Darwin s Artificial and Natural Selection 161 



as at rest, during the absence of a danger, as well as during 

 the approach of an enemy. 



"-Nor are we helped here by the assumption of purely inter- 

 nal motive forces, which Nageli, Askenasy, and others have 

 put forward as supplying a mechanical force of evolution. It 

 is impossible to regard the coincidence of an Indian butterfly 

 with the leaf of a tree now growing in an Indian forest as 

 fortuitous, as a lusus natures. Assuming this seemingly me- 

 chanical force, therefore, we should be led back inevitably 

 to a teleological principle which produces adaptive .characters 

 and which must have deposited the directive principle in the 

 very first germ of terrestrial organisms, so that after untold 

 ages at a definite time and place the illusive leaf markings 

 should be developed. The assumption of preestablished 

 harmony between the evolution of the ancestral line of the 

 tree with its prefigurative leaf, and that of the butterfly with 

 its imitating wing, is absolutely necessary here, as I pointed 

 out many years ago, but as is constantly forgotten by the 

 promulgators of the theory of internal evolutionary forces." 



Weismann concludes, therefore, that for his present pur- 

 pose it suffices to show " that cases exist wherein all natural 

 explanations except that of selection fail us," and he then 

 proceeds to point out that even the natural selection of Dar- 

 win and of Wallace also fail to give us a reasonable explana- 

 tion of how, for example, the markings on the wings of the 

 Kallima butterfly have come about. The main reason that 

 he gives to show that this is the case rests on the difficulty of 

 the assumption that the right variations should always be 

 present in the right place. Here "is the insurmountable 

 barrier for the explanatory power of the principle [natural 

 selection] for who, or what, is to be our guarantee that the 

 dark scales shall appear at the exact spots on the wing where 

 the midrib of the leaf must grow? And that later dark 

 scales shall appear at the exact spots to which the midrib 

 must be prolonged? And that still later such dark scales 



