Popular Science Monthly/Volume 5/August 1874/The Chain of Species II
|THE CHAIN OF SPECIES.|
WHEN Wolf, Goethe, Oken, and Geoffroy St.-Hilaire began to tell us that the method of the creation of living creatures is an evolution, it was far from satisfactory. To comprehend the proposition in the first place was exceedingly difficult. It was almost incomprehensible, indeed, to minds tutored in the anthropomorphic notions of spasmodic and arbitrary special efforts on the part of some Demiurgus. Educated to see in Nature what were called evidences of design, meaning plan and purpose according to our finite ideas of design, we could not rise to the conception of the continuous action of universal law; and every thing not easily construed by our preconceived teleology was settled by the convenient doctrines of miracles and cataclysms. In another way, also, the world was not prepared for the proposition; for, in the second place, the proofs were hidden away in the still undiscovered facts of homology. The science of morphology was yet to be created. Not yet was it known that Bryant's solemn verses—
The globe are but a handful to the tribes
That slumber in its bosom"—
are as applicable to the genera and species of all living creatures as to the individuals of the human race; that the organic forms now extant are in simplest truth insignificant, both as to numbers and varieties, when compared with those which have preceded them and which have perished forever.
No wonder, then, that the new-fledged doctrine of evolution soon went out of fashion when even the great disciples of the great leaders just mentioned, Lamarck and the elder Darwin, had no better explanation to offer than the hypothesis of transmutation. Yet it ought not to be forgotten that their principal opponents were not devout professors of religion and theology, to the really qualified of whom, it must have been indifferent; but Voltaire, Frederick II., Cuvier, and Agassiz, men whom no one ever suspected of any profound knowledge of theology, or of special reverence for its deductions.
But now the mists are clearing away, just as, according to the logic of things, we should expect. For there is evolution in human thought and comprehension, as in all things else. Yet the how—the question of the method—the process of the development of life—still confronts us; and the recent labors of Charles Darwin, Wallace, Voght, Haeckel, Cope, and others, have taught us that the answer is not to be jumped at by mere speculation, nor by hasty inference from the sparse and ill-digested facts of natural history now in our possession. The full solution of the mystery still lies in the future, and is to be reached only after the collection and comparison of a mass of data overwhelming to contemplate. In the mean time, confusion of ideas and differences about words characterize all our controversies. How men will misunderstand one another!
Too false t' unite us, or control;
And for the word itself we fight
In bitterness of soul."—Wordsworth.
It is usual to state that there are two theories of the origin of species, of the production of organic forms, namely, the theory of special creations, and the theory of natural selection. But the statement stands in need of criticism. The supposition of a special genesis, whether by some assumed ab extra influence, in other words, miraculous interference, or by some influence working ab intra, is a virtual begging of the question, a virtual admission that we never can follow the chain of causation. And this, because at some stage of the process the battle is to be stopped; at some step of the argument our mouths are to be shut, if not with a miracle after the manner of the Sunday-school teachers, which has at least the merit of piety about it, then with the more formidable obstruction of an inexplicable fact or property. And we are actually told, by one of this turn of thinking, that "to bother ourselves about these inexplicable facts is as irrational as to discuss the politics of the moon."
But, leaving special genesis aside, let us consider a little more closely the doctrine of natural selection. This, in fact, is not a theory of the origin—of the genesis of species. As M. Quatrefages has remarked, it is not a theory at all, for it explains nothing, accounts for nothing, and is not therefore truly an hypothesis in philosophy. Natural selection is not a cause, but the discussion of it draws attention to the chain of causes at the bottom of which we must look for the solution of our problem. As we all know, it is a notion taken from the selection exercised by the hand of man in the rearing of domestic plants and animals. Man, by the conscious or unconscious selection of that which best suited his wants or caprices, has educed and displayed many varieties of living creatures; and in like manner it is suggested that Nature, by holding on to the fittest in the struggle for life, has herself made selection of the innumerable forms we see. In a general way, this principle has always been recognized; and in past ages it has been always remarked that the varieties discovered are well adapted to exist, and that, if they were not so adapted, they would perish. But we have yet to see that these variations are always the fittest, or that the fittest comparatively always survive.
But, indeed, so far as Mr. Darwin puts forth a theory at all, it is not that natural selection is the cause of species; but that slight accidental variations occur from some unknown or inexplicable cause; and that by natural selection the fittest of these will survive.
There is not so much difference after all between the two so-called theories as it is common to believe; the inexplicable accident brings them together at last. It is only the using of different words for the same notion. Variation, by slight accidental degrees, is quite as unphilosophical as the production of species, by decided saltus or springs, from some innate miraculous cause. Suppose, then, we drop the question of the origin of species and the cause of variation, and try to observe and understand the different stages of the growth and evolution of such living creatures as the earth is filled with, whether fittest or unfittest: would not this be much more consistent with the vocation of science? In itself it will be a delight, whether it solves anything or not.
Assuming matter and its law or properties—and remember, again, that without them it would not be matter—how, and by what steps, and stages, and degrees, has it put on the myriad forms of life?
We will not enter into those deeper speculations of philosophy which range every thing in unity or duality—which divide Nature into matter and force—or look upon the cosmos as one substance under two aspects, static and dynamic; and regard matter, in mathematical language, as composed of points in space and time; or, using purer ontological phrases, as centres of force or motion under the cosmical relations of time and space. Nor is it necessary to draw comparisons between these conceptions. They are but doctrines of method, and for certain purposes one may use either. Neither is this the place to dwell upon the forces, their correlation, and their unity; nor upon the principles of physics as a science. All this will be assumed as generally understood.
Matter we have; now life!
Suppose every thing prepared—a home, the earth, fitted to receive the invited guest; whence the mysterious power, vitality? The altar is prepared, the wood cleaved, and the sacrifice laid thereon: how was the fire from heaven invoked?
Recollect this maxim of even the old scholastic philosophy: that, having matter and form—that is, Nature and its laws—we are not to search outside of these, for we need no other factors to account for all the metamorphoses this basement matter may assume.
The first appearance of organic life—the stealing of this first fire from heaven—this is the easiest step in the whole process; easiest, naturally, because nearest to the inorganic kingdom, which is so much less complicated than the organic; easiest, because, as we advance, the factors which enter into the calculation and bear upon the result become so numerous and obscure that we never can know when we have discovered the half of them; much less their interminable combinations. And yet, as to organic life in general, is it not confessed that, if we could only account for the existence of the cell, of that first morsel of colloid matter, we should have the key to all its mysteries?
Very well. What is a cell? Or, expressed in other words, what is that drop—that particle of matter, called now by that same old fashion of supplying phrases when ideas fail—protoplasm? What is protoplasm?
For aught we know, there may be monads or gemmules of organic creatures, as conjectured by Mr. Spencer and Mr. Darwin, there may be a peculiar substance endowed with life as a property, as conjectured by Mr. Huxley, there may be these atoms of organic life—the bases of organizations; and organized creatures may be definite arrangements of these, for aught we know. But really, except as a provisional theory, used, as we see it in the notion of Pangenesis put forth by the great naturalist, merely to aid in rising to other conceptions, there is very little need for such a supposition. Especially is it to be used guardedly. For, while put forth expressly in analogy to the atomic theory in chemistry, which is an aid to grasp the law of definite proportions, it is to be feared that many will so lean upon the crutch, they may never learn to walk. We know that in chemistry this is true; that many possessed of feeble powers of abstraction rest in the doctrine of atoms as the final fact; as in religion feeble minds stop at the forms and images used, and fail to comprehend the Deity taught and concealed thereby.
Rightly understood, the doctrines of Protoplasm, of Gemmules, et id omne genus, if they aid little, can do little harm. For, to the physiologist, there remains the great fact that organization is life. In, through, and by means of organization, or, if you prefer it, an organism, is matter endowed with life. So far as physiology, and therefore natural science and physics, is concerned in this matter, life does not exist without organization. Now, then, what is the first, the simplest form of organization—the primordial type of organic creatures? It is a cell. For, notice what is really meant by an organism—an organized creature. It is a creature that has functions dependent upon organs or parts. There is, then, in the very simplest organism already a manifestation of Von Baer's great law in biology—differentiation. Without differentiation there is no organization, and without organization, again, no life.
It is impossible to stop here to dwell upon the organization of a cell, and the proofs of it in unicellular creatures. This has been more fully treated of in some of my earlier productions. Still, a glance at this question—how? whence?—was promised.
See, then, this drop of colloid matter—this protoplasm—this cell. It can scarcely be called protoplasm until there is organization; and, if so, then it is what the older anatomists called a cell. Can there be colloid matter without organization? Both chemistry and physiology answer in the affirmative. It may and does so exist in abundance. But it yet remains to be shown that the substance itself, and all the other necessary external circumstances, can meet without producing or exhibiting life, Not that our experiments have ever shown a single instance of the fact. But it has never in the failures been shown that every necessary concurrent circumstance was also applied. Far have every one of the experiments been from the least pretense to a perfect repetition of the exact circumstances which in the beginning did actually witness the genesis of the germs of life. Now that we have these germs we think it easier to understand their successive reproduction than their primal genesis. How far this is from the fact we have already noted.
When we have a morsel, a drop of nitrogenized colloid matter, we can easily comprehend how the attacks of oxygen will cause the evolution of those forces which again will cause a difference of functions in different parts; which, again, by this very differentiation become organs. Without a differentiation there would be no relation of the parts; no polarity; no motion; no circulation; no duplication; no increase—the best evidence of the presence of organic life. In our most ordinary notion of a cell there is all of this; and this motion, this polarity, this circulation, can be caused by oxygen alone, attacking a suitable compound. A circulation, which is but a repetition of rhythmical motion, once set up, organization is complete. Endow this organization with continuity, or the power of repeating itself, which the rhythmical circulation and polarity are capable of doing; endow it with the power of inspiring other colloid and crystalloid atoms with like vibrations, attracting them into its own mass, and then ejecting them again, arranged in form like to the original cell, which it will continue to do from habit, and you have living creatures.
Comparatively simple as this is, we are not so much concerned at present with the origin of life as with its metamorphoses. Having life in the shape of cells, and the first must be hypothetical, how does it advance? This is biological science.
The advance of life is also simple. It progresses by characteristics which must distinguish all organization, whether of organic or of inorganic elements—cosmical, chemical, or social. It is by aggregation, as Mr. Spencer has it—by a compounding. By compounding, and by differentiation; these are the two great laws.
The primordial cell, by holding on to the new broods of cells as they seek to escape from the parent hive—by retaining them and giving them a new division of labor for the common family—compounds and increases the energy of the common organism.
Every living creature, as we now know these creatures, is a compound. Simplicity is nowhere. Even the simplest the microscope can show is probably already a compound of many removes from the beginning.
The law of compounding is not at all mysterious. If the first force of reproduction, the genesis of individuals, exhibits itself in the form of an evolution—a budding forth, a repetition of the form and structure of the parent cell—the law of compounding first presents itself as an arrestation of the law of gemmation. The old cell fails in force to throw off the new one; or, by another law, equally in force, adhesion takes place between the contiguous surfaces of the old and the new, and the extrusion to complete independence cannot be effected. This is the simple rule extending up through all the shining ranks and files of life. In the higher forms this law presents itself in the form of involution; but in lower creatures it is but an adhesion—an anchylosis. In all it is a failure of a perfect evolution—a failure perfectly to develop and reproduce a separate cell.
Now that we have the laws, let us see if we comprehend their application in Nature's workings. Really we might go to mathematics, and take from geometry, not only illustrations, but the very definitions of biology. Geometry, first and simplest of the sciences, begins its definitions with a point; from a point it proceeds to a line; from a line to a surface; from a surface to a solid. In biology, beginning with a cell, which is the physiological point or unit; the next development is to an axis, a line of cells, the type of all baculate structures. This baculus revolved upon itself, phyllate structure—a biological surface, the type of all organisms having one depth of cells. The next step is to the biological solid. This is made by an involution, a folding down of the surface upon itself, constituting a creature of two laminæ—two tiers of cells.
Passing on to another class, typically displayed for instance in hepaticæ, we find that another involution has taken place. This time the phyllum of two tiers of cells is folded upon itself, constituting one normally of four layers. Here first occur those curious openings into the centre of the structure known as stomata, rendered necessary, of course, from the fact that two of the strata are internal, and but for these contrivances would be cut off from that direct contact with atmospheric air which is necessary to the life of external living creatures. The next and the last evolution which takes place in the ascending development of plants is the folding upon itself of this leaf of four thicknesses of cells, so as to make a sheet consisting essentially of eight thicknesses of cells—the type of all the so-called higher plants. In this respect exogens and endogens do not differ; their only real difference being the atrophy and suppression of one cotyledon of the former to constitute the latter. In exogens there may be, in addition to these, various subordinate adhesions, but no further involutions of the whole creature. These may be called topical, as affecting only portions of the compound structure; and among endogens, from the rolling up of the single phyllate cotyledon, these adhesions must be innumerable. Otherwise there would be no stems to plants of this class. A careful analysis will always enable us to trace the original layers, and, wherever reduced to a simple leaf, to find the law invariable.
It is manifest that all these creatures live externally. The leaf is the type of all; and every metamorphosis is some modification of a leaf. Even creatures of a single cell may be regarded as diminutive leaves; and all leaves are compounds of simple cells. The point is, the true manner of organization or of life. The grand peculiarity of all of them is that the great disturber—life-destroyer and life-giver, atmospheric oxygen—must come in direct contact with each and every cell. Organisms living in this way are called plants—a term which has no scientific meaning or value, since it indicates no relation to other creatures. All other living creatures constitute but one other grand kingdom; animals, another unscientific term. Unscientific as the terms are, it is generally supposed that we know pretty well to what they apply. We understand these are the first two branches from the main root of organic life, springing from the same original germ, and expanding into two great trees, never uniting nor mingling their boughs any more. It is easy to see the correlation of these two; the true distinction between them.
The so-called plant never loses the type or plan of the original leaf, of the primitive cell. It always remains phyllate, and living, as it were, cell by cell, in external relations to the air and the sources of nutrition. The so-called animal is more complicated. It differentiates completely the points, or spots, or organs of aëration and of nutrition; devotes one part of the organism to nutrition, and another to oxygenation. This is not all. Thus far probably all cells agree. But in animals the organs and functions of nutrition, at least, are in some fold of the tegument or sarcode, so that they store away their food in a special receptacle, and carry it about. This is as perfectly true of the most elementary amoeba as of the elephant. This view of the ground of classification has been rejected by naturalists—by Dr. Carpenter among others; but this was done years ago, inconsiderately, and without the aid of recent advances in biology.
The amœba, although a mere drop of jelly, improvises a pocket, or stomach, for the reception of its food, which, for the time being, is differentiated to nutrition. The so-called plant, on the contrary, has its mesentery, as well as its apparatus for aëration, external to the organism. The animal involutes a part of its investing tegument; takes the mesentery, at least, into the inside of the body; and, in the higher orders, the lungs also.
The scientific relations of the two kingdoms are well indicated, therefore, in the terminology which classifies the one as Exothens—external livers; and the other as Endothens—internal livers.
As to their life, and the elements of organization, they are precisely the same. The same chemical constituents, the same protoplasm, the same basement of cell-growth, and the same compounding of cells, characterize the development of both. In short, there is but one set of organic creatures. They are all animals, or all plants, as you may please to call them.
Before this simple distinction into endothentic and exothentic, all the difficulties, of discriminating in special cases as to which of the two great kingdoms has the best claim to a particular species, vanish. Those organisms of the boundary-line which could never be located, because sometimes apparently one, and sometimes the other, as motion and volition, seemed to be present or absent, now readily take their places. For instance, the aëthalium—to show the impossibility of a definite boundary—is sometimes observed as a flying vibrio, then a crawling amœba-like drop of sarcode; then, in the still condition, a greenish spherical cell like some protococus; even this mysterious creature need no longer be regarded as amphibious. In every stage it is decidedly exothentic.
What, then, is the distinction, and how does the evolution advance? Let us go back a little. We observe that in plants evolution advances by a folding down of the creature upon itself—an involution. A strip of paper may well represent that type which consists of a single tier or layer of cells, as in ulvaceæ; or even baculate types, as confervæ. This is our biological surface.
Fold it down upon itself, the two surfaces coming in contact soon adhere, and then we have, as we have seen, a creature of two tiers of cells. This begins to look like reducing a loose collection, or a mere association of cells, to a consolidated organization.
Fold the sheet again, and you have another solid, a creature of four tiers of cells, with a distinct axis of growth. This is the type of all the higher cryptogams.
Fold it once more, and you have a type, normally, of eight tiers—the type of the phanerogams.
It is to be noted that every folding develops a new axis of growth at right angles to that of the preceding type, the folding being really the mechanism employed for the evolution of the axis. Involutions do not stop here, for no surfaces come together without a tendency to adhere, especially in the fœtal stage of life when all the elements are plastic; and it is at that period that all variations occur. But, as already noticed, no further involutions, except these three, affect the axis of the whole creature and change the type. To this simple fact of the adhesion of organic surfaces all the innumerable morphological variations may be referred. Upon it depends the success of the surgeon's skill, as well as the analysis of the speculative physiologist. Adhesions of the edges of leaves account for their shapes and sizes. Adhesion by their surfaces causes many wonders, among them the evolution of all carpels, seeds, and fruits. Sometimes, also, there is adhesion of surfaces confined to a single leaf, as in the iris. But these adhesions are all topical, if I may use the word, and do not affect the type and general axis.
Now, this involution, by which the progress of evolution takes place, may, as remarked, and strange to say, be described as a failure of development—a failure to unfold into the original type, constituting, by the very failure, a new and higher type. If we watch the development of any leaf from venation, we observe that in some way it was folded down upon itself, or rolled up with its fellows of the same bud, and before coming to adult age it expanded into the recognized form of its order. Let but that expansion fail, and the evolution may be said to fail. A happy failure; for to this we owe the production of grain and fruit—the food of man. Adhesions take place within the buds, which change the leaf-buds into flower-buds. The change is always made in the embryo, while still plastic and capable of being moulded into new forms.
It would be delightful to follow the immortal Linnæus, Wolf, Goethe, and the grand army of enlightened living botanists, who have illustrated this beautiful transcendental history of leaves, and flowers, and fruits; but time forbids at present. Suffice it to say that, so far as the vegetal kingdom is concerned, the doctrines of involution have become the common property of the scientific world.
Let us pass on to the so-called animal kingdom.
And first, as to the bridge we cross. True, it is somewhat the fashion to tell us that here is a great gulf fixed, and no crossing was ever possible. Yet Nature found a bridge somewhere, and we ought not to despair of finding at least some remains of the abutments.
To illustrate this passage from the vegetal to the animal plan of structure, take a hollow India-rubber ball, which may very well represent a cell enlarged a few thousand diameters. It is a perfect image of an external living creature, and is also typical of one stage of the development of the ova of the lower orders of animals—probably of all animals. But it is the adult form of plants. The vital functions of such a cell are all within; and there is no communication with the external world, except by osmotic action. Through this same cell wall—this same external coat—and by means of it, nutrition and aeration are both carried on. This is an exothen.
Now, by pressing the finger upon a part of the elastic coat, a portion of it sinks in, and you have a cup—a cup with double walls and a space between them. This is an involution; and by means of such an involution as this Nature transforms a plant into an animal.
Recently-published observations of Kowalewsky declare that he has seen this sort of transformation actually take place in the growth of the embryo of a creature as high in the scale as the Ascidians. Doubtless, in Nature, the true process was a failure at one point to fill out the rounded fullness of the ball; some contraction, some atrophy, some failure to grow at a certain spot, while the balance of the creature continues its development, until a wall arises around the constriction and becomes permanent, further growth only increasing the introversion—precisely such a process as we may witness at the hands of a potter when he places a ball of clay upon his wheel. First he flattens the top of his ball; then, as he continues to press upon the point, the adjacent clay rises around it; and next you have a cup. And such a cup-like cell is the type of all endothentic creatures; that is, of all animals.
Naturalists, it is true, regarding only flagrant forms, confine the term, cœlenterate to one class; those that are permanently open-cupped. But, strictly, all differentiate a part of the investing tegument into a cup, for the purpose of carrying about their nutrition; and, however much the cup may be extended and contorted—drawn out into a tube and folded away into some convenient receptacle, and puckered, and tucked, and furnished with a thousand little pockets supplementary—it is all but an extension of that same original internal fold or cup. Finally, in the highest animals, all the principal vital functions are found severally the office of some pocket of the integument involuted and shoved out of sight and out of harm's way into the great cavity.
Having obtained for our animal kingdom the cupped cell as a type, we have plain sailing for some time.
In the outset, we may subdivide the kingdom into the occasionally cupped and the permanently cupped. Those that present the cup as occasion serves, or only in one stage of their evolution, are exemplified in the Rhizopoda. The permanently-cupped include not only those always open-mouthed—the cœlenterata of Mr. Huxley—but, as we have seen, necessarily all of the higher classes.
Observe, again, the India-rubber ball. First, we have the cup. Now, this open cup is mouth—is stomach—is vent—is every thing. Every thing that goes into the creature enters here; all that comes out finds exit by means of this common gap. Soon Nature, so to speak, finds this plan poor economy, and divides off one side, or edge of the cup, for one purpose, another for another purpose, and we find one corner or edge of it devoted to the entrance of nutrition, another to the exit of the débris. Nor is this mere speculation. Creatures are actually so constituted, and are seen to develop to this type from the ovum. We often see, in cœlenterates, as in sea-anemone, a tendency to the same thing. Now, this very fact of voiding indigesta at one angle of the mouth, while the other is receiving a fresh supply of food, produces a constriction in the unused middle region; and the consequence is that here the lips approximate one another, and finally, adhering at the point of contact, a permanent perinæum-like septum is formed.
This adhesion completed, nothing more is wanting to exhibit the type of the higher animalia; for here is a complete alimentary canal, however short, and a dorsal and a ventral aspect. This is an animal of one segment, or cell, type of both the cœlenterata and molluscoida of Mr. Huxley, and of all mono-segmental creatures.
Henceforward, this type is the unit of animal organisms, as a simple cell is the unit of primary vegetal life. For want of a better term such a creature may be called a mono-segmentary, consisting of but one segment or unit; while all others are polysegmentarian—consisting of more sections than one.
Nature retains her habits very tenaciously. If we have seen her rising to higher and higher types of vegetal structure by adding cell to cell; duplicating cells by division and holding on to them by adhesion; extending them by increase upon one axis into a baculus; upon two axes into thalli; and folding them again into fronds, leaves, and fruit we no less see her handling this new unit of organization in a similar manner. Ay! the parsimonious builder again goes through precisely the same means of progress. She works by compounding; by multiplication of segments; by gemmation; by the evolution of higher types; by a failure to expel the simple segments; until, by adhesions in fœtal life, a more complex creature is formed of multiplied powers.
Here, again, we come to great gulfs and faults in the strata, which naturalists tell us cannot be bridged over. There remain still the three highest types of organic creatures to be accounted for, namely, the annulosa, or articulata the mollusca, and the vertebrata. Our most eminent recent naturalists regard all these as unconnected with preceding forms, and unconnected with one another. Some of the very recent, as Haeckel, endeavor to show a consistent chain here by connecting directly molluscoids with vertebrates; dropping out of the series entirely the two great classes of mollusca and articulata, and leaving their evolution unaccounted for. True, there can be no objection to leaving them out of the chain, if they have no place in it. If we can arrive at vertebrates and man directly from the holothurius, well and good. It would be analogous to other proceedings of Nature, as in the separate evolution of the whole grand vegetal kingdom from the first living germs, in a direction exactly opposite, as it were, to animal evolution. But we should also account for the genesis of the two eccentric classes, and connect them in some way with primordial types.