In this brief sketch of some broad distinctions among the masses of animals we have a hint of the basis of their classification. ANIMAL LIFE IN ORDERLY ARRANGEMENT Classification is really only a sorting out of things into groups of the same kind. It may be artificial, according to fancy or convenience, or it may be by discovery of nature's inevitable development. It has been done crudely ever since men began to show curiosity about the things around them. They spoke of animals of the land, of the water, and of the air; of those that lived on vegetable fare as different from the flesh eaters; and in a more particular way they recognized various obviously like and unlike groups within the larger ones. All these distinctions were made on external appearance or behavior, and closer observation presently showed bad combinations, such as placing bats with birds simply because both flew, or whales with fish because both lived in water. Slowly it became evident that the only proper way to classify animals was by putting together those of like structure, and this could be accomplished only by intense comparative study of the interior anatomy of their bodies. Even here, however, progress was limited until the great light from the idea of organic evolution fell on biological science, by which it was perceived that the true criterion by which the proper place of any animal could be determined was its line of descent--a matter wherein the student of fossils could render, and has rendered, vast assistance. In other words a real, natural classification is according to ancestry, just as human relatives are grouped into families according to their known descent from the same forefather. In this evolutionary light zoölogists have now perfected, at least in respect to its larger divisions, a classification of the animal kingdom which is generally accepted, and is followed in this book. It proceeds, reading downward, from the simpler and older forms of animal life to the more complex and more recent forms.