differing from each other, display some remarkable attributes
of organization which may be looked upon as common
characters. ,
It is well known that the great class of warm-blooded
quadrupeds eoRetithted by Linnæus, under the name of
Mammalia, has, two distinctive characters. It is understood
as implied by the term, th a tth e tribes to which? it belongs,
are those which bring forth their offspring in a mature state;
in that state in which the young animal ceasing to be organically
united to the body of its mother» is fit to enter upon
its independent existence, and to be entrusted to the spontaneous
impulses of its own instinct. The offspring of mam .
mifers, unlike that -birds and amphibious animals and the
lower orders of the creation, passes at ofice from the state of
foetal or uterine existence, to that of separate life “and locomotion.
A second feature considered to h e not less essential
to animals comprised in the class of mammalia,-:consists
in the endowment of a double heart and a double vascular
system, with more ample organs for breathing and evolving
heat. To the universal conjunction of their leading character
of structure, no exception was for a long .time beliëved
to exist, and it would have been deemed in the highest !de-
gyee~ improbable that such a fact: should ever have .then
discovered.
It must however .now be admitted, that a remarkable
deviation from the general character of the mammalia has
taken place in the marsupial tribe, of which Terra Australis
is well known to be the principal abode. JNfature has fitted
these animals for a sort of double gestation, having- provided
the females with a pouch formed by a fold of the abdominal
tegument and supported by marsupial bones. The pouch
encloses the organs of suckling and receives the embryos,
which are transferred to it, as yet little developed, from the
uterus ; and in this new receptacle they are subjected to different
and peculiar modes of vitality, until at a later period
the; offspring becoming more robust and capable, of temporary
separation from the pouch, begin to accustom themselves
to the open air and the exercise of their instinctive
faculties, but take refuge at the least alarm, in their former
retreah I t is not determined in what precise manner the
embryo is conveyed from the uterus into the pouch, though
the remarkable discovery of peritoneal canals in the female
kangaroo by Michael Owen has seemed likely to afford an
explanation. ; Nothing however can b e more obvious than the
fact that the existence of..this;marsupial structure, in providing
for the premature removal, o f.the embryo. from the
uterus, constitutes an important deviation from the essential
character of the mammalia. A still greater, deviation is however,;
th a t which has ;been .noted ; in anothçr tribe, bearing
some remarkable analogy to the marsupial animals:,. I allude
to'the paonotremes, r crejrtujès: which," though warm-blooded
.quadruped s^are provided with a system c f respiration so* anomalous,
that naturalists have diff ered widely from ; each other
as,tosthe. Station to be assigned to them in the system of organized
beings. M. Cuvier placed; the monotremes at the end
of th e ed^Otata, b u t this was with|reference * merely to their
^organs of mastication,- \ Authors who direct,'their .attention
to the productive] system, and particularly M. Lesson, place
them as afseparate class between birds and reptiles) as an
intermediate?$lipk which- connects - these departments of the
animal kingdom: to the mammalia. The. monptremes are
provided with cloacae analogous in every respect .to the
cloacae of birds j .they have no abdominal pouch, but the presence
of marsupial bones * displays ; in , their organization
thp -rudiment of this-gtructure. Their, reproduction appears
?to; be ovo-viviparus,.. and the envelope of the membranous
;>egg.:, being tom . is- supposed i to afford an exit to
the foetus, which may then be considered as hatched.* Their
manner -of lactation has only been- the theme of doubtful
conjectures. .iTheir skeleton; is equally anomalous, and it
displays a clavicle, common to the two shoulders, similar in
structure to that of birds.
I t is impossible to determine or even to conjecture what are
,the circumstances. in the; physical state of the Austral land,
which require the existence o r favouri the developement of
.structures so . peculiar ; in other words, how it comes to pass
* M. Lesson, tom. 4.