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JOURNAL OF MAMMALOGY 
‘^English spotting’’ in rabbits, and black-eyed white-spotting and yellow 
in mice) to make us chary about drawing any general conclusions con- 
cerning the occurrence of evolution purely by loss mutations from type. 
The generality that does appear is rather the . widespread occurrence 
in this order of similar variations both under domestication when the 
animals are saved and bred, and in the wild, when usually only the 
stuffed specimens are preserved. There is implied in this similarity, 
which in certain cases amounts to a homology, a similarity in that 
part of the organism which is responsible for the variations, i.e. the 
germ plasm. It may be that we know in these days as little concerning 
the causes of variation as did naturalists in the days of Lamarck or of 
Darwin. We do at least know where the causes are to be sought, 
and, once having arisen by a mysterious occurrence called mutation, 
we have learned something of the manner in which the variations are 
inherited, and by a process of inference have been able to localize still 
more exactly the region of change. The only permissible generality, 
then, concerns a general similarity in the germ plasm and probably 
in the individual chromosomes of these many species of rodents. But in 
one case the similarity between species has been found to be more 
than general. It has been found to be quite a specific similarity. 
If we examine this case in detail we find that in two distinct, inter- 
sterile species, mice and rats, two similarly appearing variations have 
occurred, albinism and pink-eye. In rats the genes for these varia- 
tions are finked with a strength of about 21 per cent, which is possibly 
slightly in excess of the actual. In mice the linkage between these 
genes in something less than 15 per cent, which is based on observations 
of 6700 animals raised solely for the purpose of determining this link- 
age and is probably reasonably accurate. In terms of the chromosome 
hypothesis these facts mean that these two genes are present in the 
same chromosome in rats and mice, in rats at a distance of 21 units 
apart, in mice at, a distance of about 15 units apart. The difference 
in location is so small that for practical purposes we can say that they 
are located at homologous points in the two species. 
In guinea-pigs where both of these variations occur, there is incom- 
plete evidence, but the data which Doctor Wright has supplied indicate 
that pink-eye and albinism in guinea-pigs are probably not finked and 
may therefore be determined in different chromosomes. This does 
not prove, however, that these variations in guinea-pigs are not the 
same as those in mice and rats. It may mean that the chromosome 
which contains both genes in the more primitive Muridse may in the 
