Creation by Evolution/Evolution as Shown by the Advancement of the Individual Organism
EVOLUTION AS SHOWN BY THE DEVELOPMENT OF THE INDIVIDUAL ORGANISM
By Ernest William MacBride
Professor of Zoölogy, Imperial College of Science and Technology, London; Vice President of the Zoölogical Society of London
According to the theory of evolution as it is applied to zoölogy the fundamental likenesses or homologies among animals are the expressions of blood relationship. We believe, for example, that all the various species which belong to the cat tribe—such as the domestic cat, the leopard, the jaguar, the puma, the lynx, the lion, and the tiger—are the descendants of a primeval species of cat, just as the dog, the fox, the jackal, and the wolf are the descendants of a primeval species of dog. Furthermore, we believe that the primeval dog and the primeval cat were distant cousins, and that millions of years ago both were represented by one species of primitive carnivore, from which both have been derived.
Now the human body resembles the body of a higher ape just as the body of the cat resembles that of the dog; and if our ideas as to the relationship between cats and dogs are sound it must follow that apes and men have been gradually developed out of one and the same ancestral species. But we assume that evolution proceeds very slowly—that great changes require millions of years—so that direct evidence of it is impossible to obtain. To get such evidence, indeed, we should require the notes of some immortal angelic observer who had watched the process of evolution for ages and had left us a record of his observations! All our evidence must therefore be indirect and circumstantial, and it may be divided into three main groups—(1) evidence from the systematic relations of allied species, (2) evidence from fossils (palaeontology), and (3) evidence from the development of the individual (embryology). The evidence comprised under the first two heads is presented elsewhere in this book; it is my task to present the evidence or argument from embryology.
The embryological argument rests on a daring hypothesis, which was first clearly expounded by Haeckel in what he termed the jundamental law of biogenetics, which he enunciated as follows: Every animal, in its growth from the egg to maturity, recapitulates the history of the race. Therefore, when we find in the embryo, during its growth, stages that resemble animals lower in the scale of life, this resemblance is regarded as evidence that the animal whose embryo we are studying was derived from ancestors that resembled those stages.
When we take a broad survey of the animal kingdom we find that it can be divided into great primary divisions termed phyla. The bodies of all the members of a phylum are constructed on the same plan. A good example of a phylum is provided by the Arthropoda, the jointed animals. This group comprises all the insects, spiders, mites, scorpions, centipedes, lobsters, crabs, shrimps, barnacles, and water-fleas; in fact, it includes three-fourths of the species of animals now living on the globe. All these animals have, outside their flesh, a hard, shelly skeleton, which is divided into joints by zones of thinner shell, as otherwise the animal could not bend itself or move. All likewise possess several pairs of limbs, which, like the body, are encased in similar armour and similarly provided with joints or hinges. Each phylum is divided into classes, the members of which agree in their fundamental plan. Thus the Arthropoda (if we exclude certain smaller groups) are divided into three classes (1) the Insecta, in which there is a pair of feelers in front of the mouth and in which the body consists of three parts, the head, the thorax, and the abdomen; (2) the Arachnida (the scorpions, spiders, and mites), in which there are no feelers, but only a little pair of pincers in front of the mouth, and in which none of the legs are changed to jaws; and (3) the Crustacea, in which there are two pairs of feelers in front of the mouth and several jaws. These three broad classes are divided into orders, the orders into tribes, the tribes into families, the families into genera, and the genera into species. The Malacostraca form an order of Crustacea in which the body is divided into just twenty segments, and this order is classified into two main divisions, the Macrura, or the longtails (the shrimps, prawns, and lobsters), in which the abdomen is long, stretched out, and the Brachyura, or the shorttails (the crabs), in which the abdomen is reduced to a useless vestige and carried under the rest of the body. But there is an intermediate tribe, the Anomura, which includes the hermit crabs. A member of this tribe has an abdomen of moderate length, and many of them have the extraordinary habit of thrusting it into an empty snail shell, in consequence of which habit it has become curved and asymmetrical. Now no one who has studied animals of this type doubts that they are modifications of the type having a straight, symmetrical abdomen. If we trace the course of the development of these twisted-bodied crabs we find that when they are young they swim in the sea like little shrimps and that during this stage of their lives the abdomen is perfectly straight; so that the life history confirms the conclusion to which we were led by the study of the adult form — that the twisted body is a modification of a straight one. These crabs with the aborted abdomen must have been developed from animals — such as the shrimps and lobsters — in which the abdomen is long and normal; and so we find that in their young stages they are very much like little lobsters. If these crabs were specially created why should they begin life like lobsters?
In fact, we may lay down the general law that any member of an order or tribe that differs in structure from the other members will show in its young stages a close agreement in structure with the type prevailing in that order or tribe.
A splendid example of this law is provided by the Indian cat-fish Clarias. The cat-fishes are widely distributed over North America, Europe, and Asia, and their general appearance is familiar to most persons. With one or two exceptions all are river fish and are quite devoid of scales, having naked dark-brown or black skins, flat heads, and broad mouths. From both lips and from the corners of the mouth protrude long, slender rods called barbels, with which these fish probe the mud for the worms and other small animals on which they live. It is the fanciful comparison of these barbels to the whiskers of a cat that suggested the name cat-fish. Now one of the genera of Indian cat-fish called Clarias has learned to emerge from the water and breathe air, and in order to do this it has developed above its gills two curious tree-like organs, which are richly supplied with blood vessels. As Figure 1 shows, however, this fish is essentially like other cat-fish when it is young, but as it grows older two little buds appear above the gills on each side, which develop into the tree-like organs of the adult. Can anyone seriously doubt that Clarias has been developed from an ordinary cat-fish? And if we admit this must we not also admit that, here, at least, the young animal recapitulates the past history of the race?
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In recent years the law of recapitulation has been proved by experimental evidence. It has been found that the black and yellow salamander of Europe (Salamandra maculosa) is capable of slowly altering its colour as it grows up; it makes its colour harmonize with that of its surroundings. If these young salamanders are confined in yellow boxes the yellow spots on their skins enlarge in size as they grow to maturity. If they are confined in black boxes the yellow spots diminish in size and many of them disappear. These changes are in some degree passed on to the offspring, for if two “yellowed” salamanders are mated together they produce young that are much yellower than their parents were at the corresponding stage of development, and if these young continue to live amid yellow surroundings they become almost entirely yellow when they are fully mature. Now if the offspring of “yellowed” salamanders are reared in black boxes they steadily become yellower for the first year of their lives, thus recapitulating the experience through which their parents passed; after that, and then only, does the effect of the black environment begin to tell on them, for the yellow spots begin to diminish in size.[1]
The young forms termed larvae, which by their structure and habits repeat ancestral conditions, live freely in the world and earn their own living. The most familiar example of a larva is the tadpole of the frog, which, by its gills and tail recalls the fish-like ancestors of the frogs. There is another type of young animal which is known as an embryo (Greek εν (εμ), in, βρύειν, grow). This type, during its development, is sheltered and fed either within an egg shell or in the womb of the mother. A good example of this type is the chicken, of which the greater part of the development is completed within the egg shell. The young form in this type derives its food from the yolk in the egg, which it slowly digests as it grows, and from the “white,” or albumen. Another variety of embryo is sheltered within the womb of the mother and obtains all its nourishment from the maternal blood. The embryos of all the higher warm-blooded mammals, such as those of dogs, horses, and cattle, as well as the human embryo, are of this kind.
Now let us consider how the embryonic and the larval types of development are related to one another. Was the original form of development embryonic or larval?
When we closely examine the life histories of animals we discover that there is an embryonic and a larval phase in all, though these phases are of extremely different lengths in different animals. No animal deposits naked eggs; an egg-shell, though it may be thin and elastic, is always formed, and the egg always includes some yolk, so that there is always a period during which the young animal develops as an embryo before it bursts the egg-shell and begins as a larva to earn its own living. Furthermore, an animal is not hatched in exactly the form of the adult; it attains this form only after a period of growth, and during this period it may properly be termed a larva.
The salamanders show us clearly that the embryonic phase is a secondary modification of the larval phase—in a word, that the embryo is a larva which is provided with shelter and food.
There are two species of European salamander, the black and yellow salamander (Salamandra maculosa), commonly known as the fire salamander, which inhabits the plains and lower reaches of the valleys, and the black salamander (Salamandra atra), which is found in Alpine pastures. Both species bring forth the young alive, but the young of the fire salamander are provided with long, feathery gills and gill slits and pass the first six months of their life in water, whereas the young of the black salamander are devoid of gills and gill slits at birth and are ready to take up the parental habit of life on land.
If now we open the womb of a pregnant black salamander we shall find in it a number of embryos with long, feathery gills, and if these embryos are thrown into water some of them will survive and develop like the young of the fire salamander. The gills are an adaptation to life in water, and when we find them in an embryo we may be sure that the embryo was once a larva and that the larval phase is therefore the primary one.
Similar examples could be adduced from almost every group of animals. We have already alluded to the well-known tadpole larvae of frogs and toads. There is, to mention one more animal, a small West Indian tree frog (Hylodes) which produces a few large eggs from which emerge not tadpoles but little frogs, which resemble their parents. If, however, we dissect off the membranes of developing eggs we find within them tadpoles, complete, with their characteristic tails.
The earliest stages in development are the most delicate and vulnerable, and it is these which first become embryonic; the latest stages in development, which represent comparatively recent ancestral history, are always larval. In human development, as we all know, the baby is an embryo for nine months before birth, and after it is born the child may be justly termed a larva until the beginning of puberty. The mental powers are not fully developed until the child reaches the age of about fifteen years.
During the period of its life within the womb the human embryo develops a large organ like a sucker, which is closely pressed against the wall of the womb and which enables the tiny baby to suck nourishment from its mother’s blood. This sucker, which is called the placenta, is developed from the belly of the embryo, which is thereby distorted out of shape. Now no one imagines that some ancestor of man went about through life with a placenta protruding from its under surface; the placenta is a secondary outgrowth to enable the embryo to live in the womb. Such “secondary” changes are known as falsifications of development; they may be likened to interpolations made by some later writer in an ancient historical document. But during the time that the embryo carries this extraordinary appendage, protruding from its under surface, its upper surface passes through a most interesting series of changes. Its mouth at first resembles that of a shark, and the nostrils, as in the shark, are connected with the edges of the mouth by grooves. Then the head grows to be like that of the tadpole, and, just as in the young tadpole, this head is divided from the body by a narrow neck, which has nothing to do with the neck of the young child. Along the sides of the neck there are two series of gill slits, and, just as in the tadpole, these become covered by flaps of skin that grow back from the head and join the trunk. The neck indentation is thus obliterated, and the head passes without a break into the trunk, just as it does in the older tadpole. The blood vessels at the sides of the gill clefts resemble exactly those of the tadpole. There are four of them on each side, and, with accurate imitation of the tadpole, the third on each side drops out. The salamander retains the four throughout life, but its near cousin, the newt, drops out the third, as does the frog. Thus the story of man’s development from a water animal and his gradual closing up of his gill clefts is accurately repeated in the womb, and the distortion of this story by the development of the placenta is easily recognised. We find the same history if we study the development of the young lizard within its mother; but here no placenta is developed, and the egg is afterward laid, but development has begun long before that. So by comparing the life histories of different animals belonging to the same phylum we can separate the secondary accretions from the original story and thus recover the true ancestral history.
To return to human development: As this proceeds the limbs grow out and the embryo comes to resemble an ordinary four-footed animal, but the fingers and toes are at first webbed like those of a frog. At this stage there is a well-developed tail, and later there is a complete covering of hair, resembling the hairy skin of an ape. At birth the big toe is widely separated from the other toes, just as is the big toe of an ape, and the legs curve inward at the ankles, so that when the child is held upright only the outer edge of the sole rests on the ground. This arrangement of the legs is identical with that seen in the leg of an ape; it is an adaptation that makes it easy to press the soles of the feet against opposite sides of a branch in climbing.
The repetition of ancestral phases in life histories has been compared to memory. If we learn tennis when we are young and have no time to practice it until middle life we shall nevertheless find that when we resume the game a part of our skill comes back to us, that, in a word, we remember it. So when salamanders have grown up in a yellow box and have become yellowed in consequence, their young "remember" the parental experience and strive to turn yellow in spite of being enclosed in black boxes. The tadpole represents a memory of the time when the ancestors of the frog lived like fish. But all development does not reflect purely ancestral experience. Just as memories become blurred and sometimes confused with the lapse of time, so secondary modifying factors tend to render the ancestral record of development illegible. The organs of a larva are used, broadly speaking, in much the same way as the ancestors used them, but the corresponding organs of an embryo are not used, and so these tend to be imperfectly developed and to degenerate into mere sketches of the ancestral organs. Larval life, which represents ancestral habits, may become excessively dangerous, and the larva, in self defence, may be driven to adopt another mode of life, as when the young of the May-fly learn to live in water and develop secondary gills. But these secondary modifications are usually peculiar in each kind of animal, and the fundamental ancestral record can be deciphered only by comparing a large number of life histories, just as the historian extracts the truth from ancient documents by stressing the points in which they agree and discounting those in which they differ.
The most direct proof of evolution is certainly afforded by fossils. When we study the fossils preserved in a great mass of strata, overlying one another like the pages in a gigantic book, we find some of these fossil animals gradually changing as we pass up the series. In the Devonian strata we find numerous remains of fish with fins consisting of a central axis beset with two rows of branches like a feather. As we reach the base of the Carboniferous strata these give place to newts with five-fingered hands and feet. An eminent palaeontologist (Prof. D. M. S. Watson, F.R.S.) who has studied this series of fossils, arrived at the conclusion that the fourth finger on the hand represented the axis of the feather, that the fifth finger was the sole remaining branch on one side, and that the first three fingers were the branches on the other side. A German embryologist, who was totally ignorant of Dr. Watson's conclusion, studied the early development of the wing of the common fowl. Here, in the youngest stage, five columns of condensed tissue, representing the five fingers, can be made out, though in the adult bird only two and the trace of the thumb remain. From a study of the growth of these rudimentary fingers the German arrived at the same conclusion that was reached by Dr. Watson from his study of the fossils, namely, that the fourth finger represents the axis of the fin. Thus, when the opportunity is afforded, the conclusions drawn from embryology are confirmed by palaeontological evidence. Other examples of the same confirmation could be given, but their citation would involve detailed descriptions of anatomy. As, however, these confirmations are increased in number, our confidence in the truth of the embryological record grows; and this confidence is of the utmost importance to us for tracing the history of evolution, because this ancestral record, repeated in embryonic development, gives us the only means that we possess of tracing the history of life back to its beginnings, or rather of completing the imperfect story told by fossils.
The earliest stages in this history in the human embryo are blurred out of all recognition, but if we examine the earlier history of the tadpole or, still better, that of the newt, a primitive amphibian, we can get a general idea of the history of life from the beginning. The egg, as we have seen, is a single cell in its essential structure, the same as a whole group of single-celled animals termed the Protozoa. This cell divides into many cells, which cohere together and form a little hollow ball called the blastula. Similar little balls, in which the cells have acquired green colouring matter, roll about in the waters of our ditches. The blastula becomes converted into a hollow cup called the gastrula by the pushing in of the cells at one end. The opening of the cup is called the blastopore. This opening is retained throughout life as the anus or vent of the animal, and above the anus the tail grows out. The outer layer of the cup forms the skin; the brain and the nervous system are at first mere thickenings in this layer. The inner layer forms the lining of the stomach; the rudimentary backbone is only a ridge or folding of this inner skin along its upper surface. The mouth is formed as a new opening in front; the gill slits are clefts at the sides of the throat; the eyes grow out as buds on the brain; the nose and ears are pits in the skin, and behold! we have before us no longer a swimming cup but the beginning of a tadpole, which is really a very primitive type of fish.
A study of the development of jointed animals (the Arthropoda), of the Mollusca (clams, oysters, and snails), and of the Echinodermata (starfish and sea-urchins) leads us to similar startling and fascinating results. We find that worms, arthropods, and mollusks arose from a common ancestor and diverged from one another owing to their adoption of different habits; and, strangest of all, that backboned animals, the class to which we ourselves belong, diverged from the same root as the starfish, sea-urchins, and sea cucumbers, which constitute the class Echinodermata.
Comparative anatomy and systematic zoölogy take us only a little way, for we have no reason to assume that the ancestral forms of animals have persisted unchanged to the present day. The evidence from fossils is best, but fossils preserve only the hard parts, and the earliest fossils thus far found are already far advanced in evolution. But every animal begins its development in the egg, which is a single cell, comparable in structure to the lowest forms of life known to us, and as it grows to the adult form it sketches in broad outlines the whole story of its evolution.
REFERENCES
There are unfortunately no short, comprehensive books on embryology, and the reader who wishes to pursue the subject further must have recourse to large treatises, in which it is dealt with in detail. For the development of the invertebrates we recommend Textbook of Embryology (vol. 1), Invertebrates, by E. W. MacBride; Macmillan & Co. For the development of the vertebrata we recommend The Embryology of the Vertebrata (this book does not include the frog) and The Frog, both by A. Milnes Marshall; for The Embryology of the Chick, Frank R. Lillie. Text books of embryology by Clement Heisler and by Bailey and Miller deal with the embryology of man.
- ↑ All biologists are not agreed as to the sufficiency of the evidence for the inheritance of acquired characters.—Ed.