CAUSE OF GERMINAL VARIATIONS 



341 



pairs. Under the microscope it has been seen that the paired 

 chromosomes have occasionally exchanged places. This is not a 

 certainty, because, under the microscope, both members of a pair 

 of chromosomes look exactly alike. But it can be seen that they 

 become twisted. The experimental work makes the possibility 

 seem highly plausible that some of the genes from one chromo- 

 some of the fruit fly have crossed over to the other. 



(d) Decided changes in environment. Doctor Tower of the Uni- 

 versity of Chicago collected 40,000 potato beetles, grouped them 

 into colonies, put them in glass cages, and subjected them to varying 

 degrees of heat and moisture. He found that the change in environ- 

 ment had no effect upon that generation, but the eggs laid by these 

 beetles produced offspring of a different color, which continued to 



parent 



temperature 



temperotticne 

 high humility 



. ^rtuoSc 



pcxrent/ jnelanothora>c rubicund 



Certain scientists are trying to prove that environmental conditions can produce changed 

 characters in organisms, which will be transmitted to their offspring. Tower subjected parent 

 potato beetles to certain varied conditions of temperature and humidity. The offspring showed 

 new characters which they in turn transmitted. Some of Tower's mutants are shown in the 

 above diagram. 



breed true as long as he kept the heat and moisture the same. 

 Tower has been able to produce mutants by environmental 



