52 EVOLUTION AND ANIMAL LIFE 



acteristic is precisely one of the criteria largely used in deter- 

 mining species limits in nature, naturalists call the artificially 

 produced kinds by another name than species; they call them 

 races or varieties, meaning by this to indicate obvious struc- 

 tural and functional differences. Thus artificial selection, while 

 a factor in determining the extent and character of the modifi- 

 cation of many kinds of animals and plants, is not considered a 

 factor in the determination of natural lines of descent. Its 

 value in this regard lies in the clew it gives to natural processes 

 of the same kind. 



Selection by nature among the variations which appear is 

 made possible only by several other factors or actually existent 

 conditions. One is the "prodigality of production' or the 

 constant tendency to overpopulation due to reproduction by 

 multiplication or in a geometrically progressive ratio. Every 

 mature female or hermaphroditic plant or animal produces, 

 at least in the condition of eggs or germ cells, more than one 

 new individual like itself. (There are a very few exceptional 

 cases, compensated for, however, in other ways.) Most produce 

 many new individuals and some reproduce enormously. Cer- 

 tain fishes lay millions of eggs; so do certain oysters; many 

 insects produce thousands of young; many plants produce 

 myriads of seeds. But not all can grow up: there is neither 

 room nor food for all. There must inevitably be a selection by 

 active or passive, guided or fortuitous, means. 



It is a necessary assumption, for the effectiveness of the 

 natural selection factor, that this selection is actually based on 

 the fitness or advantage of some of the variations as compared 

 with others. The trying out or determination of the advantage 

 of these variations comes about as an inevitable active or passive 

 competition for life among the overabundant^ appearing new 

 individuals. This is the "struggle for existence," and the 

 " survival of the fittest ' ' is the expression of the assumed fact 

 of the success of the individuals advantageously (i. e., most 

 fitly) varying. The unfit and the less fit are assumed to com- 

 pose the thousands and hundreds of thousands who must die 

 where only tens or hundreds can live at one time. 



But if natural selection, which is, so far, obviously one 

 of but individuals alone, is to produce new species and control 

 descent lines, it has to depend on a further factor, one named by 

 a familiar word, but not at all explained by it, namely, the factor 



