Chap. VI. 



ON HETEROSTYLED PLANTS. 



255 



In some of these families the heterostyled condition 

 must have been acquired at a very remote period. 

 Thus the three closely allied genera, Menyanthes, 

 Limnanthemum, and Villarsia, inhabit respectively 

 Europe, India, and South America. Heterostyled 

 species of Hedyotis are found in the temperate regions 

 of North and the tropical regions of South America. 

 Trimorphic species of Oxalis live on both sides of 

 the Cordillera in South America and at the Cape of 

 Good Hope. In these and some other cases it is not 

 probable that each species acquired its heterostyled 

 structure independently of its close allies. If they 

 did not do so, the three closely connected genera of 

 the Menyanthese and the several trimorphic species of 

 Oxalis must have inherited their structure from a 

 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 



