gemmule may unite or conjugate in the ovum with particles which are not perfectly equivalent to it, but only very closely related to it. Thus a variation may affect a considerable number of related cells at the same time, or a variation in any part may cause in succeeding generations the variation of homologous parts, thus producing what Darwin has called correlated variation. We can also understand how it is that when any part of a complicated organ varies, variations in other parts of it are also soon presented for the action of natural selection, so that an harmonious readjustment is soon established.

According to this view we must believe that all the characteristics which are established as true racecharacteristics, as hereditary peculiarities of the species, are transmitted by the ovum, which has in itself the power to develop, when excited by a proper stimulus which may or may not be due to impregnation, into a new individual of the parent form.

New variations, on the other hand, are produced through the agency of gemmules thrown off from cells like those in which the variation appears.

Gemmules may penetrate to all parts of the body, and they may thus give rise to bud-variation and to analogous changes; or they may penetrate to an ovarian ovum and give rise to variation without fertilization: but as these phenomena depend upon chance, they are comparatively rare, while the aggregation of the gemmules in the male cell and their transmission by impregnation are normal processes.

According to this view, the male element is the originating and the female the perpetuating factor; the ovum is conservative; the male cell progressive. Heredity or adherence to type is brought about by the ovum;

variation and adaptation through the male element; and the ovum is the essential, the male cell the secondary, factor in heredity.

The various hypotheses which we have noticed have little in common, and it is therefore interesting to note that they all present points of resemblance to the one which is here advanced, and that this alone has features in common with them all.

Like Aristotle and the ancients, we must believe that the two reproductive elements play widely different parts. Like Bonnet and Haller, we see that the structure of the adult is latent in the egg.

The mode of origin and transmission of the gemmules is essentially like Darwin's conception, and we must acknowledge that Buffon's view of the part played by his organic molecules was very near the truth.

The analogy upon which Haeckel lays so much stress is readily explicable by our theory, for since each stage in the evolution of the species has been impressed by gemmules upon the egg, it is, in truth, only natural that the developing organism should mirror the ancestral history of its species; and, finally, our view of the origin of the properties of the ovarian egg is identical with that given by Jäger in his explanation of reversion.

An honest attempt to reason from the phenomena of nature can hardly fail to result in the discovery of some little truth, and I think we may hope that all these points of agreement with hypotheses which are manifestly inadequate can only be due to the presence in them all of some portion of the true light of nature.

Mivart, who believes with Darwin that natural selection has been a great but not the exclusive means through which organisms have been modified, has attempted in Chapter xi. of his book on the Genesis of Species to

indicate some of the requisites of a true theory of the origin of specics. This valuable and instructive book is well worthy of careful study, and most students will find in it much material for reflection. Mivart has no explanation of his own to offer, and some of the characteristics of the explanation which he believes in, but does not furnish, are conspicuously absent in the present attempt as well as in Darwin's work; but it is interesting to note that many of the conditions which he enumerates are complied with by our theory of heredity, and by no other explanation which has ever been proposed. Thus he says (p. 244) that "It is quite conceivable that the material organic world may be so constituted that the simultaneous action upon it of all known forces, mechanical, physical, chemical, magnetic, terrestrial and cosmical, together with other as yet unknown forces which probably exist, may result in changes which are harmonious and symmetrical, just as the internal nature of vibrating plates causes particles of sand scattered over them to assume definite and symmetrical figures when made to oscillate in different ways by the bow of a violin being drawn along their edges. The results of these combined internal powers and external influences might be represented under the symbol of complex series of vibrations (analogous to those of sound and light) forming a most complex harmony or a display of most varied colors.

"In such a way the reperation of local injuries might be symbolized as a filling up and completion of an interrupted rhythm. Thus also monstrous aberrations from typical structure might correspond to a discord, and sterility from crossing be compared with the darkness resulting from the interference of waves of light.

"Such symbolism will harmonize with the peculiar

reproduction, before mentioned, of heads in the body of certain annelids, with the facts of serial homology, as well as those of bilateral and vertical symmetry. Also as the atoms of a resonant body may be made to give out sound by the juxtaposition of a vibrating tuningfork, so it is conceivable that the physiological units of a living organism may be so influenced by surrounding conditions (organic and other) that the accumulation of these conditions may upset the previous rhythm of such units, producing modifications in them-a fresh chord in the harmony of Nature—a new species. . . It seems probable, therefore, that new species may arise from some constitutional affection of parental forms—an affection mainly if not exclusively of their generative system."

According to the view which I have presented a new variation is caused in essentially the manner which Mivart suggests as probable. The accumulated influence of surrounding conditions, organic and inorganic, does upset the previous rhythm of the physiological units of the living organism, and causes them to give rise to gemmules, and the tendency of the corresponding units of the offspring to vary, is directly due to this constitutional affection of the parental forms.

I have spoken of the egg as containing material particles of some kind to represent each of the hereditary congenital peculiarities of the race. According to this view the egg of one of the higher animals must be a wonderfully complex structure. At first sight it would. seem as if it must be as complicated as the adult animal, but a little thought will show that this is by no means the case.

In the first place, there are many structures which enter into the formation of the body without being part of its actual living substance. Nearly every living

body consists in part of structures which are in no sense alive, but which are built up by the formative activity of the living protoplasm. The shell of a snail or of an oyster is purely inorganic, and although it is built up by the animal, and is necessary to its existence, it is no more a part of the living substance of the animal than the shell which is picked up and inhabited by a hermit crab. It is true that the oyster's shell is formed by the animal, as part of itself, but the shell does not grow, like living tissues, by the absorption and transformation of nutriment, but by the crystallization of the amorphous mineral matter which is poured out by the living cells of the mantle; and microscopic examination shows that it is not an organized tissue made up of cells, but an aggregate of purely mineral crystals.

Since this is the case it is clear that it is not the shell itself, but a tendency to build the shell, which is hereditary, and is contained in the egg; and an illustration will serve to show that the inheritance of the tendency involves much less complexity in the structure of the egg than the inheritance of the thing itself would imply.

A bee inherits a tendency to build up a comb of wax, and to fill the cells of this comb with honey.

The comb and the honey are due to the vital activity of the bee, just as the shell is the result of the vital activity of the oyster; but the statement that the bee's egg contains something which corresponds to the structural organization to which the tendency is due, is certrinly not equivalent to a statement that the actual comb, filled with honey, is represented in the egg. This is just as true of structures which are built up, inside the body, by its vital activity, as it is of those which are built up in the same way outside the body.

When we take into account all structures of this kind