FACTS,

OBSERVATIONS, AND CONJECTURES
RELATIVE TO
THE GENERATION
OF
THE  OPOSSUM
   OF NORTH-AMERICA.
IN A LETTER FROM PROFESSOR BARTON
TO
MONS. ROUME, OF PARIS.
        PHILADELPHIA:
PRINTED BY THOMAS AND GEORGE PALMER,
         116, HIGH-STREET.
1806. 
 
FACTS,  OBSERVATIONS,  &c>
Dear  Sir,

           IN looking over my list of correspondents^
I find that I am indebted to you a letter.  I cannot think
of writing a mere formal letter of apology, for my long
silence; and, therefore, I shall contrive to send you
something that may, at least, amuse you.

   You and  I have often talked together, and speculated,'
about the Generation of the Opossum of North-America
(the Virginian  Opossum of Pennant;  my Didelphis
Woapink*),  I think I informed you, when I had the

  * There is not a little confusion concerning the  nomenclature
of the different species of Didelphis, in  the writings of Linnxus,
Gmelin, and other naturalists.  See the  articles " Didelphis mar-
supialis," and " D. Opossum," in the Systema J\Tatur<e, as published
by Linnaeus himself, and by Gmelin.  I have, therefore, thought 
4.        Facts, Observations, and Conjectures]

pleasure of seeing you in Philadelphia* that I had, for
several years, been  engaged in an extensive  series of
experiments  and observations relative  to  this curious
animal;  this  " prodigiosum animal," as Benzoe calls
it*.   The result of my inquiries will be communicated
to  the public, in two memoirs,  the second (and most
difficult) of which is nearly finished.


   In the first of these memoirs, I shall detail, at length,
the general natural  history of the  animal;  examine its
place in the system; its food j  its manners; its geogra-
phical range through the continent, &c, &c.   I  shall,

it most adviseable to impose a new, and  more determinate, name
upon the animal, which has been the subject of my experiments.
The specific name  of marsufrialis is not very happily applied to
any particular species of Didelphis,  since most  of the  specie's
of this singular genus  are furnished with the marsufiium, or ab-
dominal sack.   I object to Dr. Shaw's  specific name, Virginica,
(taken from Mr. Pennant), because it implies, that our Opossum
is restricted to, or especially common in, Virginia ; whereas this
animal is nearly equally common in  every part of the  United-
States (east of the Missisippi), from the latitude of 40 to that of
25, and even much further south.  The name  Woafiink,  which I
have chosen,  signifies  "  white-face."   I should, perhaps, have
preferred the Tuscarora or Cheerake names, Chera, or Seequa, but
that t know not the precise meaning of these appellations. I may
add, in this place, that the specific name of " dorsigera," which
Linnsus has applied to another species of didelphis (the Merian
opossum of Pennant) is likewise exceptionable; for I have disco-
vered,  that my Didelphis Woapink often carries her young ones
upon her back.
  * Lib* ii. p. 215* 
       relative to the Generation of the Opossitm.     $
also, particularly notice the periods of the intercourse of
the sexes, and shall pursue the female through the whole
progress of what I call the uterine gestation, which com-
prehends a period of between twenty-two and  twenty-
six days.

   The other memoir will commence with the second
term of  gestation,  which I  call  the marsupial gesta-
tion.  This,  which dates its beginning  from the first
reception of the embryons from the uterus, into the mar-
supium,  bourse, or  pouch,  is much longer than  the
uterine gestation, and comprehends, even in a physiolo-
gical point of view, by far the most interesting era in the
history of the animal.  I have been so fortunate as to as-
certain the size  and weight of several embryons imme-
diately after their exclusion from the uterus.   One of
them weighed only one grain!   The weight of  each of
the six other young ones was but little more than  this.

   The young opossums,  unformed and perfectly sight-
less  as they are at this period, find their way to the teats
by the power of an  invariable, a determinate  instinct,
which may, surely,  be considered as one of the  most
wonderful that is furnished to us by the science of  na-
tural history*.  In  this new domicilium,  they continue
  * It is not true, as has been often asserted, that the mother, with
her paws, puts the young ones into the pouch.  In my first  me- 
 6       Facts, Observations, 2nd Conjecturef,
 for about  fifty da\fs, that is  until they  attain the size
 of a common hous-e-mouse (Mus musculus), when they
 begin to leave the teats occasionally, but return to them
 again, until they are nearly of the size of rats (Mus rattus),
 at which time they see*n to be no longer necessarily sup-
 ported by the milk o* the mother, but eat meat and ve-
 getables of various k*Uds.

   The  female Didelphis Woapink sometimes produces
 sixteen young ones at a birth.   I have actually seen this
 number, attached to the teats; but never a greater num-
 ber*.   When they are first excluded from the uterus,
 they are not only very small, but very obscurely shaped.
 The place of the future eyes is  merely marked by two
 pale-bluish specks :  we see no ears;  in short,  the ani-
 mal is a mere mishaped embryon.  Its mouth, which is
 afterwards to become very large, is,  at first, a minute
 hole, nearly of a triangular form, and just of a sufficient
 size to receive the teat, to which the  little creature ad-
 heres so firmly, that it is scarcely matter of surprise, that

moir, I shall show, to the satisfaction of every one, that the com-
mon opinions, on this subject, are altogether erroneous.
  * I have been informed, that female opossums have been seen
with more than sixteen young ones, of the same birth,   I cannot,
however, place implicit dependence upon this information, especially
as I have never seen an opossum with viore than sixteen teats. 
       relative to the Generation of the Opossum.     %
Beverley* and other writers have asserted, that the young
are originally produced in the marsupium,  where they
grow fast to the teats: an opinion very generally adopted,
in many parts of the United-States.

   It is not true, that the young cannot be detached from
the mother, without the loss of blood.  I can assert
the contrary from many experiments,  made upon  em-
bryons weighing nine grains, and upwards.     I have
fully satisfied myself as to all  the various circumstances,
both in the structure and in the exertions of the minute
animal, which enable it, while yet a mere speck,  as it
were, of living matter,  to cling so firmly to the fountain
of its support.

   It is truly an interesting task to pursue  the  various
steps in the progressive evolution of the parts of the
young opossum, while  in the  marsupium, and especially
so long as it is necessarily attached to the  teat.  It is
natural to  suppose, that the all-careful hand of Nature
first evolves those parts, which are the most immediately
important  to the animal.  In this supposition we are not

  * " The young ones (says this writer)  are bred in this false
Belly, without ever being within the true one.  They are form'd
at the Teat,  and there they grow for several  weeks together into
perfect Shape," &c.  The History of Virginia, &c, page 136.
London: 1722. 
 %       Facts, Observations, and Conjectures,
 mistaken.   It is a long time before the embryon has any
 occasion for  the senses of sight and hearing:  but  a
 mouth  and the powers of deglutition,  as well as  of
 breathing, are necessary to it, immediately after its ex-
 clusion  from the uterus.   Accordingly, its mouth and
 nostrils  are open; and, for a long time, all the air which
 it respires  is received through, and passes out of, the lat-
 ter channels.   The stomach seems to perform its diges-
 tive office  in the embryon immediately after its first at-
 tachment to the teat*; and the wonderful little didelphis
 is by no means the inanimate or the passive being some
 physiologists  and naturalists Jiave represented  itf,

   The toes of the fore-feet of the new-born embryon-
 opossum are  furnished with  sharp  and hard  nails,  or
 claws:  but this is not the case with the hind-feet.  The
 latter are,  for some  weeks, of little use to the animal;
 but by means of the former it is enabled to cling, most
 firmly, to  the teat;   and especially to the hair, in the

  * In an opossum weighing only forty-one  grains,  I have seen
the stomach very considerably distended with a white matter,  or
milk.  But the  milk that is afforded to the embryons, for  a few
days after their  first reception into" the marsupium, is nearly pel-
lucid, or transparent.
  t Mr. Pennant says they adhere  to the teats " as if they  were
inanimate, till they arrive at a degree of perfection in shape, and
attain sight,  strength, and hair: after which they undergo  a sort
of second birth." Arctic Zoology. Vol. i. page 84. 
      relative to the Generation of the Opossum.     9
marsupium, immediately around the teat.  I cannot sup-
pose, with the respectable Mr. E. Home,  of London,
that the viscous fluid which surrounds the body of the
embryon, when it is first excluded from  the uterus, is
of any service in facilitating its attachment to the teat*.

  There is one instance of the evolution  of the parts of
the embryon-opossum, which has greatly surprised me,
and seems, with many other facts, to show, that Nature
will, for a long time at least, confound our endeavours
to unravel her rete mirabile of final  causes.  In an em-
bryon-opossum,  weighing only sixty or eighty grains,
and entirely destitute of the senses of sight and  hearing,
you may observe, with the naked eye, the marsupium
of the female distinctly formed, and even count the num-
ber of the teats.

   The humane and ingenious conjecture of Buffon, con-
cerning the preservation of human embryons, or at least
fetus, far from being arrived at their? last stage of growth,
has received some confirmation from my experimentsf:
  * Speaking of the Kanguroo, Mr. Home says, " It would seem
probable, that the mouth of the foetus is originally  attached to the
nipple by means of the gelatinous substance contained in the ute-
rus."  Observations on the Mode of Generation of the Kanguroo.
  f « Personne n'a observe la duree de la gestation de ces ani-
" maux, que nous presumons etre beaucoup plus courte que dans
" les autres ; St comme c'est un exemple singulier dans la Nature
                          B 
10       Facts, Observations, and Conjectures,
but I cannot,  at present,  detail these experiments.  I
shall only observe, that an opossum-embryon,  or fetus,
which weighed  sixty-seven  grains,  lived upwards of
thirty hours after I had detached it from the teat.   An-
other, which weighed one  hundred and sixteen grains,
lived  thirty-eight hours, at which  time I killed it, by
putting it into  spirits.

  At the end of about  fifty or fifty-two days, from its
first reception  into the  pouch  (the period varies some-
what, even among the different individuals of the same
birth), the eyes  of the young  begin to open.   At this
period, and for a short time before, it is capable of re-
taking the teat, after having been separated from it, by
the hand, or otherwise.

   The growth of the young opossum, while in the mar-
supium, and under the  immediate  care of its mother, is
pretty rapid.   I  have found, that the same embryon has
increased, in weight, five hundred  and thirty-one grains
" que  cette exclusion precoce, nous exhortons ceux qui sont a
" portee de voir des saiigues vivans dans Itur p ivs natal, detacher
" de s. voir combien les  femelles portent de temps, 8c combitn de
" temps encore  apres la naissance les petits restent attaches a la
" mamtlle avant que de s'en separer;  cette obsci-vution, curieuae
" par t He-mem e, pourroi! devenir utile^ en nous indu-uunt peut-etre
" quelque mouen de conserver la vie aux enfans venus avant le terme."
Histoire Naturelle, 8cc., &c Tom. xxi. p. 171, 172.  A Paris;
1765. 
      relative to the Generation of the Opossum.    11
in sixty days : that is, at the rate of almost nine grains
daily.  But, as you may readily imagine, its increment,
in bulk and weight,  is much greater one day than ano-
ther.   The animal attains to nearly its full growth in
about five months; but never, I believe (in our latitudes,
I mean), procreates the first year of its existence.

   Possibly, I have  been  relating nothing bat what is
familiarly known to you.   The following fact, however,
will, I flatter myself, be entirely new to you ; and if the
relation of it should  give you half of the pleasure  that
the discovery of it did me, I am persuaded, that this
letter will not be altogether unacceptable to you.

   On the 14th of May, I purchased a female opossum,
with seven young ones.   They were, at this time, about
the size of rats, two-thirds grown; and subsisted partly
upon  their mother's milk, and partly upon  meats and
vegetables.  Of course,  the  period of their  necessary
connection with the mother was at an end.

   On the 21st of the month, that is, at the expiration of
seven complete days, upon  looking into  the box which
contained the animal, I found, that the mother had just
excluded from her uterus, seven embryons, the smallest
of which  scarcely weighed one grain;  another barely 
12      Facts, Observations, and Conjectures,
two  grains;  and the remaining  five (taken together)
exactly seven grains.

  You, my dear Sir, who are by no means a stranger
to the  enthusiasm that is inspired by the contemplation
and study of Nature, will readily imagine what were my
sensations on the discovery of this unexpected new family
of didelphides.   The fact, which I was so fortunate as
to witness, is, in my opinion, one of the most interesting
in the whole science of zoology ;  and, so far as I know,
it has never been noticed by any naturalist but myself.

  You will, I doubt not, immediately attach to this fact,
its proper and full value.  We are no longer, it appears
to me, at a loss to comprehend the final intention of Na-
ture in furnishing the opossum with a pouch for the re-
ception of the tender embryons, excluded, as we have seen
they are, from the uterus, in a very unformed state.  Nature
has determined, that the female didelphis shall produce,
at least, two Jitters of young ones, in the course of the
same year.  Superfetation (I should, perhaps,  in strict
propriety, say uterine superfetation) is wholly incompa-
tible with the  established laws of the  economy of the
didelphis,   But Nature, always provident,  wastes  no
time.  While, therefore, the first litter of young opos-
sums are fast approaching to their adult or more indepen- 
       relative to the Generation of the Opossum.     13
dent state, the mother accepts the ardour of the male;
she  is impregnated;  and after a gestation which is not,
I think, remarkably short,  if we consider the small size
of the embryons when they are excluded from  the
uterus*, the marsupium is destined to perform the office
of a  second, I was going to say a more important, uterus;
just at the time when the first litter have attained such a
size, that they are no longer (one or two of them, at the
utmost) capable of taking refuge in her pouch;  and
when, being now provided with teeth, and the requisite
strength, they are not necessarily dependent  upon their
mother.

   But even after the second litter has been received into
the marsupium,  the  young of the  first  litter, if any of
them be living, still continue with the mother, who does
not yet withdraw from  them her useful attentions, and
assistance.   They are no  longer,  indeed,  permitted to
take the milk secreted  by her breasts;  but she sedu-

  * Buffon, in the passage which  I have quoted from his work,
has very properly observed, that the uterine gestation of his Sa-
rigue is very short: short, indeed, when we compare  this first
gestation  with that of the marsupium.  But I have shown, that
the female didelphis carries her young in utero between twenty-
two and twenty-six days, which is no inconsiderable period, if we
reflect on the very small size (somtimes/es* than one grain) of the
embryons, when they are dislodged from the uterus : for the weight
of our female opossum is often, at least, eighteen pounds. 
14     Facts, Observations, and Conjectures, 8V.
lously watches them, and even conveys them, while they
cling to her back and tail, for considerable distances,
through the woods,  &c.

    -It         afc,          «d*,         ^          *&,
    vjv         *a*          ^v         vjv          -yi*
         *         *         *         *

  But it is time to put an end to this long letter.  Be-
lieve me, I shall be truly glad if it afford you any infor-
mation, or amusement.

  With the genuine regard of a Naturalist, I remain,
my dear Sir, your friend, &c,
                 BENJAMIN  SMITH  BARTON.
  Philadelphia, August, 1805.
To Mr. Roume, Farts.