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Still more opposed to the hypothesis of evolution is the remarkable fact that the changes which take place in the developing egg are not such as would lead directly to the formation of the adult animal. In most cases a circuitous or indirect path is followed, and this indirect path leads at first towards the adult form of lower members of the group.
This, the most suggestive fact of. modern embryology, may perhaps be made clearer by an illustration. .
Let us try to compare the growth of an egg into an adult animal with the growth of some manufactured product in the hands of its maker.
The evolutionist view of the development of an organism may be illustrated by the manufacture of a yarn base-ball. A boy, wishing to make a yarn ball, procures, if possible, a small rubber ball, and winds his yarn onto this until the desired size is reached, the only changes during the growth of the ball being the change of size and of material.
The observed facts of embryology show that the development of an embryo does not take place in any such way as this. It may, however, be illustrated by the growth of a steam-ship in the hands of the builder, who first lays down an indefinite skeleton, and outlines in a vague way the more prominent features, before any of the details are finished. In order to make the illustration perfect, however, we must imagine the builder to commence work upon his steam-ship by laying out the skeleton of a big triareme; we must imagine him to carry this some stages towards completion, and to put into it certain contrivances, such as rowers' benches, which are of no use in a steam-ship. We must imagine that he then abandons his plan, tears down his benches, and uses the material to make a deck; that he changes the shape and proportions of his hull a little to fit it for sailing instead of rowing, that he puts in masts and spars, and makes everything ready for a ship's rigging; that he then changes the shape of his hull once more; tears out part of his cabin, puts in bulkheads, coal bunkers, and an engine and boiler; shortens his masts, alters-his rigging, and finally converts his unfinished ship into a finished steamer.
This is not by any means a forced illustration, .but a very fair outline of the development of an animal. In nearly every case we find that the development of the embryo as a whole, or else the development of certain organs, takes place in this roundabout, indirect way, and repeats, usually in an imperfect manner, the structure of a_ related but lower animal.
As an example, we may refer to the history of the blood-vessels of a mammal. The breathing organs of the lower vertebrates are gills on the sides of the neck, and the venous blood is driven from the heart through a series of branchial arteries to the gills, where it is aerated and conveyed into a series of branchial veins which carry it, not back to the heart, but to the various organs of the body. In a mammal there are no traces of gills at any stage of development; the adult animal breathes by lungs, and the blood which has been aerated in the lungs goes back to the heart before it is distributed throughout the body. Now the early stages in the development of the bloodvessels of a mammal would, if carried out to completion, lead to the formation of the system found in fishes.
The mammalian embryo has no gills* but it does have branchial arteries and veins, and its blood at first follows tlid same course that it follows in a fish. It is plain that the fish-like circulation is not an outline or sketch of that of a mammal; that it is not a necessary stage in the formation of the latter, for the branchial vessels are soon, in part pulled down and destroyed, and in part profoundly modified, in order to conform to the mammalian type.
Cases of this kind are almost universal, and the law of resemblance between the early stages of higher animals and the adult condition of lower animals is a fundamental law of embryology.
It is obvious that the hypothesis of evolution of a perfectly formed germ contained in the egg, is utterly irreconcilable with this law, and we may therefore state with confidence that this hypothesis is refuted by the observed facts of embryology.
We must not forget, however, that there were other less superficial forms of the evolution hypothesis, and that these cannot be disproved so easily.
Buffon, for instance, held that the embryo is built up by the union of organic particles which are given off from every part of the body of the parent, and which, assembling in the sexual secretions, assume in the body of the offspring positions like those which they occupied in the parent. This is essentially an evolution hypothesis, but it is logically complete, since it accounts for the production of successive generations without the necessity for assuming that they were all contained in embryo in the body of a remote ancestor. Microscopic examination cannot overthrow this hypothesis, for a failure to discover these organic particles with any particular magnifying power does not, of course, disprove their existence any more than a failure to see them without a microscope.
Although Bnffon's hypothesis does not account for the fact that development is indirect in most cases, that the egg does not build up the adult animal in the simplest way, but takes a roundabout circuit, this fact is not directly opposed to his hypothesis, for we can easily conceive that after an indirect method of development has been established it might be perpetuated by Buffon's organic molecules, provided these are given off by the parent organism at all stages of its life, and not simply after it has reached its final form.
There is, however, another class of phenomena of even greater importance—the phenomena of variation.
Buffon's hypothesis accounts for the resemblance between the child and the parents, but we now know that the child is not exactly like its parents or even midway between them, that animals and plants are born with a tendency to vary, that this variation may affect any part of the body, and that by the selection of these congenital variations the most profound changes of hereditary strueture may be produced.
The fact of congenital variation is as profound, as universal, and as characteristic of living things as the fact of heredity, and the constant appearance of new variations is as fatal to Buffon's hypothesis of evolution as it is to that of Bonnet.
With the growth of the modern science of morphology these hypotheses have been abandoned and the hypothesis of epigenesis almost universally accepted in their place.
This hypothesis, first brought into notice by the researches of Harvey and Wolff on the development of the chick, has gradually assumed a more definite shape with the progress of embryology, and has been especially modified by the growth of the cell theory.
In its modern form it may, for convenience of discussion, be divided into two parts—a statement of the observed facts, and an explanation of the origin of the phenomena.
So far as it is a statement of facts, it cannot be called an hypothesis, for it simply affirms that the egg is optically an ordinary unspecialized cell; that it gives rise, during the process of segmentation, in a manner which is identical with ordinary growth by cell division, to a number of cells which gradually become specialized for certain functions, and are set apart as the foundations of the various organs of the body; that the repetition of this process gives rise, at last, to the perfect body of the mature animal; that the reproductive elements which are to give rise to the next generation, originate, like all the cells of the body, by cell division during the process of development, and that they are simply cells specialized for the reproductive function as other cells are specialized for other functions. Every one who has the slightest acquaintance with modern biology will accept thiS statement, not as an hypothesis, but as an observed fact, and will agree that between this and the old evolution hyp'othesis there can be but one choice.
The old hypothesis of evolution, however, claimed to be something more than a statement of fact, for the presence of the germ within the egg accounted for the wonderful properties of the egg itself.
We are at once compelled to ask, then, how, on the hypothesis of epigenesis, has the egg acquired these properties? If it is simply an unspecialized cell; if, as Gegenbauer states, "the egg is nothing more nor less than a cell; the egg-cell does not differ from other cells in any essential points" (Comp. Anat., Bell's Trans., p. 18), how can the egg of a horse develop into a horse, while another cell, which "does not-differ from it in any essential points," develops into a bee or an alligator or an oyster?
Nothing in nature is more marvellous than the devel