The Gentleman's Magazine/Volume 271/September 1891/Zoological Retrogression< The Gentleman's Magazine | Volume 271(Redirected from Zoological Retrogression)
Perhaps no scientific theories are more widely discussed or more generally misunderstood among cultivated people than the views held by biologists regarding the past history and future prospects of their province—life. Using their technical phrases and misquoting their authorities in an invincibly optimistic spirit, the educated public has arrived in its own way at a rendering of their results which it finds extremely satisfactory. It has decided that in the past the great scroll of nature has been steadily unfolding to reveal a constantly richer harmony of forms and successively higher grades of being, and it assumes that this "evolution" will continue with increasing velocity under the supervision of its extreme expression—man. This belief, as effective, progressive, and pleasing as transformation scenes at a pantomime, receives neither in the geological record nor in the studies of the phylogenetic embryologist any entirely satisfactory confirmation.
On the contrary, there is almost always associated with the suggestion of advance in biological phenomena an opposite idea, which is its essential complement. The technicality expressing this would, if it obtained sufficient currency in the world of culture, do much to reconcile the naturalist and his traducers. The toneless glare of optimistic evolution would then be softened by a shadow; the monotonous reiteration of "Excelsior" by people who did not climb wouId cease; the too sweet harmony of the spheres would be enhanced by a discord, this evolutionary antithesis—degradation.
Isolated cases of degeneration have long been known, and popular attention has been drawn to them in order to point well-meant moral lessons, the fallacious analogy of species to individual being employed. It is only recently, however, that the enormous importance of degeneration as a plastic process in nature has been suspected and its entire parity with evolution recognised.
It is no libel to say that three-quarters of the people who use the phrase, "organic evolution," interpret it very much in this way:—Life began with the amœba, and then came jelly-fish, shell-fish, and all those miscellaneous invertebrate things, and then real fishes and amphibia, reptiles, birds, mammals, and man, the last and first of creation. It has been pointed out that this is very like regarding a man as the offspring of his first cousins; these, of his second; these, of his relations at the next remove, and so forth—making the remotest living human being his primary ancestor. Or, to select another image, it is like elevating the modest poor relation at the family gathering to the unexpected altitude of fountain-head—a proceeding which would involve some cruel reflections on her age and character. The sounder view is, as scientific writers have frequently insisted, that living species have varied along divergent lines from intermediate forms, and, as it is the object of this paper to point out, not necessarily in an upward direction.
In fact, the path of life, so frequently compared to some steadily-rising mountain-slope, is far more like a footway worn by leisurely wanderers in an undulating country. Excelsior biology is a popular and poetic creation—the real form of a phylum, or line of descent, is far more like the course of a busy man moving about a great city. Sometimes it goes underground, sometimes it doubles and twists in tortuous streets, now it rises far overhead along some viaduct, and, again, the river is taken advantage of in these varied journeyings to and fro. Upward and downward these threads of pedigree interweave, slowly working out a pattern of accomplished things that is difficult to interpret, but in which scientific observers certainly fail to discover that inevitable tendency to higher and better things with which the word "evolution" is popularly associated.
The best known and, perhaps, the most graphic and typical, illustration of the downward course is to be found in the division of the Tunicata. These creatures constitute a group which is, in several recent schemes of classification, raised to the high rank of a sub-phylum, and which includes, among a great variety of forms, the fairly common Sea Squirts, or Ascidians, of our coasts. By an untrained observer a specimen of these would at first very probably be placed in the mineral or vegetable kingdoms. Externally they are simply shapeless lumps of a stiff; semi-transparent cartilaginous substance, in which pebbles, twigs, and dirt are imbedded, and only the most careful examination of this unpromising exterior would discover any evidence of the living thing within. A penknife, however, serves to lay bare the animal inside this house, or "test," and the fleshy texture of the semi-transparent body must then convince the unscientific investigator of his error.
He would forthwith almost certainly make a fresh mistake in his classification of this new animal. Like most zoologists until a comparatively recent date, he would think of such impassive and, from the human point of view, lowly beings as the oyster and mussel as its brethren, and a superficial study of its anatomy might even strengthen this opinion. As a matter of fact, however, these singular creatures are far more closely related to the vertebrates—they lay claim to the quarterings, not of molluscs, but of imperial man! and, like novelette heroes with a birth-mark, they carry their proofs about with them.
This startling and very significant fact is exhibited in the details of their development. It is a matter of common knowledge that living things repeat in a more or less blurred and abbreviated series their generalized pedigree in their embryological changes. For instance, as we shall presently remind the reader, the developing chick or rabbit passes through a fish like stage, and the human fœtus wears an undeniable tail. In the case of these ascidians, the fertilized egg-cell, destined to become a fresh individual, takes almost from the first an entirely different course from that pursued by the molluscs. Instead, the dividing and growing ovum exhibits phases resembling in the most remarkable way those of the lowliest among fishes, the Lancelet, or Amphioxus. The method of division, the formation of the primitive stomach and body-cavity, and the origin of the nervous system are identical, and a stage is attained in which thee young organism displays—or else simulates in an altogether inexplicable way—vertebrate characteristics. It has a notochord, or primary skeletal axis, the representative or forerunner in all vertebrata of the backbone; it displays gill-slits behind its mouth, as do all vertebrated animals in the earlier stages only or throughout life; and, finally, the origin and position of its nervous axis are essentially and characteristically vertebrate. In these three independent series of structures the young ascidian stands apart from all invertebrated animals, and manifests its high descent. In fact, at this stage it differs far more widely from its own adult form than it does from Amphioxus or a simplified tadpole.
Like a tadpole, the animal has a well-developed tail which propels its owner vigorously through the water. There is a conspicuous single eye, reminding the zoologist at once of the Polyphemus eye that almost certainly existed in the central group of the vertebrata. There are also serviceable organs of taste and hearing, and the lively movements of the little creature justify the supposition that its being is fairly full of endurable sensations. But this flush of golden youth is sadly transient: it is barely attained before a remarkable and depressing change appears in the drift of the development.
The ascidian begins to take things seriously—a deliberate sobriety gradually succeeds its tremulous vivacity. L'Allegro dies away; the tones of Il Penseroso become dominant.
On the head appear certain sucker-like structures, paralleled, one may note, in the embryos of certain ganoid fishes. The animal becomes dull, moves about more and more slowly, and finally fixes itself by these suckers to a rock. It has settled down in life. The tail that waggled so merrily undergoes a rapid process of absorption; eye and ear, no longer needed, atrophy completely, and the skin secretes the coarse, inorganic-looking "test." It is very remarkable that this "test" should consist of a kind of cellulose—a compound otherwise almost exclusively confined to the vegetable kingdom. The transient glimpse of vivid animal life is forgotten, and the rest of this existence is a passive receptivity to what chance and the water bring along. The ascidian lives henceforth an idyll of contentment, glued, head downwards, to a stone,
- The world forgetting, by the world forgot.
Now here, to all who refer nature to one rigid table of precedence, is an altogether inexplicable thing. A creature on a level, at lowest, immediately next to vertebrated life, turns back from the upward path and becomes at last a merely vegetative excrescence on a rock.
It is lower even than the patriarchal amœba of popular science if we take psychic life as the standard: for does not even the amœba crawl after and choose its food and immediate environment? We have then, as I have read somewhere—I think it was in an ecclesiastical biography—a career not perhaps teemingly eventful, but full of the richest suggestion and edification.
And here one may note a curious comparison which can be made between this life-history and that of many a respectable pinnacle and gargoyle on the social fabric. Every respectable citizen of the professional classes passes through a period of activity and imagination, of "liveliness and eccentricity," of "Sturm und Drang." He shocks his aunts. Presently, however, he realizes the sober aspect of things. He becomes dull; he enters a profession; suckers appear on his head; and he studies. Finally, by virtue of these he settles down—he marries. All his wild ambitions and subtle æsthetic perceptions atrophy as needless in the presence of calm domesticity. He secretes a house, or "establishment," round himself, of inorganic and servile material. His Bohemian tail is discarded. Henceforth his life is a passive receptivity to what chance and the drift of his profession bring along; he lives an almost entirely vegetative excrescence on the side of a street, and in the tranquillity of his calling finds that colourless contentment that replaces happiness.
But this comparison is possibly fallacious, and is certainly a digression.
The ascidian, through a pronounced case of degradation, is only one of an endless multitude. Those shelly warts that cover every fragment of sea-side shingle are degraded crustaceans; at first they are active and sensitive creatures, similar essentially to the earlier phases of the life-history of a prawn. Other Cirripeds and many Copepods sink down still deeper, to almost entire shapelessness and loss of organization. The corals, sea-mats, the immobile oysters and mussels are undoubtedly descended from free living ancestors with eye-spots and other sense-organs. Various sea-worms and holothurians have also taken to covering themselves over from danger, and so have deliberately foregone their dangerous birthright to a more varied and active career. The most fruitful and efficient cause of degradation, however, is not simply cowardice, but that loathsome tendency that is so closely akin to it—an aptness for parasitism. There are whole orders and classes thus pitifully submerged. The Acarina, or Mites, include an immense array of genera profoundly sunken in this way, and the great majority of both the flat and round worms are parasitic degeneration forms. The vile tapeworm, at the nadir, seems to have lost even common sensation; it has become an insensible mechanism of evil—a multiplying disease-spot, living to that extent, and otherwise utterly dead.
Such evident and indisputable present instances of degeneration alone would form a very large proportion of the catalogue of living animals. If we were to add to this list the names of all those genera the ancestors of which have at any time sunk to rise again, it is probable that we should have to write down the entire roll of the animal kingdom!
In some cases the degradation has been a strategic retrogression—the type has stooped to conquer. This is, perhaps, most manifest in the case of the higher vertebrate types.
It is one of the best-known embryological facts that a bird or mammal starts in its development as if a fish were in the making. The extremely ugly embryo of such types has gill-slits, sense-organs, facial parts, and limbs resembling far more closely those of a dog-fish than its own destined adult form. To use a cricketing expression, it is "pulled" subsequently into its later line of advance.
The comparative anatomy of almost every set of organs in the adult body enforces the suggestion of this ovarian history We find what are certainly modified placoid fish scales, pressed into the work of skull-covering, while others retain their typical enamel caps as teeth. The skull itself is a piscine cranium, ossified and altered, in the most patchy way, to meet the heavier blows that bodies falling through air, instead of water, deliver. The nasal organ is a fish's nasal organ, constructed to smell in water, and the roof of the mouth and front of the skull have been profoundly altered to meet a fresh set of needs in aerial life. The ear-drum, in a precisely similar way, is derived from a gill-slit twisted up to supplement the aquatic internal ear, which would otherwise fail to appreciate the weaker sound-waves in air. The bathymetric air bladder becomes a lung; and so one might go on through all the entire organisation of a higher vertebrate. Everywhere we should find the anatomy of a fish twisted and patched to fit a life out of water; nowhere organs built specially for this very special condition. There is nothing like this in the case of a fish. There the organs are from the first recognizable sketches of their adult forms, and they develop straightforwardly. But the higher types go a considerable distance towards the fish, and then turn round and complete their development in an entirely opposite direction.
This turning is evidently precisely similar in nature, though not in effect, to the retrogression of the ascidian after its pisciform or larval stage.
If the reader can bear the painful spectacle of his ancestor's degradation, I would ask him to imagine the visit of some bodiless Linnæus to this world during the upper Silurian period. Such a spirit would, of course, immediately begin to classify animated nature, neatly and swiftly.
It would be at once apparent that the most varied and vigorous life was to be found in the ocean. On the land a monotonous vegetation of cryptogams would shelter a sparse fauna of insects, gasteropods, and arachnids; but the highest life would certainly be the placoid fishes of the seas—the ancient representatives of the sharks and rays. On the diverse grounds of size, power, and activity, these would head any classification he planned. If our Linnæus were a disembodied human spirit, he would immediately appoint these placoids his ancestors, and consent to a further analysis of the matter only very reluctantly, and possibly even with some severe remarks and protests about carrying science too far.
The true forefathers of the reader, however, had even at that early period very probably already left the seas, and were—with a certain absence of dignity—accommodating themselves to the necessities of air-breathing.
It is almost certain that the seasonal differences of that time were very much greater than they are now. Intensely dry weather followed stormy rainy seasons, and the rivers of that forgotten world—like some tropical rivers of to-day—were at one time tumultuous floods and at another baking expanses of mud. In such rivers it would be idle to expect self-respecting gill-breathing fish. Our imaginary zoological investigator would, however, have found that they were not altogether tenantless. Swimming in the pluvial waters, or inert and caked over by the torrid mud, he would have discovered what he would certainly have regarded as lowly, specially modified, and degenerate relations of the active denizens of the ocean—the Dipnoi, or mud-fish. He would have found in conjunction with the extremely primitive skull, axial skeleton, and fin possessed by these Silurian mud-fish, a remarkable adaptation of the swimming bladder to the needs of the waterless season. It would have undergone the minimum amount of alteration to render it a lung, and blood-vessels and other points of the anatomy would show correlated changes.
Unless our zoological investigator were a prophet, he would certainly never have imagined that in these forms vested the inheritance of the earth, nor have awarded them a high place in the category of nature. Why were they living thus in inhospitable rivers and spending half their lives half baked in river-mud? The answer would be the old story of degeneration again; they had failed in the struggle, they were less active and powerful than their rivals of the sea, and they had taken the second great road of preservation—flight. Just as the ascidian has retired from an open sea too crowded and full of danger to make life worth the trouble, so in that older epoch did the mud fish. They preferrcd dirt, discomfort, and survival to a gallant fight and death. Very properly, then, they would be classed in our zoologist's scheme as a degenerate group.
Some conservative descendants of these mud-fish live to-day in African and Australian rivers, archaic forms that have kept right up to the present the structure of Palæozoic days. Others of their children, however, have risen in the world again. The gill-breathing stage became less and less important, and the air-bladder was constantly elaborated under the slow, incessant moulding of circumstances to the fashion of a more and more efficient breathing-organ. Emigrants from the rivers swarmed over the yet uncrowded land. Aldermanic amphibia were the magnates of the great coal measure epoch, to give place presently to the central group of reptiles. From these sprang divergently the birds and mammals, and, finally, the last of the mud fish family, man, the heir of the ages. He it is who goes down to the sea in ships, and, with wide-sweeping nets and hooks cunningly baited, beguiles the children of those who drove his ancestors out of the water. Thus the whirligig of time brings round its revenges; still, in all age of excessive self-admiration, it would he well for man to remember that his family was driven from the waters by fishes, who still—in spite of incidental fish-hooks, seines, and dredges—hold that empire triumphantly against him.
Witness especially the trout; I doubt whether it has ever been captured except by sheer misadventure.
These brief instances of degradation may perhaps suffice to show that there is a good deal to be found in the work of biologists quite inharmonious with such phrases as "the progress of the ages," and the "march of mind." The zoologist demonstrates that advance has been fitful and uncertain; rapid progress has often been followed by rapid extinction or degeneration, while, on the other hand, a form lowly and degraded has in its degradation often happened upon some fortunate discovery or valuable discipline and risen again, like a more fortunate Antæos, to victory. There is, therefore, no guarantee in scientific knowledge of man's permanence or permanent ascendency. He has a remarkably variable organisation, and his own activities and increase cause the conditions of his existence to fluctuate far more widely than those of any animal have ever done. The presumption is that before him lies a long future of profound modification, but whether that will be, according to present ideals, upward or downward, no one can forecast. Still, so far as any scientist can tell us, it may be that, instead of this, Nature is, in unsuspected obscurity, equipping some now humble creature with wider possibilities of appetite, endurance, or destruction, to rise in the fulness of time and sweep homo away into the darkness from which his universe arose. The Coming Beast must certainly be reckoned in any anticipatory calculations regarding the Coming Man.
- H. G. WELLS.