Popular Science Monthly/Volume 19/July 1881/Degeneration II
IN groups of the animal series, both nearly allied to the crustacean class and far removed from it in structure, equally interesting and often curious examples of degradation may be found. The class of insects and the nearly related group, including the mites, spiders, and scorpions as its representatives, number in their ranks instances of degraded and degenerate forms. Among the insects which are parasitic in habits a notable absence of wings is discernible, and this latter want is seen even in those cases in which one sex alone of a particular insect species assumes the habit in question. An excellent illustration of such a fact, and also of the extreme modification of form which may accompany the degeneracy of highly organized animals, is found in the history of the insects collectively known as Strepsiptera, and of which the genus Stylops is the best-known example. The male stylops (Fig. 13, a) is an active insect, possessing a
single pair of wings. These wings are the hinder pair; the front pair being represented by a pair of twisted organs (w), which illustrate wing-degeneration, possibly through disuse. Both males and females as they leave the egg are small, active, six-legged beings (d, e), which crawl about on the bodies of bees. Carried into the hive, the young stylops behave like the proverbial viper, injuring the community which gives them shelter by boring their way into the bodies of larval or infant bees. Here the young stylops, casting their skin, become in the larval interior sluggish, footless grubs. Each possesses a mouth, small jaws, and a digestive system of simple construction. Meanwhile, bee-development progresses; and, as the larval bee passes through its chrysalis state with its stylops-lodger contained in its interior, the latter thrusts the front extremity of its body from between two of the hinder body-segments of the bee. Then the male stylops, undergoing development in this position, becomes the winged insect (a) and passes into the world. The female stylops (c), on the other hand, remain in their places on the bees. They undergo but a slight change of form, persisting as mere sac-like bodies (c), without legs or digestive system (b), and develop in their interior the eggs from which succeeding generations of stylops will be produced. Such a case of absolute degeneracy is all the more remarkable in view of the facts that it is limited to one sex alone, and that the free-winged males of stylops are as highly organized as most of their neighbor insects.
The class of the spiders (Arachnida) offers collective examples of degeneration and retrogression, which show how large numbers of animals may acquire lower characters, contrasting with the higher phases to which other members of their class have attained. The mites and ticks have unquestionably originated from the same root-stock as the spiders and scorpions. The development of the two groups proves this much. But, while the latter animals have advanced to a high complexity of organization, the mites and ticks have degenerated into parasitic forms—or at least exemplify beings which, first attaining a respectable rank in their own series, have certainly not advanced upon that rank. Many of the mites, however, exhibit well-marked degeneration. Only on the hypothesis of sweeping retrogression can we account for the singular and anomalous condition in which a certain harmless mite, named Demodex folliculorum (Fig. 14), spends its existence. This mite inhabits the sacs or follicles of the human skin at the sides of the nose. It is a minute, worm-like animal, possessing eight degenerate rudiments of legs, and a thoroughly rudimentary structure in other respects. Here parasitism has denuded the
|Fig. 14.—Demodex (magnified).||Fig. 15.—Linguatulina.|
animal of wellnigh every attribute of its Arachnidan character, and has left it in a condition analogous in many respects to sacculina itself. Of the equally curious Linguatulina (Fig. 15), inhabiting the "frontal sinuses" or forehead spaces of dogs, wolves, horses, and sheep, the same remark holds good. The body here is thoroughly worm-like in shape (b, c), and a digestive and nervous system are to be enumerated among the possessions of the organism. But not even the rudiments of legs are to be perceived, although the mouth bears certain apologies for the appendages proper to that region in the mite and spider class. Yet the young linguatulina (a) exactly resembles the early form of the mites. It possesses two pairs of jointed limbs, and certain style-like organs pertaining to the mouth. There is thus the clearest evidence that linguatulina is a degraded animal. It is the degenerate descendant of a free-living and apparently four-legged—or it may be eight legged—ancestor; and its further history seems to afford a clew to the causes of its retrogression. For the four-legged larvæ of linguatulina escape, while still within the egg, from the nose of the dog or sheep host which has harbored their parents. Received along with food into the body of the hare or rabbit, the larval being liberates itself. From the rabbit's digestive system it bores its way through the tissues to the liver, thus reminding one strongly of the similar migrations of the embryo tapeworm. In the liver further changes ensue. Frequent moltings become the order of the day, and at length they assume a worm-like aspect and remain thus, still imperfect, until, by transference to the body of dog, wolf, or sheep, and by passage to the frontal sinuses, they acquire perfection of their life-functions. If the history of these beings teaches us anything concerning their past, it points to a free and active state as their original condition, and to the probable acquirement, first, of a lodgment in the digestive system of one animal as a relatively simple parasite; and, secondly, of a further modification of habit, transferring at once its perfection and completed degradation to the forehead cavities of a second host.
But the conditions which make for the degeneracy of an animal are, as we have seen in the case of the barnacles, not always associated with a parasitic habit. Mere fixation, as we have observed, secures the disappearance of useless organs, such as organs of motion and sense-organs, which, being possessed by the young form, clearly indicate that the ancestry of the animals in question has at any rate been capable of leading to better things than the descendants represent in their existent persons. The sea-squirts, or ascidians, besides serving as a text for the derivation of vertebrates, and for abnormal ways in the animal chemistry which imitates the plant's work, have been selected as fruitful objects of discussion by those biologists who find in the idea of degeneration an explanation of knotty points in natural history. For the same voice that proclaims the fact that a sea-squirt—which is a mere rooted bag with a double neck (Fig. 16)—begins life as a free-swimming, tadpole-like larva (Fig. IT, 5), tells us in the same breath that there must have been retrogression and degeneration from an active condition to produce the sac-like adult state. The assertion that the youthful sea-squirt, moreover, possesses first a rod-like body—called the notochord (Fig. 17, n)—only found besides in the young of vertebrate animals, is also to be taken as implying the superiority of ascidian infancy to sea-squirt maturity. And, when it is added that the elderly squirt wants the sense-organs and nervous cord which the larva possesses, it may well be argued that sheer degeneracy of habit and structure can alone account for the sweeping transformations which mark the phases of ascidian life-history. Thus it is matter of sober, natural-history fact that a sea-squirt larva, of all invertebrate animals, is the only being that possesses organs and parts proper to the young vertebrate or to the adult form of one lower vertebrate in
|Fig. 16.—Sea-Squirt.||Fig. 17.—Development of Sea-Squirt.|
particular. This adult is the little fish known as the lancelet, which, in the relative simplicity of its organization, makes a nearer approach to the poor or sea-squirt relations of the vertebrates than any other fish.
The fact of vertebrate and sea-squirt relationship is worth dwelling upon, because the topic unquestionably presents one with a common point of view, whence a survey of the higher development, evolution, and progress of the vertebrates, and a view of the degeneracy and retrogression of the sea-squirts, may best be obtained. Reveling in the freedom of its early life, the larval sea-squirt—presenting, as already noted, a striking resemblance to the tadpole of the frog, in its backbone, its nerve-system, and its breathing-sac, or modified throat—ultimately settles down. Like the youthful barnacle somewhat, the young sea-squirt attaches itself to a stone or shell by the suckers with which nature has provided its head. Then succeeds the disappearance of the tail, with its backbone and its nerve-cord, and the body itself soon assumes the sac-like shape that betokens the mature ascidian character. The outer skin becomes tough and leathery, and develops the cellulose which, by biological right, we should expect to find in plants alone. Then succeeds the fuller formation of the gill-sac or breathing-chamber, and of its neighbor compartment, which receives the effete water of respiration to be ejected by the second mouth of the sac like body. The eye of the larva likewise disappears, and all that remains to the adult ascidian is a nerve-mass, called by courtesy the "brain," and which serves to regulate the few acts that mark the placid and rooted existence of the race. Attention has been recently directed in a special manner to the resemblance which exists between the eye of the larval sea-squirt and that of vertebrates—a statement to be taken along with that which conversely declares the unlikeness of the ascidian eye to that of all other invertebrate animals. It is matter of fact that the chief parts of the eye of a vertebrate animal grow inward as developments from the skin, and unite with an outgrowth from the brain. This outgrowth forms the retina, a nervous network of the eye, whereon the images of things seen are duly received for transmission to brain and sensorium. Now, in invertebrate animals the retina is formed from the skin-layer. This latter method of growth, it has been remarked, is a perfectly natural one. It was to be expected that, as the retina is to be affected in the discharge of its duty by light rays, it should form on the surface of the body where the light-rays fall. In the vertebrate, and in the sea-quirt larva, the retina, on the contrary, forms away below the skin-surface, and grows outward from the brain. Why is this so? Professor Ray Lankester maintains that because the ascidian larva is perfectly transparent, the light-rays pass through to its brain-eye, and thus give rise to sensations of sight. Hence, if the original and primitive vertebrate animal or root-stock were like the larval sea-squirt, as we suppose it to have been, its body would be transparent, and its eye or eyes, situated on its brain, would receive light-rays through its clear body. But, as the evolution of the vertebrate race proceeded, the tissues became firmer and denser. By "natural selection"—or, in other words, by the exercise of accommodating power to function the eyed region of the brain would tend to grow more and more toward the body's surface, to receive the rays of light. As development, therefore, proceeded, the mode of growth of the vertebrate eye would be adapted to the exigencies of its new surroundings. Thus, to-day, the vertebrate eye grows from without inward, because light-rays strike naturally on the outer surface of the body. But it likewise grows from within outward as well, because of the ancestral and hereditary tendencies which cause it to repeat in the individual growth the passage to the surface it had to make in the evolution of the race. If one might add a suggestion to such an explanation, it would consist in an endeavor to account for that affinity between brain and outer surface of body which we see to exist. Why the brain should grow outward, as it does in eye, ear, and nose likewise, to connect with the body's surface, and so to form organs of sense, is plain enough. We must bear in mind that the brain itself is formed from the outer layer or epiblast of the larva, and from the same layer which develops into the skin. Brain and skin, to begin with, arise from the same layer. Hence, before even the matter of eyes falls to be considered, the affinity of the skin-layer and the nervous system is a fact worth noting. It is this truest of relationships which may reasonably enough explain, not merely why the sense-organs arise from the skin-surface, but also why the brain grows outward to meet with the structure to which it is so near akin.
Degeneration of a very pronounced kind thus accounts for the peculiarities of sea-squirt structure to-day. The case of ascidian retrogression is likewise the more interesting, seeing that its reverse side is that of progressive evolution and development of the highest forms of life the existing world knows. It is, therefore, important to note in passing that the possibilities of development may include degeneration of a very marked type, along with progressive evolution of equally pronounced kind. The category of life's extension includes, in fact, many possibilities which at first sight might appear of most unlikely kind; and, among these possibilities, that of extreme degeneration is by no means the least notable as an element in inducing the material variety of life we behold in the animal and plant worlds of to-day. The list of causes which lead to the degeneration of living beings includes, however, other fashions of producing retrogression than by fixation and parasitic habits, and operates in different ways upon organisms of varied structure and social or biological rank. Changes in food and feeding may thus accomplish degeneration and induce physiological backsliding of the most typical description. It is a familiar fact that the animal organism is of relatively higher nature than the plant, seeing that the animal frame can, as a rule, feed upon and build up its tissues from organic or living matter only. Animals, in other words, demand the substance of other animals or of plants, or of both combined, as a necessity of their commissariat arrangements. Plants, on the other hand, are specially constructive and elaborative in their feeding. They build up from the non-living matters around them—carbonic acid, water, ammonia, and minerals—the tissues of their living bodies. They "transubstantiate" this nonliving matter into living tissue; and the verdant tints of spring, the full glory of the summer's blossom, or the mellow ruddiness of autumn's fruits, represents, each in its way, the result at once of the plant's constructive chemistry and of the elaboration into living matter of the inorganic materials of air and soil around.
The animal frame, therefore, presents us—amid exceptions to the above rule in both animal and plant series—with relatively greater complexity of organs and tissues than the plant-body presents. This statement simply reëchoes what commonplace observation daily demonstrates. Hence, it may be a natural enough inference that whatever causes tend to bring the animal feeding nearer in type to that of the plant will tend to simplify animal structure, and so to produce retrogression and degeneration of the animal type. Many animals are thus known to develop chlorophyl, or the green color we see characteristically in every leaf. Through the combined operation of this green color either singly or aided by the leaf-protoplasm—and the action of light, plants decompose the carbonic acid of the air, as every schoolboy knows, and, retaining the carbon to aid in the formation of
starch, set free the oxygen, which thus returns to the atmosphere, and is welcomed by the animal hosts. The hydra, or common fresh-water polyp (Fig. 18), many animalcules, and certain worms of a low type, possess this chlorophyl. Like dishonest manufacturers, they seem Fig. 19.—Rotifera. to have infringed the patent-rights of the plant to elaborate this green color. And it is no longer matter of theory, but ascertained fact, that these green animals are capable, like the plants, of absorbing carbonic acid—usually a fatal gas to the animal constitution—and of elaborating starch therefrom like their plant neighbors. Thus a simpler mode of feeding, obviating the necessities of animal existence in the way of digestive apparatus, has apparently led to the simplification of structure. Degeneration has followed in the worms just mentioned, as the result of their imitation and acquirement of vegetative powers of feeding; and it is probable that other alterations in the way of dietary, of less sweeping character than that just mentioned, will affect, in like retrogressive fashion, the animal constitution.
Some of the most curious cases of degeneration known to us illustrate the total disappearance of digestive apparatus even in some beings, in which, as in the stylops already mentioned, one sex becomes retrogressive while the other sex remains structurally fully developed. Such a case is illustrated by the males of those remarkable organisms, the Rotifera, or "wheel-animalcules" (Fig. 19). These minute creatures, inhabiting our fresh waters, may be desiccated and dried, and revived, on the application of moisture, many times in succession. But in their ordinary existence, and in the details of their structure, the "wheel-animalcules" present details equally interesting with their exhibition of "potential vitality." The female animalcules possess a complete digestive system, a set of water vessels, a nervous ganglion, and other belongings; but their partners are decidedly inferior creatures, since their digestive system becomes totally abortive, while in size the males are likewise far excelled by the lady rotifers. How this degeneration and disappearance of digestive apparatus and the inferiority of size have been produced in the male rotifers may be a matter regarding which difference of opinion will certainly exist in biological minds. The fact that retrogression is here illustrated, however, can not be questioned. It may also be added that, in all probability, the extreme development of the function of perpetuating the species and the extraordinary fertility of production witnessed in these animalcules, may satisfactorily account for the abrogation of digestion in favor of reproduction. Thus, to the other causes of degeneration in animal life and structure, we may append that which takes origin from the extreme or excessive development of one function over another. Physiological development in one direction, overstepping the natural and ordinary limits, runs concurrently with destruction of life's equilibrium, and naturally tends to produce degeneration and simplification of other organs and other duties of life.
How far the theory of degeneration we have thus briefly discussed may be applied in explanation of the peculiarities of animal structure, remains as a task for the future of biology to satisfactorily determine. Possibly the corrections which the future of every hypothesis carries with it may be many and sweeping. The deductions and inferences we extract from a study of degeneration to-day may perchance be falsified by the higher and newer views of the to-morrow of biological science. But enough has been said to show that, even in a cursory review of the doctrine of degeneration and retrogression, many phases of living histories become theoretically plain; and it argues hopefully for the correctness and value of the doctrine before us that it has, so far as it has been logically pursued, fitted compactly and harmoniously enough with ascertained facts and with received views of the origin of animals and plants. That higher forms of life than the sea-squirt and insect race are by no means exempt from the influence of retrogressive change is an observation worth noting at the close of our researches. We know, for instance, of lowly structures in shell-fish life appearing in the midst of highly organized frames. A mussel, a cockle, or an oyster, whose early development runs in parallel lines to that of the snail and whelk class, is nevertheless esteemed less highly organized than the latter. The mussel or oyster tribe possess no head; the snails and their allies, as every one knows, not merely exhibit a well-developed head, but have that extremity provided with eyes, tentacles or feelers, and other addenda of the front region of the animal body. Hence it is more than probable that the mussel, headless and inclosed in its shell and possessing relatively little interest in the affairs of the outer world, is an example of a degenerated type of mollusks. The mussels and their relations stand, in fact, at the opposite extreme of development in this respect from those well-known mollusks, the cuttle-fishes. In these creatures, the tendency to head-development—or what Professor Dana calls "cephalization"—reaches its maximum, as any one may readily enough suppose on looking at an octopus or squid, with its great head, its enormous eyes, and its nerves massed together to form a brain inclosed in a kind of skull. Even as compared with the earlier cuttle-fishes—whose shells, under the name of ammonites and the like, we find fossilized in large numbers—the squids and cuttles of to-day present, in the extreme development of head, a noteworthy advance. Thus, while the one molluscan tribe of mussels and their neighbors has degenerated and gone to its own lowly place in the series, other groups, starting on an equal footing, have advanced, and, through progressive evolution, have produced those higher manifestations of molluscan life that teem in the seas of to-day. Even among the vertebrate animals we meet with examples of degenerative tendency which are not so easily explicable as the foregoing illustrations. In most snakes only one lung is fully developed, as a rule, the companion organ being rudimentary and degenerate. In birds, the egg-producing organs are similarly developed on one side only. How degeneration should be thus partial and affect one half of an animal's frame, so to speak, is very hard to discover. External conditions of life and the influences of surroundings could apparently possess little effect in inducing such an unsymmetrical retrogression of parts. Most probably we shall find the solution of such conditions to exist within the operation of some deep-seated law of the living constitution, and in the effects of that law in molding, or even contorting, the animal frame.
It constitutes one of the chief glories of biological science, as pursued among us to-day, that its studies are of far-reaching order, and lead, as the results of their natural extension, to the consideration of fields of thought often widely removed from the original topic which interests the reader. The present subject of degenerative changes, regarded as part and parcel of the living constitution, can readily be shown to possess applications far removed from zoology and botany, and extending into the most intimate spheres and phases of human history itself. Degenerative change in human tissues is medically symptomatic of very many of the ills to which flesh is heir. Tissues and organs degenerate in individual animals, as animal frames retrogress in their entirety. Cells retrograde and fibers degenerate in our bodies, just as the sea-squirt's frame exhibits, as a whole a universal, physiological backsliding. Nor may many of our diseases alone be esteemed mere examples of degeneration affecting our tissues. The termination and decline of life itself and the age that really "melts in unperceived decay" are, in reality, examples of natural degeneration also. The decline of existence is largely a retrogression of structure. There can be no such thing as a really "green old age," any more than we can speak of "the sere and yellow" of the autumnal leaf as imitating the verdant nature of the spring blossom. Nay, stranger still is it to discern that the full flush of life's vigor is accompanied by degenerative changes as typical as those which mark life's decline. For every tissue wastes as it works; and cells degenerate, die, and are cast off from every surface and tissue of our frames as the natural result of living and being. "Generally speaking," says a writer, in discussing the degeneration of human tissues, "those parts which live most slowly are those of which the duration is the greatest, and in which there is consequently the least frequent change. Of the exuviation of epidermic structures en masse—a process altogether comparable to the fall of the leaf—we have striking examples in the entire desquamation of serpents, the molting of the plumage in birds, and the shedding of the hair in mammalia; and, in the shedding of the antlers of the stag, we have an example of the exuviation of a highly organized and vascular part, which periodically dies, and which, being external, is cast off entire. 'What means all this,' says Sir James Paget, 'but that these organs have their severally appointed tissues, degenerate, die, are cast away, and in due time are replaced by others, which in their turn are to be developed to perfection, to live their life in the mature state, and to be cast off?'" And, again, the same high authority remarks that "it is, further, probable that no part of the body is exempt from the second source of impairment; that, namely, which consists in the natural death or deterioration of the parts (independent of the death and decay of the whole body) after a certain period of their life. It may be proved, partly by demonstration and partly by analogy, that each integral or elemental part of the body is formed for a certain natural period of existence in the ordinary conditions of active life, at the end of which period, if not previously destroyed by outward force or exercise, it degenerates and is absorbed, or dies and is cast out; needing, in either case, to be replaced for the maintenance of health." To these weighty words we may lastly add the opinion of Dr. Carpenter, who remarks that, "when the adult type has once been completely attained, every subsequent change is one rather of degeneration than of development, of retrogression rather than of advance."
Degeneration is thus an invariable concomitant of life. So far from being in any way an abnormal phase of living action, it is seen to be as natural a process for living beings to retrogress—wholly, as we have seen in some cases, or partly, in others—as it is for them to develop and advance. And what is thus undoubtedly true of the individual man or other animal is no less so of the race. "Buried civilizations" are by no means unknown; extinct culture is an archæological fact; the decline and fall of nations are matters of history. May not these things be likewise explained as a part of that wide theory of life which regards even the highest interests of man as lying within the operation and sway of causes which mold his physical organization? If this notion be accepted, then is the idea of degeneration as a normal phase of life rendered still more feasible and plain. Reaching to the individual and to the species as well; extending and including in its scope the lowly organized as well as the higher being; affecting one group or class lightly, and influencing another wellnigh to the complete exclusion of progress, we find degeneration and retrogression to be numbered among the stern realities of existence. And no less clearly and forcibly may we trace the truly natural place of degeneration in our own physical history: since, as physiology teaches and daily experience declares, not an action is wrought or a thought conceived without the presence of change and decay of tissue a process this which, limited in early life by progressive growth and by development, at last comes in our latter days to assume the reins of government, and in time to dissipate our energy and substance into the nothingness of physical and corporate extinction.
The philosophy of biology, however, may, in conclusion, be found to point out to us that the subject of degeneration, while treating of a powerful factor in modifying the living form, yet possesses a favorable aspect in relation to progress and evolution. High authority in matters biological may be found for the statement that degeneration is really a result of progress, that it is dependent on high development, and that, while it simplifies the living being, "it produces the same effect as differentiation, for it leads to variety in form." Thus there is a kind of evolution and progress inseparable even from degeneration itself. For the retrogression may in itself lead to variety and change, and in due time such variety may be the starting-point of new and higher developments. So, likewise, we are reminded that reduction and degeneration of some parts may proceed contemporaneously with the higher development of others, with the total result of perfecting the organism and of evolving a higher type of structure. The degeneration of a frog's tail is in reality a feature of its higher type as compared with its tailed friends, the newts and salamanders. The disappearance and reduction of the tail which the young crab possesses is a chief reason why we esteem the crab, whose body is all head and chest, a higher animal than the lobster or prawn, with head, chest, and tail complete. The degeneration of the "outside" gills of the Alpine salamander's young, which never have access to water, is not a mark of inferiority but of superiority; it is, in reality, the casting off of the old or larval and aquatic characters and the putting on of the new and higher features of the land-animal. Even the degeneration of human structures—the modification of the tail which early human existence exhibits, and of muscular structures well developed in lower life—are no proofs of inferiority, but are evidences of superiority in ourselves. Thus, even in the great work of evolving higher races out of the lower, to degeneration much is owing for its aid in repressing larval characters and the structures which belong to lower existences. While progressive evolution develops the great tree of life, extends each branch, clothes it with verdure, and expands each blossom, it is degeneration which lops the worn and aged stems, prunes the weakly foliage, trims the budding growths, and so directs and molds the outlines of the organic whole. It is to evolution and progress that the world of life largely owes its forward march. But hardly less is the debt of gratitude due by the living hosts to degenerative change and retrogression which, though stern and oft times cruel in their ways, nevertheless mark wisely and well the pathways of life, and prevent the useless and weak from cumbering the ground.—Gentleman's Magazine.