EVOLUTION TODAY 443 



tion as a process only in the individual, however, and it remains 

 for us to explain how these changes become a part of the sjoecic^s. 

 That they are of the heritage is easily shown. Use and disuse of 

 functional capacity in the hereditary bodies, the g(>nes, has been 

 proposed as an explanation of changes in the functional capacity 

 of these minute bodies. This may account for the gradual devel- 

 opment of characters through a long succession of generations 

 under continued or progressive environmental conditions, and 

 when characters have attained great development in response to 

 conditions of the external environment they may well have at- 

 tained such importance in the internal environment that their 

 persistence will be necessary. Future work in evolution must 

 take into account these things. The difficulties of experimental 

 work are great, but the fact of evolution is so well established 

 that we may expect confidently to solve the riddles of evolutionary 

 method in the future. 



REFERENCES 



Morgan, T. H., Evolution and Adaplaliori, 1903. 



Thomson, J. A., Heredity, 1909. 



OsBORN, H. F., The Origin and Evolution of Life, 1918. 



Morgan, C L., Emergent Evolution, 1922. 



Eldridge, S., The Organization of Life, 1925. 



Kepner, W. a., Animals Looking into the Future, 1925. 



Noble, E., Purposive Evolution, 1926. 



LiNDSEY, A. W., "Factors in Phylogenetic Development," American Natu- 

 ralist XLI, 251-265, 1927. 



Jennings, H. S., "Diverse Doctrines of Evolution," etc.. Science LXV, 19-25, 

 1927. 



Washburn, M. F., "Purposive Action," Science LXVII, 24-28, 1928. 



