Fish

These animals:

• breathe through gills,
• have a smooth skin or covered with scales
• have fins.

This degradation in organic structure and in the number of faculties manifested in the total collection of animals, we see that fish must necessarily be placed in the fourth rank, that is, after the reptiles. They have, in fact, an organic structure still less perfected than that of the reptiles, and thus, more distant from that of the most perfect animals.

Their general shape, lack of a narrowing between the head and the body (to form a neck), and the different fins serving them in place of limbs result from the influence of the dense milieu in which they live and not from the degradation in their organic structure.

But this degradation is no less real and impressive, as we can see by examining their interior organs. That structure is such that it forces us to assign fish a rank after that of the reptiles.

In fish we do not find any more the respiratory organs of the most perfect animals; that is, fish lack a true lung and in place of this organ have only gills or vascular pectinate folds, placed on the two sides of the neck or the head, a total of four on each side.

The water which these animals take in by the mouth passes between the layers of gills, bathing the numerous blood vessels located there. Since this water is mixed with air or contains air in solution, the air, although in small amounts, works on the blood in the gills and brings about the benefits of respiration.

Then the water goes out sideways through the gills, that is, by the holes opened up on the two sides of the neck.

Now, note that this is the last time that a respiratory fluid will enter by the animal’s mouth to reach the respiratory organ.

These animals, as well as those of the later ranks, do not have a trachea, a larynx, a real voice (even those called grondeurs), eyelids on their eyes, and so on. Here we see organs and faculties lost, ones which we do not find again any more in the rest of the animal kingdom.

However, the fish still are a part of the group of vertebrate animals. But they are the last of it, and they bring to a close the fifth level of organic structure, being, along with the reptiles, the only animals which have the following:

–a vertebral column; –nerves ending in a brain which does not fill up the skull; –a heart with one ventricle; –cold blood; –finally, an entirely internal ear.

In addition, the fish display in their organic structure an oviparous reproductive system.

They have a body without mammary glands, a body shaped very appropriately for swimming, fins which are not completely analogous to the four limbs of the most perfect animals, a very incomplete and remarkably modified skeleton (barely outlined in the last animals of this class), a single ventricle in the heart, cold blood, gills in place of a lung, a very small brain, a sense of touch incapable of understanding the shape of bodies, and apparently without a sense of smell, because odours are transmitted only through the air. It is obvious that these animals, for their part, strongly confirm the degradation in organic structure which we have undertaken to follow through the full extent of the animal kingdom.

Now we are going to see that the main division of fish shows us, in the fish called bony, the most improved among them, and in the fish called cartilaginous, the least improved. These two points confirm, within this very class, the degradation in the organic structure. For the cartilaginous fish with their soft cartilaginous parts designed to make their bodies stronger and to make their movements easier show us that among them the skeleton ends, or rather that with them nature started to sketch out the skeleton.

As we still follow the order in the direction opposite to that followed by nature, the eight final genera of this class must include the fish whose brachial openings, without an operculum or a membrane, are only holes on the side or under the throat. Finally, the lampreys and the hag fish must come at the end of the class, because these fish are extremely different from all the others on account of the imperfection of their skeletons, and their bare viscous bodies, lacking lateral fins, and so on.

Observations on the Vertebrates

Although the vertebrate animals manifest among themselves great differences in their organs, all their organic structures appear to be formed on a common plan. As we go up from the fish to the mammals, we see that this plan has been improved from class to class and that it has stopped completely only in the most perfect mammals. But we also notice that in the course of its improvement, this plan has undergone numerous modifications, some even very significant, on account of the influence of the environmental conditions of the animals, as well as of the habits which each race has been compelled to acquire according to the circumstances in which it found itself.

Hence, we notice, on the one hand, that if the vertebrate animals are very different from one another in the condition of their organic structure, the reason is that nature began to carry out her plan for them only in the fish, later improved it in the reptiles, brought it closer to its completion in the birds, and finally succeeded in bringing it completely to a conclusion only in the most perfect mammals.

On the other hand, we cannot avoid the recognition that, if the improvements in the planned organic structure in vertebrates do not illustrate everywhere, from the most imperfect fish right up to the most perfect mammals, a regular modulated gradation, the reason is that the work of nature has often been changed, thwarted, and even turned away from its direction, by the influences which remarkably different, even contrasting circumstances have exerted on the animals exposed to them in the course of a long succession of generations.

Disappearance of the Vertebral Column

At this point of the animal ladder, we find the vertebral column completely done away with. Since this column is the basis for all true skeletons and since this framework of bone constitutes an important part of the organic structure of the most perfect animals, all the invertebrate animals we are going to examine in succession thus have an organic structure even more degraded than it is in the four classes which we have just reviewed. Also from now on, the supports for muscular action will not be placed any more on interior parts.

Moreover, none of the invertebrate animals breathes by lungs with chambers or has a voice (and thus an organ for this faculty). Finally, they appear, for the most part, to lack real blood, that is, the essentially red fluid in the vertebrates which derives its colour only from the intensity of its animalisation and, above all, which establishes a real circulation. Surely it would be an abuse of language to give the word blood to the colourless thin fluid which moves slowly in the cellular substance of polyps? Will it then be necessary to assign a similar name to the sap in plants?

In addition to the vertebral column, in invertebrates is also lost the iris, which characterizes the eyes of the most perfect animals. For, among the invertebrate animals, those with eyes do not have them clearly equipped with irises.

Similarly, the kidneys are found only in the vertebrate animals, fish being the last ones in which we still encounter this organ. From here on, there is no more spinal marrow, no more large sympathetic nerve system.

Finally, a very important observation to note is that in the vertebrates, and mainly towards the end of the animal scale which displays the most perfect animals, all the essential organs are isolated, or each one has an isolated location in as many particular spots. We will soon see that the reverse of this has taken place, as we move towards the other end of the same scale.

It is thus evident that the invertebrate animals all have a less perfect organic structure than those which possess a vertebral column; the organic structure of mammals displays a vertebral column, a feature which indicates the most perfect animals with respect to all the affinities, and is, without dispute, the true pattern of the most perfect organic structure.

Let us see now if the classes and the large families which are part of the numerous series of invertebrate animals display also, in a comparison of their groups, an increasing degradation in the complexity and the perfection of the organic structure in the animals which constitute them.