526 MANUAL OF ELEMENTARY ZOOLOGY 



next generation does not inherit the peculiar features of these 

 individuals, which therefore gradually cease to appear in the 

 species. Thus in a cold country those members of a 

 species of mammals which had not thick fur would either 

 die of cold or be so enfeebled that they could not compete 

 with the rest in the struggle for food or mates, or in a 

 herd of wild horses pursued by wolves the slowest would 

 be killed, so that the next generation would be descended 

 from those members of the species which were best clad 

 or swiftest, as the case might be, and would of course 

 inherit their peculiarities. The result of this process 

 would be the selection by Nature of certain individuals 

 to breed, just as a breeder selects sheep with thick wool 

 or cattle with certain muscles well developed, and, breeding 

 from these in preference to the rest, alters his breed of 

 sheep or cattle. 



Evolution by natural selection depends upon three 

 factors : variation, the struggle for existence, and heredity, 

 (i) We have seen that all animals are variable. It is true 

 that there is great doubt of the heritability of the 

 " acquired " variations x produced in the individual by the 

 action of its surroundings at various stages of its existence, 

 but there are other variations of whose inheritance there is 

 no doubt. These are known as germinal variations or 

 mutations, and are due to inborn tendencies which arise in 

 the germs in some way which is not as yet understood. 

 It is often urged that variations are too small to give an 

 effective advantage in the struggle for existence, but muta- 

 tions may be so great that in regard to them this difficulty 

 does not exist. It may be that evolution takes place by 

 the inheritance of such mutations alone. (2) The struggle 

 for existence involves, not merely reaching a certain 

 standard of fitness to cope with the surroundings, but a 

 competition between individuals, because, while the off- 

 spring are always more numerous than the parents, the 

 total number of individuals in the species does not as a 

 rule increase, being already as many as the conditions of 



1 Small acquired variations due to the effect of the environment 

 during growth are known as fluctuations. Thus one of a litter of young 

 being better nourished in the womb may be larger and stronger, but 

 will not transmit those qualities to its offspring. 



