THE CONQUEST OF THE LAND
The Amphibia are the oldest and the lowest group of vertebrates that
are able to lead an active existence on land, and the characters
which distinguish them most definitely from the fishes are all to be
interpreted as adaptions to the new mode of life. One of the most
obvious external differences between the two groups is in the structure
of the extremities, the fish having fins, while the amphibian has limbs
constructed on the same general lines as our own arms and legs. The
fish's fin is to be regarded as an extremity with a very great number
of fingers or toes. It has the function of a paddle, and is obviously
useless whether for supporting or propelling the body on land. The
first obvious necessity for a land existence is some mechanism by
which the limb can be alternately pushed forward and, being fixed to
some solid object, drawn upon, so as to pull the body after it. A
different arrangement of bones and muscles, so as to give a much more
complex lever system than that of a fin, and some kind of clawing
arrangement at the end, were thus necessary. The similarity in the
limbs of all the land vertebrates is very striking, as is indicated by
the comparison of human and a frog's limbs on Fig. 84. In each case
there is a single bone in the upper arm or thigh, which is attached
to a bony girdle in the trunk. There are two elements in the forearm
and in the lower leg respectively, below which, in either case, is
a group of small bones constituting a complex joint at the wrist or
ankle. Then follows the set of five bones in the foot or hand, to each
of which is attached a jointed finger or toe. We have no reason to
believe that this particular arrangement, and the particular number
of digits, was arrived at except by accident. Once arrived at,
however, the arrangement was adhered to with considerable strictness.
For although the number of digits is in some groups--in the birds
especially--reduced, the primary design is almost always readily
recognisable. The second function of the limb, that of supporting
the body, was developed very slowly. In the amphibians and reptiles,
and even in the lower mammals, the legs are comparatively weak and
sprawling, and the creature crawls on the belly.
[Illustration: Left hind leg of frog.
Left leg of man.
Left arm of man.
Left arm of frog.
FIG. 84.]
[Illustration: FIG. 85.--Development of the Frog.]
The second great change which required to be made was of course in
the method of breathing. An ordinary fish, when taken out of the
water, dies of suffocation, because its gills become inefficient for
respiration as soon as they become dry. An entirely new type of organ
had therefore to be evolved, and this occurred on the same lines as
in the Dipnoi, by the development of a pair of sacs from the upper
part of the digestive canal, in which the blood is made to circulate,
and which are kept filled with air taken direct from the atmosphere.
It is of course very well known that an ordinary amphibian is not
a lung breather throughout its whole life. The metamorphosis of a
gill-breathing tadpole into a lung-breathing frog, illustrated in
Fig. 85, is a phenomenon with which everyone is familiar. And this
condition, in which a change in the mode of life is made by each
individual in the course of its development, is the typical one. But
the modern amphibians include types ranging from completely water to
perfect land forms. Some, like the Austrian Olm (Fig. 86), are gill
breathers throughout their whole life. One which is normally of this
type, the Axolotyl, illustrated in Fig. 87, can be made to acquire
lungs and assume a land mode of life. Others, which normally make
the metamorphosis, can be prevented from doing so by being confined
to the water, and complete their life-histories in the condition of
gill breathers. In still other forms (_e.g._ the Cœcilians, Fig. 89)
the change is made before the young creature leaves the egg, and the
independent life is commenced in the condition of a land animal.
Correlated with the development of the lungs is a change in the
structure of the nostrils, from the condition of blind sacs, as they
occur in the fishes, to that of air passages, communicating with the
upper part of the alimentary canal, and thence with the lungs.
[Illustration: FIG. 86.--The Olm--_Proteus anguincus_.]
[Illustration: FIG. 87.--Axolotyl. The ordinary gill-breathing form,
and the artificially developed land form.]
[Illustration: FIG. 88.--Amphibians--The Fire Salamander.]
The living forms of the Amphibia differ considerably from the types
which constituted the group in those long-distant ages when it was
in the heyday of its prosperity. The latter forms were characterised
especially by a system of armour-plating over the head, which
frequently extended under the breast and even covered the greater part
of the lower surface, and which appears to have formed a protection
against the multitude of sharks which populated the waters in which the
amphibians partly lived. The armour-plated type has long ago become
extinct, but it is in it, rather than in the modern forms of Amphibia,
that we must look for the direct ancestor of man. A fossil of this
earlier group is shown on Fig. 90.
[Illustration: FIG. 89.--The Ceylon Cœcilian, _Ichthyophis glutinosa_,
with eggs.]
[Illustration: FIG. 90.--Fossil amphibian, _Brachiosaurus_, from the
Permian.]
Our next group in the order of Evolution is that of the reptiles, the
main differences between which and the Amphibia are of the nature of
more complete adaptions to a life on land. The reptiles have in fact
completed the conquest of the land which was undertaken by the previous
group, and many of them, living as they do in dry and hot deserts, are
as independent of the water as any form of animal life. Thus whereas
the amphibian has a thin skin, which is kept moist by the secretions of
numerous skin glands, the reptile has a body-covering of scales, which
form an effective protection against a too rapid loss of moisture.
Evidently with the same object, the reptile egg is enclosed in a hard
and resistant shell. Correlated with the change in the skin is a
much more perfect development of the lungs, for while the amphibian
breathes to a considerable extent through its thin moist skin, this
method of assisting respiration is not available to the reptile.
Again connected with the improvement of the respiratory process,
there is a partial development of a septum dividing the ventricle or
pumping chamber of the heart. The value of this division of course
lies in the fact that the purified and oxygenated blood from the lungs
is prevented from mixing with the venous blood from the body. The
course of the blood is from the body to the right auricle, thence to
the right ventricle, and thence to the lungs. The pure blood from the
lungs returns to the left auricle, passes thence to the ventricle on
the same side to be pumped to the general circulation. The disadvantage
of a single ventricle, such as occurs in the Amphibia, and the
advantage of the regular double circulation, such as that in man, are
sufficiently obvious. The division of the ventricle into two chambers
is less complete in the lizards and snakes, very nearly perfect in the
crocodiles.
The reptiles, like the Amphibia, are 'cold blooded,' by which is meant,
not that their blood is necessarily cold, but that its temperature
varies with that of the surroundings, while that of the blood of the
mammals and birds is practically constant.
A very important feature of the reptiles, which they possess in common
with the mammals and the birds, is that the embryo produces two
membranous outgrowths called respectively the amnion and the allantois,
which completely envelop it, and which have important functions in
connection with nutrition, respiration, and excretion during the period
when the young creature is enclosed in the egg. It is, of course, not
until we reach the higher mammals that these membranes assume their
greatest importance.
[Illustration: FIG. 91.--Imaginative European landscape in the
Cretaceous period, with reconstructions of typical reptiles.
Plesiosaurus (swimming, up to 50 ft. long).
Three Ichthyosaurians.
Pterodactyls (flying).
Iguanodon.
Megalosaurus (40 ft. long).
Rhamphorhynchus.]
For a considerable time in the world's history the reptiles were the
dominant vertebrate class, and in the chalk period especially they were
represented by a great variety of forms, and by a number of species of
colossal stature, one at least of which was over a hundred feet long.
In those times the reptiles were by no means all condemned to crawl on
their bellies, for they included a large number of marine forms,
comparable to the porpoises and whales among the mammals, and flying
forms whose aspect must have resembled, and been equally terrifying
with, that of our mythical dragons. A few reconstructions of these are
shown in Fig. 91.
[Illustration:
Photo: Berridge.
FIG. 92.--Reptiles--A Chameleon.]
[Illustration:
Photo: Berridge.
FIG. 93.--Reptiles--Indian Python.]
[Illustration: FIG. 94.--Reptiles--Turtle and Tortoises.]
[Illustration: FIG. 95.--Reptiles--Alligator.]
All living reptiles, with a single exception, belong to the four
comparatively modern types of the lizards, snakes, tortoises, and
crocodiles, of which examples are illustrated in Figs. 92 to 95. None
of these are closely related to the mammals or birds. For the common
ancestor of all these types we must go back to some primitive reptile
form. Fortunately such a type is represented at the present day in a
single peculiar species found in New Zealand, which bears the Maori
name of the Tuatara. It was formerly found commonly on the mainland,
but is now confined to a few small islands in the Bay of Plenty,
North Island, where it enjoys Government protection. It is, as the
illustration in Fig. 96 shows, a lizard-like creature, and reaches
a length of about two feet. It lives in burrows near the shore, and
feeds on small animals that are left behind by the tide. The Sphenodon,
as zoologists have named it, has apparently been preserved owing
to the absence of competition by the mammals, and by adopting the
rather curious mode of life just described. In all its features, but
especially in the primitive condition of its vertebræ, it is very much
lower than any other living reptile, and it connects the higher groups
with the Amphibia. Many closely related fossil species are known, one
of which is shown in Fig. 97.
[Illustration: FIG. 96.--The Tuatara, _Sphenodon punctatus_.]
[Illustration: FIG. 97.--_Homeosaurus pulchellus._ A fossil early
reptile from the Jurassic.]
To the lay mind the distinctions between the Amphibia and the reptiles
are not very obvious, and indeed in the older classifications the
former group was not separated from the latter. The differences between
a reptile and a bird, on the other hand, are very striking. It might
therefore be regarded as a matter for surprise that zoologists now make
the greater distinction between the Amphibia and the reptiles, grouping
the former in one great class with the fishes, the latter in a second
great section with the birds. But in fact there are many fundamental
points of agreement between reptiles and birds, and it is impossible
to doubt that the latter have sprung from a reptilian stock. Indeed, a
most interesting connecting link is known, in the fossil Archiopteryx
shown in Fig. 98, of which only two specimens have been found, and
which is the only creature of its type of which we have any record. In
all its skeletal features, the Archiopteryx is reptilian, and it would
undoubtedly have been classed as a new type of reptile but for the
obvious and unmistakable traces of feathers. From what particular class
of reptiles the birds have sprung is not known.
The birds have assumed the position of almost unquestioned masters
of the air, but like other great groups they show possibilities of
evolution in other directions wherever opportunity offers, and types
like the kiwi and the penguin shown in Figs. 100 and 101 have forsaken
their native element--the one for the land, the other for the water.
The birds agree with the mammals in the development of a four-chambered
heart, in their warm blood, in their external covering for the skin,
and in the development of arrangements and instincts for the parental
care of the young. Their line of Evolution has thus been to some
extent parallel to that of the mammals. On the other hand, they differ
obviously in the structure and function of their fore limbs, in the
absence of a diaphragm, and in their special methods for the care of
the young, and there can be no doubt that the two groups have had quite
different origins in the reptile class.
[Illustration: FIG. 98.--Archiopteryx, a fossil lizard-like bird.
Photo: Underwood.]
[Illustration: FIG. 99.--Eagle, with prey.]
[Illustration: FIG. 100.--The Kiwi, Apteryx. A practically wingless
running bird.
Photo: W. P. Dando.]
[Illustration: FIG. 101.--The King Penguin. A highly specialised diving
bird.
Photo: W. P. Dando.]