82 'APPENDIX. 



fulness of the imitation, but on numerous other factors, 

 such as the frequency and sharp-sightedness of the 

 enemies of the species, the fertility of the species, their 

 frequency and persecution in earlier developmental 

 stages, and so forth, in brief, on their need of protec- 

 tion on the one hand and on their other means of pro- 

 tection on the other. 



Now all this cannot be exactly calculated in any 

 given case, and it will be better, instead of haggling 

 about individual cases concerning which we can never 

 judge with certainty, to take the position adopted in 

 the text and say : Since the utility of the initial stages 

 must be assumed unless we are to renounce forever the 

 explanation of adaptation, let us then take it for 

 granted. No contradiction of facts is involved in this 

 assumption ; in fact, even individual variations exist 

 whose eventual utility can be demonstrated, for ex- 

 ample, the invisible differences enabling Europeans of 

 certain constitutions to resist the attacks of tropical 

 malarial fevers, or the differences of structure, like- 

 wise not directly visible, which enable palms from the 

 summits of the Cordilleras to withstand our winter 

 climate better than palms of the same species from 

 along the base-line of the mountains ; and so on. 



VII. THE ASSUMPTION OF INTERNAL EVOLUTIONARY 



FORCES 



Definite variation was not only postulated in the 

 last decade by Nageli and Askenasy, but has also been 

 repeatedly set up in recent years by various other 

 authors. The Rev. George Henslow, in his book The 

 Origin of Species Without the Aid of Natural Selec- 

 tion, 1894, regards the variations occurring in the state 



