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 /usus nature. 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 preéstablished 
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 
M 
