backboned animals, called mammalia. When our survey is extended to other
divisions of this class, additional laws of organic relationship are
discovered. If in a series of evolving generations the line of
modification proceeding from a terrestrial animal like a cat to
semi-aquatic and marine types substantially like an otter and a seal should
be carried further, it will inevitably lead to forms possessing characters
such as those displayed by whales and the related porpoises, dolphins, and
narwhals of the order cetacea. In their make-up all of these animals
clearly possess the general characteristics of mammals, and they
constitute collectively another limb which has sprung from the same stock
as the carnivora, although at an earlier time. This we believe because of
their plan of body and because their peculiar organization fits them even
more perfectly than the seals for aquatic existence that is their only
possible mode of life. In the case of the whales the bony framework of the
fore limb is again like that of the cat's leg, although the whole
structure is a flexible finlike paddle. The hind limb has disappeared as
an efficient organ, but the significant fact is that small rudiments of
hind limbs are present just where corresponding structures are placed in
the seal. These vestiges cannot be reasonably accounted for, unless they
are the degenerate hinder limbs of a remote four-footed ancestor.
Furthermore the unborn whale possesses a complete coat of hair, which is
afterwards replaced by blubber; but hair is a thatchlike coat to shed
rain, as the way the hairs lie on a terrestrial mammal indicates. We are
therefore forced to conclude that whales have originated from four-footed
animals walking about on land, because no opposed explanation gives so
reasonable an interpretation of the observed facts.
Another group of familiar animals materially reinforces the results
already established. After what has been said, it will not be difficult to
perceive the meaning of the resemblances among mice of the house and
field, and of rats and rabbits and squirrels. All of them possess heavy
curved gnawing teeth, or incisors, and lack the flesh-tearing or canine
teeth. They agree in many other respects which distinguish them as a
separate natural order of the mammals called the rodentia. Again we find a
highly aberrant form in the flying squirrel, which leads toward an order
with another plan of body. This animal is a true rodent, which lengthens
its leap from branch to branch by means of a fold of skin stretching
between its fore and its hind limbs. It is an animated aeroplane, and it
shows in part how bats have originated. The wing of a bat is an elastic
membrane stretching not only between the two legs of one side, but also
between the greatly lengthened "fingers" of the fore limb. But the bones
of arm, wrist, and fingers are almost precisely the same in number and
relation as in walking forms. The fact that this peculiar wing adheres to
a plan belonging to the anterior legs of walking or climbing types has no
reasonable explanation save that of evolution.
The well-known group of hoofed animals, including horses and cattle, is
also valuable for our present purposes, as well as in a later connection
when the evidence of fossils is described. The elephant possesses five
toes armed with well-developed nails or hoofs. A tapir has four or three
toes, and it would seem that its ancestor had had five toes, of which one
or two had been lost. A rhinoceros possesses three toes, and its foot is
constructed internally like the elephant's with the outer elements absent.
The horse comes last with one large toe and hoof, but on either side of
the main bones of this digit are vestiges of what must have been toes in
its ancestors. Among the even-toed forms the hippopotamus has four which
reach the ground, with a vestige of a fifth, so this animal has apparently
descended from a typical mammal with the full number along a different
line from that taken by the odd-toed forms. A pig has a cloven hoof, made
up of what we may call the third and fourth members of a series of five
digits, but the second and fifth fingers and toes are present, though they
are withdrawn from the ground so as to be no longer functional; this
animal seems to have proceeded further along the same line taken by the
hippopotamus. A deer, with still smaller rudiments at the sides of its
double foot, leads in the comparative series to the camel with a cloven
hoof devoid of any such relics.
We must pass with only brief mention the lower orders of mammalia, like
the insect-eating forms to which armadillos and ant-bears belong. Of
greater interest are the pouched mammals like the kangaroo and opossums,
which live almost exclusively in the Australian realm. The kangaroo is
endowed with a head somewhat like that of a goat, and well-developed hind
legs that enable it to make leaps of astonishing length. Some of its
relatives, such as the bandicoot, are like rats, or like bears, as in the
case of the wombat. The Tasmanian wolf is another true marsupial, even
though divergent adaptation has brought it to resemble the carnivora of
the dog tribe in general appearance and in special structures like the
teeth. Finally at the very bottom of the mammalian scale are two small
forms living in the Australian faunal region. The duckbill or
_Ornithorhynchus_ is the better known animal, with its close fur, webbed
feet, and flattened ducklike beak, while its only other near relative, the
_Echidna_, is somewhat similar to the spiny hedgehog in external
appearance. A unique peculiarity of these two forms is that they produce
eggs much like those of reptiles and birds, and this fact, together with
others of a structural nature, brings the whole group of mammals near to
the lower classes of the Vertebrata.
Looking back on the several orders of mammals, it will be seen that the
last mentioned are much less differentiated or specialized in their
general organization. Above the level of the egg-layers and the pouched
mammals, the higher orders branch out in different directions and reach up
to various levels of the scale of animal organization.
The foregoing structural evidences of organic transformation in the past
histories of cats and seals and whales insistently recall the analogies of
the locomotive and the ship employed at the outset. All these animals,
like the mechanical examples, have come to differ in their derivation from
the same original parents, and their lines of descent have diverged so as
to fit the products of evolutionary modification to diverse circumstances.
Even the vestigial organs of animals have their counterparts in the
machines. The cowcatcher was a large and important structure in the early
days of railroading, but it has become relatively useless with the
decrease of grade crossings and the construction of more complete lines of
fence. The structure still persists, sometimes in a greatly reduced form.
Even more obvious is the change of structure in the case of masts of
vessels, which originally bore the sails for propelling the ship. When
steam engines were employed to give motive power, masts did not disappear.
They now provide the derrick supports of trading steamers; in battleships
their function is changed to that of fighting tops and signal yards. Even
the poles carried by canal boats to bear windmills must be regarded as the
reduced vestiges of masts originally constructed to carry sails; and their
adaptive evolution, like that of countless structures in animals, has been
accomplished by degeneration.
* * * * *
The birds are another class of backboned animals which exhibit identical
principles of relationship. A heron has long legs and wide-spreading toes,
which keep its body out of the water as it stalks about the marshes where
it seeks its food; its bill is a long slender pincers. Compare it with an
eagle; the latter has a short and heavily hooked beak to tear flesh, while
its stout legs bear strongly curved talons to hold its struggling prey.
Swimming birds like the swan and duck and loon possess feet which are
constructed in general like those of the former examples, but they are
webbed and shortened to serve as paddles. In the penguin we find a
counterpart of the seal among mammals; its feathers are much reduced and
its fore limbs are no longer wings enabling the animal to fly, but they
are paddles which it uses when it swims in pursuit of fish. Finally the
ostrich and wingless bird of New Zealand--the _Apteryx_--have wings that
are useless vestiges, which, in the latter case, are hidden under the
brushlike feathers covering the body. It is unnecessary to add more
examples, for even these few illustrations establish exactly the same
principles of relationship and evidences of evolution that are to be found
in the series of mammalia.
Reptiles also are grouped, like the mammals and birds, as variations about
a central theme. An ordinary lizard is perhaps the nearest in form to the
remote ancestor from which all have sprung. Some lizards are long and very
slender, with all four limbs of greatly reduced size. Others, which are
still true lizards, have lost the hind limbs, or even all the legs, as in
the "blind worms" of England. One step more, and an animal which has
progressed further along a similar line of descent would be a snake. Just
as whales as a group are derivable from forms which resemble types
belonging to another order, so snakes as an order are to be regarded as
more radically altered derivatives of some four-footed lizardlike
creature. Alligators are very much like lizards in general form, and their
order is a diverging branch from the same limb. Finally the evolution of
turtles from the same ancestors is intelligible if we begin with a short
stout animal like the so-called "horned toad" of Arizona, and proceed to
the soft-shelled tortoise of the Mississippi River system; the
establishment of a bony armor completes the evolution of the familiar and
more characteristic turtle.
Frogs and salamanders constitute another lower class, called the amphibia,
whose members are gilled during the earlier stages of development. An
adult frog is essentially a salamander without a tail and with highly
developed hinder limbs. The salamanders differ as regards the number of
fishlike gill clefts that they all possess in their young stages, but
which disappear entirely or in part during later life. In comparison with
the lizard as a typical reptile, a salamander is more primitive in all of
its inner organic systems, while in its nearly continuous body, with head
and tail gradually merging into the trunk, it also displays a somewhat
simpler form of body.
The fishes are the lowest among the common vertebrates, and they offer an
abundance of independent testimony as to the truth of the principles of
comparative anatomy. The common shark is perhaps the most fundamental
form, with a hull-like body undivided into head, trunk, and tail, and from
it have originated such peculiar variations as the hammerhead and skate.
Among fishes with true bones, a cod or trout is the most typical in
general features. Without ceasing to be true bony fishes, the trunk-fish
and cow-fish are adapted by their peculiar characters of spine and armor
plate to repel many enemies. The puff fish can take in a great amount of
water, when disturbed, so as to become too large to be swallowed by some
of its foes, illustrating another adaptive modification for self-defense.
The wonderful colors and color patterns of the tropical fish of the reef,
or of the open water forms like the mouse-fish of the Sargossa Sea, often
render them more or less completely hidden from the foraging enemy. A
flounder looks like a fish which was originally symmetrical, but which had
come to lie flat on its side upon the bottom, whereupon the eye underneath
had left its original place to appear on the upper surface. The difficult
and unusual conditions of deep-sea existence have been met by fishes in
two ways; some forms possess luminous frilled and weedlike fins, which
lure their prey to within easy reach of their jaws, while others have
enormous eyes, so as to make use of all possible rays of light in their
pursuit of food organisms. But all of these diverse forms are true
_fishes_, possessing a common heritage of structure which demonstrates
their unity of origin.
The brief review of backboned animals has shown how comprehensive are the
principles of relationship. The families and tribes of each order, such as
the carnivora, are like branches arising from a single limb; the orders in
their turn exhibit common qualities of structure which mean that they have
grown from the same antecedents, while even the larger divisions or
classes of mammals, birds, reptiles, amphibia, and fishes, possess a deep
underlying theme whose dominant motif is the backbone, which proves their
ultimate unity in ancestry. The greater and lesser branches have reached
different levels, for the fish is clearly simpler in its make-up than the
highly specialized bird. But the great fact is that structural evidences
demonstrating the reality of genealogical affinities are displayed by the
entire series of vertebrates; although they differ much or little in many
or fewer respects they have one and the same ground-plan.
* * * * *
The lower animals devoid of backbones, and therefore called invertebrates,
are not so well-known except to the student of comparative anatomy,
because they are not so often met with, and because they are usually very
small or microscopic; but in many respects their importance to the
evolutionist surpasses that of the vertebrates. Their structural plans are
far more varied, and they range more widely from higher and relatively
complicated organisms to the unitary one-celled animals. A knowledge of
some of them is essential for our present purpose, which is to learn how
sure is the basis for the principles of relationship and how complete is
the structural evidence of evolution.
Worms are represented in the minds of most people by the common earthworm
or sandworm. The body in either case is made up of a series of segments or
joints which agree closely throughout the animal in external appearance
and in internal constitution. A section of the digestive tract, a pair of
nerve centers, two funnel-like tubes for excretion, and similar blood
vessels occur in each portion.
Precisely similar features are displayed by the crustacea, which seem to
be so different. Every one is familiar with the appearance of lobsters and
crabs. Even in these animals the body is composed of segments, but these
are not like one another, nor are they freely movable throughout the body.
Five are fused in all crustacea to make a head; in lower members of the
order the eight succeeding segments are free, but in the lobster they are
joined together and united with the head. The hinder part of this animal
is a long abdomen whose segments remain more primitive and independent.
But in a crab, the whole plan has been modified by the shortening and
broadening of the head-thorax, and by the reduction of the abdomen, which
is also turned under the anterior part of the body. The internal organic
systems are constructed upon a worm plan with modifications. Nearly every
one of the segments bears one pair of appendages, which can be referred by
their forked nature to the two-parted, oarlike flaps of sandworms, but the
appendages of crustacea have departed from their prototypes in functional
respects and in details of structure. They are variously feelers, jaws,
legs, pincers, and swimming paddles, evolved to serve different purposes,
just as the limbs of the vertebrates we have described have become
variously arms, wings, flippers and paddles in apes, bats, seals, and
whales.
Butterflies, beetles, bees, and grasshoppers seem at first sight to be
entirely different, even though they agree in being more or less
segmented. But all of them have heads with four pairs of appendages of the
same essential plan, middle thoracic regions of three segments more or
less united, bearing three pairs of legs and usually two pairs of wings,
while the hinder part is a freely jointed abdomen without real limbs. In
these respects the countless varieties of insects agree so that they also
like crustacea of various kinds seem to have been derived from wormlike
animals with more simply segmented bodies. Indeed spiders and scorpions
and their relatives of the group arachnida prove for similar reasons to be
derivatives of the same original stock, and own cousins of the insects.
In nearly every one of the invertebrate branches we find representatives
which interest us chiefly because they appear to have reached their
present condition by retrograde evolution. Barnacles are really crustacea,
but they have lost their eyes as well as some other structures that are
most useful in animals with a free existence, because they have adopted a
fixed mode of life, which has also brought about the loss of the original
freely jointed character of the body. A tapeworm as an example of internal
parasites is an extremely degenerate form which lacks a digestive tract,
because this is superfluous in an animal which lives bathed in the
nutrient fluids of its host. Comparing it in other respects with other low
wormlike creatures, it appears to be a relative of peculiar simple worms
with complete organization and independence of life. All these degenerate
forms enlarge our conception of adaptation by adding the essential point
that progress is not always the result of evolution. Indeed we have
learned this in the case of vestigial and rudimentary structures of higher
forms like whales, and now we find that entire animals may degenerate as a
result of changes no less adaptive than progressive modifications.
Passing by other invertebrate groups made up of species arranged like
higher animals in smaller and larger branches according to their degree of
fundamental similarity, we arrive at a place in the scale occupied by
two-layer animals without the highly developed and clearly differentiated
organic systems of the forms above. The fresh-water animal _Hydra_
exemplifies the creatures of this level, where also we find sea-anemones
and the soft polyps which form corals and coral reefs by their combined
skeletons. _Hydra_ is an animal to which we must return again and again as
we study one or another aspect of organic evolution. In general form it is
a hollow cylinder closed at one end, by which it attaches itself, while at
the upper end, surrounded by a group of tentacles, is the mouth which
leads to the central cavity. The wall of this simple body is composed of
two layers of cells, between which there is a gelatinous layer rarely
invaded by cells. The inner layer lines the central space into which food
organisms are thrust by the tentacles, and it is concerned primarily with
digestion. The outer layer comprises cells for protection and sensation
primarily. Cells of both layers have muscular prolongations which by their
operation enable the whole animal to change its form and to move from one
place to another.
It may seem that such an animal is totally unlike any of the higher and
more complex types. In certain respects, however, it is identical with the
other forms inasmuch as it performs all of the eight biological tasks
demanded by nature. It is also similar in so far as its inner layer, like
the innermost sheet of cells in higher forms, is concerned with problems
of taking and preparing food, while the protective outer layer resembles
in function the outermost covering of all animals higher in the scale.
Beyond these a still more fundamental agreement is found in its cellular
composition.
At the lower end of the animal scale are organisms which consist of one
cell and nothing more. _Amoeba_, to which we must refer again and again,
is an example of this group which possesses an overwhelming importance to
the comparative student because the origins of all the characteristics of
animals higher in the scale are to be found within it. _Amoeba_ itself
is a naked mass of protoplasm, about 1/100 of an inch in diameter,
enclosing a nucleus. Its form is not constant during activity, for
fingerlike processes called pseudopodia are pushed out tentatively in many
directions to be followed as circumstances direct by the materials of the
whole cell body. Other protozoa differ in possessing constant forms, or in
having constant vibratile processes, or shells of some kind, while in
still other cases like individuals combine to make colonies which are more
or less definite and permanent. Here at the very foot of the organic scale
are found animals which seem to be entirely different from those above.
Upon examination they, like _Hydra_, prove to be the same as regards the
number and kind of functions they perform, but in structural regards their
evolutionary relation to all higher animals is indicated solely by the
fact that they are cells composed of protoplasm. Nevertheless the
principle which states that resemblance means consanguinity still holds
true, for cellular constitution is a unique possession of things of the
living world,--something which demonstrates the common origin of all
living things just as truly as the "cat-_ness_" of our first series of
examples reveals for a smaller group the significance of likeness and the
nature of the basic law of comparative anatomy.
* * * * *
Employing a figure of speech, we have climbed down the animal tree from
the higher regions where the mammals belong. Having reached the very foot
of the trunk we are in a position to review and summarize the evidences
which we have discovered all about us as we have descended. The various
examples we have mentioned and the groups to which they belong clearly
occupy different places in the scale which begins with the protozoa and
extends upward to the most complicated and differentiated animals. _Hydra_
takes its place above the protozoa for obvious structural reasons; worms
belong to a still higher zone, surpassed by the more complex jointed
animals like crustacea and insects. Far above these are the vertebrates,
among which we have already demonstrated the occurrence of different
grades of organization, from the fish up to the higher amphibia and
reptiles, and beyond in two directions to the diverging birds and mammals.
The basic characteristics of every group in a high position may be traced
back to some one or another of the divisions at a lower level, so that the
general sequence of the structural levels from low to high becomes
intelligible as the order of their evolution.
To my mind the rudimentary and vestigial structures of animals are in
themselves proof positive of a natural history of change. The few
illustrations can be reinforced by countless examples offered by every
group of living animals. If such structures have not evolved naturally by
degenerating from more efficient counterparts in ancestors of earlier
times, and if they have been specially created, they are utterly
meaningless and their very existence is unreasonable. If common sense is
to be employed, they demonstrate evolution.
Everywhere throughout the whole series animals place themselves in a
treelike arrangement, for in their respective levels they occur like
leaves at the ends of the lines of descent which have led up to them and
which are comparable to the branches and limbs arising from the trunk of a
tree. Thus the major and minor divisions of animals do not follow in the
order of the rungs of a ladder, even though they must be assigned to
different levels according to the complexity of their construction. The
summary given above, namely, that the occurrence of lower and higher
levels reveals an order of evolution, is amplified and not contradicted by
the statement that the species of animals are group in a treelike
arrangement. It is the task of the evolutionist, provided with all the
facts of comparative anatomy and dealing only with the various species as
separate leaves, so to speak, to reconstruct the now invisible but not
unreal twigs and branches and limbs of the animal tree, and to show how
they have diverged at one time or another as they have grown and spread to
produce the species of the present day. This he may do in so far as he may
find sufficient materials to enable him to employ the methods of
comparative anatomy and the great natural principle established by this
method--that essential likeness means consanguinity.
* * * * *