ORIGIN OF THE LIVING MACHINE: ADAPTATION 351 



1. The direct effect of the environment acting upon animals and 

 plants, modifying them, generation after generation. 



2. New physical needs, necessitating new conditions of life; 

 these new conditions producing changes in the animals them- 

 selves. 



3. Use and disuse. It is a well-known fact that the use of 

 any organ causes it to increase, and the failure to use it causes 

 it to decrease in size and in efficiency. Lamarck supposed that 

 the arms of birds became wings through continued use in this 

 direction, and that the hind legs of snakes were lost because 

 they were not used. This has been the most universally recog- 

 nized of the Lamarckian factors. 



4. The transmission of these acquired characters to posterity. 

 Lamarck assumed, as everyone else assumed in his day, that 

 any characteristics possessed by an animal or a plant might be 

 transferred to its offspring. Hence any of the changes produced 

 by the environment, by new physical needs, or by use and disuse, 

 would be transmitted to the offspring, and, therefore, the next 

 generation would have the body modified by the habit and 

 environment in which the first generation lived. This would 

 result in a constant modification of organisms, producing evolu- 

 tion. 



There were certain other factors in Lamarck's conception 

 which, though really part of the original theory, are not com- 

 monly included under the term of Lamarckian factors. One 

 of these was cross breeding, i. e., breeding together of individuals 

 of different varieties, or perhaps even of different species, the 

 result being an offspring different from either parent. A second 

 was isolation, a suggestion that certain individuals became 

 separated from the rest, and they and their offspring, being 

 obliged to breed together, produced types in an isolated locality, 

 which developed along lines different from those taken by other 

 members of the same species in other parts of the world. 



Although these Lamarckian factors are several in number, 

 it will be seen that there is one common phase. In all it is 



