POPULATION PEOBLEMS 193 



hria) has in a few years gained a majority over the 

 typical form in some parts of England ; a dark variety 

 of Rice-bird (Oryzivora) is superseding the typical form 

 in the West Indies. We know of a number of these 

 changes occurring at present in wild nature, and we 

 know what is done by analogous methods in domestica- 

 tion and cultivation. 



But besides strength, quickness, cleverness, armour, 

 weapons, inconspicuousness, and so on, there is another 

 quahty that has a great deal to do with survival, and 

 that is fertihty. Variants with valuable life-saving 

 qualities will become the dominant type of the species 

 more rapidly if they are also more prolific than their 

 neighbours. Other things equal, low birth-rate may be 

 a serious handicap. The success of a strain or species 

 in short, depends in part on its relative fertility. 



So, as every one knows, a great many animals are 

 prodigiously fertile. A cod has several milhons of 

 eggs ; if these all developed into codlings and these 

 into codfish there would soon be no more fishing, and 

 that would be the end of the world. There is a star- 

 fish, called Luidia — and not a very common one — 

 which has 200 miUion eggs. Huxley calculated that if 

 the descendants of a single green-fly all survived and 

 multiplied, they would, at the end of summer, weigh 

 down the population of China. An oyster may have 

 60 miUion eggs, and the average American yield is 16 

 millions. If all the progeny of one oyster survived 

 and multiplied, its great-great-grandchildren would 

 number 66 with 33 noughts after it, and the heap of 



N 



