RESULTS OF IRREGULARITIES 37 



of Cyclops have 6 chromosomes, others 12. A very large num- 

 ber of cases of this type are known. It appears that changes 

 in the number of sets of chromosomes present plays an impor- 

 tant role in bringing about differences of type and variety, in 

 both w^ild and domesticated animals and plants. 



There is another method by which changes in the structure 

 of the genetic system, changes in the grouping of the genes, 

 produce genetic variations and add to the variety in the 

 organic world. In any given species or variety, the genes are 

 grouped into a certain definite number of chromosomes; in 

 man 48, in the Jimson weed 24, in the fruit fly 8, in the horse 

 worm 2, and so on. This grouping of the genes has an effect 

 on inheritance, for all the genes that are grouped in a single 

 chromosome tend to go together from parents to descendants; 

 and similarly all the characteristics that depend on these genes 

 that are grouped together tend to go together in inheritance. 

 It is this tendency of characters dependent on genes in the 

 same chromosome to go together that is called linkage. 



Now, it sometimes happens that a single chromosome be- 

 comes broken into two, thus adding one to the number of 

 groups of genes. And sometimes two chromosomes belonging 

 to the same pair, or two belonging to different pairs, become 

 united, so decreasing the number of groups, or chromosomes, 

 into which the genes are united. Such changes have been 

 observed in organisms under cultivation in the laboratories. 

 They are now producible on a large scale by subjecting organ- 

 isms to radiations. These changes produce alterations in the 

 method of inheritance in the organisms in which they occur, 

 for they alter the linkage, or tendency of characteristics to go 

 together to the same individual. 



And when we compare related species or varieties, we often 

 find that they have differing groups of chromosomes, such 

 that one could have been derived from another in one of these 



