ORGANIC EVOLUTION — THE FACTORS 95 



have already dealt, and to which therefore we need not 

 devote much space. He writes — 



" For now observe the fact which here chiefly concerns 

 us, that the survival of the fittest can increase any 

 serviceable trait, only if that trait conduces to pros- 

 perity of the individual, or of posterity, or of both, in 

 an important degree. There can be no increase of any 

 structure by natural selection unless, amid all the 

 slightly varying structures constituting the organism, 

 increase of this particular one is so advantageous as to 

 cause greater multiplication of the family in which it 

 arises than of other families. Variations which, though 

 advantageous, fail to do this must disappear again. 

 Let us take a case" (pp. 11-2). 



He then proceeds to discuss the numerous qualities 

 which in a herd of deer conduce to survival — keenness 

 of scent, sight, and hearing, excellence of digestion 

 through superiority of teeth, gastric juice, stomach, &c., 

 agility, sagacity, power of resisting flies, cold, and so 

 forth. Founding his argument apparently on the as- 

 sumption that an individual of the herd which is 

 superior as regards one quality will be inferior as 

 regards the others^ he comes to the conclusion — " If 

 these other individuals severally profit by their small 

 superiorities, and transmit them to equally large numbers 

 of descendants, no increase of the variation in question 

 can take place, it must soon be cancelled " (p. 13). 

 In other words, interbreeding must soon swamp the 

 specified variation. 



The fallacy that underlies Mr. Spencer's argument is 

 this. Excellence in one quality is not usually found dis- 

 sociated with excellence in other qualities. The vigorous 

 in one respect may be, and commonly are, vigorous in 

 other respects also. Moreover, the qualities are not 

 isolated — keenness of sight in one deer, keenness of 



