xv THE PHILOSOPHY OF ZOOLOGY 617 



Natural Selection. According to Darwin's theory of Natural 

 Selection, nature, i.e., the conditions under which the organism 

 exists, selects certain variations as they arise, very much as the 

 breeder or the gardener selects variations in domestic animals or 

 cultivated plants. Let us see how this selection is carried on. 

 We have seen that there is going on, on all sides, a struggle for 

 existence. It is at first difficult to realise the intensity of this 

 struggle, for there is little appearance of it on the surface. If we 

 consider, however, that a large proportion of living things prey on 

 living things of other groups, and when we bear in mind the 

 extremely small proportion which, in most cases, the surviving 

 individuals of any group bear to the number of young produced, 

 we come to understand that this struggle for existence must be 

 general and intense. 



Now in the case of a species living under tolerably uniform and 

 stable conditions as regards climate, food-supply, and the like, the 

 effects of this struggle would be the survival of the fittest. Of the 

 young produced only a small proportion (in most cases) reach 

 maturity; some of these surviving forms have survived, perhaps, 

 because they have happened to escape being preyed upon by 

 enemies, while others have succumbed ; but there can be little 

 doubt that, in the long run, such individuals will survive as are 

 best fitted to cope with the conditions to which they are subjected 

 such as are swiftest, let us say, in escaping pursuit ; or such as, 

 by their special shade of colour or the nature of their markings, 

 elude the observation of an enemy ; or such as, by reason of their 

 thicker covering, can better endure extremes of cold. Such 

 surviving individuals would, it is assumed, transmit their special 

 properties to their progeny, and there would thus be a gradual 

 approximation towards a better adaptation of the species to its 

 surrounding conditions by virtue of this " survival of the fittest." 



Let us suppose the conditions to change. Gradual changes in 

 climate and other conditions are known to take place owing to 

 subsidence or elevation of the land. But conditions might be 

 changed in many other ways: some animal or plant previously 

 used as food might become exterminated ; or a new enemy might 

 find its way into the district inhabited by the species. Then such 

 individuals as presented variations which enabled them better to 

 cope with the new surroundings would have the advantage over 

 the others, and would have a much better chance of surviving, and 

 leaving progeny. The useful variations thus produced and trans- 

 mitted to the progeny would tend to increase, generation after 

 generation, until a form sufficiently distinct to be regarded as a new 

 species had become developed from the original one. 



The process of survival of the fittest has a reverse side, which 

 has been termed the elimination of the unfit. Of the varieties 

 that appear some are less completely adapted to their surroundings 



