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 Buffon'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 structure 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 hypothesis 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

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 seems to 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, has been slowly evolved from an ancestral rhizopod, and since the ovum of a horse is homologous with a rhizopod, or is morphologically equivalent to it, we 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