312 BOTANY. [Ch. XVI. 



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 hermaphrodite animals, such as snails, which 

 require the concourse of two individuals, although each pos- 

 sesses both the sexual elements. The difference is that in the 

 case of the primrose it is perfect fertility, and not simply fertility, 

 that depends on the mutual action of the two sets of in- 

 dividuals. 



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 between 

 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 " illegitimately " 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 researches 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 f rather than a selected quality. The result of his 

 work on heterostyled plants is of importance as showing that 

 sterility is no test of specific distinctness, and that it depends 

 on differentiation of the sexual elements which is independent 

 of any racial difference. I imagine that it was his instinctive 

 love of making out a difficulty which to a great extent kept him 

 at work so patiently on the heterostyled plants. But it was 

 the fact that general conclusions of the above character could 

 be drawn from his results which made him think his results 

 worthy of publication. 



* 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 incidental 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. 



