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opment of each egg into its proper organism, and if it is true that the egg which is to give rise to a man differs in no essential point from that which is to give rise to an insect, we may conclude that the mystery is too great to be fathomed by our intelligence, and we may fairly ask what possible explanation can, on this hypothesis, be given of the wonderful properties of the egg.
The answer which has been given, and which seemsto have been thought satisfactory by many students, is this:
We know, from a mass of evidence which is constantly and rapidly increasing , and to which each new observation adds cumulative weight, that the various forms of life have been slowly evolved, during long ages, from older and simpler forms; that as we trace back the history of any two animals or plants we find evidence that in the past they had for a common ancestor a species which had not yet acquired the distinctive features of either of them; that a little farther back we trace this species to an ancestor with still wider relationships.
Every day the evidence grows stronger to show that more complete knowledge will ultimately prove that the same thing is true of still larger groups; that families, classes and orders of organisms have been formed in the same way by gradual modification and divergence; that complete knowledge of the ancestry of any organism would lead us back through simpler and simpler forms to a remote unspecialized unicellular ancestral form. It is unnecessary to review in this place the evidence for this conclusion, for the fact that it is fully accepted by those best qualified to judge of its truth, is perfectly familiar to all students.
Now it is said, and the explanation is pretty generally accepted, that since any particular organism, a horse for instance, lias been slowly evolved from an ancestral rliizopod, and since the ovum of a horse is homologous with a rhizopod, or is morphologically equivalent to it, Ave have in the gradual phylogenetic evolution of the horse species from an unicellular ancestor, a satisfactory explanation of the ontogenetic development of the individual horse from an unicellular ovum.
As soon as attention is fairly fixed upon the subject, the weakness of this explanation becomes so evident that I take the liberty of making the following quotation from a well-known authority, in order to show that the explanation has been soberly advanced. In making the extract from Haeckel's writings I am not actuated by a desire to attack his views, for the same idea can be found, expressed pretty definitely, in the works of many other writers, and this particular selection is simply a matter of convenience.
Haeckel says: "Until recently the greatest students of embryology, Wolff, Baer, Remack, Schleiden and the whole school of embryology founded by them, have regarded the science as exclusively the study of individual development. Far otherwise to-day, when the mysteries of the wonderful history of the development of individual organisms no longer face us as an incomprehensible riddle, but have clearly revealed their deep significance: for the changes of form which the germ passes through under our eyes in a short time are, by the law of inheritance, a condensed and shortened repetition of the corresponding changes of form which the ancestors of the organism in question have passed through in the course of many million years. To-day, when we lay a hen's egg in an incubator, and in twenty-one days see the chick break out of it, we no longer gaze in dumb wonder on the marvellous changes which lead from the simple egg to the two-layered gastrula: from this to the worm-like and skulless germ, and from this to later stages which repeat, essentially, the organization of fish, amphibian, reptile, until at last we have a perfect bird. On the contrary, we unravel from this history the corresponding series of ancestral forms, which have led up through the amoeba, the gastraea, the worms, the acrania, the fishes, the amphibia and the reptiles to the bird.
"The series of changes in the hen's egg gives us an outline sketch of the series of ancestors. This ancestral or phylogenetic significance of the phenomena of ontogeny or individual development is up to the present tune the only explanation of the latter." (" Gesammelte Populare Vortrilge," II., p. 103.) "Any one who accepts the law that individual development is a recapitulation of the evolution of the species icill find it simply natural that the microcosm of the ontogenetic cell-tree should be the diminutive, and in part distorted, reflection of the macrocosm of the phylogenetic genealogical tree of the species." (" Gesammelte Populare Vortrage," II., p. 68.)
No one can set too high a value upon the scientific law here expressed—that individual development is a recapitulation of the history of the evolution of the species. It must be regarded as one of the greatest generalizations of modern science, but I do not think it is possible to agree with Haeckel that with its discovery the mystery of individual development has clearly revealed its deep significance, and no longer faces us as a riddle.
It may be true that it is "simply natural" that the egg of a horse should recapitulate the ancestral history of horses, and the egg of a bird the ancestral history of birds, but the statement that this is the case is in no sense an explanation of heredity. For that matter it is "simply natural" that a bird's egg should give rise to a bird, and a horse-ovum to a horse, but no one would accept the statement as an explauation.
We have in the natural selection of variations a true explanation of the manner in which an unicellular rhizopod has been slowly and gradually modified by an almost infinite number of slight changes, extending through countless millions of generations, into a bird. The change is one of the most wonderful of the phenomena of nature, but it is in no sense a mystery, for the skill of the breeder may even now, by the employment of the same means, produce similar results, only on a much smaller scale; by the methodical selection of congenital variations an organism may be, in a few generations, slightly modified in any desired direction, and we can fairly and truly affirm that we understand the evolution of birds from their unicellular ancestors; but the resemblance between the evolution of birds from these remote ancestors by natural selection, and the development of an individual bird from an unicellular ovum, is simply an analogy. It is true that it is an analogy of the greatest significance, but we must not lose sight of the fact that the means by which the end is accomplished—the natural selection, through a long series of generations, of congenital variations—is absent in the second case. If the epigenesis hypothesis is true, if the egg is simply, like the rhizopod, an unspecialized cell; if the egg of a bird does not differ from the egg of a star-fish in any essential points, we must acknowledge that the mystery of individual development is not only as yet unsolved, but absolutely insoluble.
The student at the sea-shore may collect at the surface, with his dip-net, three similar transparent spherical eggs. Each of these is, optically, simply a nucleated cell, and each when placed under the microscope will soon be seen to pass through almost exactly the same changes, giving rise by division to a spherical layer of cells. Yet if these three eggs are placed together in a tumbler of water and exposed to identical conditions, one may at last become a star-fish, another a crustacean, and another a vertebrate. Similar things under similar conditions cannot give rise to widely different results, and there seems no escape from the conclusion that these three eggs are not similar, or even essentially alike, but that one of them is a potential star-fish, another a potential crustacean, and a third a potential vertebrate. That there is in each of them a something which separates it very widely from the other two, and determines its future history.
The hypothesis of epigenesis proves, then, on careful analysis to be as unsatisfactory as the speculations of Bonnet and Buffon, and we must acknowledge that we are as yet unable to picture to ourselves the hidden significance of the phenomena of individual development, without returning to some modification of the old evolution hypothesis.
The attempt to escape this necessity, and to hold fast to the hypothesis of epigenesis, has given rise within recent years to much ingenious speculation, and an examination of some of the published papers will help, rather than retard, our argument.
Among these, one of the most ingenious and suggestive is Hacckel's paper, "Ueber die Wellenzengung der Lebenstheilchen oder die Perigenesis der Plastidule." The following extract (" Gesammelte Popularo Vortrage," II., pp. 66-72) will, I hope, give a sufficiently clear statement of his views:
"In order to penetrate still farther into the mechan