238 COLLEGE BOTANY 



iarities and differences between forms of life, both past and 

 present, but the explanation of these similarities and differences 

 presents many problems which have given rise to several theories 

 of evolution. Although evolution was partially recognized by 

 a few scholars during the periods of ancient and medieval his- 

 tory, it has made its greatest advance within the last century. 

 These various theories and explanations can be studied to the 

 best advantage in connection with the workers by whom they 

 were advanced. 



Lamarck. This very noted French 

 naturalist (Fig. 107) advanced the theory 

 of use and disuse in 1801. This theory 

 assumed that the environment was con- 

 stantly changing and that animals made 

 certain efforts in accordance with desires 

 which resulted in modifications. For 

 example, the giraffe, by continued efforts 

 to reach the foliage of the trees, gradually, 

 FIG. 107. Lamarck. through many generations, developed a 

 long neck. Snakes, preferring to travel by gliding move- 

 ments, had no very great need for legs and continued dis- 

 use led to their gradual disappearance. Since plants could 

 not make conscious efforts, he assumed that they were un- 

 consciously modified by the environmental factors. Of 

 course, this theory depended on the assumption that the off- 

 spring would inherit characters acquired by the parents 

 previous to the birth of the offspring. However, it is very gen- 

 erally believed that acquired characters are not inherited by 

 either plants or animals. The next great step in the study of 

 organic evolution was made by Charles Darwin. 



