Chap. VI. ON HETEKOSTYLBD PLANTS. 259 



Plants which are already well adapted by the structure 

 of their flowers for cross-fertilisation by the aid of 

 insects often possess an irregular corolla, which has 

 been modelled in relation to their visits ; and it would 

 have been of little or no use to such plants to have 

 become heterostyled. We can thus understand why 

 it is that not a single species is heterostyled in such 

 great families as the Leguminosce, Labiatse, Scrophu- 

 lariacese, Orchidese, &c., all of which have irregular 

 flowers. Every known heterostyled plant, however, 

 depends on insects for its fertilisation, and not on the 

 wind ; so that it is a rather surprising fact that only 

 one genus, Pontederia, has a plainly irregular corolla. 

 Why some species are adapted for cross-fertilisation, 

 whilst others within the same genus are not so, or 

 if they once were, have since lost such adaptation 

 and in consequence are now usually self-fertilised, I 

 have endeavoured elsewhere to explain to a certain 

 limited extent.* If it be further asked why some 

 species have been adapted for this end by being made 

 heterostyled, rather than by any of the above specified 

 means, the answer probably lies in the manner in 

 which heterostylism originated, — a subject immedi- 

 ately to be discussed. Heterostyled species, however, 

 have an advantage over dichogamous species, as all 

 the flowers on the same heterostyled plant belong to 

 the same form, so that when fertilised legitimately by 

 insects two distinct individuals are sure to intercross. 

 On the other hand, with dichogamous plants, early or 

 late flowers on the same individual may intercross ; 

 and a cross of this kind does hardly any or no good. 

 Whenever it is profitable to a species to produce a 



"Tlio l!:ffccts of Cross and Self-fertiliBation,' 1870, p. HI. 



