THE VARIETIES OF FERTILISATION. 319 



In no case is it logical to say that sucli ari'angements are to 

 prevent self-fertilisation. We may well ask why are a 

 comparatively few plants thus provided for, and yet the 

 vast majority are not. If, however, we regard them as 

 results of differentiation brought about by the stimulus of 

 insect agency — so that in certain places hypertrophy has set 

 in and rendered the flov/er hercogamous, in others the 

 androecium is so stimulated and its development so hurried 

 on that the flower becomes protandrous, or its pollen so 

 highly differentiated as to become like that of a distinct 

 species, — we have a reasonable interpretation for these 

 phenomena. Moreover, not one of them is absolute or stable. 

 Thus a hercogamous Orchid can become self-fertilising ; * 



* Since the above was in type, Mr. H. N. Ridley has read a paper, 

 at a meeting of the Linnean Society (Feb. IG, 1888), on "The Self- 

 fertilisation of Orchids," in which he arrives at the same conclusions 

 as Mr. H. 0. Forbes (see above, p. 253, note), finding- that the process 

 is effected in several ways, especially, perhaps, by the degeneration of 

 the rostellum. Moreover, the Orchids which he discovered to be capable 

 of fertilising themselves are not only the most numerous in individuals, 

 but are also the most widely dispersed of the genera to which they 

 respectively belong. He also corroborates Mr. Forbes's observations, that 

 Orchids set but a small percentage of their fruit, although fully exposed 

 to the visits of insects. 



Mr. H. Veitch has also contributed a valuable paper on the 

 " Hybridisation of Orchids," in which he appears to corroborate M. 

 Guignard's observations in every particular (see above, Chap. XVIII.). 



The reader will take note of the significance of the fact that when 

 Mr. Darwin published his work on " The Fertilisation of Orchids," it 

 was thonght that no flowers could equal them in theii- remarkable 

 adaptations for securing the benefits of intercrossing by insect agency, 

 and in their methods of "preventing self-fertilisation." Yet, of all 

 flowering plants, evidence now tends to show that they set the least 

 amount of seed, even when fully exposed to insects ; while the order 

 has furnished materials for two important papers on the many forms 

 and ways by which self-fertilisation is secured in different genera. 



