676 Studies in the Theory of Descent. 



cal sense. But as soon as we attempt to do this 

 it is seen that, in the first enthusiasm over the 

 newly discovered principle of selection, the one 

 factor of transformation contained in this principle 

 itself has been unduly pushed into the background, 

 to make way for the other more apparent and 

 better known factors. 



I have for many years insisted that the first, and 

 perhaps most important, or in any case the most 

 indispensable, factor in every transformation, is the 

 physical nature of the organism itself?* 



It would be an error to believe that it is entirely 

 the external conditions which determine what 

 changes shall appear in a given species ; the 

 nature of these changes depends essentially upon 

 the physical constitution of the species itself, and 

 a modification actually arising can obviously be 

 only regarded as the resultant of this constitution 

 and of the external influences acting thereon. 



But if an essential or perhaps even a preponde- 

 rating share in determining new characters is to be 

 undoubtedly ascribed to the organism itself, for 

 a mechanical representation of organic develop- 



14 "Uber die Berechtigung," &c., Leipzig, 1868. In this 

 work will be found briefly laid down the theoretical conception 

 of variability here propounded somewhat more broadly. [In 

 the last edition of the " Origin of Species " Darwin stages, with 

 respect to the direct action of the conditions of life as pro- 

 ducing variability, that in every case there are two factors, 

 " the nature of the organism and the nature of the conditions." 

 6th ed. p. 6. R. M.] 



