CHAPTER III. Controversy on the Zoological Position

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).

Note brightness of eye when on the alert.

Plate 6]

Note end of top lid “cocked” for catching sound. Eye comparatively inert.

Plate 6]