Chapter XII: , which, according to Kowalewsky and others, give origin to the (2)

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The origin of Vertebrates from a Palæostracan stock, as put forward in this book, gives no indication of the systematic position {494}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, {495}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.

_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 Ammocoetes. 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 {496}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 coelom 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 idiosyncrasy of the observer. At his will he decided whether any appearance was coenogenetic or palingenetic, {497}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 {498}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 {499}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."

{501}BIBLIOGRAPHY AND INDEX OF AUTHORS

GENERAL INDEX

[_The numbers in dark type refer to illustrations_]

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The Origin of VertebratesChapter XII: , which, according to Kowalewsky and others, give origin to the (2)

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