A few characters are common to one group of species and other features
are common to a second group. The most striking of these features is the
character of the main fold in the molariform teeth. In one group the
fold transversely crosses the crown of the tooth and in the other it
extends scarcely halfway across. No specimen is intermediate in this
respect. These two groups, furthermore, are separated geographically by
an important barrier, the arid belt that starts in the northeastern
littoral of Brazil (Ceará), and that extends south and southwesterly,
more or less accompanying the São Francisco River in the Plateau, to
about 20° S. _Proechimys_ is thought not to inhabit this arid belt. At
the latitude of 20° S the conditions become more suitable for
_Proechimys_, especially along the rivers which flow eastward, but there
the Plateau is replaced by mountains: the Serra Geral at the west, and
Serra da Mantiqueira at the south; these ranges are bare of forests at
higher elevations. Two groups of species of _Proechimys_ are, therefore,
kept geographically isolated: one group lives in southeastern Brazil,
and the other lives in a large area to the west which starts at 21° S in
Paraguay and Brazil and widens northward and includes, farther west,
central and northern Brazil and all the South American countries
above 21° S, as well as Central America northward to southern Nicaragua.
The two groups which are here treated as subgenera may be designated as
follows:
_Trinomys_--main fold deep: aristiforms well-developed on the rump and
outer thighs; tail no less than 75 per cent of length of head and body;
skull without ridges across the parietals; no conspicuous groove for
transmission of nerve inside infraorbital foramen; molariform teeth
decreasing in size from premolar to third molar; 1 to 3 counterfolds in
the molariform teeth.
_Proechimys_--main fold shallow: aristiforms not developed on rump and
outer thighs; tail less than 75 per cent of length of head and body;
groove for transmission of nerve present in infraorbital foramen of
several subspecies; molariform teeth increasing in size from premolar to
second molar; 2 to 5 counterfolds in molariform teeth.
Most of these characters vary but do not overlap. Subgeneric rank is
here accorded to the two groups of _Proechimys_ characterized
immediately above.
The primary cause of the subgeneric differentiation is thought to have
been geologic changes in the continental area. As already pointed out
(see Paleontology), decreasing humidity in the Central Plateau of Brazil
may have caused a migration southwestward of one or more of the species
along with the forests. Once isolated geographically, the species
probably differentiated at an accelerated rate.
The fact that a much larger number of subspecies occupies the larger
geographic range of the subgenus _Proechimys_ would not be sufficient to
prove that this subgenus, _Proechimys_, is nearer to the primitive group
than _Trinomys_, the subgenus occupying the smaller range with fewer
subspecies. The paleontological evolution of the rodents, however,
consistently points to teeth with a larger number of counterfolds (as
seen in _Proechimys_) as the primitive condition. The extension of the
main fold, tending to set apart one lamina in each upper molariform
tooth, seems to be a specialization; reduction in the size of the head
and body, increase in length of tail and decreasing size of molars
posteriorly also may be specializations. The main point, however, is to
establish if _Trinomys_ is a relic group rather than a "differentiated"
one. If an intermediate form were known which connected _Trinomys_ with
one species of _Proechimys_ more than with another or even if _Trinomys_
itself more closely resembled one of the groups of species of the
subgenus _Proechimys_ than it did another, we would assume that
divergence and selection accounted for the subgeneric variation. The
lack of any such connecting link favors the first idea, namely that
_Trinomys_ differentiated rapidly with the aid of geographic variation.
If _Trinomys_ is, as I am inclined to consider it, the result of
"differentiation," its subgeneric features are to be admitted as "new"
and therefore the most primitive species in the genus should be found in
the subgenus _Proechimys_.
It is a matter of common sense to admit the two groups considered above
as subgenera rather than genera. Since the two structural plans were
established they would, and do, act as different sources of variation.
On the other hand, the morphological differences do not give the two
groups an amount of morphological differences that would justify full
generic rank for each.