CONTROVERSY ON THE ZOOLOGICAL POSITION
Vertebrate animals are now divided into five classes--Fishes, Amphibia,
Reptiles, Birds, and Mammals--distinguished from one another by such
obvious characters that no zoological knowledge is required to classify
any normal member of one of these great groups. When, however, the
position of _Ornithorhynchus_ first came under discussion, Amphibia
were usually included with Reptilia, making four classes only.
To Shaw, the first zoologist to handle a platypus, the animal was
obviously a mammal, since it had a furry covering; he placed it in
the lowest Linnean Order, Bruta, which included what we know now as
the Edentata, or toothless mammals (though not all are toothless),
anteaters, sloths, and the like. Shaw had only a stuffed skin, and
could know nothing of the anatomy. Yet he was fully seized of its
remarkable character, as may be judged by his reference to Buffon’s
aphorism in the passage quoted in the preceding chapter.
Blumenbach (1800) made anatomical observations on the bones contained
within the skin sent him by Banks, but his paper is not available in
Australia. The first account of the anatomy of specimens preserved in
spirit is that by Home (1802), who received a male and female from Sir
Joseph Banks, both probably sent by Governor King.
Home was an anatomist, not a systematist; in his first paper he does
not concern himself with the systematic position of the animal whose
anatomy he was describing. He notes the remarkable character of the
organs of reproduction both in male and female, and remarks (1802, p.
81):--
“This structure of the female organs is unlike anything hitherto met
with in quadrupeds; since, in all of them that I have examined, there
is the body of the uterus, from which the horns go off as appendages.
The opossum differs from all other animals in the structure of these
parts, but has a perfectly formed uterus; nor can I suppose it
wanting in any of the class Mammalia.”
Finding that these organs were not comparable with those of mammals,
Home was led to examine the corresponding parts in birds and reptiles,
and found most resemblance to the organs of ovi-viviparous lizards. He
concludes (p. 82)
“There is therefore every reason to believe that this animal also is
ovi-viviparous in its mode of generation.”
Later in the same year Home examined the anatomy of the echidna (Spiny
Anteater), which Shaw has described in 1792 as _Myrmecophaga aculeata_.
He at once recognized its close affinity to the platypus, and included
it in the same genus as _Ornithorhynchus hystrix_ (1802, p. 356). He
sums up on p. 360 as follows:--
“The peculiar characters of the _Ornithorhynchus_, as a genus, or
more properly a tribe of animals, are:
“The male having a spur upon the two hind legs, close to the heel.
“The female having no nipples.
“The beak being smooth, while the rest of the animal is covered with
hair.
“The tongue having horny processes, answering the purposes of teeth.
“The penis of the male being appropriated to the passage of semen;
and its external orifice being subdivided into several openings, so
as to scatter the semen over an extent of surface, while the urine
passes by a separate canal into the rectum.
“The female having no common uterus; and the tubes, which correspond
to the horns of the uterus in other quadrupeds, receiving the semen
immediately from the penis of the male.
“These characters distinguish the _Ornithorhynchus_, in a very
remarkable manner, from all other quadrupeds, giving _this new tribe_
a resemblance in some respects to birds, in others to the Amphibia;
so that it may be considered as an intermediate link between the
classes Mammalia, Aves, and Amphibia....
“Between it and the bird, no link of importance seems to be wanting.”
In a later paper (1819, p. 237) he returns to this linkage with birds:--
“The mode of formation of the ova in the ornithorhynchi, constitutes
the intermediate link between that of the American opossum and the
bird.”
This talk of “links” between classes, long before the advent of
Charles Darwin, is interesting. Home may not have used the word in an
evolutionary sense, but it seems likely that he did. In reading these
early discussions on the platypus, one is naturally struck by the
implied acceptance of the evolutionary idea which permeates them.
Home’s opinion was, then, that the genus _Ornithorhynchus_ constituted
a new tribe of animals, to which, however, he gave no designation.
This opinion was based chiefly upon the structure of the organs of
reproduction in the female, and a brief explanation of the importance
of these organs may prove helpful to general readers. In birds and
reptiles the female organs consist of a pair of ovaries placed on the
dorsal wall of the body cavity, one on each side of the middle line.
In close relation to the ovaries are the wide funnel-shaped mouths of
a pair of tubes, the oviducts, which receive the ova shed from the
ovaries, invest them with additional coats, albumen, shell-membranes,
and shell, and pass them backwards to the cloaca--the common terminal
chamber, into which the alimentary canal and urinary organs also open.
In birds, the gradual development of a large-yolked egg, and the danger
to the mother involved in the production of such large eggs in both
oviducts simultaneously, has led to the reduction of the right ovary
and oviduct to functionless vestiges, those of the left side alone
remaining functional.
In the higher mammals the organs are essentially the same, but with
certain marked differences. There is no longer a cloaca, the terminal
aperture of the food-canal being quite separate from that of the
urinogenital organs. Both ovaries and both oviducts are functional,
since the mammalian ovum is very small. But, except in a few lower
forms, the oviducts are no longer separate tubes; a varying amount of
fusion has taken place, the fused portion being the uterus, or womb.
This may be a single cavity, its double origin indicated only by the
horns of which Home speaks and into the apices of which open the upper
portions of the oviducts; or it may be more or less divided by a septum
and thus afford more obvious evidence of its double nature. Within this
uterus the young develop and are nourished until birth, being attached
to the maternal wall by a placenta. Where no placenta exists, as in
most marsupials, the young are born in a very immature condition; they
become attached to an external teat, and are nourished by milk, which
they cannot suck but which is impelled into them by muscular reaction
on the mammary glands of the mother.
Turning again to _Ornithorhynchus_, we find that the oviducts open
separately into a cloaca, that the left ovary and oviduct alone are
functional, and, as will be shown later, that a shelled egg is laid,
which is hatched outside the body of the mother. This last fact was
not, of course, known to the naturalists who first discussed the
position of the platypus in zoological systems. In all these features
the platypus agrees with birds and reptiles, rather than with mammals.
It must also be held in mind that its mammary glands, the outstanding
characteristic of the mammalian class, had not yet been discovered; it
was not until 1824 that Meckel announced their discovery--which was
not, even then, accepted by all zoologists.
It is hardly to be wondered at that zoologists were in doubt as to
the systematic position of this weird animal. Etienne Geoffroy (1803,
p. 125) included it with the echidna in a new order, Monotremata,
characterized by:--“Digits clawed; no true teeth; a common cloaca
opening to the exterior by a single orifice;” but he did not give any
precise indication as to where this order should be placed. Tiedemann
(1808) evaded the difficulty by placing it in an appendix. Lamarck
(1809) created a new class, Prototheria, for platypus and echidna,
deciding that they were not mammals, for they had no mammary glands and
were probably oviparous; they were not birds, for their lungs differed,
and they had no wings; and they were not reptiles, for they possessed
a four-chambered heart. Illiger (1811) placed them in a division
Reptantia, intermediate between reptiles and mammals. Blainville
(1812), on the other hand, was convinced that they were mammals, though
belonging to a separate order, Ornithodelphia. He was the first to
indicate their many close points of agreement with the marsupials, and
gave a long list of mammalian resemblances.
Etienne Geoffroy was convinced from the outset that the platypus was
oviparous, and was not a mammal. He entered into a controversy with
Blainville which lasted for many years. Van der Hoeven and Latreille
were on his side, while Cuvier and Meckel supported Blainville. Isidore
Geoffroy, in his article on _Ornithorhynchus_ in the _Dictionnaire
Classique d’Histoire Naturelle_ (1827) includes the animal in Mammalia,
with a query, and gives a good summary of the position at that date,
just after Meckel’s discovery of the mammary glands. He supports
his father’s views, and argues that, even if the glands discovered
by Meckel are true mammary glands (which he doubts), this does not
necessarily make _Ornithorhynchus_ a mammal. He cites (p. 395) his
father, who claims (1826) that the so-called mammary glands are
homologous with the flank glands of shrews. They differ remarkably
in structure from any known milk-producing gland, and particularly
from those of marsupials, to which one would expect to find most
resemblance. Besides, how can one conceive of an animal with a horny
beak sucking, especially when the supposed milk gland has no teat? It
may be a scent-gland; or perhaps its secretion serves to coat the fur
and render it less permeable by water. The crux of the whole matter
is--Does _Ornithorhynchus_ lay eggs, or does it not?
Meckel reported the discovery of the mammary glands in 1824, but it
was not until 1826 that his full description of them was published.
The glands lie between the main abdominal muscles and the muscular
sheet (_panniculus carnosus_) lining the skin. They are two in number,
each consisting of a considerable number of tubules which open by
minute pores upon a small area of skin; there is no teat. They are
large during the suckling period, but become very much reduced after
it is over, and this accounts for their being so long overlooked. We
now know that they are of the nature of modified sweat-glands, and so
differ from the mammary glands of mammals in general; there was thus
some justification for Geoffroy’s doubts. He returns to the charge
again in 1829, and first traverses the conclusions he had reached and
published in 1827. Monotremes, he maintained, were not mammals, as was
thought, and as was apparently indicated by the identity of several
important organs, e.g. limbs, hair, lungs, blood-vessels, heart with
two ventricles, closure of the chest by a diaphragm, etc. They must
be removed from the Mammalia, because their reproductive organs and
several other parts placed them amongst the oviparous classes. They
could not be included in the birds, because they had neither wings nor
feathers. Nor could they be placed amongst the reptiles, for their
blood was warm, and their lungs were enclosed in pleurae, and shut
off from the abdomen by a diaphragm. Still less were they fish, which
breathe by means of gills. They were therefore mammals without the
mammalian character, without the special organization of viviparous
animals, without all the consequences and functions of an apparatus
capable of producing a placental foetus. Rejected on all sides, they
did not belong to any recognized class of vertebrates; they must have a
special class to themselves.
In spite of all this, Meckel presumes to discover mammary glands, and
announces the fact in 1824. For two years scientific men were kept
in suspense; then in 1826 appeared his wonderful description in which
Blainville found confirmation of the notions which he had published in
1808. But Meckel writes (1826, pp. 159-160):--
“I thought, and I said, that the new elements introduced into this
controversy were not what they had been announced to be. It is true
that a richly glandular apparatus had been discovered on each side of
the abdomen of an _Ornithorhynchus_; but it was a little too hasty to
decide and declare that it was a mammary gland. I examined the facts
again in a specimen preserved in spirit which formed part of Baron
Cuvier’s collections; I saw a glandular structure, but simple, and
merely composed of a considerable number of ampullae with long necks.
There were none of the characters which distinguish a milk gland.”
Geoffroy proceeds to announce the discovery of eggs, the sad story of
which will be told in another place, and concludes (p. 164):--
“We may to-day regard it as certain that the first zoological
division, comprising vertebrate animals, should henceforth be divided
into the five following types: Mammals, Monotremes, Birds, Reptiles,
and Fishes.”
Despite these strenuous efforts, however, the monotremes were generally
considered to be mammals, although the actual method of reproduction
was not definitely known until 1884, and although there are still some
doubtful points about the lactatation.
In this connection Etienne Geoffroy’s final desperate effort to
disprove the mammary nature of the glands may be mentioned. He compared
them to the so-called mammary glands of whales and porpoises, which
secrete not milk but mucus. This coagulates on being ejected into the
water, and is then devoured by the young. Unfortunately, just as he had
elaborated this hypothesis, he had the ill-fortune to secure a nursing
porpoise, and found that the glands secreted milk. Nevertheless, this
endeavour may account for a statement by Verreaux which we shall have
to consider later.
The production of milk by _Ornithorhynchus_ was definitely recorded by
Maule (1832) and by Bennett (1833).
[Illustration: MALE PLATYPUS WATCHING INTENTLY.
Note brightness of eye when on the alert.
_Plate 6_]
[Illustration: FEMALE PLATYPUS LISTENING INTENTLY.
Note end of top lid “cocked” for catching sound. Eye comparatively
inert.
_Plate 6_]