Criticisms of Theory of Natural Selection. 337 



survival of the fittest becomes the winnowing Tan, 

 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 theoretically competent to explain the facts, without 

 our having to postulate the operation of unknown causes producing 

 variations in determinate lines, or not further than is stated in para- 

 graphs i 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 Professors Asa Gray and Nageli, maintain the 

 existence of such causes, to substantiate their belief by indicating them. 



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