THE MECHANISM OF HEREDITY 61 



white fowls with colored races, for instance, gives the 

 ratio 13 white : 3 colored. 



It is evident, if one runs over these examples and works 

 out all the possibilities involved, he will find that two 

 white races of sweet pea, when crossed, will give purples 

 in the F l generation, a white race of guinea-pigs, crossed 

 with a black variety, will give all agouti, etc. Such curi- 

 ous results are actually obtained. They are quite simple, 

 and their whole heredity may be visualized by the use of 

 the same chromosome scheme as given above. Of course, 

 some of them require the assumption of differences in 

 more than two allelomorphic factors, but this can be done 

 by remembering that additional factor pairs follow the 

 same mathematical scheme as do one or two pairs. 



No matter how satisfactory it would be to have all 

 biological facts interpreted with a primer simplicity, 

 the truth is that animals and plants are complex organ- 

 izations. Probably only the tiniest fraction of the germ 

 cell constitution of any organism has ever been analyzed 

 through Mendelian methods, yet in the pomace fly 

 Drosophila melanogaster, in which there are only four 

 pairs of chromosomes, Morgan and his associates have 

 traced the hereditary transmission of well over one hun- 

 dred factors, each of which has one or more functions to 

 perform in the development of characters in the adult. 

 It is obvious that with such a large number of characters 

 and such a small number of chromosomes, a single chro- 

 mosome must carry many factors. This conclusion 

 granted, it would seem as if any of these groups of fac- 

 tors carried by a single chromosome would necessarily 

 behave as single factors ; in other words, they would enter 



