1 14 Studies in the Theory of Descent. 



mal or, to express myself generally, why, from a 

 given starting-point, the development of a par- 

 ticular species cannot now attain, even under the 

 most favourable external conditions, any desired 

 goal ; and why, from this starting-point, given 

 courses of development, even when of conside- 

 rable latitude, must be restricted, just as a ball 

 rolling down a hill is diverted by a fixed 

 obstacle in a direction determined by the posi- 

 tion of the latter, and depending on the direction 

 of motion and the velocity at the moment of 

 being diverted. 



In this sense I agree with Askenasy's " fixed" 

 direction of variation ; but not if another new 

 physical force directing variation itself is thereby 

 intended. 7 The explanation of the phenomena 

 does not appear to me to require such an admission, 

 and, if unnecessary, it is certainly not legitimate. 

 According to my view, transmutation by purely 

 internal causes is not to be entertained. If we 

 could absolutely suspend the changes of the ex- 

 ternal conditions of life, existing species would 

 remain stationary. The action of external inciting 

 causes, in the widest sense of the word, is alone able 

 to produce modifications ; and even the never- 

 failing " individual variations," together with the 

 inherited dissimilarity of constitution, appear to 



7 I expressly insist upon this here, because the notice of 

 Askenasy's thoughtful essay which I gave in the " Archiv fur 

 Anthropologie " (1873) has frequently been misunderstood. 



