Chapter 6

Like the fundamental principle of comparative anatomy in its sphere, the Law of Recapitulation, formulated as a summary description of the foregoing and similar facts, is one that holds true throughout the entire range of embryology and for every division of the animal series, however large or small. We have discussed its broader application, and now we may take up some of the more or less special cases mentioned in the earlier section of the present chapter, to see how it may work in detail.

The flounder was noted as a variant of the fish theme which seemed to be a descendant of a symmetrical ancestor because its structural plan was like that of other bony fishes. If this be true, and if in its development a flounder must review its mode of evolution as a species, the young fish ought to be symmetrical; and it actually is. The grotesque skate and hammerhead shark were demonstrated to be derivatives of a simpler type of shark; their embryos are practically indistinguishable from those of ordinary dogfish and sharks.

Among the jointed animals a wealth of interesting material is found by the embryologist. All crabs seemed to be modified lobsterlike creatures; to confirm this interpretation, based solely upon details of adult structure, young crabs pass through a stage when to all intents and purposes they are counterparts of lobsters. Even the twisted hermit crab, which has a soft-skinned hinder part coiled to fit the curve of the snail shell used as a protection, is symmetrical and lobster-like when it is a larva.

Among the insects many examples occur that are already familiar to every one. The egg of a common house-fly hatches into a larva called a maggot; in this condition the body destined to become the vastly different fly is composed of soft-skinned segments very much alike and also similar to the joints of a worm. Comparative anatomy demonstrates that the fly and all other insects have arisen from wormlike ancestors, whose originally similar segments later differentiated in various ways to become the diverse segments of adult insects; the embryonic history of flies of to-day corroborates these assertions, in so far as every individual fly actually does become a wormlike larva before it changes into the final and complete adult insect. The other kinds of insects are equally striking in their life-histories. All beetles, such as the potato bug and June bug, develop from grubs which, like the maggots of flies, are similar to worms in numerous respects. Butterflies and moths pass through a caterpillar stage having even more striking resemblances to worms. All the larvæ of insects are therefore like one another, and like worms also, in certain fundamental characters of internal and external structure; so the conclusion that the whole group of insects has arisen by evolution from more primitive ancestors resembling the worms of to-day is based upon mutually explanatory details of comparative anatomy and embryology.

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Let us now turn back to some of the earlier pages of the embryological record which we passed over in order that we might translate the later portions dealing with more familiar and intelligible structures like gills. Before the egg of the frog becomes an elliptical mass of cells, it is at one time a double-walled sac enclosing a central cavity; in this stage it is called a _gastrula_. Tracing back the mode of its formation, we find that it is produced from a hollow sphere of fewer cells that are essentially alike; this stage also is so important that the special term _blastula_ is applied to it. Still earlier, there are fewer cells--128 or thereabouts, 64, 32, 16, 8, 4, 2, and 1. In other words, the starting point in the development of the frog is a _single biological unit_; this divides and its products redivide to constitute the many-celled blastula and the double-walled gastrula. All the other animals we have mentioned begin like the frog, as eggs which are single cells and nothing more; they too pass on to become blastulæ and gastrulæ, similar to those of the frog in all essential respects, particularly as regards the nature of the organs produced by each of the two primary layers, and the mode of their formation. Does the occurrence of blastulæ and gastrulæ and one-celled beginnings mean that the higher animals composed of numerous and much differentiated cells have evolved in company from two-layered saccular ancestors which were themselves the descendants of spherical colonies of like cells, and ultimately of one-celled animals?

Comparative anatomy has asserted that this is so, as we have already learned, for it finds that adult animals array themselves at different levels of a scale beginning at the bottom with the protozoa, continuing on to the two-layered animals like _Hydra_ and jellyfish and sea-anemones, and then extending upwards to the region of the more complicated invertebrates and vertebrates. It was difficult perhaps to believe that these successive grades of organic structure indicated an order of evolution, because it seemed impossible that an animal so simple as a protozoan could produce offspring with the complex organization of a frog or a cat, even in long ages. But development delivers its evidence relating to this matter with telling and impressive force. How can we doubt the possibility of an evolution of higher animals from ancestors as simple as _Hydra_ and _Amoeba_ when a frog and a cat, like all other complicated organisms, begin individual existence as single cells, and pass through gastrula stages? If we deny it, we contradict the evidence of our senses, for the development is actually accomplished by the transformation of a single cell into a double-walled sac, and of this into different and more intricate organic mechanisms. The process _can_ take place, for it _does_ take place. Not until the investigator becomes familiar with a wide range of diverse animals and the peculiar qualities of their similar early stages, can he estimate the tremendous weight of the facts of comparative embryology. Were the statement iterated and reiterated on every page and in every paragraph, there would be no undue emphasis put upon the astounding fact that the apparently impassable gap between a one-celled animal like _Amoeba_ and a mammal like a cat is actually compassed during the development of the last-named organisms from single cells. The occurrence of gill-slits in the embryos of lizards, birds, and mammals now seems a small thing when compared with the correspondences disclosed by the earliest stages of development. But in spite of their complexity, all the changes of "growing up" are explained and understood by the simple formula that the mode of individual development owes its nature primarily to the hereditary influence of earlier ancestors back to the original animals which were protozoa.

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Embryology as a distinct division of zoölogy has grown out of studies of classification and comparative anatomy. Its beginnings may be found in medieval natural history, for as far back as 1651 Harvey had pointed out that all living things originate from somewhat similar germs, the terse dictum being "Ex ovo omnia." By the end of the eighteenth century many had turned to the study of developing organisms, though their views by no means agreed as to the way an adult was related to the egg. Some, like Bonnet, held that the germ was a minute and complete replica of its parent, which simply unfolded and enlarged like a bud to produce a similar organism. Even if this were true, little would be gained, for it would still remain unknown how the germinal miniature originated to be just what it was conceived and assumed to be. Wolff was the originator of the view that is now practically universal among naturalists, namely, that development is a real process of transformation from simpler to more complex conditions.

The subject of comparative embryology grew rapidly during the nineteenth century as the field of comparative anatomy became better known, and when naturalists became interested in animals, not only as specific types, but also as the finished products of an intricate series of transformations. When life-histories were more closely compared, the meaning of the resemblances between early stages of diverse adult organisms was read by the same method which in comparative anatomy finds that consanguinity is expressed by resemblance. The great law of recapitulation, stated in one form by Von Baer and more definitely by Haeckel in the terms employed in the foregoing sections, was for a time too freely used and too rigidly applied by naturalists whose enthusiasm clouded their judgment. A strong reaction set in during the latter part of the nineteenth century, when attention was directed to the anachronisms of the embryonic record and to the alterations that are the results of larval or embryonic adaptation as short cuts in development. Nevertheless, it is not seriously questioned, I believe, that the main facts of a single life-history owe their nature to the past evolution of the species to which a given animal belongs.

Nowadays the problems in this well-organized department are concerned not only with more accurate accounts of the development of animals, but also with the mechanics of development, with the relative value of external and internal influences, and above all with the physical basis of inheritance. It is clear that the factors that direct the development of a wood frog's egg so that it becomes a wood-frog and not a tree-toad must lie in the egg itself, as derivatives from the two parent organisms. Weismann and his followers have proved that a peculiar substance in the nuclei of the egg and its daughter-products contains the essential factors of development, whatever these may be. Experiments dealing with the phenomena of heredity in pure and mixed breeds have largely confirmed Weismann's doctrine, and they have prepared the way for a deeper investigation of the marvelous process of biological inheritance.

However much he may be interested in the details of embryological science, the general student of natural history is more concerned with the bearing of its primary laws upon the great problem of evolution. In the foregoing brief review of the fundamental facts and principles of this subject, the purpose has been to show how the phenomena of development are viewed by men of science, and how they take their place in the doctrine of organic evolution. And it has also been made plain that comparative anatomy and comparative embryology support and supplement one another in countless ways and places, although each in itself is a complete demonstration that evolution is a real and a natural process.

III

THE EVIDENCE OF FOSSIL REMAINS

Few natural objects appeal to the interest and imagination of the student with more force than the fragments of animals and plants released from the rocks where they have been entombed for ages. Our lives are so brief that it is impossible for us to comprehend the full duration of the slow process which constructed the burial shrouds of these creatures of long ago. We try to picture the earth and its inhabitants as they were when lizards were the highest forms of animals, and we wonder how life was lived in the dense forests of the coal age. Science can never learn all about the ancient history of the earth and of the organisms of bygone times; yet it has been able to accomplish much through its endeavors to reconstruct the past, for its method is one by which sure results can always be obtained whenever there are definite facts with which it can work. In our present study of evolution we reach the point when we must examine the testimony of the rocks, and the results and methods of that department of knowledge called palæontology, which is concerned with fossils and their interpretation.

The word "palæontology" means literally the "science of living things of long ago." It deals directly with the remains of animals and plants found as fossils, and it interprets them through its knowledge of the way modern animals are constructed and of the changes the earth's crust has undergone. A skull-like object may be found in a coal field and may come into the hands of the palæontologist: from his acquaintance with the head skeletons of recent types he will be able to assign the extinct creature which possessed the skull to a definite place in the animal scale and to understand its nearer or wider affinities with other animals of later times and of earlier epochs. In doing these things palæontology employs the methods of comparative anatomy with which we have now become familiar. In the performance of its other tasks, however, palæontology must work independently. It is necessary to know when a fossilized animal lived, not that its time need be measured by an absolute number of a few thousands or millions of years antedating our own era, for that is impossible. But the important thing is to know its relative age, and whether it preceded or followed other similar animals of its own group or of different divisions. The rocks themselves must be understood, how they have been formed and how they are related in mineralogical nature and in historical succession. Palæontology also deals with a number of subjects that are not in themselves biological, such as the combination of circumstances necessary for the adequate preservation of fossil relics. In so far as it is concerned with physical matters, as contrasted with strictly biological data, it is one with geology. Indeed, the investigators in these two departments must always work side by side and render mutual assistance to one another in countless ways, for each division needs the results of the other in order to accomplish its own distinct purposes. It must be evident to every one that it is impossible to understand the meaning of fossils and the place of the testimony of the rocks in the doctrine of evolution without knowing much about the geological history of the earth and the influences at work in the past. For these reasons palæontology differs somewhat from the other divisions of zoölogy where direct observation gives the materials for arrangement and study; in this case the individual data, that is, the fossil fragments themselves, can be made available only through a knowledge of their exact situations, of the reasons for their occurrence in particular places in the rock series and of the way rocks themselves are constructed and worked over by natural agencies. Our task is therefore twofold: certain physical matters of a geological nature must first be investigated before the biological facts can be described.

No doubt most people feel justified in believing that the whole doctrine of evolution must stand or fall according to the cogency of the palæontological evidences. Plain common sense says that the owners of shelly or bony fragments found in the deeply-laid strata of the earth must have lived countless years ago, and if the evolutionist asserts that primitive organic forms of ancient times have produced changed descendants of later times, it would seem that fossil evidence would be supremely and overwhelmingly important. It is true, of course, that this evidence is peculiarly significant, because in some ways it is more direct than that of the other categories already outlined. But it must not be forgotten that the doctrine is already securely founded upon the basic principles of anatomy and embryology. Science must treat the data of this category by different methods and must view them in different ways. Therefore we are interested in palæontology because of the way it tells the story of evolution in its own words, and because we are justified in expecting that its account should include a description of some such order of events as that revealed by the developing embryos of modern organisms and that demonstrated by the comparative anatomy of the varied species of adult animals.

It is true that palæontology gives direct testimony about the evolutionary succession of animals in geologic time. But we now know that embryology is even more direct in its proof that organic transformation is natural and real; while at the same time there is a completeness in the full series of developmental stages connecting the one-celled egg with the adult creature that must be forever lacking in the case of the fossil sequence of species. If paragraphs and pages are missing from the brief embryonic recapitulation, whole chapters and volumes of the fossil series have been lost for all time. The investigators whose task it has been to decipher the story of the earth's evolution have had to meet numerous and exasperating difficulties which do not confront the embryologist and anatomist who study living materials. Nevertheless the library of palæontological documents is one which has been founded for over a century, and it has grown fast during recent decades, so that consistent accounts may now be read of the great changes in organic life as the earth has altered and grown older. And in all this record, there is not a single line or word of fact that contradicts evolution. What definite evidence there is tells uniformly in favor of the doctrine, for it is possible, in the first place, to work out the order of succession of many of the great groups of animals, and this order is found to be the same as that established by the other bodies of evidence. Secondly, some fossil groups are astonishingly complete, so that the ancient history of a form like the horse can be written with something approaching fullness. Finally, the remains of certain animals have been found so situated in geological ways, and so constructed anatomically, that the zoölogist is justified in denoting them "missing links," because they seem to have been intermediate between groups that have diverged so widely during recent epochs as to render their common ancestry scarcely credible.

With these general results in mind, we must now become acquainted with such subjects as the interpretation of fossils, the causes for the incompleteness of the series, the conditions for fossilization, the forces of geological nature, and other matters that make the fossils themselves intelligible as scientific evidence.

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Many views have been entertained regarding the actual nature of the relics of antiquity exhumed from the rocks or exposed upon the surface by the wear and tear of natural agencies. In earliest times such things were variously considered as curious freaks of geological formation, as sports of nature, or as the remains of the slain left upon the battle-ground of mythical Titans. Some of the Greeks supposed that fossils were parts of animals formed in the bowels of the earth by a process of spontaneous generation, which had died before they could make their way to the surface. They were sometimes described as the bones of creatures stranded upon the dry land by tidal waves, or by some such catastrophe as the traditional flood of the scriptures. In medieval times, and even in our own day, some people who have been opposed to the acceptance of any portion of the doctrine of evolution have actually defended the view that the things called fossils were never the shells or bones of animals living in bygone times, but that they only simulate such things and have been created as such together with the layers of rock from which they may have been taken. If we employed the same arguments in dealing with the broken fragments of vases and jewelry taken from the Egyptian tombs or from the buried ruins of Pompeii, we would have to believe that such pieces were created as fragments and that they were never portions of complete objects, just because no one alive to-day has ever seen the perfect vessel or bracelet fashioned so long ago. Common sense directs us to discard such a fantastic interpretation in favor of the view that fossils are what they seem to be--simply relics of creatures that lived when the earth was younger.

Until this common sense view was adopted there was no science of palæontology. Cuvier was the first great naturalist to devote particular attention to the mainly unrelated and unverified facts that had been discovered before his time. He was truly the originator of this branch of zoölogy, for he brought together the observations of earlier men and extended his own studies widely and surely, emphasizing particularly the necessity for noting carefully the geological situation of a fossil in rocks of an older or later period of formation. His great result was the demonstration that many groups of animals existed in earlier ages that seem to have no descendants of the same nature to-day, and also that many or most of our modern groups are not represented in the earliest formed sedimentary rocks, although these recent forms possess hard parts which would surely be present somewhere in these levels if the animals actually existed in those times. But the meaning of these facts escaped Cuvier's mind. He was a believer in special creation, like Linnæus and all but a few among his predecessors, and he explained the diversity of the faunas of different geological times in what seems to us a very simple and naïve way. In the beginning, he held, when the world was created, it was furnished with a complete set of animals and plants. Then some great upheaval of nature occurred which overwhelmed and destroyed all living creatures. The Creator then, in Cuvier's view, proceeded to construct a new series of animals and plants, which were not identical with those of the former time, but were created according to the same general working plans or architectural schemes employed before. Another cataclysm was supposed to have occurred, which destroyed the second series of organisms and laid a new covering of rocks over the earth's surface for a subsequent period of relative quiet; and so the process was continued. By this account, Cuvier endeavored to reconcile the doctrine of supernatural creation and intervention with the obvious facts that organisms have differed at various times in the earth's history. Although he saw that animals of successive periods displayed similar structures, like the skeleton of vertebrates, which testified to some connection, Cuvier could not bring himself to believe that this connection was a genealogical one.

Mainly through the influence of the renowned English man of science, Charles Lyell, the students of the earth came to the conclusion that its manifold structures had developed by a slow and orderly process that was entirely natural; for they found no evidence of any sudden and drastic world-wide remodeling such as that postulated by the Cuvierian hypothesis of catastrophe. The battle waged for many years; but now naturalists believe that the forces, of nature, whose workings may be seen on all sides at the present time, have reconstructed the continents and ocean beds in the past in the same way that they work to-day. The long name of "uniformitarianism" is given to Lyell's doctrine, which has exerted an influence upon knowledge far outside the department of geology. Darwin tells us how much he himself was impressed by it, and how it led him to study the factors at work upon organic things to see if he could discern evidence of a biological uniformitarianism, according to which the past history of living things might be interpreted through an understanding of their present lives.

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