Reptiles also are grouped, like the mammals and birds, as variations about a central theme. An ordinary lizard is perhaps the nearest in form to the remote ancestor from which all have sprung. Some lizards are long and very slender, with all four limbs of greatly reduced size. Others, which are still true lizards, have lost the hind limbs, or even all the legs, as in the "blind worms" of England. One step more, and an animal which has progressed further along a similar line of descent would be a snake. Just as whales as a group are derivable from forms which resemble types belonging to another order, so snakes as an order are to be regarded as more radically altered derivatives of some four-footed lizardlike creature. Alligators are very much like lizards in general form, and their order is a diverging branch from the same limb. Finally the evolution of turtles from the same ancestors is intelligible if we begin with a short stout animal like the so-called "horned toad" of Arizona, and proceed to the soft-sh.e.l.led tortoise of the Mississippi River system; the establishment of a bony armor completes the evolution of the familiar and more characteristic turtle.

Frogs and salamanders const.i.tute another lower cla.s.s, called the amphibia, whose members are gilled during the earlier stages of development. An adult frog is essentially a salamander without a tail and with highly developed hinder limbs. The salamanders differ as regards the number of fishlike gill clefts that they all possess in their young stages, but which disappear entirely or in part during later life. In comparison with the lizard as a typical reptile, a salamander is more primitive in all of its inner organic systems, while in its nearly continuous body, with head and tail gradually merging into the trunk, it also displays a somewhat simpler form of body.

The fishes are the lowest among the common vertebrates, and they offer an abundance of independent testimony as to the truth of the principles of comparative anatomy. The common shark is perhaps the most fundamental form, with a hull-like body undivided into head, trunk, and tail, and from it have originated such peculiar variations as the hammerhead and skate.

Among fishes with true bones, a cod or trout is the most typical in general features. Without ceasing to be true bony fishes, the trunk-fish and cow-fish are adapted by their peculiar characters of spine and armor plate to repel many enemies. The puff fish can take in a great amount of water, when disturbed, so as to become too large to be swallowed by some of its foes, ill.u.s.trating another adaptive modification for self-defense.

The wonderful colors and color patterns of the tropical fish of the reef, or of the open water forms like the mouse-fish of the Sargossa Sea, often render them more or less completely hidden from the foraging enemy. A flounder looks like a fish which was originally symmetrical, but which had come to lie flat on its side upon the bottom, whereupon the eye underneath had left its original place to appear on the upper surface. The difficult and unusual conditions of deep-sea existence have been met by fishes in two ways; some forms possess luminous frilled and weedlike fins, which lure their prey to within easy reach of their jaws, while others have enormous eyes, so as to make use of all possible rays of light in their pursuit of food organisms. But all of these diverse forms are true _fishes_, possessing a common heritage of structure which demonstrates their unity of origin.

The brief review of backboned animals has shown how comprehensive are the principles of relationship. The families and tribes of each order, such as the carnivora, are like branches arising from a single limb; the orders in their turn exhibit common qualities of structure which mean that they have grown from the same antecedents, while even the larger divisions or cla.s.ses of mammals, birds, reptiles, amphibia, and fishes, possess a deep underlying theme whose dominant motif is the backbone, which proves their ultimate unity in ancestry. The greater and lesser branches have reached different levels, for the fish is clearly simpler in its make-up than the highly specialized bird. But the great fact is that structural evidences demonstrating the reality of genealogical affinities are displayed by the entire series of vertebrates; although they differ much or little in many or fewer respects they have one and the same ground-plan.

The lower animals devoid of backbones, and therefore called invertebrates, are not so well-known except to the student of comparative anatomy, because they are not so often met with, and because they are usually very small or microscopic; but in many respects their importance to the evolutionist surpa.s.ses that of the vertebrates. Their structural plans are far more varied, and they range more widely from higher and relatively complicated organisms to the unitary one-celled animals. A knowledge of some of them is essential for our present purpose, which is to learn how sure is the basis for the principles of relationship and how complete is the structural evidence of evolution.

Worms are represented in the minds of most people by the common earthworm or sandworm. The body in either case is made up of a series of segments or joints which agree closely throughout the animal in external appearance and in internal const.i.tution. A section of the digestive tract, a pair of nerve centers, two funnel-like tubes for excretion, and similar blood vessels occur in each portion.

Precisely similar features are displayed by the crustacea, which seem to be so different. Every one is familiar with the appearance of lobsters and crabs. Even in these animals the body is composed of segments, but these are not like one another, nor are they freely movable throughout the body.

Five are fused in all crustacea to make a head; in lower members of the order the eight succeeding segments are free, but in the lobster they are joined together and united with the head. The hinder part of this animal is a long abdomen whose segments remain more primitive and independent.

But in a crab, the whole plan has been modified by the shortening and broadening of the head-thorax, and by the reduction of the abdomen, which is also turned under the anterior part of the body. The internal organic systems are constructed upon a worm plan with modifications. Nearly every one of the segments bears one pair of appendages, which can be referred by their forked nature to the two-parted, oarlike flaps of sandworms, but the appendages of crustacea have departed from their prototypes in functional respects and in details of structure. They are variously feelers, jaws, legs, pincers, and swimming paddles, evolved to serve different purposes, just as the limbs of the vertebrates we have described have become variously arms, wings, flippers and paddles in apes, bats, seals, and whales.

b.u.t.terflies, beetles, bees, and gra.s.shoppers seem at first sight to be entirely different, even though they agree in being more or less segmented. But all of them have heads with four pairs of appendages of the same essential plan, middle thoracic regions of three segments more or less united, bearing three pairs of legs and usually two pairs of wings, while the hinder part is a freely jointed abdomen without real limbs. In these respects the countless varieties of insects agree so that they also like crustacea of various kinds seem to have been derived from wormlike animals with more simply segmented bodies. Indeed spiders and scorpions and their relatives of the group arachnida prove for similar reasons to be derivatives of the same original stock, and own cousins of the insects.

In nearly every one of the invertebrate branches we find representatives which interest us chiefly because they appear to have reached their present condition by retrograde evolution. Barnacles are really crustacea, but they have lost their eyes as well as some other structures that are most useful in animals with a free existence, because they have adopted a fixed mode of life, which has also brought about the loss of the original freely jointed character of the body. A tapeworm as an example of internal parasites is an extremely degenerate form which lacks a digestive tract, because this is superfluous in an animal which lives bathed in the nutrient fluids of its host. Comparing it in other respects with other low wormlike creatures, it appears to be a relative of peculiar simple worms with complete organization and independence of life. All these degenerate forms enlarge our conception of adaptation by adding the essential point that progress is not always the result of evolution. Indeed we have learned this in the case of vestigial and rudimentary structures of higher forms like whales, and now we find that entire animals may degenerate as a result of changes no less adaptive than progressive modifications.

Pa.s.sing by other invertebrate groups made up of species arranged like higher animals in smaller and larger branches according to their degree of fundamental similarity, we arrive at a place in the scale occupied by two-layer animals without the highly developed and clearly differentiated organic systems of the forms above. The fresh-water animal _Hydra_ exemplifies the creatures of this level, where also we find sea-anemones and the soft polyps which form corals and coral reefs by their combined skeletons. _Hydra_ is an animal to which we must return again and again as we study one or another aspect of organic evolution. In general form it is a hollow cylinder closed at one end, by which it attaches itself, while at the upper end, surrounded by a group of tentacles, is the mouth which leads to the central cavity. The wall of this simple body is composed of two layers of cells, between which there is a gelatinous layer rarely invaded by cells. The inner layer lines the central s.p.a.ce into which food organisms are thrust by the tentacles, and it is concerned primarily with digestion. The outer layer comprises cells for protection and sensation primarily. Cells of both layers have muscular prolongations which by their operation enable the whole animal to change its form and to move from one place to another.

It may seem that such an animal is totally unlike any of the higher and more complex types. In certain respects, however, it is identical with the other forms inasmuch as it performs all of the eight biological tasks demanded by nature. It is also similar in so far as its inner layer, like the innermost sheet of cells in higher forms, is concerned with problems of taking and preparing food, while the protective outer layer resembles in function the outermost covering of all animals higher in the scale.

Beyond these a still more fundamental agreement is found in its cellular composition.

At the lower end of the animal scale are organisms which consist of one cell and nothing more. _Amoeba_, to which we must refer again and again, is an example of this group which possesses an overwhelming importance to the comparative student because the origins of all the characteristics of animals higher in the scale are to be found within it. _Amoeba_ itself is a naked ma.s.s of protoplasm, about 1/100 of an inch in diameter, enclosing a nucleus. Its form is not constant during activity, for fingerlike processes called pseudopodia are pushed out tentatively in many directions to be followed as circ.u.mstances direct by the materials of the whole cell body. Other protozoa differ in possessing constant forms, or in having constant vibratile processes, or sh.e.l.ls of some kind, while in still other cases like individuals combine to make colonies which are more or less definite and permanent. Here at the very foot of the organic scale are found animals which seem to be entirely different from those above.

Upon examination they, like _Hydra_, prove to be the same as regards the number and kind of functions they perform, but in structural regards their evolutionary relation to all higher animals is indicated solely by the fact that they are cells composed of protoplasm. Nevertheless the principle which states that resemblance means consanguinity still holds true, for cellular const.i.tution is a unique possession of things of the living world,--something which demonstrates the common origin of all living things just as truly as the "cat-_ness_" of our first series of examples reveals for a smaller group the significance of likeness and the nature of the basic law of comparative anatomy.

Employing a figure of speech, we have climbed down the animal tree from the higher regions where the mammals belong. Having reached the very foot of the trunk we are in a position to review and summarize the evidences which we have discovered all about us as we have descended. The various examples we have mentioned and the groups to which they belong clearly occupy different places in the scale which begins with the protozoa and extends upward to the most complicated and differentiated animals. _Hydra_ takes its place above the protozoa for obvious structural reasons; worms belong to a still higher zone, surpa.s.sed by the more complex jointed animals like crustacea and insects. Far above these are the vertebrates, among which we have already demonstrated the occurrence of different grades of organization, from the fish up to the higher amphibia and reptiles, and beyond in two directions to the diverging birds and mammals.

The basic characteristics of every group in a high position may be traced back to some one or another of the divisions at a lower level, so that the general sequence of the structural levels from low to high becomes intelligible as the order of their evolution.

To my mind the rudimentary and vestigial structures of animals are in themselves proof positive of a natural history of change. The few ill.u.s.trations can be reinforced by countless examples offered by every group of living animals. If such structures have not evolved naturally by degenerating from more efficient counterparts in ancestors of earlier times, and if they have been specially created, they are utterly meaningless and their very existence is unreasonable. If common sense is to be employed, they demonstrate evolution.

Everywhere throughout the whole series animals place themselves in a treelike arrangement, for in their respective levels they occur like leaves at the ends of the lines of descent which have led up to them and which are comparable to the branches and limbs arising from the trunk of a tree. Thus the major and minor divisions of animals do not follow in the order of the rungs of a ladder, even though they must be a.s.signed to different levels according to the complexity of their construction. The summary given above, namely, that the occurrence of lower and higher levels reveals an order of evolution, is amplified and not contradicted by the statement that the species of animals are group in a treelike arrangement. It is the task of the evolutionist, provided with all the facts of comparative anatomy and dealing only with the various species as separate leaves, so to speak, to reconstruct the now invisible but not unreal twigs and branches and limbs of the animal tree, and to show how they have diverged at one time or another as they have grown and spread to produce the species of the present day. This he may do in so far as he may find sufficient materials to enable him to employ the methods of comparative anatomy and the great natural principle established by this method--that essential likeness means consanguinity.

No evidence of evolution could be more significant and interesting than the results provided by the comparative study of development. In the first place it is an obvious fact that every living thing changes in the course of its life-history, and if as an adult it occupies a high place in the animal scale, its embryological transformation is more elaborate and intricate than in the case of a lower form. Every one knows that organisms do develop, and yet I believe that few appreciate the tremendous significance of the mere fact that this is true, while still fewer are aware that the peculiar and characteristic early stages through which an animal pa.s.ses in becoming an adult are even more striking than the fact of development itself. We shall learn something of these earlier conditions in the development of some of our most familiar animals, but at the outset nothing can be more important than an appreciation of the first great lesson of this department of natural history--namely that organic transformation is real and natural. We do not need to employ the methods of formal logic to know that in growing up a human infant undergoes the changes of childhood and adolescence, that kittens become cats, and that an oak tree is produced by an acorn, for we know these things directly by observing them. It is natural for development to take place under normal conditions, and if it does not, then something has interfered with nature.

Inasmuch as "growing up" is accomplished by the alteration of an organic mechanism with one structure into an individual with a changed plan of body, it is in essence the actual process of evolution which the comparative study of grown animals of to-day demonstrates in the way we have learned. The study of animal structure discovers the process of evolution because the most reasonable interpretation of the similarities and minor differences exhibited everywhere by the various groups of animals is that descent with adaptive and divergent modification has taken place; the result is reached by inference, it is true, but by scientific and logical inference. With development it is otherwise. No reasoning is necessary to tell us that organic transformation is a real and a natural process. We see it everywhere about us and we ourselves have come to be what we are by a natural history of change. Can we consistently deny that it is possible for a species to alter in the long course of time when a few brief weeks are sufficient for the new-laid egg of the fowl to develop into a fledgling? Many indeed strain at the gnat of the longer process in the past when without hesitation they recognize the real and obvious fact of individual development in a brief period.

I have said that development is a "natural" process. We employ this word for the familiar and everyday occurrence or thing; it does not imply that everything is known about the object or phenomenon, because science knows that complete and final knowledge is impossible. We say that it is natural for rain to fall to the earth, and we speak of the law of gravitation according to which this takes place as a natural principle, but it may not have occurred to many to inquire _what_ makes rain fall and _why_ do ma.s.ses of matter everywhere behave toward one another in the consistent manner described by the law in question. Sunshine is natural, but we do not know _why_ light travels as it does from the sun to the earth, and this is another question which, like the inquiry into the ultimate cause of the familiar and natural phenomenon of gravitation, has not yet been answered. But it is still regarded as natural for the rain to fall and for the sun to shine. In the same way does science view development, denoting it natural because it is an ordinary everyday matter. And we are under no more obligation to postulate supernatural control for the changing forms in the life-history of a chick or a cat than we need to a.s.sume that gravitation and the radiation of light demand immediate supernatural direction. The embryology of no form is fully understood or described or explained, but no intelligent person would be willing to a.s.sert that because complete knowledge is lacking, it is unnatural for organic transformation to take place during growth. Whatever may be the ultimate origin and nature of the directing powers behind gravitation and development and other phenomena, we have no concern with such matters because they cannot be handled by scientific methods and one belief about them is on the same plane with any other. Our task is to deal with the everyday phenomena of life and the production of living species.

It is not necessary to go far afield to find an animal which will introduce us to the general principles of embryology. In the present instance as in the case of comparative anatomy almost any form will disclose the meaning of development, for animate nature is uniform and consistent in its methods of operation throughout its wide range. We shall begin with the familiar frog which every one knows is a product of a tadpole; pa.s.sing on to the chick we will learn more facts that will enable us to formulate the main principle of comparative embryology in definite terms; we will then be prepared to extend our survey so as to include somewhat less familiar facts and animals that are even more significant than the first ill.u.s.trations.

If we should visit a woodland pond in early spring, we would find somewhere among the leaves and sticks in the water large ma.s.ses of a clear jellylike consistency enclosing hundreds of little black spheres about an eighth of an inch in diameter. These are the egg ma.s.ses and eggs of a common frog. Watching them day by day we see the small one-celled egg spheres divide into more and more numerous portions which are the daughter-cells, destined to form by their products the many varied tissues and organs of the developing larva and adult frog. After three or four days the egg changes from its globular form into an oval or elliptical ma.s.s, and from one end of this a small k.n.o.b projects to become a flattened waving tail a few days later. On the sides of the larger anterior portion shallow grooves make their appearance and soon break through from the throat or pharynx to the exterior as gill-slits. Shortly afterwards the little embryo wriggles out of its encasing coat of jelly, develops a mouth, and begins its independent existence as a small tadpole, with eyes, nasal and auditory organs, and all other parts that are necessary for a free life. Thus the one-celled egg has transformed into something that it was not at first, and in doing this it has proved the possibility and the reality of organic reconstruction.

The tadpole breathes by means of its gills, and it is at first entirely devoid of the lungs which the adult frog possesses and uses. When we speak of the larval respiratory organs as gills we imply that they are like the organs of a fish which have the same name; they are truly like those of fishes, for the blood-vessels which go to them are essentially the same as in the lower types and they are supported by simple skeletal rods like the gill-bars of the fish. In a word, they are the same things.

The animal feeds and grows during the months of its first summer, and hibernates the following winter; with the warmth of spring it revives and proceeds further along the course of its development. Near the base of the tail two minute legs grow out from the hinder part of the body, and while these are enlarging two front legs make their appearance a little behind the gills. The tadpole now rises more frequently to the surface where it takes small mouthfuls of air. Meanwhile great changes are effected inside the body where the various systems of fishlike organs become remodeled into amphibian structures. A sac is formed from the wall of the esophagus, and this enlarges and divides to form the two simple lungs. The legs increase in size, the tail dwindles more and more, the gills close up, and soon the animal hops out on land as a complete young frog. From this time on it breathes by means of its lungs instead of gills, even though it returns to the water to escape its foes, to seek its prey, and to hibernate in the mud of the lake bed during the winter months.

All these changes are familiar and natural, but until science places them and similar facts in their proper relations their significance is lost to us. The tadpole is essentially a fish in its general structure and mode of life, even though its heritage is such that it can develop into a higher animal. When it does become a frog it proves beyond a doubt that there is no impa.s.sable barrier between fishes and amphibia. Our earlier comparison of the structures of these two cla.s.ses of vertebrates led to the conclusion that the latter had evolved from antecedents like the former, and had thus followed them upon the earth; now that sequence seems to have some connection with the method by which a tadpole, obviously not a fish but nevertheless actually fishlike, changes into a frog, a member of a higher cla.s.s of vertebrates. This method is employed by developing frogs apparently because it follows the ancestral order of events, and because, so to speak, the only way a frog knows how to become a frog is to develop from an egg first into a fishlike tadpole and then to alter itself as its ancestors did during their evolution in the past. We begin to see, then, that in addition to the impressive fact of development itself, the mode of organic transformation is far more conclusive evidence of evolution, because it reveals an order of events which parallels the order established by comparative anatomy as the evolutionary sequence.

However it is well to review some of the changes by which a chick comes into existence before attempting to comprehend fully the fundamental principle of development that the tadpole's history discloses to us. The egg of a common fowl is certainly not a chick. Within the calcareous sh.e.l.l are two delicate membranes that enclose the white or alb.u.men; within this, swung by two thickened cords of the alb.u.men, is the yellow yolk ball enclosed by a proper membrane of its own. In the earliest condition, even before the alb.u.men and the sh.e.l.l are added and before the egg is laid, on one side of the yolk-ma.s.s there is a tiny protoplasmic spot which is at first a single cell and nothing more. The hen's egg is relatively enormous, but nevertheless, like that of the frog, it starts upon its course of development as a single unitary biological element--a cell.

During the earliest subsequent hours the first cell divides again and again to form a small disk upon the surface of the yolk. Soon the cells along the middle line of this small sheet become rearranged to make an obvious streak or band, and about this line a simple tube is constructed which is destined to become the future brain and spinal cord. The whole disk continues to enlarge by further division of its const.i.tuent elements so that it encloses more and more of the yolk ma.s.s, but the little chick itself is made out of the cells along the central line of the original plate, from which it folds at the sides and in front and behind so as to lie somewhat above and apart from the flatter enclosing cell layers which partly surround the yolk.

At the sides of the primitive nerve-tube small blocks of cells arise to develop into primitive muscles and other structures. As nourishment is brought to the embryo from the surrounding layers enclosing the nutrient yolk, one system after another takes its shape and builds its several parts into organs which can be recognized as elementary structures of a chick. Among the more interesting ones are small clefts or slits formed in the side walls of the rudimentary throat or pharynx. Blood-vessels go forward from the simple heart to run up through the intervening bars exactly as in the tadpole and the fish. In brief, the young chick possesses a series of gill-slits, for these structures are the same in essential plan and relations as the clefts of tadpoles and fishes. Does this mean that even birds have descended from gill-breathing ancestors?

Science answers in the affirmative, because evolution gives the only reasonable explanation of such facts as these. The case seems different from that of the frog, because gills are used by the tadpole, but gill-slits and gill-bars can have no conceivable value for the chick as organs concerned with the purification of the blood. None the less, if the transition from a gilled tadpole to the adult with lungs means an evolution of amphibia from fishlike ancestors, then the change of a chick embryo with gill-clefts into the fledgling without them is most reasonably interpreted as proof that birds as well as amphibia have had ancestors as simple as fishes.

As development progresses four small pads make their appearance; two of these lie on either side of the body back of the head and the other two arise near the posterior end. They are far from being wings and legs, but as day follows day they become molded into somewhat similar limbs, as much alike in general plan as the four legs of a lizard; subsequently the ones at the front change into real wings and the hinder ones become legs.

Meanwhile the internal organs slowly transform from fishlike structures into things that display the characteristics of reptilian counterparts, and only later do they become truly avian. Last of all the finishing touches are made, and the whole creature becomes a particular kind of a bird which picks its way out of the sh.e.l.l and shifts for itself as a chick.

Only a few of the countless details have been mentioned which demonstrate the resemblance of the successive stages first to fishes, and later to amphibia and reptiles. We have a wide choice of materials, but even the foregoing brief list of ill.u.s.trations shows that the order in which the stages follow is the one which comparative anatomy independently proves to be the order of the evolution of fishes, amphibia, reptiles, and birds.

Why, now, should it be necessary for a developing bird to follow this order? The answer has been found in the immense array of embryological facts that investigators have verified and cla.s.sified, that all tell the same story. It is, that birds have arisen by evolution from ancestors which were really as simple as the members of these lower cla.s.ses. It seems then that the only way a bird of to-day can become itself is to traverse the path along which its progenitors had progressed in evolution.

Stating its conclusions precisely, science formulates the principle in the following words: _individual development is a brief resume of the history of the species in past times_, or, more technically, _ontogeny recapitulates phylogeny_. To be sure, the full history is not reviewed in detail, for the chick embryo does not actually swim in water and breathe by means of gills. Only a condensed account of evolution of its kind is presented by an embryo during its development; as Huxley and Haeckel have put it, whole lines and paragraphs and even pages are left out; many false pa.s.sages of a later date are inserted as the result of peculiar larval and embryonic needs and adjustments. But in its major statements and as a general outline, the account is a trustworthy natural doc.u.ment submitted as evidence that higher species of to-day have evolved from ancestors which must have been like some of the present lower animals.

Coming now to the mammalia, it might seem that we have reached forms so highly developed that they would not exhibit the same kind of developmental history, but would have their own mode of growing up. This is not so, for like the adult fish, the larval tadpole, and the embryo chick, an embryo of a cat or a man is at one time constructed with a series of gill-clefts and with blood-vessels and skeletal supports of fishlike nature that are everywhere a.s.sociated with gills. The embryos of wildcats and dogs, rabbits and rats, pigs, deer, and sheep, and of all other mammalia, possess similar structures. Thus they all pa.s.s through a stage which is found also in the development of reptiles, birds, and amphibia,--a stage which corresponds to the fish throughout its life.

Unless these facts mean that the great cla.s.ses of vertebrates have originated together from the same or closely similar ancestors, they are unintelligible; for we cannot see why a cat or a chick should have to be essentially fishlike at any time unless this is so. Comparative anatomy states as we have learned that the amphibia as a cla.s.s have evolved from and have out-developed the fishes, that reptiles have progressed still higher, and that birds and mammals have originated from reptilian ancestors along roads that have diverged beyond the immediate parent cla.s.s. Because the members of each cla.s.s have to pa.s.s along the same path trodden by their many varied ancestors, although at express speed, as it were, the similarity of the earliest stages in their development is explained, for during these periods they are traversing a path over which their ancestors pa.s.sed together.

The places where the developing embryos depart from the common mode show where the several divisions took leave of one another in their evolution,--a point that comes out with great clearness when the facts of mammalian development are broadly compared. The embryos of carnivora and rodents and hoofed animals are alike in their earlier development, and their agreement means a community of origin. At a certain point the cat and dog depart from the common mode, but they remain alike up to a far later stage than the one in which they are similar to the embryos of rats and sheep. The rat and squirrel and rabbit, on their part, remain together until long after they take leave of the carnivora and ungulates; while the sheep and cattle and pigs have their own branch line, which they follow in company after leaving the embryos of the other orders. The reasons for these facts seem to be that the members of the three orders exemplified have evolved from the same stock, which accounts for their embryonic similarity for a long time after they collectively come to differ from amphibia and reptiles, while the members in each order became differentiated only later, wherefore their embryonic paths coincide for a longer period. Thus the degree of adult resemblance which indicates the closeness of relationship corresponds with the degree of embryonic agreement; that is, the cat and dog are much alike and their modes of development are essentially the same to the latest stages, while the cat and horse agree only during the earliest and middle stages, and their lines diverge before those of the cat and dog on the one hand, or those of the horse and pig on the other.

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 pa.s.s 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 sh.e.l.l 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 a.s.sertions, 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. b.u.t.terflies and moths pa.s.s through a caterpillar stage having even more striking resemblances to worms. All the larvae 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.

Let us now turn back to some of the earlier pages of the embryological record which we pa.s.sed 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 ma.s.s 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 const.i.tute 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 pa.s.s on to become blastulae and gastrulae, 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 blastulae and gastrulae 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 a.s.serted 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 pa.s.s 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 impa.s.sable gap between a one-celled animal like _Amoeba_ and a mammal like a cat is actually compa.s.sed 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.

Embryology as a distinct division of zoology has grown out of studies of cla.s.sification 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 a.s.sumed 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.