LETTER XXX.
MAMMALIA. (Mammals.)
Do you remember of my talking of the vertebral column when I was describing that great artery, the aorta, to which it forms a rampart of defence? I should not have named it without explanation, but that you had only to pass your hand down your back to find out what it was. Now the vertebral column, or backbone, is one of those dominant characters which always carries along with it a train of other points of resemblance in the animals where it is found. It has been chosen, therefore, as the rallying-point of the first great group. I must tell you beforehand that there are four of these groups, four large companies, i.e., which naturalists have called by various names; as Groups, Sections, Primary Divisions and even Branches; in this case comparing them to four great branches of a tree, going off in different directions from the same trunk.
And, first of all, we have to begin with the group of the Vertebrata—vertebrata animals—vertebrata being a word which explains itself.
Of course we ourselves belong to this group. In fact, we are at the head of it; but it descends far below us. It goes on to the frog and the fish, and includes the monkey, the ox, the fowl and the lizard; for all these creatures possess the vertebral column. The frog does not appear to be very much like us at first sight; and yet, by virtue of its vertebra, it has its points of resemblance to us, which are worth the trouble of considering. Vertebrated animals are all furnished with a head, containing a brain, which gives its orders to the whole body; they have all an internal skeleton, that is to say, a system of bones linked together, forming a solid base by which all the organs are supported. I was going to add that they have all four limbs; but here the serpent glides in to call me to order, and to hiss at our childish craving for fine-drawn divisions, in perfect order, where there is an exactly proper place for everything. However, each has, without exception, a heart, with its network of blood-vessels; red blood, under its two conditions of arterial and venous; and also a digestive tube, acting, on the whole, pretty much like our own. I do not insist, mind, upon this last point, viz., that of the digestive tube; for we shall see, by-and-by, that it is a character beyond the pale of the primary groups. It is the fundamental character of the trunk itself, which necessarily exists, therefore, in all the groups; and, as I told you in my first letter, you will find it everywhere.
This is—to let you into the secret at once—the theme on which the Great Composer has based all His infinite varieties of animal life; and herein lies the uniformity of the animal creation, that startling uniformity which has given so much offence to many learned men, and which is so obvious that it will strike you of itself, I feel sure. But I reserve this subject to the end of my letters, when you will have heard all, and be able to judge for yourself.
It would be plunging back into confusion to attempt to examine all the vertebrated classes at once. After making a division you must go on. The groups have, therefore, been subdivided into five classes, which we will study in succession, only naming each now: viz. mammals, birds, reptiles, fish, and batrachians. Do not alarm yourself at this last name: it is a Greek word, meaning simply frogs.
The mammals are our immediate neighbors. Mammalia are the animals which produce milk. They bring forth their young alive, and give suck to them as soon as they are born. This was your first nourishment, my dear child, so you yourself are a little mammal.
What I said to you in the last letter about the horse, applies pretty nearly as well to all mammals. We shall not, therefore, have any great variations to notice here. Nevertheless, as these are the animals which interest us most nearly, as they are in fact our nearest of kin, so to speak, and those with whom we have the most to do, we will now pass in review the different orders of which their class is composed. I must explain to you that the classes are subdivided into orders, the orders into families, the families into genera, the genera into species; as in armies divisions subdivide into regiments, regiments into battalions, &c. It became necessary, moreover, to make use of special names, in order to make these subdivisions comprehensible, and the following are those which have been adopted.
ORDER 1. Bimana (two-handed).