404 GENERAL PRINCIPLES 



summed up in a few words. It is the fittest that survive in 

 the struggle for existence. 



819. Natural Selection. — In this brood of rabbits we have 

 imagined a process of natural selection to take place by which 

 the unfit are eliminated. Since the parent rabbits succeeded 

 they must have been fit and, therefore, the young ones which 

 resembled the parents closely would be fit also, provided they 

 Hved under the same conditions. This process would, there- 

 fore, tend to preserve the type of the parents. But conditions 

 change and a locality which at one time is most favorable for 

 a given species may become less so. Moreover, species often 

 migrate. If a locality becomes overcrowded or the food 

 scarce, or enemies too numerous, there is a special impetus 

 given to the migrating tendency. Thus a species may push 

 out into a new and quite different environment where there is 

 a different nature at the work of selecting. Suppose again 

 the rabbits: They have pushed out into colder regions, where 

 the snow lies on the ground for many months. The gray rabbit 

 would be very conspicuous against the snow and a coat of white 

 fur would be decidedly advantageous in winter. Hence in- 

 dividuals with white coats in winter might be selected here, 

 while in other regions the winter gray continues to hold the 

 advantage. The diverse conditions would thus tend to pro- 

 duce two varieties of rabbit or indeed two species, the winter 

 white and the winter gray. So long as the two kinds remain 

 connected by intermediate forms they could be only called 

 varieties, but if the intermediate forms disappear they would 

 be distinct species. This is not intended to be regarded as 

 an explanation of how the two species of rabbits originated. 

 It is simply a hypothetical case which may help one to an under- 

 standing of the method by which natural selection, acting 

 through individual variation, may produce new species. The 

 process of natural selection must necessarily be a very slow 

 one, and there are few historical records of such changes. 



