10 Heredity and Eugenics 



that en\'ironment, even in its superficial sense, is a very 

 real factor, and has played its part in every evolutionary 

 theory since. 



2. Use and disuse. — In the early part of the nineteenth 

 century, the hrst substantial explanation of organic evolu- 

 tion was proposed. Its author was Lamarck, and the theory 

 has become st}'led Lamarckism or Lamarckianism, but its 

 author called it "appetency," or the doctrine of desires. 

 It is more intelligible to the uninformed as the effect of use 

 and disuse. This explanation has been a conspicuous part 

 of e\'olutionary doctrine ever since, and in modiiied form is 

 known today as neo-Lamarckianism. The conception is 

 simple enough and has a basis of facts. It is well known, 

 for example, that use develops a muscle, and that disuse 

 deteriorates it, a deterioration that ma}' reach as far as 

 inability to function. If this effect of use and disuse be 

 applicable to aU organs and regions of the body, and certain 

 conditions of living were to change, demanding the use of 

 structures that had not been called upon to do so much 

 service before, and also excusing from such constant service 

 structures that had been very active before, one might 

 imagine changes taking place in the greater development of 

 certain structures and the less development of others. In 

 other words, change in the environment means change in the 

 demands on the structures of plants and animals, and these 

 demands are met by the active exertion of the organism. 



A well-known illustration used by Lamarck will serve 

 our purpose. A grazing animal, with an ordinary neck, 

 is placed in conditions that demand feeding upon the foliage 

 of trees. The continuous use of the neck in stretching 

 would cause it to increase somewhat in length. This 

 slight increase in length would be transmitted to the next 



