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JOURNAL OF MAMMALOGY 
gation of the units is independent, resulting in a random distribution 
of the characters of the parents among their gametes, such that when 
two units are involved the chances are equal that they will go together 
or separately. The first principle probably applies to all inheritance. 
Evidence has been brought forward to show that the factors or genes 
which represent the visible characters are not discrete but variable, 
and capable of change by selection, but this evidence has now been 
found to indicate not variability but plurality of units. The second 
principle still applies to most cases of inheritance but has been modified 
by the finding that two or more characters may not always be distributed 
independently but when entering a cross together may tend to stay 
together, and when entering a cross separately may tend to remain 
separate (in different individuals) in inheritance. This peculiarity was 
first remarked by Bateson (1906) in the case of the inheritance of 
flower color and pollen shape in sweet peas. In his experiments purple 
flower (as opposed to red) and long pollen (as opposed to round) ap- 
peared to be associated or coupled in crosses so that a marked dis- 
tortion was evident in the second generation ratio of 9:3:3:1 expected' 
on the usual hypothesis of independent assortment, in favor of the 
classes (purple-long and red-round) representing the grandparental 
combinations of these characters. The opposite phenomenon was noted 
and named repulsion. Later, Morgan in 1910 found the same phe- 
nomena while studying inheritance of certain characters which had 
arisen by mutation in the vinegar fly (Drosophila melanog aster) . He 
conceived these two exceptions to MendeFs principle of independent 
assortment as two aspects of a single phenomenon which he termed 
Linkage or associated inheritance. 
The interpretation of these events has constituted one of the great 
advances of biological science. Sutton, in 1902, suggested that the 
marked parallelism between the discreteness and assortment of unit 
characters and the behavior of the chromosomes might be due to the 
residence in the chromosomes of the determinants or genes representing 
unit characters. Immediately after Bateson’s announcement of coup- 
ling, Locke (1906) pointed out the similarity between this new mode 
of inheritance and the results which might be expected if the coupled 
characters v/ere determined in one chromosome. The development of 
this hypothesis, its proof and very important extension and generaliza- 
tion, have been the work of the American biologist T. H. Morgan, 
and of research workers associated with him, assisted more recently 
by data gathered by geneticists and cytologists working on many 
