Popular Science Monthly/Volume 63/October 1903/Highways and Byways of Animal Life
|HIGHWAYS AND BYWAYS OF ANIMAL LIFE.|
By Professor HERBERT OSBORN,
OHIO STATE UNIVERSITY.
NO matter what particular theory he may hold as to the origin and distribution of life over this globe of ours, the thoughtful naturalist or the thoughtful student in any sphere of research, if he stops to question at all, must ponder with deepest interest the problems connected with the wide dispersal of animal life and its adaptations to most diverse conditions. From darkest depths of abyssal ocean to lofty mountain peak, life abounds and even the high regions of ethereal air are traversed by one or another of the organisms which represent the great aggregate of animal life.
It is true that these limits, viewed from a certain standpoint, are very narrow, for a moment's consideration will reveal the fact that life on this sphere now, as in all time past, is confined to a comparatively thin stratum at its surface. From the deepest habitable reaches of ocean to the highest point attainable by bird (a few miles, indeed, of vertical elevation) is a slight range in the radius of the earth, and the densely populated stratum of land and sea—the stratum actually capable of supporting life continuously is, in reality, limited to a very few feet—an exceedingly thin layer on a gigantic ball. One almost trembles at the thought of how narrow the habitable limits and how slight a change in conditions of atmospheric or other physical environment might extinguish the vital spark which has characterized mother earth through untold stretches of years. Consider a moment how little below the surface any animal can live, how slightly above it is existence possible.
But my purpose here is to touch upon some of the routes of development of the shifting forms of animal life which have drifted hither and thither over sea and land in the great struggle for perpetuity and expansion.
This effort we can clearly see in the movement taking place under our own eyes, and, more largely, within historic times, as evidenced by the host of animals introduced and spread in the new world from the old, the displacement or extinction of certain forms and perhaps most emphatically in man himself, the dominant animal of the present age, whose struggle has now become not so much a struggle with other species as a struggle for dominance of race over race, or nation over nation. Glancing backward, however, over that long and varied line of animal life shown to us in fossil forms—their history incontestably preserved for us direct from the hand of creation, we must readjust our vision, enlarge our horizon to grasp the significance of the origin, distribution, adaptation and survival of life. Life is here—life has been a feature of this old earth's history through countless ages. Whence came it, what have been the paths it has followed in its development and adaptation to the varied conditions of earth, sea, land and air? It would be presumption too gross to admit of toleration to assume to fully discuss so extensive a problem within the limits of a magazine article, but an effort will be made to point out in a rapid survey some of the factors that seem to have been effective in the peopling of the earth.
First, we should observe the conditions that have existed and that had to be met in the growth of organic beings—for we should not forget that life has had to adjust itself to conditions that existed prior to its appearance, since the conditions have not been modified to accommodate its needs.
Stretches of water and great reaches of land and the atmosphere furnish the basis upon which organisms must act, and either water or air, the medium that must serve them for many of their most vital functions. Glancing over the opportunities for survival of life in a delicate, simple condition, we can hardly fail to recognize the water as the most natural element for primal life-forms. Indeed, I think no naturalist will hesitate to consider water or, at least, an extremely moist location as the necessary condition for the beginning of life. Moreover, any serious consideration of the question must force the conviction that life-forms in other less favorable locations must have reached such location by gradual modification and adaptation. For instance, we can hardly conceive of the peopling of an arid desert-region with forms such as lizards, snakes, horned toads, scorpions, beetles, etc., except by the gradual encroachment upon desert area from adjacent territory by animals which were able to adapt themselves to desert conditions. Or, to put it in another form, the change to desert conditions in previously watered area must be accompanied by the driving out or extinction of all animals unable to adjust themselves to the new conditions. Assuming, then, a primary aquatic habitat for animal life, it becomes of interest to inquire both as to the most probable point of origin and as to the direction of adaptations.
While some naturalists argue for a pelagic origin of the simplest organisms, others hold to the idea of an origin near the shore, but in either case we have evident lines of travel from such a point to occupy the surrounding space. Thus from a near shore location life might work itself shoreward, adapting itself to the variable conditions of ebb and flow of water and exposing itself to such variations of conditions as might lead to fresh-water existence—to life on the land and thence to air, and, on the other hand, a dispersal on the surface to pelagic life. Again from shore-life or the original near-shore location, there might be a contingent working into deeper seas, and still further to occupation of abyssal depths. Transfers from surface to sea bottom, and the reverse, seem probable, in fact certain, for some groups and movements of terrestrial groups of animals into water must certainly have taken place. The peopling of all habitable corners of the earth then has been a process of continual pushing out from original centers, a constant, if unconscious, effort of animal life as a whole to occupy all available space, to crowd the energy of vital force to the end of every open channel, to follow every thoroughfare and explore every by-path that might lead to nook or corner in the universe that could give support in any fashion. But seldom has any form of life started alone on its travels, and hence the crowding for place, the 'pussy wants a corner' need, the eternal jostling to get and keep that corner and its opportunities, the 'struggle for existence' that has been the dominant principle of life from the dawn of its creation. Clearly those forms most successful in adaptation to new conditions must be those that win in the race and which soonest give rise to a higher and more complex form of existence. Whether life began in a single organism the parent form for all the mighty train that followed until now, or whether numerous organisms started independently, we can hardly doubt that all were equally simple, and similar courses of modification must have affected all. Moreover, in every case, we are warranted in assuming that for all higher types of animals there was a probably common ancestral form, and distribution over the earth must have been accomplished from an initial center by succeeding generations. Further, that for each particular subordinate group, family, genus or species that now has extended distribution, we must assume dispersal from the original home of the ancestral form.
First, then, we had only aquatic life, and this element may have been densely peopled before an effort was made to move ashore or to seek dry land. All geological evidence shows enormous development of aquatic life in early times, but obviously such forms were most likely to be preserved. Land life may have been forced by the drying up of stretches of water as well as voluntary migration. But what an important change that from aquatic to terrestrial life—from water-breathing to air-breathing! What possibilities of expansion, growth and occupation in the new, untrodden sphere, in the valleys and hills of earth and in the invigorating supply of air! Up this highway have come not a few of the great groups of animals. Some of the simplest protozoans even discovered the track, and worms of various kinds have crawled to wider or sometimes to narrower possibilities. Mollusks took advantage of the path, and many of them have reached even to life arboreal, while some, apparently disheartened or diverted by seductive opportunities, have gone back to water, still retaining, however, their air-breathing organs to prove the roundabout course of their travels.
The ancestral forms of insects doubtless followed the same broad way, but so remote are these ancestors that we may best consider the insects as primitively air-breathing. Some indeed are now aquatic, but still air-breathers, and I doubt not have taken to aquatic life as a fairly modern accomplishment. Vertebrates, however, give us the greatest advance, for from the gilled fish, and early amphibian, to bird or mammal is a long and striking course. If any branch of animals is to be thought of as having had its origin on land rather than in water, it must be the insects, that is, the immediate ancestral form to all the groups of insects. Following back their ancestral line still further we should doubtless reach an aquatic animal, but one not to be recognized as in any degree insect-like in character.
But life in the open air has not confined itself to particular places or conditions. The pressure to occupy each niche of available territory is as strong here as in the water. Here too we may trace certain well-worn paths—paths that have been common to more than one group of animals and traveled independently by each. Let us mention some—subterranean, aquatic, terrestrial, arboreal, aerial. How many forms in hosts of different groups have buried themselves more or less completely in mother earth and there found, or made for themselves, all the necessary conditions for successful life. Earthworms, crustaceans, insects too numerous to mention, mollusks and, among vertebrates, frogs, snakes, lizards, turtles, birds, moles, beavers, gophers, groundhogs, badgers, etc. This for the general trend, many of these, however, taking peculiar and tortuous by-paths to reach the end desired. From terrestrial back to aquatic life has been so frequent a course that we can hardly call the road exceptional. So many insects of different groups have become aquatic in either adult or larval life that it was long held that the insects in general were derived from these aquatic groups. None, I think it safe to say, has had such an immediate ancestry, while aquatic beetles, bugs, caterpillars and even dragonflies, caddiceflies, etc., have, I firmly believe, gradually assumed aquatic habits as descendants of forms that lived on land.
We can easily believe that frogs advanced from an aquatic to a terrestrial condition, because we can actually see the process in the individual history of each, but there is reason to believe that even among certain batrachians aquatic life becomes more habitual and succeeds a terrestrial stage. Even the frog himself becomes decidedly aquatic in his old age, and for reptiles, while we often think of them as aquatic, especially such forms as the alligator and crocodile and the ancient marine saurians, in reality we should think of them as primarily land-inhabiting animals, some of which have gradually taken to aquatic life aud in the course of time become more and more confined to a watery sphere. Aquatic turtles show this most decidedly, an extreme being found in the soft-shelled turtle, which is not only a constant resident in water, but has become possessed of special organs for respiration in water, so that air-breathing is scarcely necessary. Birds that live in the water have taken the same convenient highway, and many of them have traveled it from the time of their toothed ancestors down to the present. Penguins have gone so far along this road that their wings can no longer serve for flight, while loons and grebes and auks are on the way. Gulls, albatrosses and petrels may go far out o'er billowy wave hundreds of miles from land. Ducks and geese and swans have struck the trail, and snipe, stork and heron, crane and flamingo have rolled up their trousers and are wading in. Even the fish-hawk, the osprey and the eagle find it worth while to look beneath the wave.
The aquatic habits of the beaver furnish a most remarkable example of this, and if we could trace his acquisition of this habit from the time when he must have been an ordinary terrestrial rodent with neither a paddle-tail nor a web foot, we should certainly find an interesting career. The musk-rat has not gone so far and can not reach the same goal, as he has flattened his tail in the wrong direction.
The whale—that giant of the seas—largest of mammals and indeed of all animals, has out-traveled all his relatives in reaching out into the great ocean, but we can not possibly conceive the whale to have come from any other source or to have other ancestor than a land-inhabiting mammal. We get glimpses of the mile posts he has passed in the structures shown by the manatee, the walrus, the sea lions and others. Not that these constitute in any sense his ancestral line, for that was far back in time and so far largely a lost history. But along such stages we must believe his ancestors to have passed. The manatee in its way is as strictly aquatic as the whale but hugs the shore or river mouths.
The hippopotamus is well on the way, and I would digress here to call attention to the remarkable similarity in adaptation of the sense organs of this animal to those of the alligator and crocodile. Note that the eyes, ears and nostrils are almost exactly in the same plane and so situated that they may all be above the surface of the water, while practically no part of the head or body may be visible, an admirable adjustment to avoid detection from foes or for protection against flies, mosquitoes, etc. Seals, walruses, sea lions, sea otters and, in less degree, the polar bear, the common otter and mink, all show aquatic habit fixed or growing, and even the small boy at his favorite swimming hole presents a tendency to fall into the well-worn path that leads down to amphibious quarters.
On the broad highway of terrestrial life the effort and adaptation has been to develop speed, strength, protective coverings, colors, etc., and here has been the widest and strongest struggle for supremacy that the world has witnessed. The contest for domination between the giants of old ocean—of sharks and devil-fishes, and saurians and whales, pales into insignificance compared with the battle waged between the huge terrestrial reptiles, birds, elephants, mastodons, horses, lions and man which has raged on terra firma. From this have come perfection of speed to one—power and energy to another, but, above all, intelligence and the dominance of brain. Out of this highway, too, run numerous and devious by-paths, the following of which furnishes us with a host of strange and fanciful creations—adaptations to extremes of heat and cold, moisture and dryness, latitude and altitude, plain and forest.
But, not content with solid earth, animal life finds its way upward into vegetation of various kinds and particularly in tree growth, reaches adaptations which become so fixed that life elsewhere would be an impossibility. This is especially marked in the great tangle of tropical forests. In these the animal life of the tree tops assumes a most pronounced character, which can scarcely be appreciated by one familiar only with lesser forests of temperate regions. Pictures fail to show the real density, for pictures must show the breaks and gaps to show at all. But perhaps the most dominant thought in the presence of such forest life is the superabundance of life, life everywhere, under every scrap of loose bark, every tuft of grass, on branch, and twig and leaf. In my despair of giving an adequate idea of this tropical tangle, I turn to an article in Harper's Magazine by Lafcadio Hearn on a midsummer trip to the West Indies, in which he quotes from DeRafz:
Here, then, is unlimited food, abundant moisture, warmth and light, and no wonder animal life has grown apace, multiplied and modified its form—and adapted itself to forest conditions.
Along this forest route come certain strange, peculiar molluscs, which far from the native haunts of their allies have succeeded in establishing themselves in apparently successful occupation against more active forms. Insects, unnumbered, occupying every part from solid wood of the tree heart to outermost bark or leaf, a few fishes even leave their water haunts for temporary quarters up a tree, and frogs are here to stay, while snakes, lizards, chameleons, are at home awaiting callers. To birds the trees become a most natural harbor and home to rest, to nest, to eat and die. Ungainly sloths, helpless elsewhere, are at home in the tree tops; and squirrels, opossums, coons, bears, cats, monkeys, apes, and sometimes man, find up among the branches of a tree the situation that meets pleasure or necessity.
Launching out into the air, the most difficult path to take, most hazardous and most perilous, but attempted by many different kinds of animals, is another highway. Some faint suggestion of an effort to utilize the air in locomotion is shown by the wind-blown Portuguese man-of-war, and more fully by the aeronautic spider who launches his balloon of silk for aerial flight, but no real success as traveler of the air is found until we reach the group of insects, where wings—true aerial organs of locomotion—become a conspicuous characteristic of the group. How well they have succeeded is testified by the fact that such locomotion has been in vogue with them since early paleozoic time, and, considering their size, the speed and endurance exhibited in air is equaled by no other kind of animal.
The flying-fish, driven from its native element by pursuing foe to temporary elevation in the air, is a strange abortive attempt to reach this goal, but it has taken too direct a course ever to succeed. It should have first become an air-breathing land animal before attempting the soaring act. The frogs and lizards with expanded feet for floating on the air are poor apologies for flying animals, but given time might reach that goal were not the field so fully occupied by more dominant forms. The ancient flying reptiles, known only by their fossils, reached a high degree of efficiency, if we may judge by the expanse of wing they show. In their time they were doubtless the dominant types of the air, but they left no legacy to later forms, for the wings of modern groups are formed on different plans and must have been developed de novo or regardless of the reptilian type.
So now we reach the birds—the truest, most perfect of aerial forms, the animals which, with natural organs, have come nearest to annihilating time and space—whose skill in traversing the trackless regions of aerial waste has been the constant envy of man, from early time down to Darius Green, Maxim and Langley and a host of modern inventors. 'O had I wings' in various refrains has been the lament of man till we may expect ere long that the want will be practically supplied. Some birds indeed do not seem to appreciate this gift and have sacrificed these organs to adapt themselves to water or to a speedy gait on land, but flight is the dominant mode of locomotion for the group. Some of the mammals, aside from man, have attempted to follow the lead of the birds and the bats have succeeded so well that they are practically cut off from all other modes of locomotion, while flying phalangers, flying squirrels, and others attest the effort in various groups to adopt this rapid though hazardous kind of locomotion.
Along such general lines, such open roads as these, animal life has progressed, and without such limitations as to prevent further progress or adaptations to new conditions. But I wish now particularly to call attention to certain adaptations that result in a definite limitation of the animal, a fitness to special conditions and a fitness so complete that existence under other conditions is impossible, or to put it still more broadly, a return adaptation is probably impossible. Such lines of adaptation may be looked upon as by-paths or blind alleys sought out by certain forms as presenting easier conditions for existence or into which feeble species may be crowded by the force of stronger ones. Places where certain shifts provide adequate chance for survival, albeit on a lowly plane.
Such by-paths are innumerable—almost as the nooks and crannies into which organisms may crowd—and to catalogue them would be to survey a large field of zoology. They are especially interesting and instructive as showing in most emphatic manner the factors that have been operative in modifying structure and attesting the general fact of evolution. The animals that are sedentary, domestic, subterranean, parasitic; the inhabitants of caves, deserts, manufactured products, oil, vinegar, hot springs, snow and ice, on islands, under bark, in deep sea, are illustrations of such erratic departures from normal habits. While we can review but few, these few may serve to illustrate the principles involved and some may be grouped under general heads.
Perhaps the least departure from normal, free-living conditions is presented by those animals which assume a sedentary habit. This may range all the way from a temporary anchorage in mud or on a rock to permanent attachment with most fundamental changes in form and structure of the organism. It is exhibited in some degree by almost every group of animals, and were it not that its tendency is toward limitation and restriction of powers we might look upon it as one of the main avenues of development. For minute forms the attached bell animalcules, stentors, etc., are good examples, and in sponges we find this habit of fixture a constant feature and associated with marked inferiority in symmetry and activity. Hydroids, and especially corals, show it strongly developed, though the former often present free living stages alternately with the fixed. Some worms and mollusks have assumed the rôle and it was the rule with echinoderms in early time, though modern forms have largely broken away from it, not, however, until their whole symmetry had been impressed by the results of their position. The barnacles among Crustacea have gone farthest in this direction, and their symmetry and structure have been so strongly influenced that it is not strange that earlier naturalists failed to suspect their true relationship. Mollusks, like the oyster, have also been much modified by fixation, and among insects the remarkable scale insects present extreme results in this direction. It is not mere chance that the oyster and the oyster shell bark louse have similar shape. They have both been modified, quite independently and in different locations, by the same controlling factors working on a sedentary organism. Many other insects in one stage or another illustrate this phase, but space forbids their mention.
Tunicates have traveled this road and thereby lost the rich inheritance that was theirs had they cultivated their backbone instead of allowing it to pass into 'innocuous desuetude.' Even among vertebrates we see some tendency to adopt the tied-up plan, for among the fishes the lampreys attach themselves to other fishes, the remours to the belly of the shark, the sea horse temporarily fastens to branches of coral by wrapping around them his flexible tail, the flounder rests almost fixedly at certain points, but throughout the group there is practically no permanent fixity with the degeneration it entails. It will be seen by those familiar with the forms cited that the mere fact of an animal having become habitually attached in a certain place and having lost its power of free movement has greatly affected its structure and future possibilities. It has bettered its chances for survival, but it has sacrificed all hope of progressive development. I believe it is quite safe to say that no high type of animal life can be referred in its origin to a sedentary ancestry.
Another frequent by-path is that of parasitism, and, indeed, so common is this mode of life, so prevalent in some degree or other among animals of almost every branch, that it would appear to be one of the easiest roads to travel. But this road leads inevitably to restriction of freedom and limitation of sphere—often to degeneration of some portion of the organism. Its ultimate end is extreme limitation and probable extinction. Protozoans, coelenterates, worms in great numbers, crustaceans, insects, mollusks and even some remarkable vertebrates have followed this road, and in every case where the habit has gone to any great extent it would seem impossible for them to retrace the route. Wherever the parasite has become limited to a single host or to alternate hosts, destruction of the host form means death to the parasite, and extinction of the host would mean extinction of the parasite. Loss of wings in formerly winged forms, loss of eyes and other organs of sense, loss of nervous system, loss of motion, loss of digestive organ even, in extreme cases, are the penalty they pay. 'Sans eyes, sans ears, sans nose, sans mouth, sans everything,' but actual necessities of existence and reproduction.
At first thought it may not seem so strange that wastes of desert land should present no small degree of living activity, but if we notice the conditions more carefully, we shall see that we must provide not only for the survival of the individual, but for succeeding generations of individuals, and, when we take into account the special adaptations that become necessary to permit of the development of eggs and the early stages of the different forms, it will be seen that the problem becomes much more difficult. As already hinted we can scarcely conceive of life originating under such unfavorable conditions, but must think of it as having gradually extended its range from adjacent, more habitable regions. We can see, too, that the special adaptation in this direction distinctly unfits the animal for a return to a more humid condition and, if its desert conditions were withdrawn, the probability is that it would succumb to the pressure of more active forms of life. In fact we may gather that desert forms have reached such situations as an effort to escape from the more rigid contest in regions more densely habited.
Many tribes of men have thus pushed out into arid territory, adjusting themselves as well as possible to the conditions, but always with a struggle against these special conditions that can be scarcely less severe than the struggle against stronger individuals or races that have attempted their subjugation or extermination.
Aquatic forms we may expect to be absent and still some such aquatic forms as may develop very rapidly in temporary pools of water and are otherwise adapted to long periods of desiccation have solved this problem. Birds and insects may by their ready locomotion easily take to temporary quarters under desert conditions, and some of them become fixed inhabitants, but usually with some degree of subterranean habit to protect themselves from the severity of the sun's rays, thus burrowing owls and many subterranean or nocturnal insects are characteristic of desert life. Burrowing squirrels, prairie dogs, snakes, lizards, etc., all follow the same line.
Another distinct line of adaptation is shown in the animal life inhabiting caves, a fauna so characteristic and so strikingly similar in different parts of the earth, though common origin is out of the question. Such life might be looked upon as an extreme of subterranean forms living near the surface of the ground, but there are different conditions to be met, and the results are in many cases widely different. Loss of eyes would seem to be the most frequent and, indeed, almost the first effect of such adaptation, and this modification alone would practically preclude such animals from ever attaining a successful hold upon ordinary conditions. Return to conditions of light would expose them mercilessly to the attacks of animals with ordinary organs of vision. Blind fishes, blind insects, blind crustaceans, all attest to the controlling influence of environment, and whether the animals found in these locations have come there by choice to secure certain favorable attractive conditions, to escape more dominant forms outside, or simply by chance, from being carried in streams of water into these subterranean cavities, the result has been the same for all, and their return to the struggle carried on by their ancestors out of the question. They are distinct species modified and adapted to this particular environment where survival is possible, but opportunity for progressive evolution too limited to permit of advance.
Along the path of protective devices, mimicry, adaptive coloration, form and habit have traveled a host of different forms—flies that look like bumblebees and thereby gain entrance to their nests to provide extra food for their larvæ; butterflies that look like leaves of trees; leaf insects (phasmids) so like the leaves they live upon as to be invisible to foes; bugs that look like ants; scale insects that look like excrescences on the bark of the trees they infest; edible species that have taken form and color of inedible species; toads that look like lumps of earth; frogs that look like green scum or leaf at water side; snakes and lizards in great numbers that resemble the soil on which they live; birds that so closely imitate the colors of earth, foliage or irregularities of bark as to escape observation—in fact, an unending train in varying degrees, furnishing a most fascinating field for study.
Among aquatic animals we have fishes that hug close to the bottom and acquire color and pattern which admirably protect them, and the flounder has even gone to such an extreme in this direction as to have one of its eyes transferred from its normal position to the opposite side of the head. Skates and rays have the same flattening, but retain their normal position. Other fishes resemble rocks and so perfectly as to be practically invisible. Others, like sea horses, resemble parts of sea weed, streaming in the currents of water or branches of coral to which they cling. Mollusks and crustaceans adopt this plan to make themselves inconspicuous, and the different devices shown would fill a volume. Perhaps no stranger combination is presented than in the little crab which makes its home in a mollusk shell, but not content with this protection conspires with a sea anemone to take root upon its house top and add the variety of its structure to the deception. A queer sight these anemones, nodding here and there as borne by a hidden crab.
Most animals aside from man have been content to let electricity alone, and we are inclined to think the use of this magical force in nature of very recent origin. However, some of the aquatic animals, at least, and these forms that must have had their origin in the long, long ago of geological time, have brought to their use this elusive force and present in the structure of their organs for the generation of electricity some quite remarkable parallels to the electric apparatus of human invention. In such lines we have the torpedo, a broad, flat. peculiarly spotted form common to the Mediterranean and adjacent seas capable of liberating a charge that will shock man and must be destructive to hosts of smaller animals, upon which it is doubtless used as a means of offense and defense; in the electric siluroid of Africa, which in different structure possesses the same power in greater force, and still more pronouncedly in the electric eel of South America, the shock from which is sufficient to paralyze a horse. What strange combination of circumstances has conspired to develop such a power in these animals? Evidently some condition common to their several environments, as it must have originated independently in each of the examples cited. The difference in position and structure, though all are modified from muscular tissue, is such that no common origin can be assigned to the structure in the different forms.
In the adaptations to deep sea life we have one of the greatest extremes and, since we have here one of the most remarkable series of animals and moreover one which until recent years has been unknown to science, it will not be amiss to give it more than passing notice. In the greater depths of the ocean we have conditions rivaling those of caves in the absolute exclusion of light, but very different in the medium and in the enormous pressures to which the animals are here subjected—pressures so great that when deep sea animals are brought to the surface there is an expansion of all the soft parts, an extrusion of the eyes, stomach, etc., producing most monstrous looking forms. But the passage from moderate depths to deeper and deeper points and, finally, to the abysses of mid-ocean are gradual, and we can conceive a gradual pushing off to deeper and deeper points till enormous depths are reached. Even yet, however, it is believed that the deepest reaches are uninhabited and uninhabitable, the lowest points from which life forms have been secured being far above the extreme depths that are known.
The wonderful discoveries of the 'Challenger,' 'Albatross,' 'Blake' and other deep-sea explorations, adding a new chapter in science and whole new groups of animals hitherto unsuspected, are yet so fresh in the minds of those interested in such matters that they may well serve my purpose in illustrating those special adaptations reached by animals that have pushed into apparently inhospitable regions. The passage in this case has, however, been slow. We need assume no sudden change of physical conditions tending to produce modifications of structure, but practically unaltered physical conditions and a simply crowded condition of life forms in regions already occupied, as the main factor in pushing into this hinterland of habitable zones. Here as in caves we should expect the loss of light to result in the loss of eyes, but this is not always the case, for in some of these grotesque denizens of the deep instead of the loss of eyes we find the development of marvelous phosphorescent organs which serve as a lamp to light the pathway of these strange creatures in their strange surroundings. Such adaptations can hardly be conceived as possible, save with the very gradual shifting from lighter to darker regions through the lapse of thousands and thousands of years.
In modern times, since man's domination began, another factor has appeared, and its influence in modifying animal forms and structure has been one of the most potent and striking for such as have fallen within its scope. While man's effort has been largely to exterminate the lower forms of life, especially those inimical to his interest, he has utilized others for his service, and in the process of domestication we may see compressed into brief time such changes as under natural conditions would have occupied untold years, if, indeed, they would ever have been possible, since many of these changes unfit the forms for survival under natural conditions. So potent this factor that the immortal Darwin used its results as furnishing some of the most conclusive testimony for the theory of natural selection, believing that what could be accomplished in brief time by artificial, or human, selection could be accomplished in greater time by natural selection.
To see the power of this factor we may compare the various breeds of cattle and refer all to the primitive form from which we have absolute historic evidence that they were derived. The breeds of horses are equally striking, as the immense draft horse, the Shetland pony, the thoroughbred racer and the Arabian with hosts of special strains will attest. So, too, with dogs, cats and fowls. The pigeon which furnished Darwin with so much evidence since the native rock pigeon, the extreme breeds of fantails, carriers, etc., can be seen side by side and their relationship unquestionably fixed. Now I wish particularly to call attention to the fact that many of these domesticated forms, as a result of their domestication, have been unfitted for life in other spheres, and if dropped from the fostering care of man would almost certainly suffer rapid extermination, those surviving being the ones that had been least affected by the process of domestication. A modern hog would stand a poor show, but the southern razor-back doubtless would survive, for a considerable period at least, without man's assistance. Here then is a by-path opened in very recent time and into which certain animals have been driven by man, seldom of their own choice, if ever, the confines of which have profoundly modified many and behind them the gates have been closed never to be opened again.
Pushing away from the congenial temperatures of equatorial and temperate regions, life ventures into the inhospitable frozen zones of the polar regions, and by adaptation to such clime invades the most forbidding sphere. Earth's 'warm embrace' is here a 'cold reception,' but hosts of birds, mammals, fishes and insects have become established and would doubtless suffer extinction in other apparently more congenial regions. Such is evidenced by the fact that certain formerly arctic forms have secured survival or at least a postponement of extinction in low latitudes by ascending mountain peaks or ranges, the assumption being that such forms were pushed southward during glacial periods and that some instead of retreating with the glacier took advantage of high latitudes to secure similar conditions. But while adaptations to the rigorous conditions of high latitudes must mean great hardihood, it also means small chance for progress in other lines and we do not look for progressive development in such situations. The Esquimaux and Laps will hardly produce Gladstones, Blaines, Leo XIII.'s, Bismarcks or Grants. But while extreme cold is unfavorable to life, extreme heat is equally so, and the limits in this direction arc perhaps even more strictly marked.
Nevertheless, we find aquatic animals that have adapted themselves to life in hot springs, numerous forms living in water of 100° F., while some forms occurring in hot springs of the famous Yellowstone region exist in temperatures which, except to those especially adapted, would be destructive. On the barren slopes and crests of mountains the conditions for supporting life are also very severe, especially where altitudes are such as to leave all forest growth below or to reach into the regions of perpetual snow and ice. Still such inhospitable quarters are sought out by many animals and various small mammals, many birds and insects may be found keeping up the struggle against the boundaries to life's outskirts.
Another most interesting phase of adaptation is to be noted in the community life exhibited by ants, bees, termites and some other groups. The community habit has resulted in the development of various castes, some of which have assumed the full duty of reproduction, others fitted only for the labors or defense of the colony. In some cases slavery follows this division of labor, the members of other colonies or other species being kept in captivity and utilized in carrying on the duties of the colony except of course that of reproduction. In some species we are assured this has gone so far and the slave-making species has become so dependent on the slaves that without them they will die of starvation. Community life then is an extreme specialization offering many advantages, but at the same time entailing certain limitations, cutting off the individual from any possibility of independent existence and the colony from survival, except under the conditions that have been established with the community life. Return to primitive isolated life is manifestly impossible.
These constitute some of the most striking by-paths into which life forms have been diverted either from choice or necessity, but barely a hint of the multitudinous minor lines of adaptation, the following of which results in limitation.
Such forms of insects, crustaceans, worms, etc., as have taken to living under bark and wood and become so flattened as conveniently to slip between the wood and bark in crevices scarcely permitting the passage of a sheet of paper—mollusks which bore into piles or some into solid rock—barnacles that fasten to ships and thus reach all climes—insects that thrive in such unnatural food material as tobacco, insect powder; or larvae that live in brine vats, wine vats, or, perhaps most extreme, in crude petroleum. Vinegar eels in vinegar and hosts of forms that subsist upon hams, bacon, flour, meal and other prepared foods, boots, shoes and other leather goods, and furs show the readiness with which new habits are acquired. But these habits are not so easily broken, and it is doubtful if the cheese maggot, having taken up its particular dietary furnished by man, could return to the food of its ancestors before cheeses were made.
I have spoken in places of the choice of animals for a particular sphere as if this were a conscious element, and I do not care to dispute those who maintain some such element as operative in the mutations of animal life. But, conscious or unconscious, it appears to me that the animal, even in a lowly sphere, has the power to choose in some degree or other the direction of its activity, to put itself in certain environments, and while not selecting certain changes of structure by the mere fact of selecting such environment, submits itself to inevitable changes which that environment must perforce produce. Thus, an animal may not elect to become flattened in body, but selecting narrow quarters in which flattening is necessary or advantageous this change is sure to follow.
The ancestors of snakes may not have determined upon eliminating legs from their anatomy, but by choice of habitat where legs were in the way these organs were gradually reduced and lost. But by no process of electing locations where legs are helpful can we expect the animal to restore the organ thus sacrificed. The fly that mimics a bee may never in its ancestral line have started out to make itself resemble a bee, but it may have chosen such relation to bee life that a similarity became distinctly advantageous or even necessary, and then natural selection could clearly come into operation to produce the mimicry. The ancestral whale had no glimmer of thought of launching into marine life, to trade his feet for paddles and flukes, when he first by virtue of some advantage found entrance to water desirable, but once the way was entered, selection and environment conspired to carry his descendants further and further along a path which knows no backward steps.
What fearful consequences attend these unconscious selections! Shall it be persistence or progress, mere survival or advancement—the starting on a downward path to extinction or an upward path to domination, a wanton sacrifice of rich legacies of structure or improvement of such as may be possessed?
While adaptations and progressive modifications have been the rule, there are a few striking exceptions, and we can not assert that preservation of a particular plane of development is impossible. Picture the little lamp shell, Lingula, living on unaltered from age to age, its fossils being found away back in the earliest Paleozoic time and in subsequent ages up to the present day—so much for staying at home and attending strictly to its own business. True no change—no progress—but as an example of the staying qualities nothing can be more striking.
Concluding, then, we may look in wide view at life as originating in most favorable conditions of moisture and temperature; as pushing out to occupy all favorable environments, and then, from the congestion of life in such places, as pushing out to extremes in various directions and, ultimately, by a process inherent in life itself, constantly but unconsciously striving to occupy every available niche having the remotest possible opportunities for the support of organic beings. The animal choosing its environment and the environment reacting to modify the structure of the animal.
But this pushing has been along different lines and some of these involve no such radical change of form or habit as to restrict the animal to a special environment. In such broad highways progressive evolution is still possible, and we may expect future modification, advancement, adaptation, the height to which advancement is possible depending on how fully the animal may preserve its general varied structure while reaching such perfection of organs as to enable it to dominate the forms with which it must compete for mastery.
Where the road narrows and the animal in traversing it is obliged to sacrifice some portion of its structure and to adopt some restriction of habit the result is a limitation which must ultimately mean a bar to all progressive evolution in the acceptance of a particular limited sphere within which it may survive, but to leave which means extinction. Adaptation to sedentary life, parasitism, desert, cave, deep sea, polar frigidity or extreme heat is to shut the door of progress and give over to mere survival.
We may be tempted to moralize a little, for a moment's thought assures us that man himself is, like other animals, subject to these inevitable laws, and that retrogression, degeneration, decay and extermination are as possible to him or to certain of his races as to any other form of life, but this subject widens into the great new field of sociology—one of the latest, richest and most important of the branches of biology.