360 ANIMAL BIOLOGY 



theory of the origin of species by means of natural selection. 

 According to this theory there is a process of selection 

 continually going on in nature producing results more or 

 less analogous to those produced by man by the process of 

 artificial selection. Natural selection is the outcome of the 

 struggle for existence which is ever waging in the organic 

 world. Organisms tend to multiply so rapidly that if 

 their propagation were not checked there would not be 

 space enough on the earth to support them. According to 

 Jordan a codfish may produce as many as 9,100,000 eggs 

 per year. "If each egg were to develop, in ten years the 

 sea would be solidly full of codfish." The elephant which 

 is reckoned the slowest breeder of all animals would 

 produce in 800 years, according to Darwin, 19,000,000 

 elephants from a single pair. In a few years more these 

 would increase until the entire earth would be covered by 

 elephants. 



Now it is obvious that animals do not actually increase 

 at this rapid rate. If the individuals of a species are not on 

 the increase, as they generally are not, only two individuals 

 from a single pair will on the average live to maturity. 

 Their numbers are kept down by various checks such as 

 limitation of food, climate, diseases and numerous enemies. 

 It is only occasionally when organisms are introduced into 

 a new country where for a time there is little to check their 

 increase, that the high rate of multiplication, which we 

 have described, is approximated. When rabbits were 

 introduced into Australia and New Zealand they found few 

 competitors and they multiplied so rapidly that they be- 

 came a serious nuisance and much effort has since been 

 expended to rid the country of the pest. Similarly when 

 cattle were introduced into South America by the Span- 

 iards they increased in numbers to. such an extent that 

 before many years the plains of that country were overrun 



