Criticisms of T, heory of Natural Selection. 337 
survival of the fittest becomes the winnowing fan, 
whose function it is to eliminate all the less fit in 
each generation, in order to preserve the good grain, 
out of which to constitute the next generation. And 
as this process is supposed to be continuous through 
successive generations, its action is supposed to be 
cumulative, till from the eye of a worm there is 
gradually developed the eye of an eagle. Therefore 
it follows from these suppositions (which are not 
disputed by the present objection), that if it had not 
been for the process of selection, such development 
would never have been begun ; and that in the exact 
measure of its efficiency will the development pro- 
ceed. But any agency without the operation of 
which a result cannot take place may properly be 
designated the cause of that result: it is the agency 
which, in co-operation with all the other agencies 
in the cosmos, produces that result. 
characters. These are the only causes which the theory of descent can 
consistently recognise as producing variations in determinate directions. 
(3) Inasmuch as variation presupposes the existence of parts that 
vary, and inasmuch as the variation of parts can only be in the alterna- 
tive directions of increase or decrease around an average, it follows that, 
in the first instance at all events, every variation, if determinate, must 
be so only in one or other of these two opposite directions. 
(4) In as far as variations are summated in successive generations, so 
as eventually to give rise to new structures, organs, mechanisms, &c., 
natural selection is theoietically competent to explain the facts, without 
our having to postulate the operation of unknown causes producing . 
variations in determinate lines,—or not fuither than is stated in para- 
graphs 1 and 2. 
(5) Nevertheless, it does not follow that there are not such other 
unknown causes ; and, if there are, of course the importance of natural 
selection as a cause of adaptive modification would be limited in pro- 
portion to their number and the extent of their operation. But it is for 
those who, like the late Professurs Asa Gray and Nageli, maintain the 
existence of such causes, to substantiate their belief by indicating them. 
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