96 THE NEXT GENERATION 



same season, because they are not all ready to leave the body 

 at the same time. There are from four to ten days between 

 the layings of two successive sets of eggs. 



After the laying comes the hatching ; and after the hatch- 

 ing, those young larvae of the next generation eat green things 

 in abundance and develop so fast that, within thirty-five days 

 from the time they were eggs, they have not only been 

 changed into crawling larvae but also have become full-grown 

 beetles with wings, ready to lay eggs on their own account. 



Dr. Tower learned these facts while he studied beetles 

 and carried on experiments with them in connection with The 

 University of Chicago. He knew that every kind of beetle 

 starts from a germ cell, 1 and he proposed to do what he could 

 to find out whether or not the power of germ cells can be 

 influenced in this direction or that by any change in the 

 surroundings of the parents before the next generation makes 

 its appearance. 



See how it was in the matter of color, for example. 

 Dr. Tower first secured forty thousand beetles. These were 

 sent to him from the potato fields of Massachusetts and Long 

 Island, also from Ohio and Illinois, and when they reached 

 Chicago he put them into glass cages and glass breeding 

 tanks prepared for the purpose. 



Each breeding place had its own special degree of heat or 

 of cold, and each was kept at the same temperature through 

 summer and through winter from 1893 to 1904. During these 

 years many generations of beetles lived and died, and all the 

 time Dr. Tower saw what was happening to the spots and 

 the stripes that give the creatures their color. 



When the eleven years were over, when both heat tests 

 and cold tests were ended, he found that up to a certain 

 1 All life starts from germ cells. The next chapter tells of this. 



