Popular Science Monthly/Volume 82/May 1913/A Problem in Evolution

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THE

POPULAR SCIENCE

MONTHLY

 

MAY, 1913




A PROBLEM IN EVOLUTION
By Professor WILLIAM PATTEN

DARTMOUTH COLLEGE

I. The Scope of Morphology

WHEN evolution became the accepted doctrine of the natural sciences, it was incumbent on the zoologists to construct a genealogy, or phylogeny, of the animal kingdom, one that would reveal the great highways of evolution and disclose the historic sequence in the rise of new kinds of animals, from the dawn of life to the present time, from the protozoon to man.

A complete genealogy of the animal kingdom, or even one as nearly complete as the biologist may reasonably hope to produce, would be of great value. It would represent the measure of our evidence that animal evolution had taken place. It would constitute the framework of the entire science of zoology, for at the root of every problem in anatomy, embryology, physiology and paleontology is the question of origin. It would be a moving historic picture of evolution, exhibiting the successive stages of the process and the creative value of the accompanying conditions.

The experimental methods of the laboratory and breeding-pen may measure the pliability of life under the momentary stress of artificial conditions, but only the phylogenetic history of large groups of animals, extending over immeasurably long periods of time, under various environments, can indicate the manner in which evolution actually did take place; whether it was slow or rapid, uniformly progressive or spasmodic, direct or tortuous; whether it drifted with the ebb and flow of circumstance, or opportunely threaded its way through an unyielding, but slowly changing, environment. And the manner in which evolution has taken place is the only thing that can indicate the creative value of the accompanying conditions; whether or no, or to what extent, evolution has been the product of external conditions, of natural selection, or of the fortuitous shuffling of "hereditary units"; or whether, after giving due credit to these agents, we must go still deeper and look for the primary creative factors in a universal, persistent power of growth, and in an internal, automatic adjustment of part to part and part to the whole, which go their ways in spite of the fluctuating influences of heredity, selection and external environment, moving with an increasing momentum of their own along definite paths toward definite ends that are predetermined solely by the nature of organizable materials.

In my judgment, the answer to these problems must come, if at all, from morphology, treated as the formal expression of the dynamics of organic growth, and from the history of its progress as portrayed by the embryology of the individual and the phylogeny of the race. The answer should tell us whether biology shall serve merely to record the phenomena of life, or whether, within its own sphere, it may reasonably hope to attain in some measure the dignity of prophet and master.

 

II. Missing Links in the Genealogy of the Animal Kingdom

To the layman, the most serious defect in our phylogenetic record is the absence of a connecting link between man and the apes. To the morphologist, dealing with broader aspects of the problem, it is the absence of a whole series of connecting links between the vertebrates and the invertebrates.

The evolution of the vertebrates has extended over many millions of years, from at least the beginning of the Devonian period to the present moment; but during all that time no change in the general plan of their structure has taken place. The vertebrates form an essentially continuous, united group, for the differences between the most widely separated members, as, for example, a fish and a human being, are differences in degree, not in kind; differences in the details of structure, and in the relative location and size of organs and parts of organs, or in the measure of their functions; none whatever in their serial location, in their fundamental structure, or in their mode of growth. Every important part, for example, of the digestive, excretory and reproductive systems, and of the skull, nose, eye, ear, heart and brain of a fish is readily recognized by the trained anatomist in the corresponding organs of man.

It is this broad uniformity in fundamental structure, varied by a continuous series of transitions in organic details, and the historic record of their progress by paleontology, that is the chief measure of blood relationship and community of descent.

The first vertebrates to make their appearance on the face of the earth were fishes. They are still wonderfully well preserved as fossils in the rocks of the Devonian period ; and it is perfectly clear that, when alive, they were practically identical in structure with certain fishes now living. But we have no records of true fishes from an earlier period ; from this point downwards into the abyss of time, without warning or apparent reason, the vertebrates drop from the records, although the records themselves remain, and they contain, both after that period and for an immeasurably long time previous to it, a full, even a detailed, account of nearly every known group of invertebrates. Why do the vertebrates disappear at this point? Where did they come from? What kind of invertebrates were their ancestors? How did the anatomical structures peculiar to all vertebrates originate? Here- tofore no one has been able to give even an approximately satisfactory answer to these questions. Here indeed is a great gap in the evolution of the animal kingdom. It is not merely one link that is missing; the whole middle section, perhaps two thirds of the entire animal kingdom, is either absent, or, if present, it has not been recognized and properly located. As there is no apparent resemblance between the structural plan of any known invertebrate and that of a vertebrate, there is no way of uniting the higher animals with the lower; no way of deciding what was the great trunk line of evolution.

This is a serious defect in the very foundations of the biological sciences. While it remains we are compelled to admit that, with all our boasted schemes of classification according to genetic relationships, the whole class of vertebrates hangs in mid-air over an unknown and apparently inaccessible abyss ; that we are totally ignorant of the great creative period in the evolution of the highest type of animals; that we know nothing of the way in which the fundamental structural features of man arose; that we have no basis for the interpretation of the early stages of his embryonic development; and no clue to the initial significance of a single one of his characteristic organs, such as the mouth, notochord, skeleton, lungs, jaws, appendages, heart, thymus, thyroids, pituitary body, pineal gland, sense organs and brain!

 

III. The Origin of Vertebrates Abandoned as a Hopeless Problem

During the generation following the appearance of Darwin's "Origin of Species," many attempts were made to bridge this great gap in our knowledge of evolution. The best known theories were defended by the most distinguished zoologists of their time ; but they were, after all, mere suggestions, and their authors were compelled to unite the nearest probable extremes by long arrays of purely imaginary animals. Most of the theories were mutually exclusive; none of them were based on detailed comparisons of several systems of organs; and none of them threw any light on vertebrate anatomy, or afforded even an approximate solution of the real problem.

The vital spark in these theories vanished long ago, but certain basic postulates in them have slowly petrified into the semblance of established facts and have introduced into morphology many false ideas, and a system of nomenclature that implies a knowledge we by no means possess. Many a zoologist, proud of his adherence to sound zoological principles, accepts these familiar terms as evidence that the things so named are really what the names imply, as, for example, the terms dorsal and ventral, right and left, gastrula, archenteron, blastopore and cœlomic pouches. The same subtle transformation of theory into fact is shown by the perpetuation of the view that the notochord is made of endoderin because it arises from the walls of the "archenteron"; and by the one that the primitive streak represents the closed lips of an "Uhrmund."

When real progress along the old lines ceased, the problem came to be regarded as hopeless, largely because it was assumed that the ancestors of the vertebrates were small, soft-bodied animals, unlikely to be preserved as fossils. The "practical" biologist then turned his attention to cytology, experimental evolution and genetics, and the study of morphology and phylogeny became almost as discreditable an occupation, especially in the eyes of the new school of biologists in this country, as the study of metaphysics, or the description of new species.

During this long search for the ancestors of the vertebrates, the arthropods (insects, Crustacea and arachnids), the largest and most highly organized class of invertebrates were altogether excluded, with astonishingly aggressive unanimity, from their due consideration. It is difficult to understand this impenetrable state of mind, for it did not appear to be based on any known facts, or upon any positive evidence whatever. It was apparently due to a widespread conviction that the general trend of evolution in the arthropods did not lead toward the vertebrates, that the arthropods themselves were too highly specialized to give rise to a new type, and to the fact that the hue and cry of the annelid theory was leading the chase in another direction.

 

In view of this situation, it may be readily understood that another attempt to connect the genealogy of the vertebrate stock with that of the invertebrates will now have to contend with a widespread indifference born of repeated failures; with interests already diverted into other channels; and with that first, unreasoning hostility that is the protective attitude of the mind toward any strange idea that threatens to steal away our cherished convictions.

 

IV. First Clues to a New Solution

As long ago as 1889, while working on the development of the eyes of arthropods, the author discovered that the forebrain of the embryo scorpion is gradually covered by an overgrowing fold of skin that converts the brain into a hollow vesicle. During this process, one or two pairs of eyes are transferred from the outer surface of the head to the blind end of a median tube that projects from the membranous roof of the brain.

The details of the whole process by which the eyes were transferred from the outer surface of the head to the inside of the brain were unique in the invertebrates, and so similar to what takes in the formation of the rudimentary pineal eye of vertebrates, that it clearly pointed to some intimate genetic relation between the two groups.

To test what at first sight appeared to be so improbable, a careful study of the anatomy and development of several types of arachnids was made, and, much to our astonishment, it was found that the brain of the arachnids resembled that of the vertebrates in its general shape, in its subdivision into several regions, in the general nature of the functions performed by these regions, and in the character of their appropriate nerves, ganglia and sense organs; that the arachnids possessed skeletal structures comparable, respectively, with the dermal bones, cranium, gill-bars and notochord of vertebrates; and finally it was seen that the development of the embryo and the formation of the germ layers in the arachnids, not only harmonized with, but illuminated the corresponding conditions in the vertebrates.[1]

It was evident that in their fundamental structure the arachnids resembled the vertebrates more than did any other invertebrates; and they resembled them in so many different ways that it became more and more improbable that all these resemblances could be mere coincidences, or could be reasonably accounted for as duplications of structure due to similar functions, or to environment, or to any conceivable cause other than community of origin. Nevertheless, it was hardly possible that the vertebrates came from modern air-breathing scorpions, or spiders, for the lowest vertebrates undoubtedly came from marine animals.

But the modern land arachnids are descendants of a large group of very ancient marine arachnids, the trilobites and merostomata, or giant sea-scorpions, which flourished in the early Cambrian and Ordovician periods, long before any vertebrates were known to exist. They were also found, although in rapidly diminishing numbers, in the two following periods, and often in the very same deposits in which the first vertebrates are found. Moreover, during the Silurian and Devonian PSM V82 D426 Trilobite mesonacis vermontana.pngFig. 1. Trilobite (Mesonacis vermontana Hall). Lower Cambrian. One of the marine Arachnids. periods, and living in intimate association with the declining marine arachnids and the earliest vertebrates, there was known to exist a peculiar class of animals called the Ostracoderms. Very little was known about them, for their remains were fragmentary and their meaning doubtful. Some of the species were regarded as vertebrates, others as invertebrates, and some of them showed a superficial resemblance to the sea-scorpions, as had been noticed by the older anatomists (Fig. 2). But after considerable discussion and a thorough reexamination of the available material, mainly by Huxley and Lankester, it was definitely decided that they were fishes, probably a very specialized kind; of no great importance morphologically. They were then forgotten, or at any rate their very existence remained quite unknown to many zoologists.

In view of their unfishlike appearance and their great antiquity, it is astonishing that no one suggested they might be very primitive vertebrates, dr perhaps an even more remote ancestral stock, until the author did So in his first paper on "The Origin of Vertebrates."[2]

This oversight was largely due to the fact that, at that time, zoologists firmly believed that the most primitive vertebrates were like sharks, or like Amphioxus; that is, animals that had little or no skeleton, while the Ostracoderms were, encased in an extraordinary dermal armor, in some respects quite like that of a trilobite or sea-scorpion. Something appeared to be wrong, either in the facts or in the conclusions. Could it be possible that, after all, the ostracoderms were not true fishes, but a new class of animals, a class intermediate between the fishes and the sea-scorpions? In some respects they looked as much like the one as the other, and they appeared at the right time historically to be the long-sought-for missing links between vertebrates and invertebrates. If they really were transition forms, that would fully account for the resemblance between the modern arachnids and the vertebrates, for in that case both would be derived from the same stock.

Here then, contrary to all our preconceived ideas, was a new solution of an old and very important problem, probably the most important one before the morphologist since Darwin's time. It was evident that this solution of it, if sustained, would lead to more radical changes in the classification of the animal kingdom than any that have been made since the time of Cuvier and Lamarck. Stated concisely, it was as follows: At some time toward the close of the Cambrian period the sea-scorpions probably gave rise to the ostracoderms, and the latter,

PSM V82 D427 Sea scorpion and an ostracoderm.png

Fig. 2. Semi-diagrammatic Figures of a Sea Scorpion (Eurypterid) and an Ostracoderm (Bothriolepis), showing the sequence in the location of the important functions, and the subdivisions of the body corresponding to the subdivisions of the head and brain in the higher vertebrates.

during the Silurian, to the fishes, or first true vertebrates (Fig. 3). This was an entirely new interpretation in phylogeny, but it was not inherently improbable, or contrary to any established facts; indeed, the first demands of the theory were in full accord with the known facts of anatomy, embryology and paleontology. Let us state it again in this way: In their fundamental structure, living arachnids resemble

PSM V82 D428 Genealogical tree of the animal kingdom.png

Fig. 3. Genealogical Tree of the Animal Kingdom, showing the probable genetic relations of the main phyla. The Ostracoderms, the connecting links between the vertebrates and invertebrates, are in solid black.

 

primitive vertebrates. The ancestral arachnids were marine forms, present in the oldest records we have; they nourished in the Cambrian, and were the highest type of animals in existence at that time. The ostracoderms flourished in the following, or Silurian, period and were the highest type of their time. They had some points in common with their predecessors, the marine arachnids, and also with the true fishes that appeared in the next, or Devonian, period, and which were likewise the highest type of their time. The inference is obvious, that the marine arachnids, the ostracoderms, and the fishes, represent three successive stages in the evolution of the animal kingdom, just as in the later periods the fishes, amphibia and mammals represent successive stages in the evolution of the vertebrates. If this inference is correct, then the whole creative period in the evolution of the vertebrate stock should become an open book, because the materials, both living and fossil, with which one can unravel the evolution of the arachnids, are apparently abundant and accessible.

This situation demanded careful investigation, for the issues at stake were very great. In 1889, when this problem for the first time assumed definite shape in my mind, it was apparently impossible to obtain well-preserved ostracoderms in this country, nor did the known remains in any country appear likely to yield more than the superficial details of their anatomy.

We were thus compelled to wait on opportunity, meantime, during the next ten or eleven years, giving our attention to the anatomy and embryology of living arachnids and the lower vertebrates, convinced that a careful study along these lines would ultimately yield definite evidence, one way or the other. The results fully justified this conclusion, for the longer this problem was studied, the more convincingly did it appear that the differences between these two great divisions of the animal kingdom were largely superficial and could be legitimately explained. The resemblances were fundamental; they were found in unexpected places, in independent systems of organs, and they ran through successive stages in the growth of those organs. It was clear that no other group of invertebrates resembled the vertebrates in such a variety of ways, or to the same extent, as did the arachnids; and no one has claimed that the main facts upon which these resemblances were based are not substantially as I have stated them to be.

We had demonstrated, therefore, that the marine arachnids are to be regarded as the most probable ancestors of the vertebrate stock.

 

V. The Ostracoderms

But in spite of all that, there still remained a wide gap between the arachnids and the vertebrates, and to bridge that gap we had to find

PSM V82 D430 A restored cephalapsis.png

Fig. 4. A Restoration of Cephalaspis, based mainly on a species from the lower Old Red Sandstone of Scotland. Ostracoderm.

 

some animals that were intermediate in structure between them, and which made their appearance at some time not later than the Silurian period. The ostracoderms were the only ones known to science in anyway likely to fulfil these requirements; it was, therefore, of the utmost importance to learn something more about these mysterious, extinct animals, for they appeared to contain the solution of the whole problem.

The first opportunity to test this side of the problem came in 1900-01, with a half-year's leave of absence from college duties. The prospects of success, however, were very small, especially for one trained as a laboratory morphologist and embryologist, and without experience in field geology or in paleontology. There was a bare chance that a reexamination from another point of view of the material preserved in the British Museum and other institutions, and that had already been studied by Huxley, Lankester, Traquair, Woodward and others, might reveal some suggestive details overlooked by these past masters of the subject. Failing that, it would be necessary to go into the field and dig up new material that, to serve our purpose, would have to be more perfectly preserved than any that had, by chance, been found in the preceding three quarters of a century. It was not a promising outlook, but the opportunity was gladly accepted.

We first visited the great museums of England and Scotland, and the localities where, in the early thirties, Hugh Miller unearthed the first specimens of these animals known to science, which he afterward described with such remarkable literary skill and enthusiasm in his "Footprints of the Creator," and in the "Old Bed Sandstone."

But the best material available in England and Scotland was provokingly incomplete in regard to the very structures it was most important for us to know about. However, some unexpected and suggestive details were found that greatly added to the already keen excitement of the search (Fig. 4).

It was then decided to visit the famous Silurian quarries on the Island of Oesel, in the Baltic Sea, and the museums of St. Petersburg and Moscow, where many of the Oesel fossils were preserved.

The representatives of the ostracoderms (Tremataspis) found in the Silurian rocks of the Island of Oesel are only about three inches long. But in spite of their small size, they are, in some respects, admirably preserved in a soft, fine-grained limestone; and they promised to yield important data. In fact, the specimens that were obtained there showed the presence of jointed appendages and shell-covered, stalk-like eyes. These structures were unlike those of true fishes and more like what one would expect to find in some free-swimming seascorpion. In that respect the results were highly satisfactory, and added still more evidence in favor of our first supposition (Fig. 5).

But on the whole, we felt that the results of our European expedition were incomplete, because nothing like a full account of the gross PSM V82 D432 Restoration of a tremataspis.pngFig. 5. Restoration of Tremataspis, an Ostracoderm from the upper Silurian rocks of the Island of Oesel, Baltic Sea. anatomy of any one species of ostracoderms was obtained. The organs about the mouth and the location of the principal viscera and of the nervous system were entirely unknown; and it was quite essential to obtain evidence on these points, because it had become increasingly clear that this strange class of animals could not be safely interpreted in terms of either vertebrate or invertebrate anatomy.

On my return to America it was decided to make an attack on a younger branch of the ostracoderms; one that was known to occur in the Devonian rocks of the Bay of Chaleur in Canada.

Four summers were spent in this locality, in search of specimens well enough preserved to be used for anatomical study. Fragments, or in some cases nearly the whole head, could be readily found on the beach at low tide, or by splitting open the disc-shaped nodules that had been washed from the adjacent cliff. But these specimens were generally crushed out of shape, or were so badly weather beaten and worn that they were of little value. We hoped to find in the cliff, which extended along the water front for several miles, unweathered specimens that would show not only the whole head, but the rest of the body, the nature of which at that time was entirely unknown. We accordingly examined with great care the face of the cliff as far as it was accessible; and many tons of rock were dug out of it and split open in order to locate the particular beds that contained the fossils; but the latter appeared to be very irregularly distributed, for we did not succeed in finding a single specimen in that way.

At last we found, close to the foot of the cliffs, a large piece of rock that contained several fossils of the kind we were looking for. It could not have been carried there either by the waves or by drifting ice, for evidently it had fallen quite recently from the rocks above. It did not take long to locate, about twenty-five feet above this fragment, the beds from which it came. To reach them, a path had to be cut into the face of the cliff, and the rocks overlying the beds removed with dynamite, or with pick and bar; an operation not without some danger from rocks that from time to time fell from the crumbling cliffs above. Twice, without warning, fragments weighing some fifty pounds each fell at our feet, knocking the tools, from our hands; and where they came from there were other loose ones, at least ten times as large. Our attention was reluctantly, but impartially, divided between the maintenance of a precarious footing on the face of the sweltering cliff, the threatening rocks overhead, and the treasures at our feet.

The bed was not extensive, but it proved to be literally teeming with these extraordinary animals (Bothriolepis canadensis), and by good fortune they were in a remarkably complete and instructive state of preservation; more so, perhaps, than any other fossils found heretofore (Fig. 6).

The bed had apparently formed the bottom of a shallow, brackish water-pool in which fern-like water plants had been growing, and where many millions of years ago, with the rise and fall of the tides, these specimens had been trapped, together with other species of ostracoderms and several kinds of true fishes.

The soft mud on the bottom of the pool was now turned into a fine-grained, sandy limestone, and in it the fossilized animals were preserved in the very attitudes they had assumed when they ceased to struggle out of the enclosure. One, in its death agony, had plunged into the mud with sufficient force to remain there, head down, in a vertical position. Others were arranged in horizontal series, uniformly headed in a northeast direction. Their heads were turned against a gentle current of water, as was shown by the fact that the tops of all the ferns were pointed in nearly the opposite direction.

Many of these specimens were so well preserved that the shape of the whole body, and many details on its external surface, could be readily observed; the general character and location of the principal sense organs, jaws, gills, stomach, anus and genital openings ascertained; and the neural and hæmal surfaces identified. It was also possible to determine, beyond reasonable doubt, the mode of locomotion, the mode of feeding and the nature of the food.

Of course this message from a remote past was not off-hand legible. After the scores of specimens were safely housed in the laboratory, it required nearly three years to chisel and scratch and brush away the rocky matrix in which they were imbedded, and after that many specimens had to be cut into serial sections with a diamond saw, and the sections polished and varnished to show the arrangement of the internal organs.

PSM V82 D434 Restoration of a bothriolepsis canadensis.png

Fig. 6. Restoration of Bothriolepis Canadensis; an Ostracoderm from the Upper Devonian of the Bay of Chaleur, Province of Quebec, Canada.

 

But "the labor we delight in physics pain," and patient labor is but the measure of hoped-for rewards. In this case the realization exceeded all reasonable expectations; for, if he may be counted fortunate who, by chance, finds some soothsaying relic of early life, how much more than fortunate, how exceedingly blest, is the naturalist who is permitted to lift with his own hands from some ancient storehouse of the earth a long-sought-for treasure, and to awake in it, with the chisel's kiss a spirit from the childhood era of the world!

 

VI. A Critical Period in Organic Evolution

The net result of this fortunate find was to show that the ostracoderms, as had been predicted, were neither vertebrates nor invertebrates, but a class intermediate between the two. They were, in fact, the real missing links in the animal kingdom. The posterior part of the body was membranous and decidedly fish-like in shape; but the contour of the whole animal, especially the head, the nature of the appendages, the eyes and the mode of locomotion, were more like those of the marine scorpions. The gill, or atrial, chamber, and the structure of the dermal skeleton were intermediate in character (Figs. 2 and 6).

But the most important features of all were the long sought for mouth parts, or jaws. They were paired, consisting of four separate jaws, which in chewing, or biting, moved to and from the median line, like the jaws of all known anthropods (Fig. 6, A). They were not unpaired arches moving forward and backward, as they do in all true vertebrates.

To realize the significance of this fact, it must be understood that one of the greatest differences between a vertebrate and an arachnid, or arthropod, is the position and character of the jaws and mouth. In all arthropods and arachnids, the mouth and jaws are primarily located on the same side of the body as the nervous system; food enters the alimentary canal by a passage-way in the floor of the brain; and there may be several pairs of jaw-like legs, which, in chewing, work in a lateral direction to and from the median line. In the adult vertebrates the jaws lie on the opposite side of the body from that on which the brain and nerve cord are located; the food that enters the mouth passes directly into the alimentary canal without going through a passage-way in the floor of the brain; and the jaws are two unpaired arches that work against each other in a forward and backward direction (Fig. 7, A and B).

It is evident that either we are not dealing with the same things in the two classes, or that there has been some change in their relative locations. As a matter of fact, it is part one and part the other; for we have been able to demonstrate: (1) That the nervous system of the arachnids is identical with that of the vertebrates, and lies on the same side of the body. It affords us the fixed base line of comparison. (2) That in embryo vertebrates there are at least three pairs of primitive jaws that arise from the neural surface; they are gradually shifted from their old position to a new one on the opposite side of the head.

PSM V82 D436 Diagram of the median sections of an arachnid and a vertebrate.png

Fig. 7. Semi-diagrammatic Median Sections of an Arachnid and a Vertebrate, showing the location of the principal organs and their probable relations; cerbl., cerebellum; cer. h., cerebral hemispheres; g. h., ganglion of parietal, or pineal, eye; hy., hypophysis; inf., infundibulum; ol. l., olfactory lobes; p. eye., parietal or pineal eye; s. vas., saccus vasculosus; thyr., thyroid.

(3) That the old mouth and œsophagus of the invertebrates is still present, a useless, heretofore unintelligible, rudiment, in its proper place in the floor of the brain of all vertebrates. This old passageway is there known as the infundibulum and the saccus vasculosus; it was completely shut off from the outside world by the closing of the brain tube during the process of embryonic growth. (4) The present mouth of the vertebrates is a new one; it arose through the transformation of the so-called "dorsal organ," or cephalic navel, an organ of obscure function, but one that is present in all arthropods near the place where the new mouth of the vertebrates ultimately appears. In the arthropods it may serve as an organ of attachment, and at certain stages of development affords a temporary passageway into the alimentary canal. In other words, the arachnid brain could not continue to grow in volume, and in the particular way in which it had been growing, without closing up the old mouth, and without forcing the jaws farther and farther apart, until they reached the opposite side of the head. Here they converged toward the cephalic navel, which then became a permanent opening and was utilized as a new mouth to take the place of the old one that was being closed up.

Again, in still other words, we are here dealing with a series of interlocking, internal organic adjustments that ultimately reached a condition of unstable equilibrium. A radical and comparatively rapid readjustment then took place, which led to a new condition of great stability. This situation constituted a great crisis in the evolution of this phylum, perhaps the most momentous crisis in the history of organic evolution. But these revolutionary events were brought about in an intelligible way by the cumulative action of long-established methods of growth, which can be traced through the arachnid stock up to the point where the ensuing events appear inevitable. The actual consummation of them marks the transition from the vertebrates to the invertebrates, and the beginning of a new class of animals. From the nature of the case, the transition must have taken place somewhat rapidly, and it probably occurred at some time during, or before, the Silurian period. The paired jaws of the adult ostracoderms are intelligible only on the assumption that they represent one of the early phylogenetic stages of this process.

 

VII. The Record of Paleozoic Events in Modern Vertebrate Embryos

The general way in which this metamorphosis took place is still recorded in the embryonic history of the vertebrates to the present day, for we can readily observe, in many vertebrate embryos, the shutting up of the old mouth within the brain chamber, the transfer of at least three pairs of jaws to the opposite side of the head, and their union around the new mouth.

In the embryo of the frog, for example, three pairs of rudimentary jaws may be seen after they have reached the hæmal surface and are assembled around the new mouth (Fig. 8). Ultimately the first two

PSM V82 D438 Head of an embryo frog.png

Fig. 8. Head of an Embryo Frog, showing the three pairs of primitive jaws derived from their invertebrate ancestors, and their union to form the unpaired jaws typical of the vertebrates.

pairs fuse to form the fixed upper jaw, and the third pair forms the lower jaw.

The same three pairs of invertebrate jaws are present in the embryonic stages of man. They occupy the same relative positions as in the frog, and their subsequent history is the same (Fig. 9, A). Their presence and mode of growth largely control the architecture of the

PSM V82 D438 Figures illustrating the growth of the human face.png

Fig. 9. Figures Illustrating the Mode of Growth of the Human Face. A, human embryo, a little more than a month old, showing the traces of invertebrate jaws in the mandibular, m. d., maxillary, m. x., and premaxillary, p. mx., arches. B, embryo a little less than two months old, with the preceding parts nearly united. C, mouth of adult, the contours showing its elemental structure.

human face. In rare cases they fail to unite in the normal way, giving rise to such defects in the adult as hare lip, cleft palate and open tear duct (Fig. 9, B). Even in otherwise normal faces, the presence of a pronounced "Cupid's bow" mouth, or prominent lateral lobes on the upper and lower lips, may be regarded as the last outward expression of this primitive condition (Fig. 9, C). Thus it appears that the growth of the human face is still dominated by the same forces that have been observed in the remote arachnid ancestors of the vertebrate stock.

 

There are many other organs of man, especially those in the head, as, for example, the hypophysis cerebri, thymus, thyroid and pineal eye, that have their origin in the arachnids, and it is there that we must look for a better understanding of their significance.

 

VII. Conclusion

When, in the manner indicated above, we have united the upper and lower sections of the animal kingdom to form one great phylum or highway of evolution, we observe an amazing uniformity and stability in the basic structure and mode of growth of all the animals belonging to it. The direct course of evolution within such a definitely and narrowly circumscribed path, during untold millions of years, and under widely different conditions, clearly indicates, in my judgment, that neither the haphazard shuffling of "hereditary units" in sexual reproduction, nor external environment, nor use and disuse, nor natural selection, has been the chief directing agent in organic evolution. At best, they are of secondary, or subordinate, or intermittent value. The primary and ever present creators of organic structure appear to have been a universal, persistent power of growth, rigidly controlled by the inherent nature of organizable materials; and the internal conditions successively created by growth and organic association.

  1. For a fuller description of these conditions, too technical to be repeated here, see "The Evolution of the Vertebrates and their Kin," by W. Patten. P. Blakiston & Co., Philadelphia.
  2. Q. J. Mic. Sc, Vol. 31, 1890.