DUNN — ^UNIT CHAKACTER VARIATION IN RODENTS 
139 
more speciaKzed Caviidse be represented by two chromosomes, the 
sum of which rather than either one separately may be homologous 
with the one chromosome of mice and rats. Although this will be 
recognized as speculation, there is some slight evidence that in the 
evolution of the rodents a fractionation of chromosomes may have oc- 
curred, for the mice and rats have 19 (haploid) while the guinea-pigs 
have 28. In the rabbit (Oryctolagus cuniculus) a member now judged 
too primitive for the true rodents and recently placed in the Lagomorpha 
with the others of the old rodent suborder Duplicidentata, the chromo- 
some number is probably 12. If this progressive increase in the number 
of chromosomes through the order Rodentia is found to be a fact and 
not a chance phenomenon associated with the smallness of the sample 
of four species from which our cytological evidence is drawn, it may 
furnish a very important clue to a series of evolutionary relationships 
of more than ordinary interest. 
In the concluding chapter of his recent book Professor Morgan 
(1919) has referred to the possible evolutionary significance of the lo- 
cahzation of genes as determined by the study of linkage. He has 
there reviewed some of the work on similar variations in several species 
of insects by Metz and Sturtevant, pointing out the difficulties to be 
encountered in applying this method to the analysis of species, chief 
of v/hich is the necessity of establishing the same linear order in each 
species of the genes for similar variations. A species in his point of 
view, and in this he follows De Vries, may ultimately prove to be 
a community of genes.’’ We may expect evidence of this community 
in the variations which arise from time to time within the species, 
whether they be at the time of specific value or not. Such community 
is not to be inferred from mere similarity in appearance but must rest 
on a more real homology of germinal cause. This kind of similarity 
is now apparent between Mus musculus and Rattus norvegicus, which 
have varied so far from a common type that they are now inter-sterile 
and have been placed recently in different genera. Yet they have 
retained a genetic' constitution so similar that it contains genes common * 
to both species. Whether this is due to a community of descent in 
the terms of current evolutionary theory or to relationship through 
some other cause is one of the questions which genetics, aided by the 
chromosome notation, may be expected at some time to answer. 
