BIOLOGICAL EVOLUTION 59 



the truth of the hypothesis with which he began; 1 and it was 

 given the world then only because A. R. Wallace had come to 

 similar conclusions working independently, though for a much 

 shorter time. 



Darwin started with the general theory of evolution based on a 

 recognition of the influence of heredity and environment, with 

 Malthus' doctrine of " teeming nature " and struggle for exist- 

 ence; with Lamarck's law of transmission of acquired characters; 

 and with the observed facts of variation and improvement under 

 domestication. His problem was: " Can nature, with long 

 enough time, do what man in a short time is able to accomplish by 

 use of reason and choice ? " The hypothesis of natural selection 

 with the correlative doctrine of sexual selection was the outcome 

 of his thought and years of most painstaking observation. 



There are five links in the chain of this theory of the origin of 

 species: (1) prodigality of nature; (2) struggle for existence; 

 (3) variation; (4) survival of the fittest, and (5) heredity. 2 

 Other factors recognized but not emphasized by Darwin, such as 

 " geographical isolation " have since come into prominence and 

 one, transmission of acquired characters, taken over from 

 Lamarck, has been questioned with ever increasing unanimity 

 since Weismann's experiments. 



Prodigality of Nature and Struggle for Existence. — These two 

 links are so interrelated as to call for consideration together, as 

 was done by Darwin. " A struggle for existence," he says, 

 " inevitably follows from the high rate at which all organic beings 

 tend to increase." 3 The term " struggle for existence " is used in a 

 large and metaphorical sense, as Darwin takes pains to explain, 

 and includes " dependence of one being on another, and . . . not 

 only the life of the individual, but success in leaving progeny." 4 

 This doctrine is that of " Malthus applied with manifold force to 

 the whole animal and vegetable kingdoms; for in this case there 

 can be no artificial increase of food, and no prudential restraint 

 upon marriage. . . . There is no exception to the rule that every 



1 Fifty Years of Darwinism, pp. 17 ff. 



2 Conn, The Method of Evolution, pp. 19, 20. Wallace, Darwinism, ch. I. 



3 Origin of Species, London, 1872, p. 50. 



4 Ibid., p. 50. 



