EVIDENCE FROM EXPERIMENT 155 



A very remarkable series of experiments upon the 

 Colorado Potato-beetle were made by Professor 

 Tower, of Chicago, under conditions of the greatest 

 exactitude and rigorous control, as much so as in 

 any chemical or physical laboratory; the results are 

 famous throughout the biological world. In these 

 experiments the larvse and the beetles were subjected 

 to varying conditions of heat and moisture and at 

 different stages of their development, and thus 

 several distinct races were established, differing in 

 size, colour, markings and, in one case, in breeding 

 habits; the new characters are hereditary and trans- 

 mitted from generation to generation. A remarkable 

 feature of these experiments was the discovery that 

 the beetles had a sensitive period in their develop- 

 ment, when they are particularly susceptible to the in- 

 fluence of changes in the environment. When larvse 

 in their sensitive period are exposed to altered condi- 

 tions, the adult beetles display corresponding changes, 

 but these are not hereditary and do not reappear 

 in the next generation. When, however, the beetles, 

 at the time when the eggs are maturing and getting 

 ready for fertilization, are so exposed, the beetles 

 themselves show no visible change, but the offspring 

 do and these modifications are hereditary. Tower in- 

 terprets this as being the action of external condi- 

 tions directly upon the reproductive cells, or "germ- 

 plasm," but this interpretation is disputed by others. 



Kammerer has made numbers of experiments on 

 amphibians and has succeeded in producing hered- 



