514 STRUCTURE AND LIFE HISTORIES 



cunning) will survive. In pondering this hypothesis 

 Darwin at once saw its larger application.^ There are 

 always more progeny produced by a plant or an animal 

 than there is room and food for, should they all survive. 

 Darwin showed that the descendants of a single pair of 

 elephants (one of the slowest breeders of all animals) 

 would, if all that were born survived, reach the enormous 

 number of 19,000,000 in from 740 to 750 years. ^ But 

 the total number of elephants in the world does not appre- 

 ciably increase : evidently many must perish for every one 

 that lives. There must therefore be an intense struggle 

 for existence. Darwin^ gives the following illustration: 



'' Seedlings, also, are destroyed in vast numbers by 

 various enemies; for instance, on a piece of ground 3 

 feet long and 2 wide, dug and cleared, and where there 

 could be no choking from other plants, I marked all the 

 seedlings of our native weeds as they came up, and out of 

 357 no less than 295 were destroyed, chiefly by slugs and 

 insects. If turf which has long been mown, and the case 

 would be the same with turf closely browsed by quadru- 

 peds, be let to grow, the more vigorous plants gradually 



^ "in October 1838," says Darwin, "that is, 15 months after I had 

 begun my systematic inquiry, I happened to read for amusement 'Malthus 

 on Population,' and being well prepared to appreciate the struggle for 

 existence which everywhere goes on from long-continued observation of 

 the habits of animals and plants, it at once struck me that under these 

 circumstances favorable variations would tend to be preserved, and 

 unfavorable ones to be destroyed. The result of this would be the forma- 

 tion of new species. Here then I had at last got a theory by which to 

 work." 



2 One pair of elephants produces an average of only one baby elephant 

 in 10 years, and the breeding period is confined to from about the 30th to 

 the 90th year. For illustrations of the prolific nature of plants, see 

 paragraph 173, pp. 190-191. 



* "Origin of Species" (New York, 1902 edition), pp. 83, 84. 



