336 The Theory of Natural Selection 



would have been formed in less than one year a mass of 

 protoplasm greater than the probable limits of the material 

 universe. 



The world is so constituted that it is simply impossible 

 for the descendants of any pair to survive and reproduce 

 themselves in this geometrical ratio. There are too many 

 other living things conflicting with this inner urge to multi- 

 ply. There is not standing room for all of one generation. 

 This tendency to multiply out of all proportion to the 

 possibilities of survival alarmed Malthus. It revealed to 

 him the wisdom of a providence which added famine, 

 pestilence and war to the equipment of the universe, to 

 prevent the dire disasters that might otherwise ensue from 

 an unrestricted propagation! And it was this flash of 

 insight that gave Darwin his clue to the mechanism of 

 evolution. 



Under the artificial conditions of stock raising and 

 agriculture the multiplication of any species is regulated in 

 accordance with the means of subsistence, available space, 

 shelter, and so on. In keeping a population down to man- 

 ageable dimensions the breeder also decides which of a mixed 

 group are to be the progenitors of succeeding generations. 

 The individual plants and animals under these artificial con- 

 ditions are shielded from the severities of the struggle for 

 existence which plants and animals in " a state of nature " 

 have to undergo. On the other hand, restricting propaga- 

 tion to the selected few leads to wholesale extermination of 

 hereditary lines. 



If the rate of reproduction in plants and animals were 

 a very small fraction of what it is, so that the population at a 

 given time might just barely reproduce itself in the follow- 

 ing generation, there would be no selection. The weak and 

 the strong, the slow and the swift would equally well re- 

 produce themselves and maintain themselves. It is the fact 

 that the vast majority of the generation simply must be 

 exterminated that makes possible the discriminating proc- 

 ess which Darwin came to call natural selection. 



