56 CHARLES DARWTX. 



misunderstanding is of such great interest that it 

 is desirable to consider it in detail. 



In the origin of new species by natural selection, 

 the stress of competition determines the survival 

 of favourable individual variations, and these, when 

 by the continued operation of the process they have 

 become constant, are added to those pre-existing 

 characters of the species which are inherited from 

 a remote past, and are witnesses of the operation of 

 natural selection from age to age under ever-changing 

 conditions of competition and variation. It follows, 

 therefore, that the origin of a species can only take 

 place once; for it is infinitely improbable that the 

 same variation would be independently submitted 

 under the same conditions of competition, and added 

 to the mass of inherited characters independently 

 gained in two distinct lines by natural selection 

 acting in the same manner upon the same variations 

 in the same order through all ages. Not only is 

 it inconceivable that the same species could arise by 

 natural selection from distinct lines of ancestry, but 

 it is extremely improbable that the same species 

 could arise independently in more than one centre 

 among the individuals of a changing species ; for in 

 this case, too, it is most unlikely that the same 

 conditions of competition would co-exist with the 

 same favourable variations in the areas inhabited 

 by independent colonies of the same species. 



Under other theories of evolution — direct action 

 of environment, supposed inherited effects of use and 



I 



