Amongst the Reptiles and Amphibians obtained by Mr. Darwin, in the
Voyage of the Beagle, there are several of great interest, not merely on
account of their novelty as newly discovered species, of which there are
nearly thirty, or as forming the types of genera not previously known,
or of any remarkable peculiarity of form, structure, or habit, although
in all these respects many of them are highly interesting; but more
particularly as serving to establish or confirm several points connected
with their geographical distribution.
From the structure of most of these animals and their consequent habits
of life, circumscribed as they are for the most part in their locomotive
powers, it might reasonably be predicated that they would, upon the
whole, exhibit as distinct examples of restriction, with regard to their
geographical boundaries, as any class of vertebrated animals; and that
the intervention of seas and of mountains would be sufficient to limit
the range of a species. Such is in fact usually the case; and not only
is the same species not found in the Old and New Continents, but, with
very few exceptions, not even on the opposite sides of the South
American Continent, in which range Mr. Darwin’s discoveries have
principally been made. The occurrence, however, of _Bufo Chilensis_ at
Rio Janeiro and at Buenos Ayres on the eastern, and at Valparaiso and
the Archipelago of Chonos on the western side of the continent, shows an
extent of distribution exceedingly unusual if not absolutely
unparalleled in this family. It is, however, still possible that further
and more extended researches into the characters of the animals in
question, and an examination of individuals from each locality at
various ages, may prove that there are two species, which have been
confounded with each other, and the anomaly may thus be removed.
But although the circumscribed range of a species may be accounted for
by the reasons above mentioned, and others of a restrictive nature, it
is not so easy to refer to any known or obvious cause the remarkable
fact of a whole genus, consisting of numerous species, being thus
geographically limited. Yet this is a well-known and very common
circumstance with regard to several groups of animals. In our present
researches there exists a remarkable example of this fact in the genus
_Proctotretus_, consisting, as is now known, of at least fourteen
species, all inhabiting the western coast of South America. These facts,
interesting as they are, have never been sufficiently investigated,
although, it must be confessed, there are so many anomalies in relation
to this subject, that we must despair of ever reducing the facts in
question to any thing like fixed laws.
The close approximation of the Raniform and Hyliform groups of the
Anourous Amphibia is strikingly illustrated by several new forms
obtained by Mr. Darwin, which are so perfectly osculant between the two
families, that it is difficult to assign them a decided location. And
the addition of some bufonine forms in the family _Ranidæ_, as at
present constituted, and on the other hand of some amongst the
_Bufonidæ_, which are no less raniform in their general structure and
habits, render it increasingly probable that the single character of the
presence or absence of superior maxillary teeth, must be considered as
insufficient to constitute alone a natural distinctive family character.
There are several minor points bearing upon the natural arrangement of
the Anourous Amphibians, which are illustrated by the characters of some
of the species now first described, which will doubtless at some future
time assist in the construction of a classification of these animals,
bearing at least a nearer approximation to their natural arrangement
than any that has hitherto been promulgated.
The Ophidians have been placed in the hands of Mons. Bibron, who is at
the present time engaged in completing his admirable history of
Reptiles, by the publication of those volumes which are devoted to this
order; and it must be considered a fortunate circumstance that the delay
which has taken place in the appearance of that portion of his labours,
has thus afforded the opportunity of embodying in so perfect a work, the
numerous discoveries of Mr. Darwin in this particular department of
Erpetology.