EVOLUTION 235 



place that for so long prevented a general acceptance of the belief 

 that it had i-eally occurred at all. Although much progress has 

 been made in this field of inquiry, we are still far from a complete 

 and convincing solution of the many problems which the study of 

 evolution has raised. 



Lamarck's Theory. — The first modern theory which attempted 

 to explain evolution was put forward by the great French biologist 

 Lamarck about 1809. He was much impressed by the profound 

 effect which the environment produces and noted many instances 

 of its operation, such as the vigor and luxuriance of plants grow- 

 ing in rich soil as contrasted with their stunted growth where the 

 soil is poor (Fig. 121). In the cases of several ''amphibious" 

 species he observed that leaves produced when the plant is grow- 

 ing under water are very different from those formed in the air 

 (Fig. 81). He also observed the great structural changes which 

 are brought about, in animals, by the use or the disuse of an 

 organ. Lamarck beheved that organisms possess the ability to 

 react to their environment advantageously and to modify their 

 structures and functions in such a way that success under a 

 changing environment will be attained. He never questioned 

 that all the variations which he noted were directly transmitted 

 to offspring by heredity, and he thus pictured the races of plants 

 and animals as being pushed along the evolutionary road by 

 environmental forces. 



Lamarck's explanation, although very attractive and plausible 

 in certain respects, has never won wide acceptance. Biologists 

 have as a rule not been willing to admit that an organism has 

 any innate ability to guide its reactions into a favorable course. 

 The theory has suffered still more from the absence, after inten- 

 sive search, of any very conclusive evidence that "acquired" 

 characters, such as those produced by the environment, are ever 

 inherited. There are a few biologists today who believe that in 

 certain of^its features Lamarck's theory comes nearer to explain- 

 ing the true method of evolution than any other yet suggested, but 

 as a whole it receives little support. 



Darwin's Theory. "Natural Selection." — The most notable 

 attempt to solve the riddle of evolution is the theory of Natural 

 Selection put forward in 1859 by Charles Darwin (Fig. 134) in 

 "The Origin of Species", a book which has had a very great influ- 

 ence on all human thinking. The effect of this theory in render- 

 ing the whole process of evolution plausible and understandable 



