Taxonomic classification

The Natural Basis of Classification

I mentioned in my introductory chapter that the simplest form of animal was one whose whole being was contained within a single envelope, or "skin," called a cell. Such a cell contains nothing but that strange primitive life-substance named protoplasm, condensed at one point into a nucleus, and it is precisely of such cells that the bodies of all the animals we commonly know are made up; nevertheless an immense variety of creatures still exists, especially in the plankton of the sea, that, like those at the dawn of life, consist of one cell alone. Here then we stand at the first grand division of the animal kingdom:

A. Animals consisting of a single cell—Protozoa.

B. Animals composed of an aggregation of cells—Metazoa.

This distinction, you see, is one of structure, as must be all the subdivisions that follow, if they are to be natural; and it is the clearest possible illustration of what we mean in zoölogy when we speak of "lower" and "higher" rank, for it is evident that it is a step upward, an advance from utter simplicity to greater and greater complexity, to proceed from a single-celled, all but helpless animalcule to one composed of many cells, with so vast a division of labor and extensive power of action as belong to such a combination of forces.

I do not propose to describe the Protozoa, because both of lack of space and lack of popular interest; anyone may learn about them in any good zoölogical textbook. But I do want to mention one very important point, on account of its bearing on the history of the higher animals. The protozoans reproduce their kind by simply splitting into two individuals, and these again split into another two, and so on; the process is called "fission." There comes a time, however, when the ability to do this ceases, and the protozoans of this strain will die out unless one or more of them meets with the same kind of animalcule, and the two "conjugate," or merge into one another, thus renewing their power to go on dividing.

Turning now to the Metazoa, or animals in general, we may say that they are flexible and usually motile beings, needing a supply of solid food which they convert by digestion into a fluid form, and then diffuse through their tissues. This accounts for the fact that all animals consist essentially of a tube, which in the simpler forms is very apparent. This typical tube consists of at least two layers—an outer, protective, and sensitive coat (ectoderm), and an inner, digestive one (endoderm). This two-layered condition is the limit for a few fresh-water and a vast number of marine animals therefore called "cœlenterata," of which the jellyfish and corals are examples. The two coats are separated, and at the same time connected, by a greater or less amount of a jellylike filling called the "mesenchyme." Into this intermediate mesenchyme both ectoderm and endoderm bud off cells which have certain functions—that is, they circulate the digested food, perform the creeping movements when such occur, expel the waste of the body, and most important of all, provide the germ cells by which the race is perpetuated.

Now in animals superior to the jellyfishes and the flatworms, the mesenchyme is replaced by a definite hollow tissue that produces a more efficient system of muscular, excretory, and reproductive organs. This hollow tissue is the "cœlom," and in the most advanced animals, such as the chordates, "the cœlom and its products are of the greatest importance, for they give rise to the vertebræ and the muscles, and in so doing mold the shape of the fish, amphibian, reptile, bird, and mammal."

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.

As to the names and relative order, or rank, of the subdivisions that we shall have occasion to mention, a few words are desirable. The only real fact is the individual animal. A collection of these so similar that they cannot be divided, and which will interbreed, but usually are sterile as to other animals, is termed a species. A number of species closely similar are bracketed together as a genus  (plural genera ), and this done, every individual is given a double name, as Felis leo  to the lion, the first part of which indicates its genus, and is called its "generic" name, and the second indicates its species, and is called its "specific" name. This "scientific name" is given in Latin (or Latinized Greek) so that it may be unmistakably understood in all parts of the world, for a local name in one language would mean nothing to a student speaking some other language, or perhaps speaking the same language in another country; thus the name "robin" is applied to half a dozen very different birds in separate parts of the English-speaking world, and endless confusion would result were not each animal labeled in a language understood by everybody; and this must be a dead language, so that the significance of the terms applied shall not vary in place or time.

Several similar genera may form a family ; families that agree in essential characteristics are united as orders ; orders are grouped into classes ; and finally like classes are assembled into a phylum  (Greek, "a leaf": plural phyla ), which is the largest division except the primary distinction of Protozoa and Metazoa.