Birds

These are animals:

• without mammary glands
• with 2 feet and 2 arms shaped into wings
• with feathers cover the body

The second rank belongs to the birds.

If we do not find in these animals such a large number of faculties and as great an intelligence as in the animals of the first rank, they are the only ones except for the monotremes, who have, like the mammals, a heart with two ventricles and two auricles, warm blood, a skull cavity entirely filled by a brain, and the trunk always contained in ribs.

Thus, they have qualities exclusively in common with mammalian animals and, consequently, affinities which we cannot find again in any of the animals of the later classes.

But the birds, in comparison with the mammals, display in their organic structure a clear degradation, one which has nothing to do with the influence of any sort of circumstances.

In effect, they basically lack mammary glands, organs possessed only by the animals of the first rank, which serve for a reproductive system not found in the birds or in any of the animals in the ranks which come after. In a word, they are essentially oviparous, for the system of truly viviparous reproduction, something unique to the animals of the first rank, does not recur from the second rank on and does not reappear elsewhere. Their fetus, enclosed in an inorganic envelope (the shell of the egg), hardly communicates any more with the mother and can develop within the eggshell without feeding on her material.

The diaphragm which in the mammals completely separates the chest from the abdomen (although more or less obliquely) ceases to exist in this group or is found only in very incomplete form.

There is nothing mobile in the vertebral column of birds, except the neck and tail vertebrae, because the movements of the other vertebrae in this column, not being found necessary for the animal, were not carried out and did not hinder the significant development of the sternum, which now makes such movements almost impossible.

In fact, the sternum of birds, which forms a place of attachment for the pectoral muscles which powerful movements, carried out almost continually, have made very thick and strong, has become extremely large and carinate in the centre. But this concerns the habits of these animals and not the general degradation which we are looking at. This last point is valid because the mammal which we call the bat also has a carinate sternum.

All the blood of birds passes once more through their lung before reaching the other parts of the body. Thus, they breathe completely by their lung, like the animals of the first rank. After them, no known animal has a similar system.

But here we see a very remarkable peculiarity relevant to the circumstances where these animals are found. Living more than the other vertebrates in the air, in which they rise up almost all the time and cross in all kinds of directions, the habit they have acquired of puffing up air into their lungs in order to increase their volume and to make themselves lighter has made this organ stick to the side parts of the chest cavity and has put the air held there and rarefied by the heat of the location in a position to pierce the lung and the surrounding layers and to penetrate into all parts of the body, into the interior of the large bones, which are hollow, and right into the tubes of the large feathers (1).

Nevertheless, it is only in the lung that the blood of the birds receives the influence of the air which it needs; for the air which penetrates into the other parts of the body has another purpose than respiration.

Hence, the birds, reasonably placed after the mammalian animals, display in their general organic structure a clear degradation, not because their lung presents a peculiarity which we do not find in the first animals and which is due, like their feathers, only to the habits they have acquired of ascending into the air, but because they no longer have the system of reproduction appropriate to the most perfect animals and have only the system of the majority of animals in the later classes.

It is very difficult to recognize among the birds themselves a degradation in the organic structure which is the purpose of our research here. Our knowledge of their organic structure is still too general.

Thus, up to now, one or other of the orders of this class has been arbitrarily placed at the head of it, and the class terminated in the same way with the order which someone wished to select.

Aquatic birds (like those with webbed feet), the wading birds, and the gallinaceans have an advantage over all other birds in that their young can walk and feed themselves after coming out of the egg.

The webbed-footed birds like the penguins have almost featherless wings which are oars for swimming and not for flight.

• This feature makes them similar to the monotremes and the cetaceans.

The palmipeds, wading birds, and gallinaceans must constitute the 3 first orders of birds.

The doves, passerines, birds of prey, rapaces and climbers, must form the 4 last orders.

The habits of the birds of these last 4 orders is that their newborns, emerging from the egg, cannot walk or feed themselves on their own.

Finally, if following this analysis, the climbers make up the last order of birds, since they are the only ones which have two digits at the back and two in front. This characteristic, common to them and the chameleon seems to justify our placing them close to the reptiles.