238 THE PRINCIPLES OF HEREDITY 



elimination of the unfittest. To be an effective cause of 

 evolution it must be stringent. Many stringent processes of 

 elimination, each the cause of a high rate of mortality, neces- 

 sarily cause the extinction, not the evolution, of the species 

 subjected to them. There is, therefore, a natural limit to 

 the number of instincts that may be evolved at one time in 

 a species. 1 Nature, then, by evolving memory and its corol- 

 lary reason, has discovered a way out of the difficulty ; and, 

 by supplying that which is a substitute for an infinite number 

 of instincts, has enabled animals to adapt themselves to the 

 increasing complexity of their environments, and thus to 

 achieve a higher evolution. She "is frugal in her opera- 

 tions, and will not be at the expense of a particular instinct 

 to give us that knowledge which experience and habit will 

 soon produce." 



396. But even in man, the least instinctive animal, a 



1 It is possible that an indefinite number of instincts might be evolved 

 in a species provided the conditions were not very onerous, that is, pro- 

 vided the instincts were evolved in succession one by one, or only a few 

 at a time, so that Natural Selection could establish thoroughly each 

 instinct or set of instincts before elimination for the next set began. A 

 well-established instinct (e. g. the sexual instinct), like a well-established 

 physical structure, would have a comparatively slight tendency to 

 regression, and therefore would be maintained by a relatively small 

 amount of elimination. But obviously the evolution of a number of 

 instincts by this process would be very slow. Much swifter and more 

 easy of evolution would be the single power of making mental acquire- 

 ments by means of which each individual in turn could acquire efficient 

 substitutes for a multitude of instincts. We have already noted how 

 there has been amongst the higher animals an evolution of a general 

 power of acquiring immunity against disease, combined with a particular 

 power of acquiring immunity against particular diseases. Similarly, 

 while there has been among animals an evolution of the general power 

 of making mental acquirements, there has been in different species an 

 evolution of particular powers of making mental acquirements in fixed 

 directions. Thus, the capacity of cats for being tamed, for becoming 

 adapted to a novel environment, is a manifestation of the general power, 

 whereas the innate tendency to learn to hunt small animals is an instance 

 of the evolution of the power of making acquirements in a particular 

 direction. Darwin and other writers consider the ability to breed in 

 captivity as the first condition to domestication. But, as regards the 

 higher animals, even more important is the power of making mental 

 acquirements. Otherwise the animal cannot be tamed. We are able to 

 domesticate elephants though they rarely breed in captivity. If they 

 remained as shy and savage as wild elephants we should have to shoot 

 them at sight. They could be then no more domesticated than tigers, 

 much less intelligent animals whose instincts cannot be overpowered by 

 mental acquirements. The only animals that lack great powers of 

 making mental acquirements, which we are able " to domesticate," are 

 certain insects e. g. silk-worms and these are merely kept, not 

 domesticated in any real sense. 



