2 Outline of Genetics 



and proposed simple explanations of the process. These 

 explanations called upon the direct influence of the envi- 

 ronment, but, since little effort was made to analyze the 

 process any further than this, these theories have little 

 value for us. 



The first author to provide any thoroughgoing expla- 

 nation of evolution was Lamarck, and his theory of 

 Use and disuse (1801) still commands the attention of 

 biologists. According to Lamarck, the environment 

 was important, not as a direct cause of evolution, but 

 merely as the occasion for evolutionary change. When 

 an animal came to live under changed environmental 

 conditions, possibly through migration, it encountered 

 certain new needs. These new needs stimulated in the 

 animal the desire to satisfy the needs. Following this, 

 the animal made a conscious effort to satisfy the needs, 

 and in this effort succeeded in exercising certain of its 

 organs more than before. This exercise resulted in the 

 development of the part exercised. At this point 

 Lamarck introduces his basic assumption to the eft'ect 

 that acquired characters are inherited. Whatever gain 

 is made in developing an organ through exercise, is passed 

 on to the progeny. The progeny, living under the same 

 environmental conditions and actuated by the same 

 motives, will make some further gain in the development 

 of the organ in question, and in this cumulative manner 

 the organ will eventually be developed to such an extent 

 that a new species may be said to have originated. 



The classic example, which seems rather absurd in 

 itself, but serves to illustrate Lamarck's ideas, runs as 

 follows. The horselike ancestors of the giraffe come to 

 live in a new and arid environment, such that the only 



