360 



PRINCIPLES OF ANIMAL BIOLOGY 



differentiation pi-ohably other factors enter. The most important of 

 such factors is beheved to be natural selection. 



Charles Darwin and the Natural Selection Idea. — Though Charles 

 Darwin is often popularly credited with introducing the evolution doc- 

 trine, that is not correct, since, as shown in Chap. 1, the idea of evolution 

 was already old in Darwin's time. His real contribution was the theory 

 of natural selection. This theory made evolution seem so reasonable 

 that opposition to evolution itself from intelligent people quickly fell 

 away. From this fact, and from the confusion which exists between 

 natural selection and evolution in Darwin's o\^^l writings, has no doubt 



come the popular misconception of 

 Darwin's share in promulgating the 

 evolution idea. 



The development of the natui-al 

 selection concept in Darwin's mind 

 is one of the fascinating romances of 

 biological science. Darwin had 

 come under the spell of the great 

 English geologist Sir Charles Lyell 

 (Fig. 304), one of whose principal 

 teachings was that geological proc- 

 esses of the past were essentially 

 the same as those in progress now. 

 Thi;; doctrine, which has been called 

 iiniformitarianism, means specifi- 

 cally that erosion, warping of the 

 earth's crust, rise and fall of the 

 land, volcanic action, etc., had been 

 periods of time just as they are 

 occurring now. By means of these present-day processes and no others, 

 Lyell attempted to explain the development of earth features. Darwin 

 was impi'essed with this method and was inclined to apply it to living 

 things as well. When, therefore, from 1831 to 1830 he was privileged to 

 accompany as naturalist an expedition that was traveling around the 

 world on the ship Beagle, he was already in a frame of mintl to reflect 

 present occurrences back into the past to see what they might explain. 



It was not until after his return from this voyage, however, that the 

 idea of natural selection occurred to him. As he himself says, he got it 

 from a book by Malthus, "Essay on Poi)ulation," in which it was pointed 

 out that human populations tended to increase rapidly, thus leading to a 

 struggle for existence. Darwin quickly saw in this situation a means of 

 modifying species of other organisms; for if individuals varied, and if they 

 were competing with one another, any advantage possessed by certain 





-Sir Charles Lyell, 1 797-1 S75. 



continually occurring over 



long 



