94 HEREDITY AND EVOLUTION IN PLANTS 



trate the fact that all living things, in order to flourish, 

 must be adjusted to their surroundings. 



4. Struggle for Existence. The clue to the method of 

 evolution first dawned upon Darwin in 1838, while reading 

 Malthus on "Population." Malthus emphasized the fact 

 that the number of human beings in the world increased 

 in geometrical ratio (by multiplication) , while the food sup- 

 ply increased much less rapidly by arithmetical ratio (by 

 addition). Therefore, argued Malthus, the time will soon 

 be reached when there will not be food enough for all; 

 men will then struggle for actual existence, and only the 

 fittest (i.e., the strongest, the fleetest, the most clever or 

 cunning) will survive. In pondering this hypothesis 

 Darwin at once saw its larger application. 1 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. 2 But 

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

 ciably increase : evidently many must perish for every one 

 that lives. 



^'In October 1838," says Darwirr,^" 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 3oth to 

 the Qoth year. 



