216 THE DARWINIAN THEORY 



many cases essential, to the fertility of the plant and 

 the vigour of the offspring ; and that all mechanisms 

 favouring cross, and hindering self-fertilisation, give 

 an advantage, and are hence preserved and improved 

 through Natural Selection. 



Orchids form an excellent group for the study of 

 these mechanisms of cross-fertilisation, extraordinary 

 modifications of which are found. The flowers are 

 large and conspicuous, and many of them of singular 

 form. The method of cross- fertilisation in Orchis 

 mascula we considered in detail in a former lecture.* 

 We must remember that all these details were 

 worked out by Darwin before he had himself seen 

 any insects visit the particular orchids which he 

 described, the necessary confirmation being supplied 

 in 1873 by Hermann Miiller in his work on the 

 " Fertilisation of Flowers." 



In fine, an extraordinary diversity of devices, of 

 marvellous interest, for ensuring cross-fertilisation, is 

 met with among orchids. In some the pollen cases 

 explode on being touched, and shoot their pollen at 

 the insect. Some flowers are adapted for particular 

 insects, and failure to become acclimatised is now 

 recognised in many cases as being due, not to the 

 plants being unable to live, but to the absence of the 

 proper insects which alone can effect fertilisation, 

 these as a rule being bees and butterflies. This 

 subject may well be described as the Romance of 

 Natural History, and Darwin says : " I never was 

 more interested in any subject in all my life than in 

 this of orchids." 



* See p. 146. 



