108 THE CHROMOSOMES 



chromosomes is thus of great significance, both in the 

 origin of incipient species and in providing new ' raw 

 material ' for evolution. 



The loss of portions of chromosomes from the set 

 must be assumed to have occurred in evolution to 

 compensate for the acquisition of new ' duplicated ' 

 segments ; otherwise the size of the chromosomes 

 would have increased indefinitely in the course of 

 evolution. Such loss probably takes place in two 

 stages ; the regions in question first become pro- 

 gressively inert as a result of successive mutation 

 and then get lost as a result of a ' deletion ' (deletions 

 of active regions, however small, seem to be nearly 

 always lethal in Drosophila when homozygous). We 

 know next to nothing of the physical basis of inert- 

 ness, but the mutation of an active gene to an inert 

 condition seems to differ from an ordinary mutation 

 in being irreversible. In bisexual animals duplica- 

 tion is the only way whereby the total number of 

 genes can be increased, deletion the only way it can 

 be reduced. 



