412 EVOLUTION AND GENETICS 



study of pure lines. Other theories which have been proposed to 

 supplement or replace that of Darwin are the mutation theory of 

 De Vries, Weismann's theories of panmixia and germinal selection, 

 Roux's intraselection, coincident selection, and the theory of 

 isolation. The occurrence of mutations is well established and 

 seems adequate to explain the formation of species in some cases 

 alone and in some through a process of selection of minor muta- 

 tions. Weismann's theories are now given little attention but 

 they deserve to be considered for several valuable points even 

 though they must be discarded as theories of evolution. Isolation 

 is apparently an important factor in the development of variations 

 of all kinds into specific characters. The remaining theories are 

 interesting chiefly in the fact that they emphasize the association 

 of changes with external influences. Since their authors were 

 avowedly against Lamarckian principles this tendency is doubly 

 suggestive of the inevitability of such association. Even the 

 writings of Darwin are crowded with Lamarckian explanations, in 

 spite of his early antipathy to Lamarck's views. 



REFERENCES 



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



Weismann, a., The Evolution Theory, 1904. 



De Vries, H., Species and Varieties, Their Origin by Mutation, 1905. 



Herbert, S., The First Principles of Evolution, 1913. 



Lull, R. S., Organic Evolution, 1917. 



Newman, H. H., Readings in Evolution, Genetics, and Eugenics, 1921. 



Walter, H. E., Genetics, revised edition, 1923. 



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



