ADAPTATION OF LAND REPTILES TO LIFE IN THE WATER
In the never-ceasing struggle for existence all forms of life upon the
earth, whether consciously or unconsciously, are continuously striving
for improvement; striving to flee from adverse environments, or to
adapt themselves better to those which must be endured; to escape
their enemies, or to find means whereby they may withstand them; to
find more or better food, or to prevent others from despoiling them
of what they have. There is always more or less of unrest, more or
less of discontent, if such terms may be used of the lower organisms.
It sometimes happens with groups of organisms that by reason of
unusual or extraordinary traits they become so perfectly adapted to
their environments, to their surroundings, or so easily adaptable to
changes in their environments, that they remain for long ages securely
protected and little changed. But, as with man himself, improvement is
usually the result of adversity—adversity which stimulates but does not
destroy. And the word improvement, translated into biological language,
means simply specialization, that specialization which adapts the
organism better to its mode of life, which fits it the better to excel
its less ambitious or less capable competitors. No animals or plants
are perfect; if they were, there would be no advancement, no struggle.
If all physical conditions stood still, or remained uniform, perhaps
life would stand still, but conditions never have and never will stand
still, and life must change to meet changed conditions.
Thus it is that which makes life easier, which lessens the dangers
of destruction, which insures the continued prosperity of the race,
is seized upon and utilized by all plants and animals, so far as
possible. As said long ago by Tennyson,[2] the first law of life is
not the preservation of self, but the prosperity of the race. Whatever
the causes may be whereby the offspring are better adapted to conquer
in the struggle for existence, whatever may be the laws governing
changes and specialization, whether heredity, Mendelism, mutation,
natural selection, or Lamarckism, we call the process evolution.
[2] Are God and Nature then at strife,
That Nature lends such evil dreams?
So careful of the type she seems
So careless of the single life.—_In Memoriam_, lv.
To escape from the severe competition of the overcrowding animals
of the sea, some of those creatures we call fishes long ago became
air-breathers and took possession of the unoccupied land. From among
the myriads which were driven into unbreathable water, by accident or
by their enemies, or led there in the search for more easily acquired
or better food, some survived and found that the oxygen of the air was
quite as breathable as that of the water. Steadily their progeny became
better and better adapted to the unusual life until they ceased to be
fishes and became amphibians, from which have arisen in like manner all
the reptiles and birds and mammals that live or have lived upon the
earth.
With more and better powers, developed under better opportunities, not
a few of these descendants have repeatedly sought safety from their
newly acquired enemies of the overcrowded land, or a better supply of
food in the sea; gradually, perhaps incidentally at first, as we shall
see is the case with some lizards today, but later with increased
adaptation to their new surroundings, they become truly sea or water
animals, no longer able to live upon the land. In these changed
conditions and with concomitantly changed habits they never reverted to
the primitive condition of fishes, never became water-breathing animals
again, for that would be actual retrogression, a seeming impossibility
in evolution. Nor indeed does it seem possible that a land creature
after its reversion to water life ever can return to the land again.
A fish through long ages of evolution has become well adapted to its
environments; its shape is the best for speed or varied evolutions in
the water; its teeth and mouth-organs are best suited for the food it
requires. Now it is evident that if animals of very different habits
and form should go back to the water and seek to compete with creatures
already well adapted to their surroundings, they must, so far as
possible, acquire like forms and like habits. And any improvement on
such forms and habits that their higher development permits them to
attain will of course be of advantage in their competitive struggles.
A fish makes most use of its tail fin for propulsion. It follows
that a land animal seeking to compete with it under like conditions
must acquire a tail fin or some other organ which subserves its
purpose as fully. The body fins are of little use to a fish, save for
equilibration, for preserving its position, for stopping quickly, or
for changing the direction of its movements quickly—very different
functions from those of the corresponding organs, the limbs, of higher
vertebrates. There are few better examples of predaceous, fish-eating
fishes than the common gar-pike of our rivers, fishes with a slender
body covered with very smooth scales, a strong tail, a short neck, and
long jaws armed with numerous slender and sharp teeth. Such a fish,
darting into a school of smaller fishes, by quick, sudden changes of
movement, actively opening and closing its jaws, is sure to seize some
of its sought-for prey. In a direct trial of speed with its victims it
would most likely be worsted.
There have been many animals of high and low rank which in the
past and present have gone back from a terrestrial existence to a
life in the water, finding at last a congenial home away from the
shores. Or, perhaps, like the monitor lizards of today, they have
found temporary safety in the water when hard pressed by their land
enemies, and finally found, not only protection, but an abundant
supply of easily obtainable food therein. As in every vocation of
life there have been many failures in such attempts, many partial
successes only. But not a few have found abounding and enduring
success and final prosperity—success that has led possibly to undue
adaptation to surroundings, and to the acquirement of great size,
for that has been the invariable end of water air-breathers of long
duration—specializations which finally prevented them from meeting new
exigencies. It seems to be a law of evolution that no large creatures
can give rise to races of smaller creatures; and as we shall see, the
largest sea-animals have been the final evolution of their respective
races.
There are no better examples of such success today, nor has there been
in all the geological ages, so far as we know, more perfect examples
of the adaptation of air-breathing animals to an aquatic life than the
great whalebone whales. In Eocene times their ancestors were walking
and running land animals; of that there can be not the slightest doubt,
since we cannot conceive, as did the older naturalists, of their direct
descent from the fishes while having all the essential structure of
mammals, i.e., lungs, circulatory system, manner of breeding and
rearing the young, etc. Of the living whales, or Cetacea, there are
now in existence two very distinct types, so different from each other
that some have supposed them to have been evolved from different
types of land mammals. One of these is best exemplified by the great
baleen whale, having a broad, short head and no teeth. It feeds upon
crustaceans chiefly, which are strained from the water by the great
fringe or net of “whalebone.” The other type is seen in the porpoise
or dolphin. These cetaceans have numerous, pointed and recurved teeth,
which they use as did many of the reptiles, hereinafter described, for
the seizure and retention of fishes and other swimming animals. So
great have been the changes in all these cetaceans, in the adaptation
to an aquatic life, that we are almost at a loss to conjecture from
what kinds of land animals they have descended. The great zeuglodont
whales of early Tertiary times have long been thought to be a sort of
connecting link between them and their land ancestors, and it is still
probable that they were. The forms of zeuglodont whales that have been
discovered in Africa within recent years bear so much resemblance
in their skull and teeth to the contemporary carnivores, that many
paleontologists think, with good reason, that they were descended from
them, that is, from the ancestors of all our dogs, cats, weasels,
bears, etc., of modern times. And we have much reason to believe that
future discoveries will bring further and more decisive proof of their
origin before many years have elapsed. The modern Sirenia, the dugongs
and manatees, exclusively aquatic mammals, which feed upon seaweeds at
the bottoms of shallow bays and harbors, or in the mouths of rivers,
are now known, practically with certainty, to be the descendants in
these same African regions of the earliest ancestors of our sheep,
oxen, and horses, known so certainly that they are often classed with
them, or at least with the elephants, which approach them in their
ancestral line even more closely.
A third type of living aquatic air-breathers is seen in the seals,
sea-lions, etc. They are much less highly specialized, however, than
the whales or sirenians, since they are still capable of considerable
freedom upon land, which they recurrently seek for the breeding of
their young. They still retain the primitive covering of hair, lost
almost entirely by the cetaceans and sirenians and functionally
replaced for the conservation of heat by a thick layer of blubber.
Instead of losing the hind legs and developing the tail as a propelling
organ like the whales, the seals encountered precisely the reverse
experience. The hind legs have been developed into most efficient
paddles or sculls, and the tail has been for the most part lost.
They are fish-eaters, it is true, but they do not have the long jaws
possessed by the porpoises and toothed whales.
In the sea-otters, beavers, and even the muskrats, we have examples
of less complete adaptation of land mammals to water life, the most
of them showing the beginnings at least of structural adaptations
similar to those of the seals. From an attentive examination of all
these animals, living as well as extinct, which have attained partial
or complete success as air-breathing water animals, we find certain
laws existing, if we may call them such, which we may discuss a little
in detail. As we have seen in the comparison of the whale with the
seal, the methods of adaptation have not always been the same, and some
recent writers have endeavored to classify aquatic animals under many
groups, to which they have given learned technical names, most of which
will not concern us here in dealing with the reptiles only.
Beginning with the head, we find that all those reptiles and most of
the mammals which have become aquatic fish-eaters have an elongated
skull, or rather an elongated face. The jaws are long and slender, and
the teeth are not only numerous but also sharp and slender, much like
those of the gar-pike, indeed. It is remarkable, too, that in most
such animals the external nostrils are situated, not at the extremity
of the snout, as in all terrestrial mammals and reptiles, but far back
near the eyes. In the whales this position of the nostril enables the
animals to breathe without continuous muscular exertion while floating
on the surface; that is, the nostrils are at the top of the head. In
the sirenians, on the other hand, which live habitually at the bottom
of shallow waters, coming to the surface to breathe only, the nostrils
are situated so that they are the first to emerge, that is, they are
near the front end. The crocodiles, with a more or less elongated face,
as also the Choristodera, described farther on, are exceptions, since
their nostrils are at the extremity of the snout. Both of these types,
however, notwithstanding the elongation of the face, are only partly
aquatic in habit, and in the crocodiles the breathing organs have
undergone a strange modification in accordance with habits peculiarly
their own, as will be explained later on. Whether this recession of the
nostril toward the eyes can be explained in all cases by the peculiar
breathing habits is, however, doubtful. Possibly in some cases, such as
the phytosaurs, described later, the creatures used their long beaks
to probe in the mud while breathing. Possibly the posterior position
has been in some cases rather the result of the elongation of the face,
leaving the nostrils behind in some forms, or carrying them forward in
others. Nevertheless posterior nostrils always indicate more or less
aquatic habits.
In all the earliest reptiles, as we have seen, the neck was short,
like that of their immediate progenitors, the ancient amphibians. The
shoulders were close to the skull, with not more than two vertebrae
that could be called cervical. It happens that most of the earliest
reptiles, as we know them, were more or less amphibious in habit, and
all of them were probably good swimmers; nevertheless in all likelihood
reptiles began their career as a class with a very short neck. The
earliest known distinctly terrestrial reptiles had a moderately long
neck composed of six or seven cervical vertebrae. It may therefore be
assumed with much probability that all later reptiles with a greater
or less number of cervical vertebrae are specialized animals, so far
as the neck is concerned. Most living reptiles have eight cervical
vertebrae; a few have nine, and still fewer have but five. Birds may
have as many as twenty-four, while all mammals, with two or three
exceptions, have the primitive number seven. Among extinct reptiles,
however, there were not a few with more numerous neck vertebrae, some
having the enormous number of seventy-six.
An ordinary fish has apparently no neck whatever, the trunk
being seemingly attached to the head, nearly as in the primitive
amphibians and primitive reptiles. It is evident that a movable neck
of considerable length would not only be of no use to the swiftly
swimming fish, but a positive disadvantage to it. The body is quickly
and easily turned by the powerful tail fin, and a long neck could be
of no use that the tail would not better subserve. It is therefore
of interest to learn that, as a rule, aquatic animals of all kinds
having a powerful propelling tail have also a short neck, acquired
either by the loss of neck vertebrae, or, as in the mammals, by the
shortening and coalescence of the normal number of seven. There are
very few exceptions to this rule of a short neck and a long tail.
Those strange little reptiles of Paleozoic times, the first that we
know that returned to the water, the Proganosauria, have not only a
long, flattened tail, but also an unduly elongated neck of from nine to
twelve vertebrae.
On the other hand, certain unrelated reptiles of the past, the
dolichosaurs, nothosaurs, and plesiosaurs, with a short non-propelling
tail, developed a long neck—sometimes an excessively long one in the
plesiosaurs. The turtles, some of which have attained a high adaptation
to water life, have invariably a short tail and a freely movable,
relatively long neck, a neck which Dr. Hay tells us has increased
in length from the beginning of their race by the simple elongation
of the vertebrae, as in the giraffe, and never by the addition of
vertebrae. We may then account it a rule that swimming animals with
a long neck have a short tail, and those with a short tail have a
long flexible neck. Even in the plesiosaurs there is some variation
of the length of the tail in correlation with the neck. Short-tailed
animals must necessarily propel themselves through the water by the aid
of their legs, especially the hind legs. If one watches an actively
swimming alligator he will observe that the front legs are folded or
collapsed by the side of the body, while the hind legs, much bent,
are used only slightly in propulsion. The animal swims by a marked
sinuous or serpentine movement, like that of a snake upon land,
extending throughout the tail and part of the body, at least. An animal
propelling itself by its limbs could not move sinuously, and use its
legs actively at the same time, and it is probable that the long neck
has been evolved compensatorily.
With this shortening of the neck and sinuosity of movement there is
developed in every case a long trunk as well as a long tail. The trunk
becomes more slender and cylindrical, more like that of a snake, with
an actual increase of the bones composing it, reaching the great
number of forty-three vertebrae in that most sinuous of all water
reptiles with legs, _Pleurosaurus_ of the Protorosauria. And the tail,
primitively having perhaps sixty or seventy vertebrae, may have as
many as one hundred and fifty in the more typical aquatic forms. This
elongation of trunk and tail must be of great advantage to the swimming
reptile, just as the racing scull is a more perfect type of speedy
craft than a flat-bottomed scow. Dr. Woodward has said that the fate of
all fishes, if they continue their evolution long enough, is to become
eel-like.
Not only was the tail greatly elongated in swimming reptiles, but it
was also more or less flattened. In the beginning of water adaptation
the flattening was throughout the tail, as in the living alligators and
crocodiles. As the adaptation to water life became more perfect, the
flattening became more and more restricted to the extremity; that is,
the flattening begins like that of a salamander and in the end becomes
like that of a fish, a terminal fin. And some of the actual stages in
the evolution of the fish-like fin have been observed by Dr. Merriam
in the earlier and more primitive ichthyosaurs of California. In those
animals swimming chiefly in a horizontal direction the tail fin has
become like that of fishes, that is, vertical; but in those animals
which use the tail chiefly for ascending and descending rapidly in the
water the fin is developed in a horizontal position, examples of which
are seen in the flukes of whales and sirenians.
All animals living upon the land require firm articulations between the
different bones of the skeleton, and especially between the vertebrae,
for the support and control of the body. Among aquatic animals there
is a strong tendency toward looseness of joints, with increasing
flexibility. Fishes have the articular processes between the arches
of the vertebrae feebly or not at all developed, and the centra or
bodies of the vertebrae have thick pads of cartilage between them. Firm
union between the vertebrae would restrict freedom of movement, and
firmness is not required when the body is surrounded on all sides by
water of nearly the same specific gravity as the body itself. And it is
doubtless for the same reasons that the articulations of all strictly
aquatic reptiles have for the most part become looser and less firm,
especially those between the different vertebrae.
The same looseness of articulation is also found in the ribs of
aquatic animals. In most animals, and in all those which walk erect,
like the mammals, each rib is firmly attached to the backbone by two
distinct joints, the head and tubercle, with an interval between
them. This double attachment prevents much in-and-out movement of the
ribs and gives a firm support for the attachment of the muscles of
respiration, as well as for those supporting the viscera. This firmness
is unnecessary in animals living always in the water, and the ribs
therefore in all aquatic animals tend to become single-headed and
loose. The lower or capitular articulation has been lost in part, or
almost wholly, in many cetaceans. It has been said that a whale cast up
on land will die of suffocation, not for the lack of air, for it is an
air-breathing animal like ourselves, but because it can no longer use
its respiratory muscles attached to the loosely articulated ribs; it
suffocates because the ribs collapse.
As would be expected, the greatest modifications of structure in the
adaptation of air-breathers to water life are found in the limbs. No
other parts of the body have such different functions in water and on
land as the limbs and fins. The limbs of a dog, or a cat, or a man
are feeble organs for swimming in comparison with the fins of a fish,
and if the land animal must compete with fishes to prey upon them for
food it must acquire like swimming powers. As a matter of fact, the
limbs of all typically aquatic air-breathing animals have lost nearly
all external resemblance to the legs of walking and running animals,
and have become more or less fin-like in function—fin-like in shape
and function, but never fin-like in actual structure. No creature can
go back and begin over again, any more than a man can again become a
child with all its possibilities for improvement and development. If
an animal cannot modify the organs it already possesses so as to adapt
them to new and changed uses by the aid of evolutionary forces it must
fail in the struggle. It can never acquire new material, never get new
fingers and toes, new organs or parts of organs; all its possibilities
lie in the improved and new uses it can make of the material which it
received from its ancestors.
The beginning of aquatic adaptation of the limbs lies in the membranous
webs between the toes of frogs, salamanders, ducks, seal, otters, etc.,
where the feet are used largely or entirely for propulsion through
the water, in the absence of a propelling tail. And this membrane,
in the majority of cases, is the extent of aquatic adaptation in
air-breathing animals. In those animals, however, such as most of the
reptiles described in the following pages, where the tail has developed
as the propelling organ, the limbs lose to a greater or less extent
their propelling function and become merely organs of equilibration
and control. Of the two pairs of fins of fishes it is evident that
the anterior ones have the more important equilibrational function;
the hind ones have a much less important use as guiding organs; as
a matter of fact, in not a few fishes the hind or pelvic fins have
actually migrated forward to supplement the function of the pectoral
fins. It is for these reasons that those animals best adapted of all
for life in the water—the whales and sirenians—have lost the hind legs
completely. In other tail-propelled air-breathers the hind legs have
become progressively smaller and less powerful than the front ones. In
all short-tailed water animals, however, where the legs, and especially
the hind legs, have the important function of propulsion to subserve,
they still retain the large size and firm connections with the body,
examples of which will be seen in the seals, sea-otters, marine
turtles, and plesiosaurs.
Because the legs are no longer needed for the support or propulsion of
the body in long-tailed air-breathers, their connection with the body
becomes less and less firm, long before their entire disappearance. In
animals using the legs for crawling or walking the bones of an arm and
thigh are elongated, and the joints are always well formed, permitting
varied, extensive, and firm movements. Just the reverse is the tendency
in all those animals that propel themselves by the aid of the tail in
the water, since here what is needed is broad, short limbs, not long
and slender ones.
Most reptiles have five digits on each hand or foot; the bones of the
wrist and ankle are well formed, as in mammals, and the digits are
elongate, with a very definite arrangement of the bones composing them,
as already described, never exceeding five in any one finger or toe.
In the paddles of water reptiles, as the limbs are usually called,
the bones of the first segment, that is, the humerus and femur, are
always greatly shortened in those having a propelling tail, and even
in some with a short tail, such as the seals, and in a lesser degree
in the sea-otters. On the other hand, in those animals which use the
legs chiefly for direct propulsion these bones are elongated, as
exemplified by the plesiosaurs and marine turtles. In all save the
seals and their kind, and the otters, whose legs are used rather as
sculls than as oars, the bones of the next segment, the radius and
ulna of the front pair, the tibia and fibula of the hind pair, are
always shortened, and one can tell the stage of aquatic adaptation, as
exemplified, for instance, in the plesiosaurs and ichthyosaurs by the
degree of shortening of these bones. Indeed, the first suggestion in
any crawling animal of water habits is shown in the relative lengths of
the epipodial bones, as these bones are called. Furthermore, cursorial
or terrestrial habits are suggested by the relative size of the smaller
bone of the leg, that on the little-toe side, the fibula. In birds,
pterodactyls, and most running animals, it disappears in part or
wholly. In swimming animals it tends to grow larger than the tibia, as
will be conspicuously seen in the paddle of the mosasaurs.
The bones of the wrist change in two ways: by becoming cartilaginous,
as in whales and salamanders, or by becoming more firmly ossified
and more closely united, as in the plesiosaurs. The digits always
are elongated, often extraordinarily so, either by the elongation of
individual bones or phalanges, or by the development of new bones.
These new bones, when they occur, are new growths, not the reproduction
of the old elements of fishes, and there may be as many as twenty
such new elements or phalanges in a single digit. There is one marked
exception among reptiles to this hyperphalangy, as the increased
number of phalanges is called, and that is the turtles. As we have
seen, in the elongation of the neck among turtles there never has
been an actual increase in the number of vertebrae; so also in the
elongation of the digits the normal number of three in each digit
has never been exceeded, except among the river turtles, where there
are four in the fourth digit—possibly a relic of original conditions
rather than the beginning of hyperphalangy; but the individual bones
have become greatly elongated. In living reptiles, birds and mammals
of the land, the fifth toe is always shorter than the fourth. In the
seals, the sea-otter, and to a less degree in the muskrat, the fifth
toe has become elongated. And the elongation of this toe is the first
and most decisive indication of a webbed foot of strong propelling
power among the aquatic reptiles of the past, as exemplified especially
by the proganosaurs. Finally, in one order of extinct reptiles, the
ichthyosaurs, there has been an actual increase in the number of
digits, in some to as many as nine in each paddle.
In addition to all these modifications of the skeleton, the bones
themselves tend to become softer and more spongy in aquatic animals.
The bones of the whale, as is well known, are very spongy in texture,
and those of the seals and sea-lions contain an unusually large amount
of oily matter. So, too, the bones of the extinct water reptiles—of
many of them at least—were more spongy than those of their land
relatives; and this is due in part perhaps to their lessened use as
muscular supports, in part perhaps to the necessity of a lessened
specific gravity. As a rule sea-animals need to be of the same specific
gravity as the water in which they live, or a little less. The bones
of the living sirenians, the manatees and dugongs, so far from being
light and porous, are unusually dense and solid. The sirenians live
habitually at the bottom of shallow waters, feeding upon vegetable
growths; and doubtless their bottom-feeding habits account for the
solidity of the bones. A whale would float to the top, while a
dugong would sink to the bottom, on the relaxation of all muscular
movement. And we shall see that certain reptiles in the past had in all
probability like bottom-feeding habits, because of the solidity of the
bones of their skeletons.
Many birds and fishes have a peculiar ossification of the usually
tendinous outer covering of the eyeball, called the sclerotic
membrane. These ossifications form a flattened or somewhat projecting
conical bony ring about the pupil of the eye. The individual bones
are flat and more or less imbricated plates, with some motion between
them. Accommodation for vision in reptiles, birds, and fishes is not
the simple process that it is in mammals, where it is controlled
by simple ciliary muscles which compress the lens, causing it to
assume a more spherical or a more flattened form, thus changing the
focus. In reptiles accommodation is effected by the compression of
the eyeball by means of external muscles, elongating it and causing
its front part to expand or project. The imbricated sclerotic plates
permit this expansion and contraction of the eyeball. Under great
internal or external air pressure the cornea, the only unprotected
part, must necessarily change its contour unless some compensatory
force is brought to bear to counterbalance it; and this doubtless was
the function of the sclerotic plates so commonly present in aquatic
reptiles.
Among terrestrial reptiles there are not a few examples of the
ossification of such sclerotic plates, notably among the skink
lizards. Every known form of extinct reptiles of aquatic habit had
them, and even some of the subaquatic dinosaurs, like _Diplodocus_ and
_Trachodon_. One may say with assurance that it is impossible for any
reptile to become thoroughly adapted to aquatic life without acquiring
large and strong sclerotic plates.
Most land reptiles are or were covered by horny scales or bony plates;
the pterodactyls are the only order of terrestrial reptiles with
no such covering of which we have any evidence. Such coverings are
wholly unneeded for animals living in the water. Not only are they
unnecessary, but the increased resistance to the water would be more or
less detrimental to rapid swimming. It is for these reasons doubtless
that bony plates or horny scales disappeared for the most part from the
skin of all truly aquatic reptiles and mammals.
The foregoing are the chief acquired characteristics of aquatic
air-breathing animals and especially aquatic reptiles in adaptation
to their new mode of life. The resemblances, sometimes striking,
thus brought about in animals of very different origin and remote
relationships have often been mistaken for evidences of kinship,
that is, direct inheritance from common ancestors. Such acquired
resemblances in unrelated animals are known as parallel or convergent
evolution. It has often been difficult to distinguish between
convergent evolution and direct evolution, and difficulties still
perplex and trouble the student of natural history in every branch
of life. Not till all such problems are solved can we hope to attain
the true classification of animals and plants. The whales a century
ago were considered merely breathing fishes; the ichthyosaurs until a
quarter of a century ago were supposed to be the direct descendants of
fishes; lizards and crocodiles were grouped together in a single order;
and salamanders were called reptiles not very long ago.
Perhaps the reader will be able from the foregoing to understand
and appreciate better some of the difficulties that confront the
paleontologist in his attempts to solve the problems of past life; to
understand why he sometimes makes mistakes, for he has by no means
yet learned all the permutations of the skeleton in any class of
vertebrates, and is not sure that the laws he accepts are not subject
to modifications and exceptions. If he is truly scientific he hesitates
long in prophesying or conjecturing.