EXPLANATIONS OF ORGANIC EVOLUTION 563 



more in detail. Although evolution is now an accepted fact, 

 Darwin's explanation is not accepted as being entirely correct, 

 and investigators are still endeavoring to explain evolution. 



Explanations of organic evolution. — All students of organic 

 evolution are agreed that variations, heredity, and environmental 

 influences are all concerned in the origin of species, but opinions 

 differ as to the part each plays in the process. Whatever con- 

 tributes to a better understanding of the kinds and causes of 

 variations, of the laws of heredity, and of the various ways 

 organisms are affected by their environment, contributes to the 

 possibility of arriving at the correct explanation of evolution. 



Environment and evolution. — One of the best known of the 

 early explanations of evolution put most emphasis upon environ- 

 ment. It was observed that plants and animals readily respond 

 to changes in temperature, light, moisture, food, etc. Some 

 animals have much more fur during the winter. Some change 

 their color as winter approaches. Many birds change their 

 plumage at different seasons. Plants have their structures and 

 functions modified by drought, shading, intense light, unfavorable 

 temperatures, etc. It was thought that modifications brought 

 on by environment are inheritable, and as they are passed on 

 through successive generations they may become more and 

 more pronounced, eventually becoming prominent distinguishing 

 features of new types. Such was the explanation proposed by 

 Buffon (1707-1788) in France, and by Erasmus Darwin (1731- 

 1802), an English naturalist and grandfather of Charles Darwin. 



This explanation assumes that the characters of individuals 

 brought on entirely by the effects of environment and thus not 

 due to anything which the individual inherited are transmitted 

 to future generations. In other words, it assumes that acquired 

 characters are transmitted, and therein lies the weakness of the 

 explanation, for investigations since the explanation was proposed 

 have shown quite conclusively that acquired characters are 

 seldom, if at all inheritable. 



Use and disuse of organs as related to evolution. — Lamarck 

 (1744-1829), a noted French zoologist, proposed the theory that 

 the change in the form of animals is due chiefly to the use and 

 disuse of organs. He observed that the organs of men and other 

 nimals are strengthened by use, and, if not used, they lose in size 

 and strength. Through continued disuse organs may be entirely 



