THE CLASSIFICATION OF ANIMALS
=Basis and significance of classification.=--It is the common
knowledge of all of us that animals are classified: that is, that the
different kinds are arranged in the mind of the zoologist and in the
books of natural history, in various groups, and that these various
groups are of different rank or degree of comprehensiveness. A group
of high rank or great comprehensiveness includes groups of lower rank,
and each of these includes groups of still lower rank, and so on, for
several degrees. For example, we have already learned that the toad
belongs to the great group of back-boned animals, the Vertebrates, as
the group is called. So do the fishes and the birds, the reptiles and
the mammals or quadrupeds. But each of these constitutes a lesser
group, and each may in turn be subdivided into still lesser groups.
In the early days of the study of animals and plants their
classification or division into groups was based on the resemblances
and the differences which the early naturalists found among the
organisms they knew. At first all of the classifying was done by
paying attention to external resemblances and differences, but later
when naturalists began to dissect animals and to get acquainted with
the structure of the whole body, the differences and likenesses of
inner parts, such as the skeleton and the organs of circulation and
respiration, were taken into account. At the present time and ever
since the theory of descent began to be accepted by naturalists (and
there is practically no one who does not now accept it), the
classification of animals, while still largely based on resemblances
and differences among them, tells more than the simple fact that
animals of the same group resemble each other in certain structural
characters. It means that the members of a group are related to each
other by descent, that is, genealogically. They are all the
descendants of a common ancestor; they are all sprung from a common
stock. And this added meaning of classification explains the older
meaning; it explains why the animals are alike. The members of a group
resemble each other in structure because they are actually blood
relations. But as their common ancestor lived ages ago, we can learn
the history of this descent, and find out these blood relationships
among animals only by the study of forms existing now, or through the
fragmentary remains of extinct animals preserved in the rocks as
fossils. As a matter of fact we usually learn of the existence of this
actual blood relationship, or the fact of common ancestry among
animals, by studying their structure and finding out the resemblances
and differences among them. If much alike we believe them closely
related; if less alike we believe them less closely related, and so
on. So after all, though the present-day classification means
something more, means a great deal more, in fact, than the
classification of the earlier naturalists means, it is largely based
on and determined by resemblances and differences just as was the old
classification. Sometimes the fossil remains of ancient animals tell
us much about the ancestry and descent of existing forms. For example,
the present-day one-toed horse has been clearly shown by series of
fossils to be descended from a small five-toed horse-like animal which
lived in the Tertiary age.
=Importance of development in determining classification.=--A very
important means of determining the relationships among animals is by
studying their development. If two kinds of animals undergo very
similar development, that is, if in their development and growth from
egg-cell to adult they pass through similar stages, they are nearly
related. And by the correspondence or lack of correspondence, by the
similarity or dissimilarity of the course of development of different
animals much regarding their relationship to each other is revealed.
Sometimes two kinds of animals which are really nearly related come to
differ very much in appearance in their fully developed adult
condition because of the widely different life-habits the two may
have. But if they are nearly related their developmental stages will
be closely similar until the animals are almost fully developed. For
example, certain animals belonging to the group which includes the
crabs, lobsters, and crayfishes, have adopted a parasitic habit of
life, and in their adult condition live attached to the bodies of
certain kinds of true crabs. As these parasites have no need of moving
about, being carried by their hosts, they have lost their legs by
degeneration, and the body has come to be a mere sac-like pulsating
mass, attached to the host by slender root-like processes, and not
resembling at all the bodies of their relatives the crabs and
crayfishes. If we had to trust, in making out our classification,
solely to structural resemblances and differences, we should never
classify the _Sacculina_ (the parasite) in the group Crustacea, which
is the group including the crabs and lobsters and crayfishes. But the
young _Sacculina_ is an active free-swimming creature resembling the
young crabs and young shrimps. By a study of the development of
_Sacculina_ we find that it is more closely related to the crabs and
crayfishes and the other Crustaceans than to any other animals,
although in adult condition it does not at all, at least in external
appearance, resemble a crab or lobster.
=Scientific names.=--To classify animals then, is to determine their
true relationships and to express these relationships by a scheme of
groups. To these groups proper names are given for convenience in
referring to them. These proper names are all Latin or Greek, simply
because these classic languages are taught in the schools and colleges
of almost all the countries in the world, and are thus intelligible to
naturalists of all nationalities. In the older days, indeed, all the
scientific books, the descriptions and accounts of animals and plants,
were written in Latin, and now most of the technical words used in
naming the parts of animals and plants are Latin. So that Latin may be
called the language of science. For most of the groups of animals we
have English names as well as Greek or Latin ones and when talking
with an English-speaking person we can use these names. But when
scientific men write of animals they use the names which have been
agreed on by naturalists of all nationalities and which are understood
by all of these naturalists. These Latin and Greek names of animals
laughed at by non-scientific persons as "jaw-breakers," are really a
great convenience, and save much circumlocution and misunderstanding.
AN EXAMPLE OF CLASSIFICATION.
TECHNICAL NOTE.--There should be provided a small set of bird-skins
which will serve just as well as freshly killed birds, and which may
be used for successive classes, thus doing away with the necessity
of shooting birds. The birds suggested for use are among the
commonest and most easily recognizable and obtainable. They may be
found in any locality at any time of the year. The skins can be
made by some boy interested in birds and acquainted with making
skins, or by the teacher, or can be purchased from a naturalists'
supply store, or dealer in bird skins. The skins will cost about 25
cents each. This example or lesson in classification can be given
just as well of course with other species of birds, or with a set of
some other kinds of animals, if the teacher prefers. Insects are
especially available, butterflies perhaps offering the most readily
appreciated resemblances and differences.
=Species.=--Examine specimens of two male downy woodpeckers (the males
have a scarlet band on the back of the head). (In the western States
use Gardiner's downy woodpecker.) Note that the two birds are of the
same size, have the same colors and markings, and are in all respects
alike. They are of the same kind; simply two individuals of the same
kind of animal. There are hosts of other individuals of this kind of
bird, all alike. This one kind of animal is called a _species_. The
species is the smallest[4] group recognized among animals. No attempt
is made to distinguish among the different individuals of one kind or
species of animal as we do in our own case.
Examine a specimen of the female downy woodpecker. It is like the male
except that it does not have the scarlet neck-band. But despite this
difference we know that it belongs to the same species as the male
downy because they mate together and produce young woodpeckers, male
and female, like themselves. There are thus two sorts of
individuals,[5] male and female, comprised in each species of animal.
A _species_ is a group of animals comprising similar individuals which
produce new individuals of the same kind usually after the mating
together of individuals of two sexes which may differ somewhat in
appearance and structure.
Examine a male hairy woodpecker and a female; (in western States
substitute a Harris's hairy woodpecker). Note the similarity in markings
and structure to the downy. Note the marked difference in size. Make
notes of measurements, colors and markings, and drawings of bill and
feet, showing the resemblances and the differences between the downy
woodpecker and the hairy woodpecker. These two kinds of woodpeckers are
very much alike, but the hairy woodpeckers are always much larger
(nearly a half) than the downy woodpeckers and the two kinds never mate
together. The hairy woodpeckers constitute another species of bird.
=Genus.=--Examine now a flicker (the yellow-shafted or golden-winged
flicker in the East, the red-shafted flicker in the West). Compare it
with the downy woodpecker and the hairy woodpecker. Make notes
referring to the differences, also the resemblances. The flicker is
very differently marked and colored and is also much larger than the
downy woodpecker, but its bill and feet and general make-up are
similar and it is obviously a "woodpecker." It is, however, evidently
another species of woodpecker, and a species which differs from either
the downy or the hairy woodpecker much more than these two species
differ from each other. There are two other species of flickers in
North America which, although different from the yellow-shafted
flicker, yet resemble it much more than they do the downy and hairy
woodpeckers or any other woodpeckers. We can obviously make two groups
of our woodpeckers so far studied, putting the downy and hairy
woodpeckers (together with half a dozen other species very much like
them) into one group and the three flickers together into another
group. Each of these groups is called a _genus_, and genus is thus the
name of the next group above the species. A genus usually includes
several, or if there be such, many, similar species. Sometimes it
includes but a single known species. That is, a species may not have
any other species resembling it sufficiently to group with it, and so
it constitutes a genus by itself. If later naturalists should find
other species resembling it they would put these new species into the
genus with the solitary species. Each genus of animals is given a
Greek or Latin name, of a single word. Thus the genus including the
hairy and downy woodpeckers is called _Dryobates_; and the genus
including the flickers is called _Colaptes_. But it is necessary to
distinguish the various species which compose the genus _Colaptes_,
and so each species is given a name which is composed of two words,
first the word which is the name of the genus to which it belongs,
and, second, a word which may be called the species word. The species
word of the Yellow-shafted Flicker is _auratus_ (the Latin word for
golden), so that its scientific name is _Colaptes auratus_. The
natural question, Why not have a single word for the name of each
species? may be answered thus: There are already known more than
500,000 distinct species of living animals; it is certain that there
are no less than several millions of species of living animals; new
species are being found, described and named constantly; with all the
possible ingenuity of the word-makers it would be an extremely
difficult task to find or to build up enough words to give each of
these species a separate name. This is not attempted. The same species
word is often used for several different species of animals, but never
for more than one species belonging to a given genus. And the names of
the genera are never duplicated. (There are, of course, much fewer
genera than species, and the difficulty of finding words for them is
not so serious.) Thus the genus word in the two-word name of a species
indicates at once to just what particular genus in the whole animal
kingdom the species belongs, while the second or species word
distinguishes it from the few or many other species which are included
in the same genus. This manner of naming species of animals and plants
(for plants are given their scientific names according to the same
plan) was devised by the great Swedish naturalist Linnæus in the
middle of the eighteenth century and has been in use ever since.
=Family.=--Examine a red-headed woodpecker (_Melanerpes
erythrocephalus_) and a sapsucker (_Sphyrapicus varius_) and any other
kinds of woodpeckers which can be got. Find out in what ways the hairy
and downy woodpeckers (genus _Dryobates_), the flickers (genus
_Colaptes_) and the other woodpeckers resemble each other. Examine
especially the bill, feet, wings and tail. These birds differ in size,
color and markings, but they are obviously all alike in certain
important structural respects. We recognize them all as woodpeckers.
We can group all the woodpeckers together, including several different
genera, to form a group which is called a _family_. A family is a
group of genera which have a considerable number of common structural
features. Each family is given a proper name consisting of a single
word. The family of woodpeckers is named _Picidæ_.
We have already learned that resemblances between animals indicate
(usually) relationship, and that classifying animals is simply
expressing or indicating these relationships. When we group several
species together to form a genus we indicate that these species are
closely related. And similarly a family is a group of related genera.
=Order.=--There are other groups[6] higher or more comprehensive than
families, but the principle on which they are constituted is exactly
the same as that already explained. Thus a number of related families
are grouped together to form an _order_. All the fowl-like birds,
including the families of pheasants, turkeys, grouse and quail, all
obviously related, constitute the order of gallinaceous birds called
_Gallinæ_. The families of vultures, hawks and owls constitute the
order of birds of prey, the _Raptores_, and the families of the
thrushes, wrens, warblers, sparrows, black-birds, and many others
constitute the great order of perching birds (including all the
singing birds) called the _Passeres_.
=Class and branch.=--But it is evident that all of these orders,
together with the other bird orders, ought to be combined into a great
group, which shall include all the birds, as distinguished from all
other animals, as the fishes, insects, etc. Such a group of related
orders is called a _class_. The class of birds is named _Aves_. There
is a class of fishes, _Pisces_, and one of frogs and salamanders,
_Batrachia_, one of snakes and lizards called _Reptilia_, and one of
the quadrupeds which give milk to their young called _Mammalia_. Each
of these classes is composed of several orders, each of which includes
several families and so on down. But these five classes of Pisces,
Batrachia, Reptilia, Aves and Mammals agree in being composed of
animals which have a backbone or a backbone-like structure, while
there are many other animals which do not have a backbone, such as the
insects, the starfishes, etc. Hence these five backboned classes may
be brought together into a higher group called a _branch_ or _phylum_.
They compose the branch of backboned animals, the branch _Vertebrata_;
all the animals like the starfishes, sea-urchins and sea-lilies which
have the parts of their body arranged in a radiate manner compose the
branch _Echinodermata_; all the animals like the insects and spiders
and centipedes and crabs and crayfishes which have the body composed
of a series of segments or rings and have legs or appendages each
composed of a series of joints or segments make up the branch
_Arthropoda_. And so might be enumerated all the great branches or
principal groups into which the animal kingdom is divided.
In the remainder of this book the classification of animals is always
kept in sight, and the student will see the terms species, genus,
family, order, etc., practically used. In it all should be kept
constantly in mind the significance of classification, that is, the
existence of actual relationships among animals through descent.
FOOTNOTES:
[4] The lesser group called _variety_, or subspecies, we may leave out
of consideration for the present.
[5] Some species of animals are not represented by male individuals:
and in some all the individuals are hermaphrodites, as explained in
chapter XIV.
[6] Each of these higher groups has a proper name composed of a single
word. In the case of no group except the species is a name-word ever
duplicated. Each genus, family, order, or higher group has a name-word
peculiar to it, and belonging to it alone.