20 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 
Without going into more detail it may be said that six independent 
sets of allelomorphs are known in guinea pigs which cooperate to 
determine color. The combinations of these factors determine over 
a hundred distinguishable colors. 
THE CHROMOSOMES AND HEREDITY. 
The present theory of heredity was devised to explain the results 
of experiments such as those given above. Recent studies of cells 
under the microscope have apparently brought the mechanism under 
our eyes. It has already been mentioned that proper methods of 
staining bring out a certain definite number of rod-shaped bodies, 
the chromosomes, in the cells of each kind of animal or plant. The 
reproductive cells are found to contain just half as many as the fer- 
tilized egg and the body cells. At each ordinary cell division the 
chromosomes arrange themselves in a ring, each splits lengthwise, 
and half goes to one daughter cell, half to the other. Thus all the 
body cells have a double set. In the formation of the reproductive 
cells, on the other hand, the chromosomes do not split, but the homol- 
ogous ones, derived from the sperm and egg, pair with each other and 
then separate, one going to each daughter cell. The reproductive 
cells thus get only a single set of chromosomes. It will easily be 
seen that if the hereditary units were located in the chromosomes the 
observed behavior of the latter would fully account for the laws of 
heredity illustrated above. 
Summing up, genetic experiments prove the double nature of 
individuals and the single nature of their reproductive cells in regard 
to each set of alternative hereditary factors, while the microscope 
actually shows us the chromosomes in pairs in the body cells in place 
of the single set to be observed in the reproductive cells. 
LINKAGE. 
As the study of heredity has advanced, a number of complications 
have been found. These complications, however, have only made 
closer the parallelism between the facts of heredity and the observed 
behavior of the chromosomes. The most important of these com- 
plications is the phenomenon known as linkage. A case studied by 
Prof. Castle and the writer will serve as an example. 
A few years ago a freak wild rat with yellow fur and red eyes was 
trapped on a wharf in England. Another wild rat of the same color, 
but with pink eyes, was trapped in another place. Two strains of 
yellow rats were developed which could be distinguished only by the 
color of the eyes. Each strain bred true. Crosses with normal wild 
rats showed that only one recessive unit factor was involved in each 
case. It may appear surprising that on crossing the two yellow 
