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,[26] as the horse, ass, mule, ginnus,[27] hinnus, and those which in Syria are called mules,[28] 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. [26] Animals with long hair on their tails. [27] Ginnus is the offspring of a mule and mare. Book vi. 24, 1. [28] Hemionus, perhaps the foal of a horse and wild ass, and so distinct from oreus, the foal of the he-ass and mare. 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. CHAPTER VII.