Chap. VI.] OP NATUEAL 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 inevi- 

 tably 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 in- 

 numerable other instances, can we understand the gradu- 

 ated 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 structure 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 may serve, is the sum of many inherited 

 changes, through which the species has passed during 



