CHAPTER XIII 



VARIATION IN THE NUMBER OF THE CHROMO- 

 SOMES AND ITS RELATION TO THE TOTAL- 

 ITY OF THE GENES 



The theory that the chromosomes are made up of inde- 

 pendent self-perpetuating elements or genes that compose 

 the entire hereditary complex of the race, and the impli- 

 cation contained in the theory that similar species have an 

 immense number of genes in common, makes the numeri- 

 cal relation of the chromosomes in such species of un- 

 usual interest. This subject is one that could best be 

 studied by intercrossing similar species with different 

 numbers of chromosomes, but since this would yield sig- 

 nificant results only in groups where the contents of the 

 chromosomes involved were sufficiently known to follow 

 their histories, and since as yet no such hybridizations 

 have been made, we can only fall back on the cytological 

 possibilities involved, and on the suggestive results that 

 cytologists have already obtained along these lines. 



A good deal of attention has been paid in recent years 

 to the not uncommon fact that one species may have 

 twice as many chromosomes as a closely related one. So 

 frequent is this occurrence that it seems scarcely possible 

 that it is due to chance. The implication is that the num- 

 ber of the original chromosomes has either become 

 doubled, or else halved. If the number is simply doubled 

 there would be at first four of each kind of chromosome 

 from the point of view of genetic contents. This is what 

 I understand by tetraploidy. There is some direct evi- 

 dence that doubling may occur. If a new race or species 

 is ever established in this way, we should anticipate that 

 in the course of time changes might occur in the four iden- 

 tical chromosome groups so that they would come to differ 



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