The detection of differences of systematic worth between populations of
animals, represented by skins and skulls, is a step preliminary to
deducing the factors responsible for the differences. Ordinarily the
factors which cause heritable differences have to do with geographic
isolation and adaptation to ecological conditions. When differences in
the structure of the animal are known, a person is led to speculate on
the factors which could cause them. For one thing, does the observed
degree of difference tend to isolate animals possessing the "new"
character from the other animals? It would seem to me that the isolation
once started by one of these differences tends to be accentuated with
time and the difference itself thus then becomes a factor responsible
for further differentiation.
Whether or not transition from one character to another occurs
gradually, in its geographic expression, and thus whether or not
intergradation occurs between two subspecies, can be ascertained by the
analysis of a series of population-samples appropriately distributed
geographically. If two characters of systematic worth are known to blend
in one part of the geographic range of a subgenus, and if the same two
characters are seen in two other populations, far removed geographically
from each other and without any samples of annectent populations to
provide actual evidence of intergradation, then such intergradation is
to be inferred.
The available collections of _Proechimys_ mostly were made haphazardly
with the result that there are extensive areas from which no specimens
as yet are available. Thus, actual proof of intergradation is often
lacking in areas where it almost certainly occurs. In some extensive
areas, however, many samples, from relatively regular intervals, have
been available and they provide genuine proof of intergradation. These
instances have served as a guide for estimating whether other samples
should be considered to be full species or instead merely subspecies of
the same species.
Lack of intergradation in any of the characters may be accepted as the
criterion of full species. Where two populations occupying the same
range (sympatric populations) show different qualitative characters,
they almost certainly do not crossbreed. Furthermore the characters that
distinguish such kinds of nonintergrading animals are likely to be
considered as of full specific value when detected in far distant parts
of the range of the subgenus.
In a genus that is widespread and continuously distributed, it is useful
to know which characters always distinguish full species and which ones,
sometimes or always, distinguish only subspecies, since in a population
from a small island, there is, ordinarily, less individual variation
than in a corresponding population from the mainland or a larger island;
under certain circumstances a person might be tempted to give specific
rank to the population when its characters actually are analogous to
those separating subspecies elsewhere.
Sometimes it is convenient to recognize species-groups, a systematic
category without nomenclatural status, intermediate between the species
and the subgenus. When there are two groups of species not sharply
separated, including one species whose characters overlap those of each
of the two groups, it would seem most appropriate to recognize only
species-groups instead of subgenera. When, on the other hand, the two
groups of species have mutually exclusive characters and a species with
intermediate characters is unknown, the two groups of species can
conveniently be accorded separate subgeneric rank.