260 CONCLUDING REMARKS CHAP. VI. 



of their flowers for cross-fertilisation by the aid of in- 

 sects 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 Leguminosse, Labiatse, Scrophulariaceas, Orchideae, 

 &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 large 

 number of seeds, and this obviously is a very common 

 case, heterostyled will have an advantage over dioecious 

 plants, as all the individuals of the former, whilst 



* ' The Effects of Cross and Self-fertilisation,' 1876, p. 441. 



