178 EVOLUTION AND MAN S PLACE IN NATURE 



by way of intelligence. Natural selection acts only 

 by slight modifications of structure or instinct, each 

 profitable to the individual under its conditions of 

 life. 1 The absence of reference to intelligence is 

 marked here. Survival of the fittest, taken as the 

 expression of each successive stage in the history of 

 evolution, involves organic modification of the social 

 impulse. Under natural selection, which is one of 

 the surest inductions of this age, the argument for 

 continuity of organic evolution depends on the reign 

 of passion in animal life, consequently implying that 

 intelligence has not supplied the law of progress in 

 the natural history of the earth. If, therefore, it be 

 true, as it obviously is, that under the law of natural 

 selection there has been continuity up the whole 

 scale of organic life, it follows that, as we do not on 

 this line find warrant for severance of lower orders 

 from higher, the classification of the higher mammals 

 as intelligent must rest on some other basis than is 

 supplied in the history of the common struggle for 

 existence. That struggle has uniformly shown the 

 power of passion, standing in contrast with intelligence. 

 We have now reached a point where it is needful 

 to bring man into contrast with animals low in the 

 scale. Here we must make room for development of 

 the difference between Instinct and Intelligence. 

 This will prepare for more exact conclusions as to 

 the relations of animal to human intelligence. Until 

 we have set these two more clearly in antithesis, it 

 is impossible to make way for reliable inductions. 

 Instinct cannot be left as an undefined residuum 

 into which perplexities may be cast an unexplored 



1 Origin of Species, p. 211. 



