12 INTRODUCTION [CH. 



remain the same, this struggle for existence will only weed out those 

 individuals least perfectly adapted to their environment, so that 

 the species will be kept up to a high level of adaptation to its 

 surroundings. This elimination of imperfect individuals which results 

 in the survival of the fittest is known as Natural Selection. 

 Thus we can well imagine that if white-haired individuals turned 

 up amongst hares, they would be more conspicuous and hence more 

 easily discovered by the animals which prey on hares. If however 

 the circumstances of a sp'ecies change, a different class of individuals 

 will survive. For instance, if for the greater part of the year the 

 country inhabited by the hares were covered by snow, as is the case 

 in the North of Canada, the whitest-haired individuals would have 

 the best chance, and from generation to generation would be selected 

 until the colour of the hare was totally changed. The progressive 

 modification of species by the agency of natural selection is called 

 evolution. If the modification tends towards simplification of 

 structure it is called degeneration, if on the contrary it tends 

 towards great complexity it is spoken of as differentiation. 



So far the theory shows how a species will become slowly modified 

 as its surroundings change. But it has been postulated that distinct 

 species have arisen from the same ancestors. It is of course not 

 difficult to see that if a species is distributed over a wide area the 

 conditions in different portions may vary independently of one 

 another, and hence the species may become modified in one place 

 in one direction and in another situation in a different direction by 

 the agency of natural selection. So long however as the species 

 inhabits a continuous area this tendency to split up into divergent 

 groups will be checked by inter-breeding between the sections of 

 the species which are thus becoming modified in different directions. 

 But if through geographical changes the species becomes divided 

 into groups of individuals cut off from access to another, then no 

 inter-breeding can take place and in time two species will be formed. 

 Thus when birds have been blown far out to sea and have colonised 

 a distant island they have often given rise to a new species. 

 The same result may be brought about by the sea overflowing a 

 part of the area inhabited by the species, an event which we 

 know from geology to have often occurred. The important fact to 

 be borne in mind is that at bottom the evolution of several species 

 out of one is due to the formation of colonies, and that the same 

 causes which have led to the differences between the American and 

 the Englishman have acted again and again in the world's history so 



