The Factors of Evolution 235 



ural therefore that the factors of evolution and heredity, for 

 the two processes are of necessity indissolubly united, should 

 be the center of modern biology. 



The success of Darwin in establishing the theory of evolu- 

 tion was due primarily to his explanation, by the theory of 

 natural selection, of a reasonable process by which evolution 

 could be effected, and his support of this theory with a vast 

 array of facts, and of logical arguments deduced therefrom. 

 And yet the theory of natural selection or survival of the 

 fittest antedates Darwin by more than two thousand years. 



The Greek poet-musician-naturalist, Empedocles, who lived 

 in the fifth century before Christ, fancied living creatures as 

 arising from the four elements earth, air, fire and water, un- 

 der the action of the forces of love and hate. The animals 

 first formed, " appeared, not as complete individuals, but as 

 parts of individuals, heads without necks, arms without 

 shoulders, eyes without their sockets. As a result of the tri- 

 umph of love over hate, these parts began to seek each other 

 and unite, but purely fortuitously. Thus out of this con- 

 fused play of bodies, all kinds of accidental and extraordinary 

 beings arose, animals with the heads of men, and men with 

 the heads of animals, even with double chests and heads like 

 those of the guests in the Feast of Aristophanes. But these 

 unnatural products soon became extinct, because they were 

 not capable of propagation." 1 



These crude ideas of Empedocles are interesting historically 

 because they show how early the survival idea arose in man 's 

 mind. By many other writers also, previous to Darwin, this 

 idea was expressed, notably Hume, Buffon, Kant and St. 

 Hilaire, and when Darwin's thesis itself was presented to the 

 world it appeared as two papers published simultaneously in 

 the Journal of the Linnean Society by himself and Alfred 

 Russell Wallace. 



In spite of the splendid exposition given by Darwin of the 

 theory of natural selection, and its very general acceptance 

 by scientific men of a few decades ago, the pendulum of sci- 

 entific thought is swinging today in the opposite direction, 

 and the consensus of opinion is distinctly adverse to the ac- 

 ceptance of the theory in all its important points. 



Before considering the experimental work which has largely 

 induced this reversal of opinion, let us see what the essen- 

 tial features of the theory are. Three factors are involved, 

 namely, variation, inheritance, and selection or survival. Let 

 us illustrate their co-operation to produce evolution by a con- 

 crete example from Darwin's "Origin of Species. " "The 



1 Osborn, "From the Greeks to Darwin," p. 38. By permission of the 

 Macmillan Company. 



