CHAP.Vin. GENERAL UOJSOiUSIONS. 345 



respfict to the plants belonging to the gyno-dioecious 

 sub-class, or those which co-exist as hermaphrodites 

 and females, it has been proved that they yield a 

 much larger supply of seed than they would have 

 done if they had all remained hermaphrodites ; and we 

 may feel sure from the large number of seeds pro- 

 duced by many plants that such production is often 

 necessary or advantageous. It is therefore probable 

 that the two forms in this sub-class have been sepa- 

 rated or develmjed for this special end. 



Various hermaphrodite plants have become hetero- 

 styled, and now exist under two or three forms ; and 

 we may confidently believe that this has been effected 

 in order that cross-fertilisation should be assured. 

 For the full and legitimate fertilisation of these plants 

 pollen from the one form must be applied to the 

 stigma of another. If the sexual elements belonging 

 to the same form are united the union is an illegiti- 

 mate one and more or less sterile. With dimorphic 

 species two illegitimate unions, and with trimorphic 

 species twelve are possible. There is reason to believe 

 that the sterility of these unions has not been specially 

 acquired, but follows as an incidental result from the 

 sexual elements of the two or three forms having been 

 adapted to act on one another in a particular manner, 

 so that any other kind of union is inefficient, like 

 that between distinct species. Another and still more 

 remarkable incidental result is that the seedlings 

 from an illegitimate union are often dwarfed and 

 more or less or completely barren, like hybrids from 

 the union of two widely distinct species. 



