Popular Science Monthly/Volume 8/December 1875/Progression and Retrogression
By Prof. W. D. GUNNING.
WE walk along a rocky beach when the tide is out. Twice every twenty-four hours this narrow zone is sea and twice it is land. Its tenants are, as itself, a sort of dividing zone between land and sea. The Algæ in the tide-pools will remind you of Confervæ in the ponds. The littering on the rocks will remind you of snails. The shapeless, gelatinous clumps adhering to rocks or wharf posts will remind you of garden slugs, or naked snails. We will give our attention first to these soft and shapeless chimps.
They will call up no image in the mind until the sea returns, or until you detach one of them, and drop it into a glass of sea-water. You have a Dendronotus, or a Doris, or an Eolis, or an Aplysia.
Out of the shapeless clump comes a form like that of the sing; but the slug in our captive is soon disguised, for along its back, from end to end, rises a fringe of pinkish papilla?. We have an Eolis. What does Eolis do with these papillæ? The last generation of naturalists said, "He breathes with them."
The last generation was too sparing of the knife. We cut through Eolis's back till we reach the stomach, which we find to be a mere expansion of the intestinal tube. This tube extends lengthwise through the body and lies near the dorsal, not the ventral side. It branches, and the branches branch again, and run up into the papillæ which stand out like quills on an angry porcupine. The papillæ are supplementary stomachs.
Eolis has no liver. With so much stomach it can carry on the process of digestion-without the aid of that organ, so troublesome to man and beast. A row of hepatic cells extending part way along the intestine represents the rudiment of a liver, or its vestige.
Where are the lungs? Nowhere—or, rather, everywhere. No part is specialized and set apart for aërating the blood. In the vital economy of this sea-slug, there is but little division of labor. The surface is soft tissue, covered with vibrating cilia, and currents of water, set in motion by the cilia, How around the tissue and yield oxygen to its blood.
Perhaps the gelatinous knob you detached was not an Eolis, If your knife reaches a stomach which is not arborescent, you may have a Doris. The dorsal papillæ of Doris are genuine lungs, but they breathe for only part of the body. They aerate only the blood which goes to the liver, an organ which appears now, not as a row of bile-cells, but as a well-defined gland. The foot shares the labor of the lungs, they breathing for the liver, it for the rest of the body.
Fig 1.—Doris Lacina.
In Eolis the quill-like diverticula of the stomach are placed in rows; in Doris the leaf-like, moss-like, or flower-like branchiæ are gathered into clusters (Fig, 1). Our first woodcut represents a Doris (Doris lacina), with two horn-like antenna on the head; and on the back, at the other extremity, a tuft of crimson leaves finely reticulated and deeply lobed. The second cut represents a Doris (Doris plumulata), with frond-like antennæ and a lung resembling; tufts of delicate sea-weed wrought into an eight-rayed star. Another Doris wears its lung like a brilliant flower, another like a begemmed tiara, Doris can draw his lungs into his body or throw them out at pleasure (Fig. 2),
Dendronotus may be known, as its name implies, by its branching, tree-like gills. If we leave the rocks and wharf-posts, and examine the laminaria (oar-weed), or ulva (sea-lettuce), we may find another member of this family. Aplysia is known to fishermen under the name of "sea-hare." A hump on its back calls up the image of a camel rather than that of a hare. If you make a dissection you will find that an idea has been borrowed from the camel's stomach as well as hump. Aplysia has a row of stomachs, and, what is strange, the teeth are not inserted in the mouth, but in one of the stomachs. In Aplysia, the liver is better defined than in Doris, and the leaf-like gills aerate blood for the whole body.
The classification of these naked mollusks will be as obvious now to the reader as to the observer.
In Eolis no liver, but a few bile-cells representing its rudiment, or vestige; no lung, every part of the surface respiring for itself; no well-differentiated stomach, but an arborescent intestinal tube.
Fig. 2.—Doris plumulata.
In Doris (sea-lemon), a liver; respiratory organs in the guise of crown, or star, or leaf, or tufts of sea-weed, organs which serve the liver only; a stomach.
In Aplysia (sea-hare), a better liver, respiratory organs in the form of leaves, organs which serve the whole body; many stomachs.
Eolis stands lowest, Aplysia highest. The series is suggestive of the history of organs, if not of species. It invites special attention to the lung.
In all marine animals except Cetacea, either the entire outer surface absorbs oxygen and exhales carbonic acid, or part of this surface has been differentiated for the function of respiration. In all mammals, and birds, and mature reptiles, part of an inner tissue has been differentiated and set apart for the function of respiration. External respiratory organs rise from the skin. Internal respiratory organs rise from the skin of the throat. Internal respiratory organs exist in the fish as a rudiment. External respiratory organs appear in embryotic mammals as vestiges.
The inner lung begins as a little hollow bud on the throat. This bud pushes out another and another, and so on till by continuous budding it becomes a tree-like growth, interlaced with blood-vessels. Let such a bud start from the outer surface, on the back. It will become, according to the mode of secondary budding, a little tree, or leaf, or flower of blood-vessels and vascular tissue—such a lung as adorns the back of Doris.
The history of the inner lung is indicated by fishes and amphibians. The history of the outer lung is indicated in these naked mollusks.
Eolis, which shows us the beginning of a liver, or perhaps the last stage of its reduction, seems to be prehistoric as to the gill. One part of the surface absorbs oxygen as well as another. If we leave the beach and the Eolids for mid-ocean and the Pteropods, we shall find the first shadowing forth of a gill. In the Pteropod one part of the skin is a little more vascular than the rest, and on this part the blood is more freely oxidized. Now "respiratory activity," as Spencer has shown, "aids in the development of respiratory appendages." A larger and larger surface is exposed to the water, and this larger surface, developed partly by Natural Selection, and partly by respiratory activity itself, is attained in multitudinous branchings of the mimic tree, and deep sinuosities of the mimic leaf.
But in Doris, which represents a great advance in gill development over a Pteropod, the gill is still imperfect, and as a respiratory organ it is supplemented by the creeping disk. In Aplysia the gill is carried up to perfection and aerates all the blood.
In the evolution of an organ we have hints as to the evolution of a species.
No interest can attach to such low forms of life as the Eolids unless they teach something of the methods of Nature in originating species. Readers of The Popular Science Monthly will not give their attention to mere description or anecdote. Facts they know do not pass into science until fertilized by ideas. We shall return to Eolis and its family through a study of forms which the eye, not aided by the knife, would report as far removed from them.
A mollusk is a soft, fleshy, sac-like body, with a mantle (pallium) extending from the back in two folds, right and left, around the sides. In the Bryozoan (moss-animal), whose reticulated coral incrusts many shells and sea-weeds, the molluscan type reaches down almost to the polyp. The Bryozoan has a cylindrical body with a tentacular crown. Structurally it is a mollusk, morphologically a polyp. It would seem to be a case in the organic world analogous to that in the inorganic, in which a small portion of a mineral, in crystallizing, forces a large portion of a foreign mineral into its own crystalline form and masks the structure under the shape.
The mantle performs important functions, and it will guide us along a series of transformations. Suppose that the two folds cohere along their edges. The mantle would then become a kind of sac, inclosing the body. If we call it a tunic, we might say that the animal is wrapped in its tunic, and this cohering of the tunic-folds would bring us to the order of Tunicata.
If we put the dredge down fathoms deep into the sea, it may bring from the bottom a Clavelina, most beautiful of Tunicates. In shape it is a pitcher without handle, an inch high, tapering down to a slender base, which springs from a creeping gelatinous thread. The mantle is transparent as crystal, and through it you may see, as if suspended in the cavity of the body, what seems the frilled edge of a ribbon of snow-white lace. This is Clavelina's lung. A little sac, seen through the transparent mantle and body walls, contracting and expanding with a slow and measured beat, is Clavelina's heart.
Another cloaked mollusk is Cynthia. It adheres to rocks or pebbles under a few fathoms of ocean, and has something of the form and color of a blood-peach. It is known to watermen under the name of "sea-peach." Its mantle is tough and leathery.
Another and a more interesting member of the cloaked family is the Salpa. In the structure of the heart it marks' an advance on Clavelina. Instead of a single pulsating sac, we find an auricle and a ventricle, veins and arteries. But, Nature having advanced from a single to a double heart, it would seem that she did not vet know how to vise the improvement. In the Salpa we find the heart incessantly changing its auricle into a ventricle, its ventricle into an auricle, veins into arteries, arteries into veins.
The Salpæ swim freely in the open sea and occur singly, or united in long chains or rings. They are phosphorescent, and a chain of united Salpæ appears like a writhing, fiery serpent gliding over the waves. The Pyrosomes, which are free Salpæ, congregate in vast shoals, and in their phosphorescence glare like pillars of fire, green, unearthly, elfish.
Let the edges of the mantle unite along part of their surface, but remain open at the ends. The animal now will not be completely tunicated. It will be inclosed in a kind of funnel. If, now, such a mantle be drawn out into a siphon to conduct a current of water to the gills, it would be of use to the animal in aiding respiration. The edges of the mantle having united in this way, a siphon-bearing mollusk, like the cockle or solen, would be simply a question of time. Natural Selection would bring it about.
Let the edges of the mantle not unite at all, we shall have a mollusk something like the oyster.
Remove the shell, and an oyster lies before you in irregular, ragged outline. An opening at the sharper end, which lies near the beak of the shell, is the mouth. Around the mouth are four leaf-like bodies, which hang in pairs. The heart is an advance on that of Salpa, not in structure but in behavior. It has settled down into regular work, the auricle always an auricle, and the ventricle always a ventricle. The liver is a decided advance on that of Eolis, although not yet a well-defined gland. The mantle is a fringed, veil-like membrane, whose folds are not united along their edges. Near the mouth, on the ventral side, is a portion of the surface a little tougher than the rest. This toughened surface on the oyster we shall find as significant as we found the softened vascular patch on the surface of the Pteropod.
The leaf-like bodies which surround the mouth appear as silent members. In some form or other they are present in all mollusks, and in the order of Cephalopods they reach the maximum of development, and appear as long, flexible limbs. In this order—represented by the Octopus—the molluscan type reaches the highest expression. Early in the history of life, the type had unfolded and found expression in Cephalopods of great bulk and of many species. The Cephalopods have long been a waning dynasty (Fig. 3).
Fig. 3. Octopus fulvus.
As we have met the palpi—rudimental in the oyster—in other guise in oyster's distant relatives, so we will find that toughened portion, so faintly pronounced in the oyster, expressed with greater distinctness in oysters' nearer relatives. In the mussel this toughened surface supports a bundle of fibres, which protrudes from the shell and adheres to a rock or wharf-post. In the cockle we find this same portion prolonged into a finger-like organ, which performs the office of locomotion. It is called a foot. In the teredo this "foot" has reached the maximum of development, as the palpi in Octopus. But for the rudimental palpi, we might look on the oyster as a degraded cockle or mussel. But for the toughened surface representing the cockle's foot, we might regard the oyster as a lapsed form of some ancient Cephalopod.
The mantle secretes the shell, and in all bivalves it lies through its whole extent against the shell. Now, in all mollusks, the axis of the body is at first straight, and the body is bisymmetrical. If growth were arrested at an early stage, all mollusks would look alike, and, if the embryotic mantle were to secrete a shell, all these arrested growths would appear as miniature bivalves. They would be symmetrical. But circumstances determine shapes. The mollusk which, in maturity as well as infancy, lives in the open sea, will be exposed to like conditions on either side, and will retain its bilateral symmetry. A mollusk which lies on the sea-bottom will be exposed to unlike conditions, one side being buried in mud and the other bathed in water. As a shrub which grows against a wall loses its symmetry and becomes one-sided, so a young oyster, as soon as it leaves off its roving ways, and fixes its abode on the sea-mud, must begin to develop unsymmetrically. One side and one valve of the shell outgrow the other side and valve. In the Gryphgæa, an ancient species of oyster, this over-development of one side is carried further, and, while the right valve is small and flat, the left is deep and partially rolled up. In the Gasteropods, except Chiton, this one-sidedness is carried still further. One side outgrows the other so much that the body takes a spiral form, and one valve, secreted by one fold of the mantle, appears as a spiral shell, while the other valve, secreted by the aborted fold of the mantle, appears as an operculum—a little shelly disk known under the name of "eye-stone," In the snail this one-sided development is carried to the highest pitch of asymmetry. Overgrowth of the right side forces it into a spiral, and the right valve twists around with the body it incloses, while the left valve, which, in the marine Gasteropod, we had found reduced to an operculum, is here completely eliminated.
From the cloaked clavelina to the oyster, we were led, step by step, along successive modifications of the mantle. From the oyster to the snail we have passed, step by step, along successive stages of one-sided over-development. The facts have shown that a bivalve mollusk could not have descended from a univalve. As all mollusks in early life have the axis of the body straight, and the parts symmetrically arranged on either side, we may infer that bilateral symmetry characterized the remote ancestors of the molluscan type. Now, while a mollusk is bisymmetrical or nearly so, if the mantle secretes a shell it must be in in two parts, or, as in Chiton, in many parts. The snail is the last term of our series, and its successive stages of growth should indicate the path along which Nature has moved in the evolution of the unsymmetrical Gasteropod from a symmetrical, oysterlike bivalve (Fig. 4).
Lereboullet has made out the embryology of fresh-water snail. We need not follow him into details. It will be enough for our purpose to note that from a "mulberry mass"—the egg after segmentation of the yelk—there comes a sort of hemispherical cup. The mouth of the cup changes from a circle to a long slit, and the edges of the slit unite except at one point. The embryo has now taken on the molluscan type. The aperture along the line of the slit is the opening to the sac, the mouth to the coming snail. The line along which the approximated sides of the cup have united is in the trend of a plane which divides the body into right and left sides, equal, a
Fig. 4.—Symmetry. Embryotic Snail: m, mouth; ma, mantle; c, creeping disk; in, intestine; h, heart (auricle and ventricle in line with the intestinal tube); r, remnants of yolk-cell.
and similar. The mantle has begun to form, and as a sort of cap it covers the part of the body opposite the mouth. The intestine begins in a little depression under the mantle and in line with the mouth and stomach. This depression is elongated, becomes a tube, and opens into the stomach. A few days later, traces of a heart appear as two pulsating, globular sacs, placed end to end (Fig. 5).
If development were arrested at this stage, our snail would be bi-symmetrical, and, if it had a shell, the shell would be in two equal valves, right and left. But development goes on, and now every step is a departure from right and left symmetry. First, the intestine gets a, twist. Other organs are quick to follow. Even the heart moves askance. The two chambers which, a while before, were placed end to end in line with the axis of the body, begin to change position. The receiving chamber moves obliquely to the right and downward, the distributing chamber to the left and upward. The right fold of the mantle spreads rapidly; the left, not at all. The right side of the body grows rapidly; the left remains almost stationary. The right valve of the shell grows rapidly, and twists over with the inclosed body; the left is completely aborted. Now, it is a very significant fact that the only parts which do not share this one-sided overgrowth are the head and creeping disk; and these are the parts which, not being covered by the mantle, do not become incased in the shell. Exposed to the water or the air equally on both sides, they retain their bilateral symmetry.
From a sac-like body, moving freely through the water, and thus exposed equally on both sides to the same environment, and therefore bisymmetrical, we may suppose that all mollusks have been derived. If such a free-moving body became fixed, unless as a stemmed Ascidian, its parts would be differently conditioned as to environment, and the side more favored would outgrow the other. As the first part of the snail's body to bend out of line with the axis is the intestinal canal, we infer that this bend occurred far back in the snail's ancestry. It occurs in the oyster. As the last organ to share the general twist resulting from unequal growth of the sides is the heart, we infer that displacement of this organ occurred later down in the history of the type. It does not occur in the oyster.
Fig. 5.—Asymmetry. Adult Snail: op, optic tentacle; oe, (œsophagus; en, cephalic ganglion; g, gizzard; s, stomach; l, liver; i, intestine (bent out of line with the axis of the body); h, heart (auricle and ventricle not in line with axis or intestinal tube); v, vent.
The first step toward a spiral-shelled gasteropod was taken in the first mollusk whose environment on one side was mud or rock, and on the other water. Difference of environment was the first factor in this series of evolutions. Only this would induce one-sidedness, and acting through long periods it might induce excessive one-sidedness. It might carry an oyster as far along in asymmetrical growth as the partially rolled-up oyster called Gryphæa. When asymmetry came to be of advantage to the animal. Natural Selection began and carried it to greater excess, with the aid of other factors for Nature is too rich to be limited to one or two efficient causes—carried it to the order of Gasteropods.
In this order we find Eolis, and Doris, and Aplysia. From them our studies have ranged over kindred, near and remote. From their kindred we return, prepared by what we have found to interpret them. In form, these animals do not depart from bilateral symmetry, as from their habits they should not. Each side is exposed in the same way to the same environing element. But the alimentary canal is bent out of line with the axis of the body. The reproductive system is still more askance. It is altogether one-sided. Very suggestive facts. We find one-sided growth without the conditions which induce it. These conditions must have pertained to an ancestor. The bend in the alimentary canal and the displacement of the reproductive organs have been inherited from an ancestor so conditioned in the environment as to produce overgrowth of one side. But the alimentary canal does not bend out of line so much as in the shell-bearing Gasteropods; and in Eolis—in which the last vestige of a shell has disappeared—the canal has become straight. Another suggestive fact. We find in these naked mollusks heredity and abbreviated heredity. Aplysia and Doris inherit the ancestral twist. In Eolis the heritage is cut off.
From symmetry to asymmetry, from a bivalve to a univalve, Nature has moved, closing a cycle of evolution in the snail; from asymmetry back to symmetry, from a shell-bearer to a non-shell-bearer, she is moving in the sea-slugs. In this retrogression, Aplysia has shared the least. It retains the largest shell-vestige; it has the most perfect liver; its gills cover the mantle. Eolis has been carried back the farthest. In this retrogressive movement we may find the rationale of Aplysia's many stomachs, and Eolis's branching stomach and hepatic cells. In the snail, perhaps in all Gasteropods, the alimentary canal develops isolately, a section here and another there. Now, a stomach is simply an expanded portion of the canal. Let the tract of the canal be laid in isolated openings, let these openings he elongated, each, into a tube, and let the original openings be marked as pouches along this continuous tube, and we have Aplysia's row of stomachs. It is after the pattern of the digestive tube of an embryotic Gasteropod.
In Eolis the branching alimentary canal lies along the dorsal side, not the ventral. In getting itself straight, it seems to have got itself as near the dorsal papillæ as possible. Now, these papillæ, for a long time mistaken for lungs, for a long time, perhaps, were lungs. We have found that in Doris the gills are connected only with the digestive system, and we may suppose that in some ancestral form of Eolis papilliform gills were connected with this system in the same way, that is, through the liver. Only a slight departure from the normal development would transfer the connection of a gill-bud from one part of the digestive system to another, from the liver to the stomach. If, then, it would be for the advantage of the animal to have more stomach, we can see how, by Natural Selection, all the gill-buds or papillae would, in the end, cease to respire for the liver and become diverticula of the stomach. What would become of the liver? Losing its lung, it would suffer degradation. It would abort, lapse into a few hepatic cells, and become a mere vestige.
The naked Tunicates are intelligible as initial terms of a molluscan series. The naked Gasteropods are intelligible as final terms of a descending series, as impoverished heirs of an ancient house.
We have chosen for our study these slugs of the sea to develop a phase of evolution not generally understood. Evolution does not imply an unbroken course of progression. It does not imply a tendency in every thing to become something else and better. It is determined by many factors, inner and outer, and, as Spencer has shown, "the coöperation of inner and outer factors works changes until an equilibrium is reached between the organism and its environment."
On the deep-sea bottom the environing actions remain constant age after age, and we find that in the abyssal world a number of species have remained constant since the Cretaceous epoch. On the surface of the sea and on the beach, the conditions of life have not been constant, and surface and littoral species have been more subject to change. The air is more fickle than the sea. It is now warm and now cold; now moist and now dry; now in motion and now at rest: and the aërial fauna must oppose to these outer factors a corresponding adjustment of inner factors. The fauna of this element we should find the most unstable, and so we do. The only insect known to have come down to our times from times so remote as the Cretaceous, unchanged or changed but little, is the tiger-beetle of our sea and lake shores, and the uplands of Colorado. Moreover, an insect at rest is not conditioned as an insect in the air. Let it forsake little by little its aërial life, and rest longer and longer on other bodies. In time it becomes a parasite. The structure it had acquired while in the air becomes useless. The environment being more stable, the opposing actions within are reduced, and the organism lapses into a simpler form. In the insect world we should find the largest number of retrograded species, and so we do. Fleas, bugs, the dream of which sends a shudder through our sleep, creepers in the hair, burrowers in the flesh, form a descending, series, each order carrying with it, in the form of vestiges, reminiscences of a higher state when, as winged insects, its ancestors lived in the open air.
Retrogression of this kind has affected higher orders. An amphibious mammal, taking less to the land and more to the water, would lapse in time into a simpler form. The studies of Prof. Wilder on the embryotic dugong seem to show that dugongs and manatees have descended by retrogression from some ancient hippopotamoid mammal.
Retrogression, whose rationale is not found in our studies on the Eolids, has affected still higher orders. If the elephants of our day are descendants of the mastodons and mammoths which, in Pleistocene days, possessed North America and Europe, as the investigations of Gaudry wellnigh demonstrate; if the living tigers and lions have descended from species whose remains abound in ancient caves, as is probable; if the "grizzly" of the Rocky Mountains is a modified form of the great cave-bear, once so common in Europe, as naturalists believe; if the anthropoid apes of Africa and tropical Asia are survivals from a race which spread beyond the tropics and ranked somewhat nearer to man, as the Mesopithecus of Greece and Dryopithecus of France testify out of Miocene strata, the proboscidians, carnivores, and primates have all suffered retrogression, and, at the advent of man, life having reached its zenith, animal life began a downward curve. If, in the main, the higher has followed the lower, within this cycle of progression the struggle for life would involve another cycle of retrogression. As the savage in presence of civilization often sinks to lower savagery, so a species, outstripped in the race of life, and left hopelessly behind, degenerates, and finally dies.
And as the two cycles, progression and retrogression, are involved in the life-history of the earth, so the two movements may go on simultaneously in the same species. Man himself is such a species. His brain, and its servant, the hand, have attained the utmost development. His digestive system and his foot have been modified but little from a primitive type. Progression above in that which is most distinctively human may involve retrogression below in that which is distinctively animal.