Sources of Variation 163 



darker in appearance than those who are shielded from the 

 stimulus of the ultra-violet rays. 



Another possible source of variation is conceivably pres- 

 ent within the individual as an inherent tendency to depart 

 from the ancestral pattern. There is some evidence that dur- 

 ing the age of the giant reptiles there was such a tendency 

 toward the elaboration of armors and horns, although we 

 cannot of course speak in detail of what happened as be- 

 tween one group of individuals and their immediate offspring. 



Finally it is conceivable that variations are brought about 

 by the action of the environment not upon the developing 

 organism itself but upon the germ cells in the parent. There 

 is some recent experimental evidence that such modifications 

 are actually possible. 



Dr. W. L. Tower, an American biologist, exposed potato 

 beetles during the early stages of their development to ex- 

 treme conditions of moisture and temperature, with the 

 result that the adult beetle showed distinct deviations from 

 the parental type in color and pattern. Succeeding genera- 

 tions raised from these beetles, however, under normal condi^ 

 tions, looked like the common variety, and did not show 

 any effect of the violent treatment to which their progeni- 

 tors had been subjected. That is to say, in these insects as 

 in others the conditions of development influenced the adult 

 appearance, but the effect of the environmental forces was 

 not carried on into the following generations. 



In another series of experiments. Tower subjected the 

 young beetles to extremes of temperature and moisture at 

 the time when the ovaries and spermaries were being formed. 

 In these cases the adult beetles appeared perfectly normal. 

 The progeny of these beetles, however, raised under normal 

 conditions, presented several distinct types which could be 

 related to the conditions to which the parents had been ex- 

 posed. Still more striking is the fact that these changes in 

 color and pattern were then reproduced in succeeding gen- 

 erations, indicating that a germinal change had taken place 

 under the action of the environmental forces. These results 



