BIOLOGICAL EVOLUTION 59 
the truth of the hypothesis with which he began;! 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. 
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.’’? 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. 
