102 THEORY OF EVOLUTION 



pect that those factors that are carried by the 

 same chromosome would be inherited together, 

 provided the chromosomes are definite struc- 

 tures in the cell. 



In the chromosome group of Drosophila, 



(fig. 52) there are four j)airs of chromosomes, 



three of nearly the same size and one much 



smaller. Xot only is there agreement between 



FEMALE ""-E 



Fig. j2. Clironiosonies (diploid) of D. ampelophila. The 

 sex chromosomes are XX in the female and XY in the male. 

 There are three other pairs of chromosomes. 



the number of hereditary groups and the num- 

 ber of the chromosomes, but even the size rela- 

 tions are the same, for there are tliree great 

 groups of characters and three pairs of large 

 chromosomes, and one small group of charac- 

 ters and one pair of small chromosomes. 



