THEORY OF EVOLUTION 93 



now, led a number of biologists to conclude that 

 the chromosomes are the bearers of the heredi- 

 tary units. If so, there should be many such 

 units carried by each chromosome, for the num- 

 ber of chromosomes is limited while the number 

 of independently inherited characters is large. 

 In Drosophila it has been demonstrated not only 

 that there are exactly as many groups of char- 

 acters that are inherited together as there are 

 pairs of chromosomes, but even that it is possi- 

 ble to locate one of these groups in a particular 

 chromosome and to state the relative position 

 there of the factors for the characters. If the 

 validity of this evidence is accepted, the study 

 of the cell leads us finally in a mechanical, but 

 not in a chemical sense, to the ultimate units 

 about which the whole process of the transmis- 

 sion of the hereditary factors centers. 



But before plunging into this somewhat tech- 

 nical matter (that is difficult only because it is 

 unfamiliar), certain facts which are familiar 

 for the most part should be recalled, because 

 on these turns the whole of the subsequent 

 story. 



The thousands of cells that make up the cell- 



