554 THE CAUSES OF EVOLUTION 



into existence. If animals have heritable different3es in their 

 resistance to a disease which is an important factor in their lives, a 

 more resistant race may be evolved by a selective process. If wits 

 are more important than strength in the struggle for Hfe, selection 

 may develop a more intelligent type. If concealment is of life- 

 and-death value, coloration and other features that tend to make 

 the individual resemble its surroundings will be at a premium 

 and therefore will be selected. So goes the theory, but all such 

 variations must be heritable, and therefore mutations rather than 

 fluctuations are the variations selected {cj. p. 486). 



Darwin called this process " Natural Selection," because of its 

 resemblance to the " Artificial Selection " by which breeders 

 of animals and plants select the individuals that please the fancies 

 or the necessities of man and thus bring about changes that have 

 been described as evolutionary in the preceding chapter. Herbert 

 Spencer called the same process " Survival of the Fittest," since 

 the individuals best fitted to the conditions of life are the ones that 

 survive. There must, of course, be a limit to the fitness that 

 can be built up, just as there is a limit to the efficiency that can 

 be secured by gradual improvement of a piece of machinery. The 

 speed of a deer or a wolf would be limited by the nature of the 

 bodily mechanism, intelligence by the brain cells, resistance to 

 disease by the chemico-physical reactions of which a given organ- 

 ism might be capable. But there are possibilities of further 

 evolution, as appears in the final item of the chart. 



Change of Environment. — We have spoken of the environment 

 as though it were constant. Yet great changes take place over 

 long periods of time, as when continents are made and unmade by 

 the processes of geologic evolution, or profound climatic changes 

 occur, such as the advent of an Ice Age or the change from forest 

 to desert conditions. These are probably less important than 

 changes of environment that may seem to us insignificant. The 

 introduction or destruction of a plant upon which various animals 

 feed may produce far-reaching changes in the environmental con- 

 ditions of a given species. New enemies entering a district may 

 bring new standards of selection; new parasites or disease-pro- 

 ducing organisms may put a premium upon qualities that have 

 not been hitherto selected. In the complex interplay of forces 

 there is the possibility that conditions, and therefore selection, 

 may remain stable for long periods, or that selection may sud- 



