be less strongly so in function than the species 

 in any other genus, for both forms are capable of yield- 

 ing a considerable number of spontaneously self-ferti- 

 lised seeds. Polygonum in possessing only a single het- 

 erostyled species is an extreme case; but every other 

 genus of considerable size which includes some such 

 species likewise contains homostyled species. Lyth- 

 rum includes trimorphic, dimorphic, and homostyled 

 species. 



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