128 
JOURNAL OF MAMMALOGY 
than between those located near together. The bulk of the evidence 
indicates that the loci of genes are on the same straight line in any chro- 
mosome. Numerical strength of linkage may then be a measure of 
the exact localization of the genes in the germ plasm, and it is to a con- 
sideration of this point that our whole discussion has led. For if the 
genes for unit characters can be thus localized, a direct comparison of 
species in which similar variations occur can be made on this point 
alone, even though the species cannot be crossed. 
The study of localization of the genes for unit character variations 
is attended by numerous limitations. It can only be prosecuted 
through the experimental breeding of large numbers of organisms, 
exhibiting numerous variations. It is dependent even under these 
conditions on the occurrence of hnkage, which is by no means common. 
It is a corollary of the location of genes in chromosomes, that the num- 
bers of groups of linked genes be equal to the number of chromosome 
pairs present. Where the number of chromosomes is large, and the 
number of unit variations known is small, the chances are few that any 
two characters will be found to be localized in one chromosome pair. 
Even under such limitations, linked genes have been studied in several 
insects (chiefly Drosophila) and plants, and most recently in mammals. 
The general results of these studies have been to confirm the chromosome 
theory and to increase our knowledge of the localization of genes. 
Correlative evidence has come from a brilliant series of. cytological 
investigations on the germ cells of several organisms. It has been 
established that in the cells of each species are to be found a deflnite 
number of chromosomes, characteristic for the species. This number 
in germ cells is half the number found in the somatic cells, due to the 
intervention of reducing cell divisions. The chromosomes themselves 
are in general arranged in pairs of homologues in the somatic cells and 
in the primordial germ cells, one member of each pair having come from 
each parent, and this duahty again becomes evidenced in the passage 
of one member of each pair into .the germ cells which form the next 
generation. The individual chromosomes are sometimes recognizable 
by peculiarities of shape, etc. More often their constancy is of numbers 
only. These cytological results have been made possible only by a high 
development of technique and can provide even when greatly extended 
only correlative evidence on the localization of genes. The geneticist 
or cytologist no more expects to behold the gene of which his literature 
is full than the chemist hopes to see the atom of which he speaks with 
unabated ghbness. The gene remains useful as a concept and a nota- 
tion, doubly so now that it includes an idea of spatial definition. 
