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1. The following are the principal classes which include other animals — birds, fishes, cetacea. All these have red blood. There is another class of animals covered with a shell, and called shell fish, and an anonymous class of soft-shelled animals (malacostraca), which includes carabi, carcini, and astaci; and another of mollusca, such as teuthis, teuthos, and sepia; and another class of annulose animals. All these are without blood, and the species with feet have many feet. There are no large classes of other animals; for there are many forms which are not included under a single form, but either stand alone, having no specific difference, as man, or have specific differences, but the classes are anonymous.
2. All animals with four feet and no wings have blood. Some of these are viviparous, others oviparous. The viviparous are not all covered with hair, but the oviparous have scales. The scale of a reptile is similar in situation to the scale of a fish. The class of serpents, sanguineous land animals, is naturally without feet. Though some have feet, this class is also covered with scales. All serpents, except the viper, are oviparous. The viper alone is viviparous, so that not all viviparous animals have hair; for some fishes also are viviparous. All animals, however, that have hair are viviparous; for we may consider the prickles of the hedgehog and porcupine as analogous to the hair of animals; for they answer the purpose of hair, and not, as in marine animals that are so covered, of feet.
3. There are also many classes of viviparous quadrupeds, but they have never received names. Each kind must, therefore, be taken separately , as man, as we speak of lion, stag, horse, dog , and of others in like manner. There is, however, one class of those that have a mane called lophuri, as the horse, ass, mule, ginnus, hinnus, and those which in Syria are called mules, from their resemblance, though not quite of the same form. They copulate and produce young from each other, so that it is necessary to consider well the nature of each of them separately.
4. We have now treated of these things in an outline, for the sake of giving a taste of what we are afterwards to consider, and of how many. Hereafter we will speak of them more accurately, in order that we may first of all examine into their points of difference and agreement; and afterwards we will endeavour to inquire into the causes of these things, but it will be a more natural arrangement to do so when we treat of the history of each. For it is evident from these things what they are, and what we have to demonstrate.
5 . Our first subject of consideration must be the parts of which animals are made up, for these constitute the chief and the whole difference among them; either because they have them or are without them, or these parts vary in position or arrangement, or in any of the differences mentioned before, in form, size, proportion, and difference of accidents. First of all, then, we will consider the parts of the human body; for, as every one can best understand the standard of money with which he is most familiar, so it is in other things. And of necessity, man must be the best known to us of all animals. The parts of the body are, indeed, plain enough to every one’s common sense; but, that we may not forsake our arrangement, and may have reason as well as perception, we will speak, first of all, of the organic, and afterwards of the simple, parts.
Source: Aristotle, History of Animals; translated by Richard Cresswell, M.A. (1862)