Mammals

These are animals:

• with mammary glands, 4 articulated limbs and all the essential organs of the most perfect animals
• with hair on some parts of the body

The mammals are located at one of the extremities of the animal chain displaying the most perfect animals, the most rich in organic structure and faculties.

For it is uniquely among them that we find those having the most fully developed intelligence.

If the improvement in the faculties establishes the improvement of the organs which give rise to them, as I have already said, in that case all the mammals, who are truly viviparous, therefore have the most improved organic structure, because it has been recognized that these animals have more intelligence, more faculties, and a more perfect combination of senses than all the others.

In addition, there are those of them whose organic structure comes closest to that of human beings.

Their organic structure displays a body strengthened in its parts by an articulated skeleton, more generally a complete one in these animals than in the vertebrates of the three other classes. Most mammals have four articulated limbs, extensions of the skeleton.

All have a diaphragm between the chest and the abdomen, a heart with two ventricles and two auricles, warm red blood, free standing lungs contained in the chest, into which all the blood passes before being sent to other parts of the body.

These are the only viviparous animals, for they are the only one in which the fetus, enclosed in its layers, nevertheless always communicates with its mother and develops by relying on her materials, and the young, after their birth, are nourished, for some time yet, from the milk of her breasts.

Thus, the mammals must occupy the first rank in the animal kingdom, from the point of view of the improvement in their organic structures and the large number of faculties (Recherches sur les Corps vivans, p. 15), because after them we do not find any more true viviparous reproduction, lungs contained by a diaphragm in the chest taking in all the blood which must be sent to other parts of the body, and so on.

In truth, among the mammals themselves, it is quite difficult to differentiate what belong essentially to the degradation we are examining from what is produced by environmental circumstances, ways of life, and long established habits.

However, we find even among them traces of the general degradation in organic structure, for those whose limbs are appropriate for seizing objects are more highly perfected than those whose limbs are only appropriate for movement. In fact, it is among the first that human beings, considered from the perspective of organic structure, are located. Now, it is evident that the organic structure of human beings is the most perfect and must be looked upon as the standard against which we must judge the improvement or the degradation of other organic structures in animals.

Thus, in the mammals, the three sections which divide this class (although unequally) display amongst them, as we are going to see, a perceptible degradation in the organic structure of the animals making them up.

Section 1

Unguiculate Mammals have four limbs, claws which are flattened or pointed at the end of their digits but which do not surround the digits. These limbs are, in general, appropriate for seizing objects or at least hanging from them. Among these animals are found the animals with the most improved organic development.

Section 2

Ungulate Mammals have four limbs, and their digits are entirely covered at the extremities with a round horn called a hoof. Their feet are used only for walking or running along the ground, and are not capable of being used for clambering up trees or seizing an object or prey, or to attack and rip other animals. They feed themselves only on plant life.

Section 3

Exungluate Mammals have only two limbs, and these limbs are very short, flattened, and shaped for swimming. Their digits, surrounded with skin, have neither claws nor hooves.

Of all the mammals, these are the ones whose organic structure is the least perfect. They do not have a pelvis nor rear feet; they swallow without preliminary chewing; finally, they habitually live in water, but they come to the surface to breathe air. They have been given the name cetaceans.

Although amphibians also live in water, from which they emerge from time to time to crawl around the shoreline, they actually belong to the first section of the natural order and not to the one which includes the cetaceans.

From now on, we see that it is necessary to differentiate between the degradation which comes about from the influence of the habitual environment and acquired habits and that which is the result of a less advanced progress in improvement or in the complexity of organic structure.

Also, one has to be careful about descending into consideration of detail, because, as I will show, the environments in which animals typically live, the specific locations they inhabit, patterns of behaviour enforced by circumstances, ways of life, and so on, have a large power of modifying the organs. Thus, one could attribute to the degradation we are discussing shapes of parts which are really due to other causes.

The amphibians and the cetaceans, living habitually in a dense medium, where well developed limbs would only be able to interfere with their movements, must have only very shortened limbs, that it is only as a result of the influence of the water impeding movements of very long limbs with solid interior parts, that they necessarily are made the way they really are, and that consequently these animals owe their general form to influences of the environment in which they live. But with respect to the degradation which we are seeking to recognize in the mammals themselves, the amphibians must be distant from the cetaceans, because their organic structure is much less debased in its essential parts. It requires that we bring them close to the order of unguiculate mammals, whereas, the cetaceans must form the last order of the class, since they are the least perfect mammals.

We are going to move on to the birds. But before that, I should note that between the mammals and birds there is no merging. There exists a gap to fill. Undoubtedly, nature has produced animals which almost fill up this gap and which must form a specific class, if they could be known, either in the mammals or in the birds, according to their systemic organic structure.

This has just been acknowledged by the recent discovery of two types of animals from New Holland, as follows:

Ornithorhyncus

Echidnes

Monotremes, Geoff[roy].

These animals are quadrupeds, without mammary glands, embedded teeth, or lips; they have only one orifice for the genital organs, excrement, and urine (a cloaca). Their bodies are covered with hair or bristles.

These are not mammals at all, for they do not have mammary glands and are very obviously oviparous.

They are not birds, for their lungs are not pierced and they do not have limbs shaped into wings.

Finally, they are not reptiles, for their heart with two ventricles necessarily distances them from the reptiles.

Thus, they belong to a special class.