114 The Third and Fourth Generation 



dormant through most of the Ufe-history of the 

 animal, growing rapidly to produce the eggs 

 and sperm during a very short period when the 

 animal is quite mature. If, now, the beetles 

 are kept under the conditions of excessive 

 moisture and high temperature while these 

 germ cells are undergoing rapid multiplication, 

 no effect is produced on the beetles themselves, 

 for they are already mature, but their offspring 

 are exceptionally dark in successive generations. 

 On the contrary, the young reared under these 

 same conditions, but removed before the sex 

 cells go through their period of rapid develop- 

 ment, are themselves dark, but their young 

 show no effect of the altered conditions, being 

 normally colored. It is very evident that in 

 the last case we have affected only the grow- 

 ing body; in the former case only the germ 

 cells. 



Stockard has made some very interesting 

 experiments with guinea-pigs. By putting 

 guinea-pigs into a cage,' the air of which was 

 more or less saturated with the vapor of 

 alcohol, he could habituate the pigs to the use 



