A CHAPTER OF FOUNDATIONS
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
"coelenterata," 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 "coelom," and in the most advanced animals, such
as the chordates, "the coelom 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.