second. Moreover, I shall concern myself in this volume only with our wild animals; and sheep, poultry, parrots, and all the other creatures introduced by man will not be discussed. Even our "wild" cattle, descended though they may be from the old aurochs and still preserved in a practically wild state in many parks, are thus omitted; and I would, on similar lines, have left out the fallow deer, but very interesting differences must be noted between it and its fellows.
Now, all animals, you will admit, are not alike. Take, for instance, the fox and the rabbit, the eagle and the swan, the adder and lizard, the frog and newt, the salmon and shark, the butterfly and the beetle.
Here, then, we have a dozen types of animals. By types I mean those that represent many others with which they have a good deal in common. All the above-named have one thing in common, and that is life. In fact, they are all animals. (You will sometimes see in books the word "animals" used as if it meant four-footed animals only, but this is wrong.) And if we take away the last two, all the rest have another feature in common, and that is a backbone. They are, therefore, called vertebrates, and form a sub-kingdom, or a sub-division of the entire animal kingdom. The last two have no backbone, and are called invertebrates, forming a group of sub-kingdoms. Now let us go a step further. The first two, the fox and hare, were both nourished in their early days on milk; the rest were not. The fox and hare, then, are mammals. The next two, the eagle and swan, have feathers; none of the others have such a covering. The eagle and swan, then, are birds. Mammals
