485 
a theory which cannot be imagined and which cannot be 
proved by observation. Such a theory must be inconsistent 
with itself and contrary to nature. And this is precisely 
what we have attempted to show that Natural Selection is. 
But these considerations do not seem to affect the 
loyalty of Dr. Weismann to the Darwinian hypothesis. 
And here it is curious to take note of the one point on 
which he seems to be quite confident. He admits that we 
have no means of ascertaining how much variation will 
determine the selection of variants, and then continues 
thus :— 
“ We can only say generally that selection works by the accumula 
tion of very slight variations, and conclude from this that these 
slight variations must possess selection value.” 
This seems to me to be a good specimen of the process 
known as arguing in a circle. But apart from that con 
sideration, it may be well to call to mind what Mr. Darwin 
says upon this subject. Mr. Darwin uses some very strong 
language, the purport of which is to maintain that the 
validity of his theory of Natural Selection depends upon 
the assumption that the variations which occur in nature arc 
slight in quantity. 
“Natural Selection, if it be a true principle, will banish the belief 
in any great and sudden modification of structure.” 
“Why should not nature take a sudden leap from structure to 
structure ? On the theory of Natural Selection we can clearly under 
stand why she should not ; for Natural Selection acts only by taking 
advantage of slight successive variations ; she can never take a great 
and sudden leap, but must advance by short and sure, though slow, 
steps.”—(Origin of Species, p. 156.) 
“ On the theory of Natural Selection we can clearly understand the 
full meaning of that old canon in natural history, Natura non facit 
sal turn."—(Origin of Species, p. /66.)' 
We must not suppose that Mr. Darwin meant simply to 
deny the possibility of a leap so great as to constitute