388 THEORIES OF DEVELOPMENT 



(1) "Life by its inherent power tends continually to increase 

 the volume of every living body, and to extend the dimensions 

 of its parts up to a self-regulated limit. 



(2) "The production of a new organ in an animal body re- 

 sults from the occurrence of some new need which continues to 

 make itself felt, and from a new movement which this need 

 originates and sustains. 



(3) "The development of organs and their power of action 

 are constantly determined by the use of these organs. 



(4) "All that has been acquired, begun, or changed in the 

 structure of individuals during the course of their life is pre- 

 served in reproduction and transmitted to the new individuals 

 which spring from those which have experienced the changes." 



But the man whose work is most completely identified with 

 organic evolution is Charles Darwin (1809-82), who after 

 spending many years in travel, observation, and investigation, 

 published in 1858 a paper of great scientific interest. At the 

 same time, by the arrangement of friends, Wallace, then in the 

 Malay Archipelago, published his paper, giving essentially the 

 same conclusions. In 1859 Darwin published his great work, 

 "The Origin of Species," in which natural selection was more 

 fully elaborated. 



All organisms vary. These variations may be due to envi- 

 ronmental changes or to excess of food, to the inherited effect of 

 use or disuse of parts, or to atavism, reverting to the character- 

 istics of a remote ancestor, or to reversion, a character or 

 structure found in more recent ancestors. Darwin emphasizes 

 the fluctuating or indefinite variations as of most use in natural 

 selection. Every plant or animal must struggle for existence 

 because of the vast number of other plants and animals and 

 because of conditions of environment, such as cold, heat, or 

 drouth. This struggle is threefold: (1) with its own species; 

 (2) with other species of plants or animals which may prey 

 upon it or its food, and (3) against unfa^"orable conditions of 

 climate or weather. Those \\'hich most frequently survi^'e do 

 so because of certain individual characteristics which have made 

 them able to win in this struggle for existence, or, as Spencer 

 says, "the fittest survive." Now, according to Darwin, nature 

 takes advantage of these favorable variations possessed by the 



