CHROMOSOMES AND EVOLUTION 97 



microchromosomes. In view of what has been said 

 above about spindle attachments, it is clear that the 

 microchromosomes must have arisen as duplications 

 of the spindle attachment regions of the larger 

 chromosomes. We have already seen that in some 

 and perhaps in all species of Drosophila the regions 

 round the spindle attachments are genetically inert ; 

 it seems at least possible that in other groups with 

 microchromosomes the same is the case ; if this were 

 so no upset of the ' genie balance ' would result 

 when a spindle attachment region was reduplicated 

 in the chromosome set. Perhaps in groups like the 

 Acrididae, with a remarkable constancy of chromo- 

 some number, the regions round the spindle attach- 

 ments are genetically active. The importance of 

 microchromosomes lies in the fact that, once they 

 have arisen, one or more translocations from other 

 chromosomes may (convert a microchromosome into 

 one of normal size. 



Duplication of a whole chromosome (polysomy) or 

 even duplication of a relatively large region probably 

 always upsets the ' genie balance ' and leads to the 

 production of an organism which is less viable than 

 the original one. It is obvious that the condition 

 of maximum unbalance is reached when half the 

 total chromosome set has been reduplicated (i.e. 

 when the somatic chromosome number is mid-way 

 between diploidy and triploidy).^^ It seems clear 

 that in actual fact only very small duplications are 

 likely to become established as new additions to the 

 chromosome set. That they really do so is proved 

 conclusively by the work of Bridges on the salivary 

 gland chromosomes.^* He found that a number of 

 small regions which could be recognized by the charac- 

 teristic sequence of bands were repeated or duplicated. 

 There are several possible ways in which this may 

 occur ; if we consider a region involving four bands, 

 p q r s, these may be repeated in the same chromo- 

 some in any of the following four ways : 



