156 THE PROBLEMS OF EVOLUTION 



When overfeeding was continued for several 

 months, individuals returned to a normal environ- 

 ment produced offspring in which the character 

 was evident, although it disappeared in the course 

 of a few generations. When the treatment was 

 continued for two years, the enlargement of the 

 helmet failed to disappear within the duration of 

 the experiment even though the animals were kept 

 under natural conditions. Woltereck shows that 

 other factors than food, including temperature, 

 bear upon the same type of modification through 

 their effect on the rate of metabolism and the 

 consequent degree of nourishment attained by in- 

 dividuals in a given environment. His work was 

 so carefully safeguarded with respect to the uni- 

 formity of the heritage in his experimental stock 

 and the possible interpretation of his results that 

 it furnishes one of the finest, if not the finest, avail- 

 able study of this nature. 



Sumner found that white mice reared at tempera- 

 tures from 20° to 30° C. developed longer bodies, 

 tails, ears, and hind feet than individuals reared at 

 normal temperatures.^^ When the modified mice 

 were removed to cooler rooms and bred under 

 normal conditions their offspring still showed the 

 departure from normal, although in a lessened 

 degree. ^2 Th^ percentages were recorded as fol- 



" Jn. Exp. Zool, Vol. VII, pp. 97-155, 1909. 



" Arch, fur Ent.—Mech. der Org., pp. 317-348, 1910. 



