DIFFICULTIES OF THE THEORY 205 



antenna. This antenna, when touched, transmits a sensation 

 or vibration to a certain membrane which is instantly rup- 

 tured; 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, 

 iertilisation is effected. 



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

 able 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 structure tends to be inherited, so that each modifi- 

 cation 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 its successive adaptations to changed habits 

 and conditions of life. 



Finally then, although in many cases it is most difficult 

 even to conjecture by what transitions organs have arrived 

 at their present state; yet, considering how small the propor- 

 tion of living and known forms is to the extinct and un- 

 known, I have been astonished how rarely an organ can be 

 named, towards which no transitional grade is known to lead. 

 It certainly is true, that new organs appearing as if created 

 for some special purpose, rarely or never appear in any 

 being; — as indeed is shown by that old, but somewhat exag- 

 gerated, canon in natural history of "Natura non facit sal- 

 tum." We meet with this admission in the writings of almost 

 every experienced naturalist; or as Milne Edwards has well 

 expressed it, Nature is prodigal in variety, but niggartl in 



