CHAPTER XV
FINAL REMARKS
Problems requiring investigation—
Giant nerve-cells and giant-fibres; their comparison in fishes and in arthropods; blood- and lymph-corpuscles; nature of the skin; origin of system of unstriped muscles; origin of the sympathetic nervous system; biological test of relationship.
Criticism of Balanoglossus theory.—Theory of parallel development.—Importance of the theory advocated in this book for all problems of Evolution.
The discussion in the last chapter on the "Principles of Embryology" completes the evidence which I am able to offer up to the present time in favour of my theory of the "Origin of Vertebrates." There are various questions which I have left untouched, but still are well worth discussion, and may be mentioned here. The first of these is the significance of the giant nerve-cells and giant nerve-fibres so characteristic of the brain-region of the lower vertebrates. In most fishes two very large cells are most conspicuous objects in any transverse section of the medulla oblongata at the level of entrance of the auditory nerves. Each of these cells gives off a number of processes, some of which pass in the direction of the auditory nerves and one very large axis-cylinder process which forms a giant-fibre, known by the name of a Mauthnerian fibre. Each Mauthnerian fibre crosses the middle line soon after its origin from the giant-cell, and passes down the spinal cord on the opposite side right to the tail. Here, near the end of the spinal cord, it breaks up into smaller fibres, which are believed by Fritsch and others to pass out directly into the ventral roots to supply the muscles of the tail. Thus Bela Haller says: "The Mauthnerian fibres are known to give origin to certain fibres which supply the ventral roots of the last three spinal nerves, so that their terminal branches serve, in all probability, for the innervation of the muscles of the tail-fin." They do not occur in the eel, according to Haller, or in Silurus, according to Kölliker. Their absence in those fishes, in which a well-developed tail-fin is also absent, increases the probability of the truth of Fritsch's original conclusion that these giant-fibres are associated axis-cylinders for certain definite co-ordinated movements of the fish, especially for the lateral movement of the tail.
In Ammocœtes, instead of two Mauthnerian fibres, a number of giant-fibres are found. They are called Müllerian fibres, and arise from giant-cells which are divisible into two groups. The first group consists of three pairs situated headwards of the level of exit of the trigeminal nerves. Two of these lie in front of the level of exit of the oculomotor nerves, and one pair is situated at the same level as the origin of the oculomotor nerves. The second group consists of a number of cells on each side at the level of the entrance of the fibres of the auditory nerves.
The Müllerian fibres largely decussate, as described by Ahlborn, and then become the most anterior portion of the white matter of the spinal cord, forming a group of about eight fibres on each side (Fig. 73). A few fibres are also found laterally, and slightly dorsally, to the grey matter. These giant-fibres pass down the spinal cord right to the anal region; their ultimate destination is unknown. Mayer considers that in the first part of their course they correspond to those tracts of fibres known as the "posterior longitudinal bundles" in other vertebrates. I imagine, therefore, that the spinal part of their course represents the two antero-lateral descending tracts. The second group of giant-cells, which appears to have some connection with the auditory nerves, may represent "Deiter's nucleus." The whole system is probably the central nervous part of a co-ordination mechanism, which arises entirely in the pro-otic or prosomatic region of the brain—the great co-ordinating and equilibrating region par excellence.
If we turn now to the arthropod it is a striking coincidence that in the crayfish and in the lobster the work of Retzius, of Celesia, of Allen, and of many others demonstrates the existence of an equilibration-mechanism for the swimming movements of the tail-muscles, which is carried out by means of giant-fibres. These giant-fibres are the axis-cylinder processes of giant-cells, situated exclusively in the brain-region, and they run through the whole ventral ganglionic chain in order to supply the muscles of the tail. In the ventral nerve-cord of the crayfish, according to Retzius, two specially large giant-fibres exist, each of which breaks up, at the last abdominal ganglion, into smaller fibres, which pass directly out with the nerves to the tail-fin. Allen has shown that, in addition to these two specially large giant-fibres, there are a number of others, some of which, similarly to the Müllerian fibres of Ammocœtes, cross the middle line, while some do not. Each of these arises from a large nerve-cell and passes to one or other of the last pair of abdominal ganglia. The latter fibres, he says, send off collaterals, while the two specially large giant-fibres do not. The cells which give origin to all these large, long fibres are situated in or in front of the prosomatic region of the brain, similarly to the giant-cells, which give rise to the corresponding Müllerian fibres of Ammocœtes. I do not know how far this system is represented in Limulus or Scorpio.
It is, to my mind, improbable that the Mauthnerian fibres pass out directly as motor fibres to the muscles of the tail-fin; it is more likely that they are conducting paths between the equilibration-mechanism in connection with the VIIIth nerve and the spinal centres for the movements of the tail. Similarly, with respect to the arthropod, it is difficult to believe that the motor fibres for the tail-muscles arise in the brain-region. In either case, the striking coincidence remains that the movements of the tail-end of the body are regulated by means of giant-fibres which arise from giant-cells in the head-region of the body in both the Arthropoda and the lowest members of the Vertebrata.
The meaning of this system of giant-cells and giant-fibres in both classes of animals is well worthy of further investigation.
Another important piece of comparative work which ought to help in the elucidation of this problem is the comparison of the blood- and lymph-corpuscles of the vertebrate with those of the invertebrate groups. As yet, I have not myself made any observations in this direction, and feel that it is inadvisable to discuss the results of others until I know more about the facts from personal observation.
The large and important question of the manner of formation of the vertebrate skin has only been considered to a slight extent. A much more thorough investigation requires to be made into the nature of the skin of the oldest fishes in comparison with the skin of Ammocœtes on the one side, and of Limulus and the Palæostraca on the other.
The muscular system requires further investigation, not so much the different systems of the striated voluntary musculature—for these have been for the most part compared in the two groups of animals in previous chapters—as the involuntary unstriped musculature, about which no word has been said. The origin of the different systems of unstriped muscles in the vertebrate is bound up with the origin of the sympathetic system and its relation to the cranial and sacral visceral systems. The reason why I have not included in this book the consideration of the sympathetic nervous system is on account of the difficulty in finding any such system in Ammocœtes. Also, so far as I know, the distribution of unstriped muscle in Ammocœtes has not been worked out.
One clue has arisen quite recently which is of great importance, and must be worked out in the future, viz. the extraordinary connection which exists between the action of the sympathetic nervous system and the action of adrenalin. This substance, which is obtained from the medullary part of the adrenal or suprarenal glands, when injected into an animal produces the same effects as stimulation of the nerves, which belong to the lumbo-thoracic outflow of visceral nerves, i.e. the system known as the sympathetic nervous system, which is distinct from both the cranial and sacral outflows of visceral nerves. The similarity of its action to stimulation of nerves is entirely confined to the nerves of this sympathetic system, and never resembles that of either the cranial or sacral visceral nerves.
Another most striking fact which confirms the great importance of this connection between the adrenals and the sympathetic nervous system from the point of view of the evolution of the latter system is that the extract of the adrenals always produces the same effect as that of stimulation of the nerves of the sympathetic system, whatever may be the animal from which the extract is obtained. Thus adrenalin obtained from the elasmobranch fishes will produce in the highest mammal all the effects known to occur upon stimulation of the nerves of its sympathetic system.
Further, the cells, which are always associated with the presence of this peculiar substance—adrenalin—stain in a characteristic manner in the presence of chromic salts. In Ammocœtes patches of cells which stain in this manner have been described in connection with blood-vessels in certain parts, so that, although I know of no definite evidence of the existence of cell-groups in Ammocœtes corresponding to the ganglia of the sympathetic system in other vertebrates, it is possible that further investigation into the nature and connection of these "chromaffine" cells may afford a clue to the origin of the sympathetic nervous system. At present it is premature to discuss the question further.
Finally, another test as to the kinship of two animals of different species must be considered more fully than I have been able to do up to the present time. This test is of a totally different nature to any put forth in previous pages. It is known as the "biological test" of relationship, and is the outcome of pathological rather than of physiological or anatomical research. It is possible that this test may prove the most valuable of all. At present we do not know sufficiently its limitations and its sources of error, especially in the case of cold-blooded animals, to be able to look upon it as decisive in a problem of the kind considered in this book.
The nature of this test is as follows: It has been found that the serum of the blood of another animal, when injected in sufficient quantity into a rabbit, will cause such a change in the serum of that rabbit's blood that when it is added to the serum of the other animal a copious precipitate is formed, although the serum of normal rabbit's blood when mixed with that of another animal will cause no precipitate whatever. This extraordinary production of a precipitate in the one case and not in the other indicates the production of some new substance in the rabbit's serum in consequence of the introduction of the foreign serum into the rabbit, which brings about a precipitate when the rabbit's serum containing it is mixed with the serum originally injected. The barbarous name "antibody" has been used to express this supposed substance in accordance with the meaning of such a word as "antitoxin," which has been a long time in use in connection with preventive remedies against pathogenic bacteria. Now, it is found that the rabbit's serum containing a particular "antibody" will cause a precipitate only when added to the serum of the blood of the animal from which the "antibody" was produced or to the serum of the blood of a nearly related animal.
Further, if that animal is closely related a precipitate will be formed nearly as copious as with the original serum, if more distantly related a cloudiness will occur rather than a precipitate, and if the relationship is still more distant the mixture of the two sera will remain absolutely clear. Thus this test demonstrates the close relationship of man to the anthropoid apes and his more distant relationship to monkeys in general. By this method very evident blood-relationships have been demonstrated, especially between members of the Mammalia.
I therefore started upon an investigation into the possibility of proving relationship in this way between Limulus and Ammocœtes, with the kind assistance of Mr. Graham Smith. I must confess I was not sanguine of success, as I thought the distance between Limulus and Ammocœtes was too great. Dr. Lee, of New York, kindly provided me with most excellent serum of Limulus, and the first experiments showed that the anti-serum of Limulus gave a most powerful precipitate with its own serum. Graham Smith then tried this anti-serum of Limulus with the serum of Ammocœtes, and to his surprise, and mine, he obtained a distinct cloudiness, indicative of a relationship between the two animals. This, however, is not considered sufficient, the reverse experiment must also succeed. I therefore, with Graham Smith, obtained a considerable amount of blood from the adult lampreys at Brandon, and produced an anti-serum of Petromyzon, which gave some precipitate with its own serum, but not a very powerful one. This anti-serum tried with Limulus gave no result whatever, but at the same time it gave no result with serum from Ammocœtes, so that the experiment not only showed that Petromyzon was not related to Limulus, but also was not related to its own larval form, which is absurd.
Considerable difficulties were encountered in preparing the Petromyzon anti-serum owing to the extreme toxic character of the lamprey's serum to the rabbit; in this respect it resembled that of the eel. It is possible that the failure of the lamprey's anti-serum was due to the necessity of heating the serum sufficiently to do away with its toxicity before injecting it into the rabbit. At this point the experiments have been at present left. It will require a long and careful investigation before it is possible to speak decisively one way or the other. At present the experiment is positive to a certain extent, and also negative; but the latter proves too much, for it proves that the larva is not related to the adult.
Some day I hope this "biological test" will be of use for determining the relationships of the Tunicata, the Enteropneusta, Amphioxus, etc., as well as of Limulus and Ammocœtes.
The origin of Vertebrates from a Palæostracan stock, as put forward in this book, gives no indication of the systematic position of the Tunicata or Enteropneusta. Neither the Tunicata nor Amphioxus can by any possibility be on the direct line of ascent from the invertebrate to the vertebrate. They must both be looked upon as persistent failures, relics of the time when the great change to the vertebrate took place. The Enteropneusta are on a different footing; in their case any evidence of affinity with vertebrates is very much more doubtful.
The observer Spengel, who has made the most exhaustive study of these strange forms, rejects in toto any connection with vertebrates, and considers them rather as aberrant annelids. The so-called evidence of the tubular central nervous system is worth nothing. There is not the slightest sign of any tubular nervous system in the least resembling that of the vertebrate. It is simply that in one place of the collar-region the piece of skin containing the dorsal nerve of the animal, owing to the formation of the collar, is folded, and thus forms just at this region a short tube. My theory explains in a natural manner every portion of the elaborate and complicated tube of the vertebrate central nervous system. In the Balanoglossus theory the evolution of the vertebrate tube in all its details from this collar-fold is simple guesswork, without any reasonable standpoint. Similarly, the small closed diverticulum of the gut in Balanoglossus, which is dignified with the name of "notochord," has no right to the name. As I have already said, it may help to understand why the notochord has such a peculiar structure, but it gives no help to understanding the peculiar position of the notochord. The only really striking resemblance is between the gill-slits of Amphioxus and of the Enteropneusta. In this comparison there is a very great difficulty, very similar to that of the original attempts to derive vertebrates from annelids—the gill-slits open ventrally in the one animal and dorsally in the other. In both animals an atrial cavity exists which is formed by pleural folds, and in these pleural folds the gonads are situated so that the similarity of the two branchial chambers seems at first sight very complete. In the Enteropneusta, however, there are certain forms—Ptychodera—in which these pleural folds have not met in the mid-line in this branchial region, and in these it is plainly visible that these folds, with their gonads, spring from the ventral mid-line and arch over the dorsal region of the body. Equally clearly Amphioxus shows that its pleural folds, with the gonads, spring from the dorsal side of the animal, and grow ventralwards until they fuse in the ventral mid-line (cf. Fig. 168).
As far, then, as this one single striking similarity between Amphioxus and the Enteropneusta is concerned it necessitates the reversal of dorsal and ventral surfaces to bring the two branchial chambers into harmony.
Fig. 168.—Diagram illustrating the Position of the Pleural Folds and Gonads in Ptychodera (A) and Amphioxus (B) respectively.
Al., alimentary canal; D.A., dorsal vessel; V.A., ventral vessel; g., gonads; NC., notochord; C.N.S., central nervous system.
In a mud-dwelling animal, like Balanoglossus, which possesses no appendages, no special sense-organs, it seems likely enough that ventral and dorsal may be terms of no particular meaning, and consequently what is called ventral in Balanoglossus may correspond to what is dorsal in Amphioxus; in this way the branchial regions of the two animals may be closely compared. Such comparison, however, immediately upsets the whole argument of the vertebrate nature of Balanoglossus based on the relative position of the central nervous system and gut, for now that part of its nervous system which is looked upon as the central nervous system in Balanoglossus is ventral to the gut, just as in a worm-like animal, and not dorsal to it as in a vertebrate.
There is absolutely no possibility whatever of making such a detailed comparison between Balanoglossus and any vertebrate, as I have done between a particular kind of arthropod and Ammocœtes. In the latter case not only the topographical anatomy of the organs in the two animals is the same, but the comparison is valid even to microscopical structure. In the former case the origin of almost all the vertebrate organs is absolutely hypothetical, no clue is given in Balanoglossus, not even to the segmented nature of the vertebrate. The same holds good with the evidence from Embryology and from Palæontology. I have pointed out how strongly the evidence in both cases confirms that of Comparative Anatomy. In neither case is the strength of the evidence for Balanoglossus in the slightest degree comparable. In Embryology an attempt has been made to compare the origin of the cœlom in Amphioxus and in Balanoglossus. In Palæontology there is nothing, only an assumption that in the Cambrian and Lower Silurian times a whole series of animals were evolved between Balanoglossus and the earliest armoured fishes, which have left no trace, although they were able to hold their own against the dominant Palæostracan race. The strangeness of this conception is that, when they do appear, they are fully armoured, as in Pteraspis and Cephalaspis, and it is extremely hard luck for the believers in the Balanoglossus theory that no intermediate less armoured forms have been found, especially in consideration of the fact that the theory of the origin from the Palæostracan does not require such intermediate forms, but finds that those already discovered exactly fulfil its requirements.
One difficulty in the way of accepting the theory which I have advocated is perhaps the existence of the Tunicata. I cannot see that they show any affinities to the Arthropoda, and yet they are looked upon as allied to the Vertebrata. I can only conclude that both they and Amphioxus arose late, after the vertebrate stock had become well established, so that in their degenerated condition they give indications of their vertebrate ancestry and not of their more remote arthropod ancestry.
In conclusion, the way in which vertebrates arose on the earth as suggested in this book carries with it many important far-reaching conclusions with respect to the whole problem of Evolution.
When the study of Embryology began, great hopes were entertained that by its means it would be possible to discover the pedigree of every group of animals, and for this end all the stages of development in all groups of animals were sought for and, as far as possible, studied. It was soon found, however, that the interpretation of what was seen was so difficult, as to give rise to all manner of views, depending upon the idiosyncracy of the observer. At his will he decided whether any appearance was cœnogenetic or palingenetic, with the result that, in the minds of many, embryology has failed to afford the desired clue.
At the same time, the geological record was looked upon as too imperfect to afford any real help; it was said, and is said, that the Cambrian and pre-Cambrian periods were so immense, and the animals discovered in the lower Silurian so highly organized, as to compel us to ascribe the origination of all the present-day groups to this immense early period, the animals of which have left no trace of their existence as fossils.
In consequence of, or at all events following upon, the supposed failure of embryology and of geology to solve the problem of the sequence of evolution of animal life, a new theory has arisen, which goes very near to the denial of evolution altogether. This is the theory of parallel development. It discards the old picture of a genealogical tree with main branches arising at different heights, these again branching and branching into smaller and smaller twigs, and substitutes instead the picture of the ribs of a fan, every rib running independently of every other, each group represented by a rib reaching its highest development on the circumference of the fan and coming nearer and nearer to a common point at the handle of the fan. This point of convergence, where all the groups ultimately meet, is so far back as to reach to the lowest living organisms.
This, in my opinion, unscientific and inconceivable suggestion has arisen largely in consequence of a conception which has become firmly fixed in the minds of very many writers on this subject—the conception that in the evolution of every group, the higher members of the group are the most specialized in the peculiarities of that group, and it is impossible to obtain a new group with different peculiarities from such specialized members. If, then, a higher group is to arise from a lower, it must arise from the generalized members of that lower group, in other words, from the lowest members or those nearly akin to the next lower group.
Similarly, the highest members of this latter group are too specialized, and again we must go to the more generalized members of the group. In this way each separate specialized group is put on one side, and so the conception of parallel development comes into being.
The evidence given in this book dealing with the origin of vertebrates strikes at the foundations of this belief, for it presents an image of the sequence of evolution of animal forms in orderly upward progress, caused by the struggle for existence among the members of the race dominant at the time, which brought about the origin of the next higher group not from the lowest members of the dominant group, but from some one of the higher members of that group.
The great factor in evolution has been throughout the growth of the central nervous system; from that group of animals which possessed the highest nervous system evolved up to that time the next higher group must have arisen.
In this way we can trace without a break, always following out the same law, the evolution of man from the mammal, the mammal from the reptile, the reptile from the amphibian, the amphibian from the fish, the fish from the arthropod, the arthropod from the annelid, and we may be hopeful that the same law will enable us to arrange in orderly sequence all the groups in the animal kingdom.
This very same law of the paramount importance of the development of the central nervous system for all upward progress will, I firmly believe, lead to the establishment of a new and more fruitful embryology, the leading feature of which will be, as suggested in the last chapter, not the attempt to derive from the blastula three germ-layers common to all animals, but rather two sets of organs—those which are governed by the nervous system and those which are not—and thus by means of the development of the central nervous system obtain from embryology surer indications of relationship than are given at present.
The great law of recapitulation, which asserts that the past history of the race is indicated more or less in the development of each individual, a law which of late years has fallen somewhat into disrepute, owing especially to the difficulty of interpreting the embryological history of the vertebrate, is triumphantly vindicated by the theory put forward in this book. Each separate vertebrate organ, one after the other, as shown in the last chapter, indicates in its development the manner in which it arose from the corresponding organ of the arthropod. There is no failure in the evidence of embryology, the failure is in the interpretation thereof.
So, too, my theory vindicates the geological method. There is no failure here; on the contrary, the record of the rocks proclaims with startling clearness not only the sequence of evolution in the vertebrate kingdom itself, but the origin of the vertebrate from the most highly-developed invertebrate race.
The study of the comparative anatomy of organs down to the finest details has always been a most important aid in finding out relationship between animals or groups of animals. My theory endorses this view to the uttermost, and especially indicates the study of the central nervous system and its outgoing nerves as that comparative study which is most likely to afford valuable results.
As for the individual, so for the nation; as for the nation, so for the race; the law of evolution teaches that in all cases brain-power wins. Throughout, from the dawn of animal life up to the present day, the evidence given in this book suggests that the same law has always held. In all cases, upward progress is associated with a development of the central nervous system.
The law for the whole animal kingdom is the same as for the individual. "Success in this world depends upon brains."