All text below is taken from the English translation of De animalibus by James J. Scanlan in Man and the Beasts (de Animalibus, Books 22-26. These are excerpts only, not the full text.

The Nature and Habits of Quadrupeds in General (Book 22, chapter 1)

In defining the nature of quadrupeds we reiterate the thoughts that Galen expressed in his book De spermate. All classes of four-legged animals share a common attribute; unlike humans their sperm is not affected by hours of the day and the movements of the planets and constellations but retains the original humoral complexion imposed upon it. Later on, assigning the cause for this, he says their sperm is so weak and unreceptive to change that it scarcely ever mutates, and then only on rare occasions or in an isolated portion of its makeup. He stresses that this is contrary to what happens in human sperm.

What Galen fails to mention is that the bodies of brute animals are material and earthbound, while the human body, infused with a spiritual soul, has a greater affinity with the heavens. Thus man’s body is more readily affected by celestial influences, especially since his soul is an image of his Maker and a microcosm of the larger universe. Hence, all the qualities associated with universal movers shine forth in man more than in the other animals. But the lower animals, being less solicitous and concerned about their welfare, are more dependent on the common influences of the atmosphere which are contingent on the heavens.

In the study of quadrupeds note should be taken whether they are dry and cold or moist and cold. Those with dry, cold constitutions are melancholic animals, like the cow, which conceive relatively few offspring because of their cold nature and rarely give birth to twins because of their dryness. Pay attention when cows conceive and give birth to twins in the summer because it forebodes a rainy winter. The same is true when an unusual number of cows perish, for this is the sign of a corrupting humor which has infected their living bodies and tainted them with its baneful moisture; this too is an indication of drenching rainstorms in the future.

Prognostications of the weather can also be made by watching the activities of sheep which have a moist, cold constitution, especially the old members of the flock who are even colder. When young lambs are driven to mate with older sheep during the same season and the elders have the same fervor for copulation as the younger ones, it indicates a season of abnormally hot temperatures. Since the body fluids of the lambs are inherently thin and subtle, the excessive heat has a destructive influence on their water balance and they are prone to die that year. The heat has an opposite effect on the older sheep; instead of corrupting, it thins their body fluids and they grow stronger and flourish during the year.

Beyond these, a considerable number of observations may be made about four-legged animals, but we will settle upon a final generalization. All such animals are prone to the ground, i.e. because of the weight of their head and the earthy character of their body, they tend to bear themselves in a horizontal plane, their innate heat being inadequate to maintain them in an erect posture.

The Nature of Flying Animals (Book 23)

In common with all animals birds share in the distinction of possessing a sensory soul, the identifying mark of an animal, as we have said elsewhere. In all birds the sensory soul is present in its perfected form, not just partially, but with respect to its entire potentialities. Thus, the two faculties by which the ancient philosophers distinguished animate from inanimate, viz. sense and movement, are found universally in all birds. Whichever animals have the senses that are perceived through an external medium also possess the senses that operate through an internal medium, though the reverse is not necessarily true;! there- fore, every bird is endowed with all of the senses. It stands to reason that a bird would not have the ability to fly unless it needed to reach something at a distance from itself; furthermore, it would not be moved to seek the distant object unless this was perceived through an external medium.

From the principles established in the books On the Soul, On the Movements of Animals, and On the Principles of Progressive Movement, it is evident birds are perfectly developed animals, because the entire avian class enjoys the use of all the senses and has the capacity for progressive movement. Moreover, they. have bodily organs adapted to perform these functions of a sensory soul, such as a head through which sense and motion flow from the heart; a heart to which sense and motion revert, as to a source; arid a vital spirit which is the transporting agent of each power. Clearly then birds qualify as perfect animals in the fullest sense of that term.

Nonetheless, the animal that is “bird” differs from “animal” in its most genera sense, which implies a participation in the sensation of touch as the barest minimum. Viewing birds as a class, one is struck by a commonality that encompass. es all of its species, viz. they share in all of the senses and progressive locomotion, as we have indicated. Now, “bird” and “flying animal” are interchangeable terms, in the sense that flying animal implies some kind of flying movement. The wing is the specific organ of this type of movement, and nature never provides an organ unless it is intended for a specific function; otherwise nature would be engaged in superfluous activities—a notion that cannot be seriously entertained. For example, non-motile animals that are unable to move from place to place have no organs designed for locomotion. On the other hand, “bird,” as opposed to “animal in general,” shares abundantly in the three senses that are perceived through an external medium, and in the organs necessary for progressive motion.

A bird is like a four-legged walking animal in the sense that it shares in sense and movement, but there are important points of difference. Some quadrupeds do not have every modality of sense, e.g., the mole is sightless. But every flying animal possesses sight, as well as each one of the other senses. Moreover, a bird differs from every quadruped in the number and type of its locomotion, for it has several modes of movement, viz. flying and walking, whereas the quadruped is limited to walking. Further, it differs in its style of walking because, whether it runs or proceeds at a slow pace, it relies on two legs; like man it puts down one leg after the other in walking and running, and only in leaping does it launch its forward progression from a stance with both feet planted on the ground.

Generally speaking, every bird has a bodily construction that differs markedly from the other classes of animals. Usually a bird has a rounded thorax, a globular head, a cylindrical neck, and posterior parts that taper from the chest in pyramidal shape. Though the lower or posterior parts in other animals are larger than the anterior parts, the opposite is true in a bird; its hind parts are noticeably smaller, in most cases narrowing to a point, and the anterior parts of its body occupy the greater bulk. This anatomical arrangement complements the relative weightlessness of the bird’s flesh and the light airy character of its vital spirits.

Among themselves birds display a wide spectrum of differences. The broadest differentiation of flying animals separates those with feathered wings from those with membranous wings. Feathered birds have only two wings, whereas flying animals with membranous wings sometimes have four. The bi-winged type utilizes each wing as if it were an oar; now, an oar is stroked in an up-down movement and propels the boat forward; this is precisely the way a bird flies with its two wings. Certain large birds sometimes glide in a slow descent and seem not to beat their wings at all, but the up-down motion is imperceptibly present and is less apparent: because of the leisurely pace of their gliding. When these same birds accelerate their pace to rapid soaring, the repetitive up-down movement again becomes evident.

The Nature of Aquatic Animals in General (Book 24)

In general, we may say that all aquatic animals have a humid nature that is evinced by their natural habitat and food. Each one is sustained in an environment compatible with its needs and nourished by food of the same humoral constitution as the elements from which it is constituted by reproduction. In most instances, such animals are considered to be cold-blooded, as indicated by their lack or paucity of blood. Very few of these animals have a neck, penis, vulva or mammary glands; indeed, only those which reproduce viviparously have mammilary teats located in junctures of their skin, penises and vulvae, but these organs do not protrude outside the body. The male sex organs of these mammals are hidden within the body and are extruded only at the time of copulation, as with the dolphin and whale.

It is characteristic of most fish to spend little time sleeping, and this with their eyes wide open; for they have no eyelids, but their eyes are tough and unyielding. During sleep, nothing moves but their tail, ever so slightly. The infrequency of sleep is due to the coldness of their digestive organs which dispatch very few vapors to their head.

While aquatic animals are capable of living at great depths in the sea, as well as along river banks and near seashores, the meat of those that frequent the shoreline is generally firmer, less moist, and more wholesome. The flesh of the pelagic depth-dwellers is moister and softer, unless they are of enormous size and an earthy nature, such as those called sea monsters by some authors.

Before spawning, all fish are wont to migrate and seek the location best suited for their egg-laying. At the actual time of spawning they pair off, two by two, so the eggs may be brought to fruition by the participation of the male and female. Because they are more humid and phlegmatic, flat fish grow to best size when the south wind is blowing; on the other hand, elongated fish tend to fatten better under the influence of the north wind. Assuredly the reason for this discrepancy lies in the basic differences of the two climatic conditions.

The hardness of their eyes is the reason for their generally poor vision. In terms of colors they can clearly distinguish only those at the extremes, i.e., black and white, and of the intermediate colors only the bright ones, such as flaming red and brilliant yellow. Thus, veteran fishermen avoid wearing clothes of these colors when they are fishing. For the same reason, fishing is more productive at night, especially near the time of dawn, because at this time the fish see less clearly but yet are motivated to feed.

These remarks must suffice for our overview of the fish kingdom. The reader should note that we have made no distinctions here between the species of marine dwellers and freshwater fish, nor have we classified the various kinds of sea creatures, since we have already dealt with these matters in some detail in the preceding books.

The Nature of Serpents in General (Book 25, chapter 1)

In this twenty-fifth book Of Animals we must examine the nature of serpents, which in many ways resemble aquatic animals in their bodily makeup. Like fishes, serpents have spines rather than bones, and scales on their belly in lieu of claws. In place of legs they use ribs for forward progression or crawling. Again like fishes, they have no external testicles, penises nor vulvae for copulating; rather, they possess ducts for semen and eggs, similar to those of fishes, though the two sexes do not conjoin in the style of fishes; for snakes are attached to one another, bringing into contact the ducts containing the semen and the eggs, so that when they are observed in the act of coitus they appear to have one body and two heads; during this physical contact the female discharges the eggs from her body by means of a viscous fluid which causes them to cohere, and the male pours semen over them. This type of sexual union is characteristic of every reptile classified as a true snake, but it does not apply to those which lay shell-covered eggs, such as lizards; in the latter animals the semen must be ejaculated into the female’s womb before the egg receives its hard shell, for if the semen fell on a shell-covered egg, it would be unable to penetrate the egg’s interior and would be utterly useless for conception. Therefore, like the fish, the serpent lays unfertilized eggs which are brought to fruition by contact with the male semen; consequently, the serpent’s womb is elongated like that of a fish and it discharges the eggs in a continuous outflow during coitus, like the emission of fish eggs.

The snake’s heart is located immediately beneath the head as in a fish, and like the fish heart its shape is not clearly pyramidical but more like a kidney. However, unlike the fish’s tightly restricted tongue the snake’s tongue is very long, forked and black, capable of projecting for some distance beyond the mouth, ... Moreover, in the beginning a snake's eyes consist of an undifferentiated moisture, the greater part of which is in the head but not concentrated in the ultimate locus of the eye; this moisture in the head is endowed with a formative spirit and formative power, capable of developing the eye. Hence, if the eyes of a tiny newborn snake are pierced, they regenerate, just as the eyes of a swallow regrow for the same reason. On the other hand, a snake’s tail is produced chiefly from moisture derived from food, in the same manner as a lizard’s tail; and its body is composed of like parts which require no differentiation by a forming spirit nor variation imposed by forming powers; in other words, one member is formed from another, like from like. For this reason the amputated tail of a snake grows back again, like a lizard’s tail.

Like fishes, the snake has no neck as such, but does have the capacity for turning its head while the rest of the body remains immobile. This facility is a necessary part of its endowment, so that it can look back at the long body it pulls and maneuvers. As in all things, nature does what is best.

Every serpent eats poisonous meats, as well as other meats and plants; but the more poisonous the materials it consumes, the more venomous its bite. The snake has a fancy for wine, even to the point of becoming inebriated and drowning in the beverage; of course, if the snake is venomous, the whole batch of wine thereby becomes poisonous.

Many, but not all, snakes hibernate in a dormant state during the winter. However, it is erroneous to say every serpent is cold-blooded due to the nature of its humoral complexion. On the contrary, snakes with a hot constitution do not undergo a period of dormancy. Those which sleep in the winter become lean, to the extent that their skin loosens; as soon as they awaken, they emerge from the old skin, stripping it first from the facial area, then peeling it from the whole body by forcing themselves through narrow apertures in their caves. In a certain sense, the snake rejuvenates itself by shedding its old skin, much as river crabs and birds do by molting.

Tiny Anemic Animals (Book 26)

In this final book we will devote our attention to the tiny anemic animals, i.e., the ones without blood, first considering them in general, at the risk of repeating some facts we have already discussed. These bloodless animals possess a different kind of humor that takes the place of blood. In most instances such animals are cold and, consequently, their structure is characterized by two distinguishing features. First of all, their bodies are completely segmented, as if they were composed from a series of rings that take origin from a central core. Animals that have blood are nourished by means of certain elongated tubes which are distributed through their members in linear fashion. On the other hand, the bloodless animals receive the structural composition of their body from a central axis that traverses their entire length, by means of a humor drawn to the surface in centrifugal fashion, which gives them the appearance of being composed of hard cylindrical segments. In a sense, their structure is analogous to that of serpents.

The other characteristic is an animal spirit, derived from this modified humor, a spirit both subtle and airy, accompanied by a body heat which is neither widely distributed nor a disturbing factor in their humoral complexion; for this reason also, all such creatures engage in subtle functions and usually cohabit in a quiet, communal existence; in fact, these characteristics are found in them more often than not. Due to the relative heat of the ambient air in which they thrive and the earthy character of the bodily materials diverted to their external surface, on the exteriors of their bodies there is developed a hard coating that takes the form of circumferential rings; sometimes such creatures must shed their skin, a phenomenon readily apparent in many of their species whose discarded skins are found lying on the ground. Due to the viscosity of their internal humor and the earthy nature of their external covering, these animals customarily hibernate in the winter like the bee and wasp, or they die leaving behind an egg-like state. For, if they maintained a continuous flow of nutrient fluids during the winter, their body moisture would freeze because the weak internal heat would be insufficient to withstand the cold; or alternatively, during this season when the production of phlegm is increased, it might be enough to suppress the natural heat; or again, both of these untoward events could occur. In any event, it is obviously disadvantageous for such animals to move about in the winter.

Since these animals have such a delicate moisture content, they cannot be nourished by gross earthy foods. With very few exceptions they suck in dewy fluids like bees, or the body fluids of animals like flies. A few species, such as the caterpillar and locust, gnaw on plants that are still fresh and green.

Almost all of these animals fall into three groups: those without feet; those with feet; and those with wings and feet. The apods, or footless species, have the least amount of internal heat; hence, when they are removed from a source of heat, they can scarcely function. Structurally these creatures are composed of contiguous rings in whose center is concentrated the moving heat that is directed, as from an axis, throughout all the rings of the entire body, just as the moving power of a sphere is exerted along an axis that runs through the center from pole to pole; along this linear axis runs the power of the organ that serves in lieu of a heart, with the spirit and heat of a heart, because this power flows more easily in an axial direction than in any other.

Those in this category of animals that have many legs, have all of their appendages rooted in the lower aspect of the thorax, because the sluggish heat inherent in their bodies is insufficient to cause movement by means of a secondary agency of motion, but must operate more directly by itself. When I say “by a secondary agency of motion,” I mean it operates like the heart by means of the brain, and the brain by means of the spinal cord. In animals such as these, whose body heat is fragile and weak, the limb that provides movement must be governed in a more direct, immediate way by the organ that serves in place of a heart. In proof of this contention is the fact that all such animals grow cold and become immobile at the slightest cause for fear, due to the coldness of their heart. The multiplicity of their feet is caused by the weakness innate in the individual pairs of appendages; due to their delicacy and stiff construction, the feet themselves have little power.

The fact that some have wings attached to their back is because their sparse food supply is located at a distance and their only means of transport are membranous wings whose structure is dictated by the viscosity of the humor from which they are formed. This viscosity also accounts for the several stages of development through which most of these animals pass before they attain maturity. For example, grubs develop from eggs and adults develop from grubs, as in the case of bees. Moreover, some species, like the caterpillar, transpose from grubs into another egg-like phase when we see them become cocoons; we have shown a number of individual cases in extensive detail in the foregoing books of this study.