Most of the described forms in the subgenus were initially named as
distinct full species. More recently, however, in accordance with the
ideas now prevalent in systematic work, many of the named kinds were
reduced to the rank of subspecies. Tate first made a geographic
arrangement (1935:399-400) and later (1939:177-178) provisionally
synonymised several named kinds of _Proechimys_ with _Proechimys_
"_cayennensis cayennensis_." A similar tendency was clearly displayed by
Ellerman (1940:115-122) who allocated 29 names, out of 33 (in the
subgenus, as here understood), to the species _Proechimys guyannensis_
and gave full specific rank to four other named kinds. Osgood (1944)
also had the same viewpoint; that is to say, he appeared to have the
idea that there were only two full species in the subgenus in
Brazil--admitting this orally--and consequently he synonymised some full
species where two or more occurred in the same place, thinking that he
was dealing with individual, rather than specific, differences.
Evidently the number of species in the subgenus cannot be great because
the known kinds show few patterns worthy of specific designation and
therefore the majority of the existing names should be suspected of
having no more than subspecific value. Nevertheless none of the above
writers presented real evidence in support of his arrangement.
Criteria for the recognition of full species are most easily recognized
where two or more different species live together. In the literature,
_P. goeldii_ and _P. "oris"_ were mentioned by Thomas (1912:89) as
having been collected in the same place; _P. mincae_ and _P.
canicollis_, by H. H. Smith (in Allen, 1904:440); _P._ "_leucomystax_,"
from Utiarití, by Miranda Ribeiro (1914:42) and _P._ "_longicaudatus_,"
from the same place, by Allen (1916:569) were other examples. In these,
and other alleged instances of two or more kinds occurring together,
detailed study of the specimens concerned was necessary to learn the
true facts. Also with the opportunity to compare collections from
several different places, new facts emerged. _P. longicaudatus_, as it
was conceived of by Allen, was a composite species, but in one locality,
Utiarití, Ribeiro and Allen actually were dealing with two distinct
species.
The species, or subspecies belonging to different species, living
together are: _goeldii_ and _hyleae_, at Fazenda Paraiso; _goeldii_ and
_riparum_ in Manaus; _boimensis_ and _hyleae_ in Tauarí; _leucomystax_
and _villicauda_ in Utiarití; _mincae_ and _canicollis_ in Bonda;
_gularis_ and _hendeei_ on the banks of Rio Napo ("same trap lines,"
according to P. Hershkovitz, _In Litt._). Study of samples of the above
named pairs of kinds of _Proechimys_ showed the following specific
differences: _goeldii_ is large with narrow aristiforms, has a large and
strongly built skull, with four counterfolds in one or more upper
molars: _hyleae_ is smaller, has wide aristiforms, smaller skull with
less pronounced ridges, and never has more than three counterfolds in
the upper molariform teeth; _riparum_ closely resembles _hyleae_;
_boimensis_ has thin aristiforms, small skull and no more than three
counterfolds in the upper molariform teeth in contrast to _hyleae_,
already discussed; _leucomystax_ closely resembles _boimensis_;
_villicauda_ closely resembles both _hyleae_ and _riparum_; _mincae_ is
similar to _hyleae-riparum-villicauda_; _canicollis_ has the number of
counterfolds in all molars reduced to two; _gularis_ is large, has a
strongly built and ridged skull, some upper molariform teeth with four
counterfolds and wide aristiforms; _hendeei_ closely resembles
_leucomystax_ and _boimensis_.
The evidence obtained from study of specimens where two or more species
occurred together was applied to the remaining samples and the
geographic distribution was worked out. As a result the arrangement
below was made, including all valid kinds already named and those here
newly named from Brazil. The names of kinds I do not consider as
belonging to the subgenus (and genus) are excluded. These are _Echimys
macrourus_ Jentink, not seen, and _Proechimys cayennensis hoplomyoides_
Tate (= genus _Hoplomys_). The application of names is tentative,
however, because the types deposited in Europe have not been seen. An
asterisk denotes the forms not seen by me.
_Proechimys guyannensis: arabupu, arescens, bolivianus, cherriei,
chrysaeolus, guairae, o'connelli, guyannensis*, hyleae, leioprimna,
mincae, nesiotes, ochraceus, oris, poliopus, ribeiroi, riparum,
trinitatis, urichi, vacillator*, villicauda, warreni._
_Proechimys longicaudatus: boimensis, brevicauda, elassopus,
hendeei, leucomystax, longicaudatus, nigrofulvus, pachita,
rattinus*, roberti, securus, simonsi._
_Proechimys semispinosus: amphichoricus, burrus, calidior,
centralis, chiriquinus, colombianus, decumanus, goldmani*,
gorgonae, gularis, hilda*, ignotus, kermiti, liminalis,
panamensis, rosa*, rubellus, semispinosus._
_Proechimys goeldii: goeldii, steerei._
_Proechimys canicollis._
_Proechimys guyannensis_ appears to be more plastic than any other
species. In size of animal, width of aristiforms, color and number of
counterfolds in the cheekteeth, it shows marked response to variations
in geographic conditions. _Proechimys longicaudatus_ is apparently less
plastic; only the number of counterfolds shows marked variation.
_Proechimys semispinosus_ varies much within its range. _Proechimys
goeldii_ seems to be relatively uniform. _Proechimys canicollis_ shows
relatively little variation throughout its range but probably is
divisible into two or more subspecies.
The primitive _Proechimys_ probably was large with a short tail, narrow
aristiforms, strongly built skull, and five counterfolds in each
molariform tooth. Primitiveness here is inferred from characters which
now are of general occurrence in the whole group as opposed to those
restricted in geographic occurrence.
It is a curious fact that in this genus, populations from small islands
are more primitive than populations on the mainland. Apparently a small
population restricted to a small island tends to revert to the primitive
type. The homozygous condition will tend toward a generalized genotype
and the disappearance of secondary biotypes. _P. i. iheringi_ on the
Island of São Sebastião averages larger, has thinner aristiforms, and a
stronger skull than the same subspecies on the mainland, and the
cheekteeth usually have two and three counterfolds. The same subspecies
on the mainland has no more than two counterfolds. _Proechimys
semispinosus gorgonae_ and _Proechimys semispinosus ignotus_, living on
Gorgona and San José islands, respectively, are both characterized by
large size, short tails, strong and conspicuously ridged skulls, and
cheekteeth frequently with four and five counterfolds. On the mainland,
closely related subspecies, like _P. s. panamensis_, _chiriquinus_ and
_gularis_, far less frequently have four counterfolds in more than one
or two teeth. More striking still is the population-sample of _gularis_
from the island of Llunchi, in the Rio Napo, eastern Ecuador. In it
there is a higher ratio of cheekteeth with four counterfolds than there
is in the samples from the banks of the river.
The two insular forms, _P. s. gorgonae_ and _P. s. ignotus_, referred to
as primitive in the discussion above, have wide aristiforms, which is
contrary to what would be expected in a primitive _Proechimys_.
Supposing, however, as actually seems to be the fact, that narrowness of
the aristiforms depends on an increased number of genes, we deduce that
the population from the mainland, that gave rise to the populations of
the islands, did not have all of the genes necessary to make the
aristiforms narrow. In fact the subspecies known on the mainland, near
the aforementioned islands, have wide aristiforms.
Another point which favors the idea that narrow aristiforms result from
an increased number of genes is that, generally, the aristiforms are
narrow in any species whose geographic range is extensive and relatively
uniform.
_Proechimys goeldii_ is the species which has the largest number of
characters that are judged to be primitive, and it may be the oldest
stock. _P. semispinosus_, _P. longicaudatus_ and _P. guyannensis_ may
have been derived from an early splitting of the genus or they may have
branched off the main stem at different times. _P. canicollis_, however,
seems clearly to be an offshoot of _P. guyannensis_; _canicollis_ shows
greater resemblance to _guyannensis_ than to any other species. _P. g.
vacillator_ is another close relative of _P. guyannensis_ with the
number of counterfolds almost as much reduced as in _P. canicollis_.
Conceivably, _vacillator_ is a full species, but the reduction in number
of counterfolds in the teeth more probably expresses only one extreme of
a gradient, as will be discussed below.