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 Rubiaceae 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 Boraginese and Verbenaceae, 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. 



