ch. xvl] FERTILISATION OF FLOWERS. 331 



(ii.) Polygamous, Dioecious, and Gynodicecious Plants. 



Aii.) Cleistogamic Flowers. 



The nature of heterostyled plants may be illustrated in 

 the primrose, one of the best^known examples of the class. 

 If a number of primroses be gathered, it will be found that 

 some plants yield nothing but " pin-eyed " flowers, in which 

 the style (or organ for the transmission of the pollen to the 

 ovule) is long, while the others yield only " thrum-eyed " 

 flowers with short styles. Thus primroses are divided into 

 two sets or castes differing structurally from each other. 

 My father showed that they also differ sexually, and that in 

 fact the bond between the two castes more nearly resembles 

 that between separate sexes than any other known relation- 

 ship. Thus for example a long-styled primrose, though it 

 can be fertilised by its own pollen, is not fully fertile 

 unless it is impregnated by the pollen of a short-styled 

 flower. Heterostyled plants are comparable to hermaphro- 

 dite animals, such as snails, which require the concourse 

 of two individuals, although each possesses both the sexual 

 elements. The difference is that in the case of the prim- 

 rose it is perfect fertility, and not simply fertility, that de- 

 pends on the mutual action of the two sets of individuals. 



The work on heterostyled plants has a special bearing, to 

 which the author attached much importance, on the problem 

 of the origin of species.* 



He found that a wonderfully close parallelism exists be- 

 tween hybridisation (i.e. crosses between distinct species), 

 and certain forms of fertilisation among heterostyled plants. 

 So that it is hardly an exaggeration to say that the " ille- 

 gitimately " reared seedlings are hybrids, although both 

 their parents belong to identically the same species. In a 

 letter to Professor Huxley, given in the second volume of 

 the Life and Letters (p. 384), my father writes as if his re- 

 searches on heterostyled plants tended to make him believe 

 that sterility is a selected or acquired quality. But in his 

 later publications, e.g. in the sixth edition of the Origin, 

 he adheres to the belief that sterility is an incidental \ 

 rather than a selected quality. The result of his work on 



* See Autobiography, p. 48. 



t The pollen or fertilising element is in each species adapted to produce 

 a certain change in the egg-cell (or female element), just as a key is adapted 

 to a lock. If a key opens a lock for which it was never intended it is an in- 

 cidental result. In the same way if the pollen of species of A. proves to be 

 capable of fertilising the egg-cell of species B. we may call it incidental. 



