ORDER I. _Edentata._ Toothless or with very simple teeth. Examples:
anteater, sloth, armadillo.
ORDER II. _Rodentia._ Incisor teeth chisel-shaped, usually two above and
two below. Examples: beaver, rat, porcupine, rabbit, squirrel.
ORDER III. _Cetacea._ Adapted to marine life. Examples: whale, porpoise.
ORDER IV. _Ungulata._ Hoofs, teeth adapted for grinding. Examples: (_a_)
odd-toed, horse, rhinoceros, tapir; (_b_) even-toed, ox, pig, sheep, deer.
ORDER V. _Carnivora._ Long canine teeth, sharp and long claws. Examples:
dog, cat, lion, bear, seal, and sea lion.
ORDER VI. _Insectivora._ Example: mole.
ORDER VII. _Cheiroptera._ Fore limbs adapted to flight, teeth pointed.
Example: bat.
ORDER VIII. _Primates._ Erect or nearly so, fore appendage provided with
hand. Examples: monkey, ape, man.
[Illustration: The geological history of the horse. (After Mathews, in the
American Museum of Natural History.) Ask your teacher to explain this
diagram.]
Increasing Complexity of Structure and of Habits in Plants and Animals.--In
our study of biology so far we have attempted to get some notion of the
various factors which act upon living things. We have seen how plants and
animals interact upon each other. We have learned something about the
various physiological processes of plants and animals, and have found them
to be in many respects identical. We have found grades of complexity in
plants from the one-celled plant, bacterium or pleurococcus, to the
complicated flowering plants of considerable size and with many organs. So
in animal life, from the Protozoa upward, there is constant change, and the
change is toward greater complexity of structure and functions. An insect
is a higher type of life than a protozoan, because its structure is more
complex and it can perform its work with more ease and accuracy. A fish is
a higher type of animal than the insect for these same reasons, and also
for another. The fish has an internal skeleton which forms a pointed column
of bones on the _dorsal_ side (the back) of the animal. It is a vertebrate
animal.
[Illustration: The evolutionary tree. Modified from Galloway. Copy this
diagram in your notebook. Explain it as well as you can.]
The Doctrine of Evolution.--We have now learned that animal forms may be
arranged so as to begin with very simple one-celled forms and culminate
with a group which contains man himself. This arrangement is called the
_evolutionary series_. Evolution means change, and these groups are
believed by scientists to represent stages in complexity of development of
life on the earth. Geology teaches that millions of years ago, life upon
the earth was very simple, and that gradually more and more complex forms
of life appeared, as the rocks formed latest in time show the most highly
developed forms of animal life. The great English scientist, Charles
Darwin, from this and other evidence, explained the theory of evolution.
This is the belief that simple forms of life on the earth slowly and
gradually gave rise to those more complex and that thus ultimately the most
complex forms came into existence.
The Number of Animal Species.--Over 500,000 species of animals are known to
exist to-day, as the following table shows.
Protozoa 8,000 Arachnids 16,000
Sponges 2,500 Crustaceans 16,000
Coelenterates 4,500 Mollusks 61,000
Echinoderms 4,000 Fishes 13,000
Flat-worms 5,000 Amphibians 1,400
Roundworms 1,500 Reptiles 3,500
Annelids 4,000 Birds 13,000
Insects 360,000 Mammals 3,500
Myriapods 2,000 -------
Total 518,900
Man's Place in Nature.--Although we know that man is separated mentally by
a wide gap from all other animals, in our study of physiology we must ask
where we are to place man. If we attempt to classify man, we see at once he
must be placed with the vertebrate animals because of his possession of a
vertebral column. Evidently, too, he is a mammal, because the young are
nourished by milk secreted by the mother and because his body has at least
a partial covering of hair. Anatomically we find that we must place man
with the apelike mammals, because of these numerous points of structural
likeness. The group of mammals which includes the monkeys, apes, and man we
call the _primates_.
Although anatomically there is a greater difference between the lowest type
of monkey and the highest type of ape than there is between the highest
type of ape and the lowest savage, yet there is an immense mental gap
between monkey and man.
Instincts.--Mammals are considered the highest of vertebrate animals, not
only because of their complicated structure, but because their instincts
are so well developed. Monkeys certainly seem to have many of the mental
attributes of man.
Professor Thorndike of Columbia University sums up their habits of learning
as follows:--
"In their method of learning, although monkeys do not reach the human
stage of a rich life of ideas, yet they carry the animal method of
learning, by the selection of impulses and association of them with
different sense-impressions, to a point beyond that reached by any
other of the lower animals. In this, too, they resemble man; for he
differs from the lower animals not only in the possession of a new
sort of intelligence, but also in the tremendous extension of that
sort which he has in common with them. A fish learns slowly a few
simple habits. Man learns quickly an infinitude of habits that may be
highly complex. Dogs and cats learn more than the fish, while monkeys
learn more than they. In the number of things he learns, the complex
habits he can form, the variety of lines along which he can learn
them, and in their permanence when once formed, the monkey justifies
his inclusion with man in a separate mental genus."
Evolution of Man.--Undoubtedly there once lived upon the earth races of men
who were much lower in their mental organization than the present
inhabitants. If we follow the early history of man upon the earth, we find
that at first he must have been little better than one of the lower
animals. He was a nomad, wandering from place to place, feeding upon
whatever living things he could kill with his hands. Gradually he must have
learned to use weapons, and thus kill his prey, first using rough stone
implements for this purpose. As man became more civilized, implements of
bronze and of iron were used. About this time the subjugation and
domestication of animals began to take place. Man then began to cultivate
the fields, and to have a fixed place of abode other than a cave. The
beginnings of civilization were long ago, but even to-day the earth is not
entirely civilized.
The Races of Man.--At the present time there exist upon the earth five
races or varieties of man, each very different from the other in instincts,
social customs, and, to an extent, in structure. These are the Ethiopian or
negro type, originating in Africa; the Malay or brown race, from the
islands of the Pacific; the American Indian; the Mongolian or yellow race,
including the natives of China, Japan, and the Eskimos; and finally, the
highest type of all, the Caucasians, represented by the civilized white
inhabitants of Europe and America.