XT.] SPECIFIC GENESIS. 269 



Such, then, is the singular sensitiveness of the generative 

 system. 



As to the means by which that system is affected, we 

 see that a variety of conditions affect it ; but as to the 

 modes in which they act upon it, we have as yet little if 

 any clue. 



We have also seen the singular effects (in tailed Lepi- 

 doptera, &c.) of causes connected with geographical dis- 

 tribution, the mode of action of which is as yet quite 

 inexplicable ; and we have also seen that there appears to 

 be an innate tendency in certain groups (birds of paradise, 

 &c.) to develop characters of a peculiar kind. 



It is, to say the least, probable that other influences 

 exist, terrestrial and cosmical, as yet unnoted. The 

 gradually accumulating or diversely combining actions of 

 all these on highly sensitive structures, which are them- 

 selves possessed of internal responsive powers and ten- 

 dencies, may well result in occasional repeated productions 

 of forms harmonious and vigorous, and differing from the 

 parental forms in proportion to the result of the com- 

 bining or conflicting action of all external and internal 

 influences. If, in the past history of this planet, more 

 causes ever intervened, or intervened more energetically 

 than at present, we might a priori expect a richer evolu- 

 tion of forms more radically differing one from another 

 than any which could be produced under conditions of 

 more perfect equilibrium. At the same time, if it be true 

 that the last few thousand years have been a period of 

 remarkable and exceptional uniformity as regards this 

 planet's astronomical relations, there are then some grounds 

 for thinking that organic evolution may have been ex- 

 ceptionally depressed during the same epoch. 



