Popular Science Monthly/Volume 16/March 1880/New Views of Animal Transformations
|←The Force Behind Nature||Popular Science Monthly Volume 16 March 1880 (1880)
New Views of Animal Transformations
By Edmond Perrier
|The Duty of Enjoyment→|
ONE of the results of teaching at the Museum is, that it always has considerable influence upon the teachers themselves. Forced by the nature of this institution to keep himself constantly acquainted with what is known and what is sought, with what is definitely acquired to science, and with the object of aspiration, obliged to coordinate recent with preceding discoveries, to test theories, to bind together the new material continually accumulating about the stones forming the vast edifice of science, the professor sees the lines of this structure slowly modified, he himself contributing to this result, and sometimes ends his career under the sway of other ideas than those which at first inspired him.
I confess that this has been my experience. Last year I began a series of investigations upon transformation. I had not taken sides upon this doctrine. If some general ideas had drawn me toward it, I had ever present the reiterated objections of the most illustrious French naturalists, among whom were the men I most love and venerate. But, as I proceeded with my lectures, it seemed to me that these objections were not insurmountable, that they did not touch the foundations of the doctrine, and belonged rather to the way of conceiving that the evolution of organisms has taken place. Looking not for differences but relationships among organisms, I thought I saw that a simple and general law had governed their formation, that they were derived one from another by a constant procedure, and I found myself adding further arguments to the theory of the genealogical origin of species. The law which I now have to put forward may be called the law of association ; and the process by which it works, the transformation of societies into individuals.
When we have proved that all living beings are composed of microscopic corpuscles more or less alike, when we see such corpuscles capable of leading an independent life constituting by themselves the simplest organisms, it occurs to us to compare the higher animals and vegetables to vast associations of distinct individuals, each represented by one of these corpuscles or cells. In the same animal the cells assume many different forms, having different physiological properties. These forms and properties are not modified by the vicinity of different cells. Within the organism each cell lives as if it were alone. If it were possible to isolate a cell of the human body and surround it by normal nutritive material, it would continue to live, to develop and reproduce itself, and carry on all its physiological functions exactly as before. Further, in the organism itself, the life of each cell is so independent of that of its neighbors, that we may kill all the cells of one kind without injuring the others. Claude Bernard has proved that curare poisons the elements that terminate the motor nerves, thus abolishing all movement without injuring any other part of the system and leaving sensation intact. These researches led him to the principle of the independence of the anatomical elements. Not only are the elementary individuals of organisms sometimes very dissimilar, but they preserve their personality, live their own way, and keep up with their fellow citizens the relations of good neighborhood. We may compare an animal or plant to a populous town, where each person practices a particular industry on his own account, and yet helps the general prosperity through the activity of exchange. In high organisms, a special corporation in ceaseless movement is the medium of these exchanges. The blood-globules are true traders, taking along in the liquid where they swim the complex merchandise in which they deal.
Just as we had employed all the comparisons that pedigree furnishes to express the likenesses among organisms before supposing them really to be blood relations, so we have compared organisms to societies and societies to organisms, all the while regarding these comparisons as mere fancies. On the contrary, in the last year, we have reached the conclusion that association has played an important if not exclusive part in the development of organs. We find convincing proof of this in the history of Polyps and of Worms. The connection of Worms with the Articulata is apparent to everyone, and we already see how these same Worms are related to Mollusca and Vertebrata. The theory, therefore, extends to the entire animal kingdom.
Now, what do we mean by association? When we say that animal organisms have been in great part produced by the transformation of animal societies into individuals, what do we mean by the term society? Are all societies in the way to become individuals? Many animals associate together, and their societies are sometimes admirably governed. The social manners of dogs, antelopes, beavers, and many birds are well known, while the complex and perfectly coördinated operations of societies of bees, ants, termites, are the admiration of the world. Do such societies ever become individuals? Certainly not. But there exist other animal societies in which the relations are closer—where the individuals are not only in immediate contact but in continuity of tissue with their neighbors. These societies are called colonies, but the individuals that compose them are not always indissolubly united together. They can separate from their companions, and live a long time and affirm their independence by forming new colonies. In the same zoological group of neighboring species, we find some individuals that always live solitary and others always associated, as for example the specially remarkable group of Polyps or Acalepha.
One species of this group, the brown Hydra (Hydra fusca), is common in stagnant waters and even in small garden basins. It has always excited the interest of naturalists and philosophers since Trembly made known its marvelous faculties. These Hydras ordinarily live solitary; but frequently the larger individuals are seen carrying smaller ones on the walls of their bodies. In a captured Hydra we can follow their development step by step. They are at first simple swellings,
|Fig. 1.—a, diagrammatic section of Hydra; b, Hydra viridis, showing swellings in the body-wall; c, Hydra vulgaris, with an undetached bud enlarged; d, thread-cell of the Hydra, greatly magnified.|
in the center of which there is a prolongment of the cavity of the mother's body. These swellings enlarge and soon put out tentacles, and a mouth opens in the midst of the crown formed by them. The young Hydra, like its mother, is a simple sac with its wall composed of a double layer of cells, the cavity or stomach communicating directly with the stomach of the mother, so that the contractions of the body carry all the food taken by one into the stomach of the other, and inversely (Fig. 1). The parent and child live awhile in this way, but, whenever the latter has reached a certain size, it is detached and fixes itself on some near object, where it hunts on its own account. Soon the parent and offspring are indistinguishable, and during the summer they never cease to produce new Hydras. But, sometimes, in fertile waters rich in game, each Hydra retains its progeny, the little ones grow and produce new Hydras in their turn, and thus a new colony is founded. Trembly kept a long time a Hydra that carried twenty-two young ones of four different generations — a living genealogical tree.
That which is accidental in the common Hydra is quite normal with another fresh-water species, the Cordylophora lacustris, and in most marine Hydroids, in which the colonies often consist of innumerable individuals. But then new phenomena are seen. The social life becomes complicated, and a true division of labor occurs among the members of the same colony. At first all were alike, performing the same functions in the same manner. Specialization soon begins: some hunt, others digest, others reproduce ; so that individuals that at first had no need of each other and lived united only in a careless way, become reciprocally necessary ; the society thus acquires coherence and solidarity. In the Hydractinia we count not less than seven sorts of individuals : 1. Nourishers or gasterozoids ; 2. Prehensers or dactylozoids, provided with bunches of stinging capsules ; 3. Dactylozoids without stinging capsides ; 4. Defenders ; 5. Reproducers of individuals of both sexes ; 6. Males ; 7. Females. They are different in shape as well as in function ; each taking the figure suited to its work, rising or falling in organization ; so that division of labor brings with it, as in human society, inequality of conditions. The species thus become polymorphic.
Of these seven sorts of individuals that compose a colony of Hydractinia, the nourishers alone seem capable of living by themselves. The others have neither mouth nor tentacles, the sexual individuals are reduced to simple sacs, the defenders seem to be only sharp spines, between which the polyps can hide themselves (Fig. 5). It may seem an exaggeration to attribute individuality to these different parts. It may be said that they are simply organs ; but organs of what? They are just as independent of each other, just as independent of the nourishers, as the latter can be of one another. They are, then, not organs of those Polyps. Can they be organs of the colony ? It is already understood that the colony has the character of an individual, and the transformation we seek to demonstrate is admitted. But how can a colony acquire organs? Whence can they arise except from a transformation of the individuals which compose it? We have no need of hypothesis, however, to demonstrate that these colonial organs are the equivalents of true individuals. The buds that give birth to the different sorts of individuals in a colony of Hydractinia all grow alike, and remain
alike for a long time. This is the first presumption in favor of their equivalence. But in the allied type, Podocoryne, we see the humble sac, which represents the sexual individual, replaced by a being more active, more elegant, much more elevated than the Hydra itself, by a transparent medusa, which is detached when it reaches maturity and swims actively in the water, the colony suffering no inconvenience from the change (Fig. 2). These medusæ constitute the most general
form of the sexual individuals in the group of Hydroid Polyps, but they are very polymorphic. Their form is modified from one species to another, and arrested at all stages of development. Sometimes, although completely formed, they resign their freedom and end their existence in the colony where they were born.
In one group of Polyps the Medusæ associate themselves with the reproductive individuals to form a new unit—a small, distinct colony, that might be taken for a peculiar organ curiously analogous to a
flower—with a separate chamber, and called the gonangium (Figs, 3 and 4), A step further and these strongly individualized Medusæ are seen descending to the rank of organs in more complex colonies.
All the colonies of Hydras are not fixed to submarine objects. Some of them lead a vagabond existence. They are often taken, not without reason, for simple animals analogous to the Medusæ, and called
Siphonophores (Fig, 5), They sometimes attain a large size; and the variety and profusion of the parts which compose them, as well as the brilliancy of color and incomparable beauty of their forms, have made them subjects of the profound admiration of naturalists as well as sailors (Figs. 6 and 7). Each one of these parts is the equivalent of a Hydra or of a Medusa. In one Agalma we find, as in the Hydractinia, nourishers supplied with one long tentacle, of which a single touch produces a severe burning sensation, a sort of fish-line, which in large species is capable of capturing fishes. Besides the nourishers,
|Fig. 6.—Hidden-eyed Medusa.||Fig. 7.—Gonophore of one of the Campanularida.|
are found individuals without a mouth, which are only reproducers, in the neighborhood of which are sexual individuals resembling Medusæ in form. All these individuals are fixed upon a common axis, which floats like a serpent in the water, where it is sustained by an air-vessel forming its superior extremity. Two series of sterile Medusæ appear underneath this bell, a gang of oarsmen (physales), to which the colony abandons the care of locomotion.
These various parts are in all respects too much like the Hydras and Medusæ for us to refuse them the character of individuals; the Agalma and other Siphoniferæ are true societies or colonies. But here most of the individuals can not separate themselves without danger of death; and, in certain cases, they all coördinate their movements that the colony may perform certain acts. For example, in the Portuguese men-of-war (Fig. 8) the physales frequently change their course, and then all the individuals of the colony concur in the operation. They have, then, a will which controls them—a will that can find the grounds of its decisions only in a sort of social consciousness, elevating the colony to the rank of a psychological unit. Composed of individuals each of which is equivalent to those Hydras or Medusæ that live free and isolated and sufficient for themselves, every Siphonophore must still be considered in its turn as a single animal —a true individual of a higher order. Here the transformation of the colony to an individual is manifest. The Siphonophore is an
|Fig. 8.—a, Portuguese man-of-war (after Huxley); b, Velella vulgaris (after Gosse).|
animal with organs made up of distinct animals, each having a particular function. Elsewhere we see these animal organs become less and less independent. They come together and arrange themselves around a central axis which predominates, and end by forming a being like the Porpita or Velelle, which, but for the study of neighboring types, would not be thought of as a decomposable animal.
At the present time most people consider Sea-anemones (Fig. 9) and Polyps, of the madrepores, and coral, as simple organisms—primitive individuals; while to us their origin is the same as that of Porpitæ and Velellæ—the union of three sorts of Hydroid Polyps. The admirable researches of Moseley on the Polyps of the family of Stylasteridæ furnish proof of this. If we consider only their calcareous parts, all these beings seem to be true Madrepores. The first doubt concerning their true nature was raised by Agassiz, with reference to the Millepores.
Between a Coralarian and a Hydroid Polyp the difference is considerable. One is a simple sac with tentacles, usually solid appendages of the wall of the body, that vary in number with the species, or sometimes with individuals, but are constant for each during the great part of its life. The other is formed of a stomach-like sac, open at bottom, around which are hollow tentacles, which often increase in number with the age of the Polyp (Fig. 10). These tentacles, which are free at their extremities, and united at their bases to form the wall
of the Polyp's body, open inward like the stomachal sac into a great cavity, the circumference of which is divided into cells by the soldered walls of two neighboring tentacles. On the partitions of these cells, and so within the body, the reproductive apparatus is developed; while in Hydroid Polyps it is generally on the exterior in the form of a bud. This type of structure is much more complex than that of the Hydroid Polyp, which is well represented by the Stylasteridæ. In their colonies we find the polymorphism of the Hydroida, and also the nourishers, purveyors, and reproducers. Among the Spinipora, Sporadopora, Pliohothrius, Errina, these different sorts of individuals are perfectly independent of each other: a simple vascular network distributes among them the food seized by the hunters and elaborated by the nourishers.
But with the Millipores the nourishers are the most important members of the colony, as they prepare all the nourishment, drawing around them the hunters and reproducers, but without establishing any more intimate relations. With the Astylus, the Stylaster, the Cryptohelia, this movement of concentration around the nourishers becomes pronounced; a space forms underneath; the tentacles, rendered useless by the neighborhood of the hunters, disappear, and nothing remains but a digestive sac around which the hunters perform functions exactly like those of the tentacles of a Coralarian Polyp. Each system has now a decided individuality. Another step, and the hunters, from being distinct throughout their whole length, grow together at the base and interlace with the digesters, and the reproducers follow in this movement. These different parts are, thenceforward, too near together to require a special vascular system; the vessels which unite them are simple perforations of their wall which open in the space just below the digesters, and into which the reproducers penetrate also. But this whole the most experienced naturalist could not distinguish from a Coralarian Polyp. Among Coral Polyps the individual is, then, an association of parts of different form, of which each is equivalent to a Hydroid Polyp.
A Coral Polyp with twelve tentacles is the sum of a considerable number of Hydroid Polyps — one digester, twelve hunters, and a variable number of reproducers. It is formed by the aid of Hydroid Polyps, as flowers by the aid of leaves; or, better yet, as the composite flower is formed by its florets. It is produced in the same way as the Porpita or the Velella; the formation of a colony, the division of physiological labor, the appearance of polymorphism, and the concentration of the parts so elaborated—such is the succession of phenomena which marks the transformation of Hydroida into Velellæ and Sea-Anemones. The Hydroid Polyps are the raw materials which are brought into the factory, and then fashioned and gathered together to form higher individualities.
While these morphological phenomena are taking place, others are also occurring in a physiological order. At first the associated individuals have nothing in common except nourishment, which all are capable of elaborating, but which passes from one to another so that all are equal partakers. It is just here that consolidation begins, but each polyp still preserves his personality. He has his own will, and does not share his sensations with his neighbors; we can wound or even remove one without disturbing the rest. But, in proportion as the colony becomes more coherent, sensations extend farther and farther around the polyp that experiences them. Soon all the individuals
are conscious of that which happens to any one of them, thus forming a colonial consciousness above that of the individual, and finally a single will bends all the special wills to its bidding. A new individual is now definitely constituted. Is not this the same law which presides over the transformation of savages into civilized people? Have not nations, corporations even, a consciousness and will? Do they not form great units which we designate by one word in current language?
The transformations we have followed step by step in the class of Polyps are not restricted to these animals. It is easy to show how
simple forms are again associated, in the group of Worms, to obtain the more complex forms. We find here the same laws as in studying the Polyps, Long ago, Van Beneden, Professor at the Catholic University of Louvain, affirmed that each joint of a tape-worm (Fig. 11) was
the equivalent of a Trematode worm (Fig. 12); and Douve still earlier taught that the rings of a worm, or of an insect, were considered by naturalists as equal units, formed of the same parts, having each a real individuality. The name Somites, which has been given them, shows the tendency to consider them as true elementary animals associated in colonies (Figs. 13 and 14). The power possessed by the segments of certain worms to individualize themselves and form new colonies is strong evidence in favor of this view. Polymorphism and the concentration of parts explain how a Peripatus or a Myriapod can
become a spider or an insect, how different Crustacea arise from a common stem, how from another form of colony have arisen all the Annelida, It has been often said that Echinoderms, Star-fishes, Ophiurans, were only colonies united by the head (Fig. 15). They are, at least, all colonies, but of a special nature.
Can we say as much of the Mollusca and Vertebrata, all the parts of which are so closely united, and which are the giants of creation? Are there simple forms of association which can explain the marvelous organization of these superior types of creation — as we have explained the Siphoniferæ, Coral Polyps, Echinoderms, and Arthropoda?
This is the question for our present course of lectures; but, whatever the result of our inquiries, it will not invalidate the generality of the principle of association. If, contrary to our past opinions, these higher beings are not simple individuals, we must compare them with
those primordial individuals which by combination have produced other types, and which are still found at the base of each of the great divisions of the animal kingdom. Now, how have these individuals arisen?
The Hydras and analogous organisms reply. We can cut a Hydra into as many pieces as we like, and each piece, instead of dying, continues to develop and ends by becoming a complete Hydra, It follows that these different parts are independent of each other, like the polyps forming the lowest colonies. Each cell of the Hydra is a true individual, and the Hydras are a colony of these monocellular individuals as the Siphonophores themselves are colonies of Hydras. Aptitude to social life is communicated by heredity to these cells, as it is communicated to the polyps. Each cell, each polyp, detached from the colony, is a copy of it, and his after-development tends always toward its formation. At first all the members of a colony are equally apt to reproduce; then this function is localized like the others, and pertains to some individuals, or some parts, while sexual reproduction becomes more and more important. When the society reaches a certain degree of coherence, these different parts cease to live independently of the others, and can not be separated without danger of dying.
We see still more clearly in the Sponges their colonial nature. The spongarian individual is formed of two sorts of cellular individuals, the amoeba and infusorial flagellifere, of which we find analogues living, at liberty and in isolation (Fig. 16). The flagelliferous cells of sponges present exceptional features; they are provided with a nucleus and contractile vesicle, and their unique flagellum is surrounded by a membranous collarette in the form of a funnel. All these characters are found in the Codosigœ, monocellular Infusoria, always living isolated, and are to the sponges what the Hydras are to the Siphonifera and Coral Polyps. In the Anthophysœ these cellules live in colonies, but are yet all alike. Let polymorphism step in. Let some of the associated
cells preserve the flagelliferous form, while others become amoebae, a transformation which is possible, since it constitutes one of the most frequent modes of reproduction of the amoeboid Infusoria, and the Anthophysa is transformed into a sponge. The process is always the same, whatever the nature of the assembled materials. Cells or polyps, it always submits to the same elaboration in developing new individuals. The cells, once assembled in the organism, yield easily to the changes required by the division of physiological labor, and form various organs, although these organs never become true individuals. If the individuals of a colony often descend to the state of organs, we must not conclude that the organs of an animal are always individuals that have lost their autonomy; but the animal to which they belong, though it may never have been an assemblage of individualities intermediate between its own and that of cells, is not less a colony of the latter subjected to the laws of evolution of all the others. Thus even if we can not prove that Vertebrates and Mollusks have resulted from the fusion of more simple beings that have lived an independent life, they are still colonies of cells, and the law of association has consequently lost none of its generality.
It remains the fundamental law of development in the animal kingdom, comprehending and controlling those laws of growth, of organic repetition, of economy, which have been long accepted by physiologists, explaining hitherto mysterious homologies between different parts of the body, or between different organs of the same animal; embracing in one circle all the forms of asexual generation, which are its most powerful means of creation. Resting upon the law of the division of physiological work, the importance of which was first demonstrated by Milne-Edwards, and upon that of polymorphism, which without it have only a limited and indefinite meaning, consequent on the law of division of protoplasmic masses, it has been the great producer of organization, and establishes a new link between sociology and the branches of biology that are occupied with the constitution and functions of organisms.
We now reach the ultimate elements of living bodies, the material which has served to make the most simple beings, and we ask, What is its origin? Here we are in the presence of unity; there is no longer any question of association. Most living cells are composed of four parts — a membranous envelope, a contained fluid in which is a special globule, and the nucleus, containing the nucleolus. Of these four parts only one, the contained semi-fluid, perfectly limpid or finely granular, the protoplasm, is indispensable. It is in this strange substance that life, which needs no other apparatus to manifest itself, resides. Those remarkable beings, the Monera, are formed of it alone. They are simple, homogeneous clots of a limpid jelly like the white of Qgg. This jelly has the power of movement, captures animals, digests and assimilates them, grows, and, when it has attained a certain size, divides into two or several masses, that begin anew the life of their mother, and divide like her when they have reached a certain size.
This faculty of division is an important property of protoplasm, because it governs all organic evolution. A protoplasmic mass can not exceed a determinate size. When it reaches this size, a partition forms, and, as its mass is perfectly homogeneous, as it is constantly traversed by currents that completely mingle its substance, all the resulting fragments possess the acquired or hereditary properties of the protoplasmic mass from which they came. This explains all the phenomena of heredity, by means of which each being transmits to its progeny, even in the case of sexual generation, all its specific and part of its personal characters.
From this incapacity of protoplasmic masses to exceed a certain length, it follows that all beings that are larger must be formed of several distinct masses of protoplasm — in a word, are colonies. So the generality of the law of association appears as a consequence of one of the fundamental properties of protoplasm. It constantly decomposes itself into distinct masses. These separate masses are modified, each in a particular fashion, under the influence of external agents. Hence the wonderful variety of nature is an immediate consequence of the law of association, of the. necessity imposed upon protoplasm to separate into small distinct individualities.
What, then, can be the nature of protoplasm? Struck by its homogeneity, the identity of the elements that compose it with those that form albuminoid substances, it has been taken for a mere chemical compound, and it has been boldly asked if it is not possible to produce it artificially; if man has not power to relight the torch of Prometheus and create life at will. This question, I believe, has been asked in consequence of a strange confusion of words. If it is true that the substances that form living matter are the same as those that enter into certain chemical compounds, we can not infer from this that protoplasm is one of these compounds. What characterizes a chemical compound is fixity of composition. But protoplasm changes incessantly, without modifying any of its fundamental properties. New substances are constantly entering into its mass while others are leaving it. Protoplasm is perpetually decomposing and recomposing itself. It is this, and not its chemical composition, that characterizes it. It is always in movement, and motion characterizes life.
Life is, then, only a combination of movements, or, if you please, a mode of movement of which certain substances are alone capable, and which is not without analogy with the whirling movements to which eminent physicists attribute the properties of chemical atoms. We might pursue this comparison between atoms and protoplasms, and use it to show that the latter must have been formed originally in the greatest possible number; that we seem to be powerless to reproduce them; that they appeared with a train of properties which have controlled their subsequent destiny; and that they had from the first the individuality we see in them at the present day.
- Introductory lecture to a course on Zoölogy at the Museum of Natural History in Paris, delivered March, 1879.