Chap. VI.] OF NATUKAL SELECTION. 243 



allied orchid, namely the Catasetum, is widely different, 

 though serving the same end ; and is equally curious. 

 Bees visit these flowers, like those of the Coryanthes, 

 in order to gnaw the labellum; in doing this they 

 inevitably touch a long, tapering, sensitive projection, 

 or, as I have called it, the antenna. This antenna, 

 when touched, transmits a sensation or vibration to a 

 certain membrane which is instantly ruptured; this 

 sets free a spring by which the pollen-mass is shot 

 forth, like an arrow, in the right direction, and adheres 

 by its viscid extremity to the back of the bee. The 

 pollen-mass of the male plant (for the sexes are separate 

 in this orchid) is thus carried to the flower of the female 

 plant, where it is brought into contact with the stigma, 

 which is viscid enough to break certain elastic threads, 

 and retaining the pollen, fertilisation is effected. 



How, it may be asked, in the foregoing and in 

 innumerable other instances, can we understand the 

 graduated scale of complexity and the multifarious 

 means for gaining the same end. The answer no 

 doubt is, as already remarked, that when two forms 

 vary, which already differ from each other in some 

 slight degree, the variability will not be of the same 

 exact nature, and consequently the results obtained 

 through natural selection for the same general purpose 

 will not be the same. We should also bear in mind 

 that every highly developed organism has passed 

 through many changes ; and that each modified struc- 

 ture tends to be inherited, so that each modification 

 will not readily be quite lost, but may be again and 

 again further altered. Hence the structure of each 

 part of each species, for whatever purpose it ma) 

 serve, is the sum of many inherited changes, through 



