1899] THE SCOPE OF NATURAL SELECTION 119 



in organic evolution. So far all facts point to the conclusion that 

 variations under stable conditions are definite, under unstable con- 

 ditions indefinite, and this definiteness and indefiniteness occur under 

 precisely those conditions which the theory of natural selection would 

 lead one to expect ; hence, unless definite variability can be shown to 

 occur under conditions which selection could not have produced, the 

 facts adduced by the Lamarckian School are favourable rather than 

 otherwise to the Neo-Darwinian position. 



To realise how far the theory of selection is capable of explaining 

 the facts of organic evolution, it is necessary to bear in mind the 

 postulates on which the theory is founded. 



1. It is obvious that Natural Selection can only act by preserving 

 or eliminating the complete organism. Selection must therefore be 

 organismal. This Darwin and other selectionists have clearly recog- 

 nised. 



2. As the whole organism must survive, if the favourable variation 

 or variations are to be preserved, it follows that certain minor 

 unfavourable variations may also be preserved if they happen to exist 

 in an individual which survives on account of its major favourable 

 variations. And since no individual is completely adapted to its 

 environment, it follows that there must be always a variable amount 

 of residual unfavourable variability in every organism. 



3. This residual unfavourable variability may be of considerable 

 utility under changed conditions. 



4. Complementary specialisation of parts, as Spencer has shown, 

 is favourable to successful competition, and as it is the whole organism 

 that is selected or eliminated, it follows that any weakness of one 

 specialised part, since it would disturb the balance of all, would be 

 detrimental. The more complex the organism, the more specialised 

 the structures, the more dependent one part will be on the others for 

 its existence, hence a complementary specialising tendency will be 

 favoured by selection, and therefore all struggles of one part of an 

 organism with another will be reduced to a minimum. 



It is clear that there must be some underlying criterion which 

 determines whether any given organism shall be selected or not, and 

 that criterion must be the net result of its adaptability to its environ- 

 ment. One organism may conceivably survive, by its possession of a 

 large number of small favourable variations, while another may survive 

 in virtue of a single valuable one, but in each case it would be the 

 whole value of that organism which determined its survival. This 

 fact is continually disregarded by opponents of the Neo-Darwinian 

 position, yet this selection of the organism as a whole is the funda- 

 mental postulate from which the theory of selection starts. Thus it is 

 not uncommon to read criticisms bearing on the early development of 

 some organ, in which the inadequacy of selection is supposed to be 

 proved by the writer demonstrating, or believing he has demonstrated, 



