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mother. It is plain that the child of a beardless boy could not inherit the “ soul-stuff" of a beard in the way Jäger imagines, and this fact alone is enough to show that he has not discovered the true secret of heredity.
We know, too, that reversion, the appearance in the child of the features inherited from a remote ancestor but not shared by its parents, is not at all unusual, and must be regarded as one of the leading characteristics of heredity. It is plain that if the embryonic ovum is, as Jäger states, unspecialized or “de-souled,” reversion is inexplicable. Accordingly, when he comes to discuss reversion he makes a fundamental change in his hypothesis, and holds that when the ovum divides, at a very early stage of its development, into two parts, an ontogenetic portion, which gives rise to the new organism, and a phylogenetic portion, which ultimately forms the germinative cells of its reproductive organ, the second part is not unspecialized or “ de-souled” at all, but really retains all the characteristics of the ovum which gives rise to it, and is therefore capable, like the ovum, of giving rise to a new organism.
As thus remodelled, I believe, and hope to show in the sequel, that Jäger's hypothesis is a close approximation to the truth, but it is only fair to point out that in its altered form it is not original with Jäger. The author' published almost exactly the same view in 1870 (“On a Provisional Hypothesis of Pangenesis,” Proc. Amér. Assn., 1876, and American Naturalist, March, 1877), and it had been stated as long ago as 1849 by Prof. Owen, in his paper on Parthenogenesis, although this author, in his “ Anatomy of Vertebrates," afterwards states that he now believes it to be fundamentally erroneous. It is plain, too, that in its second form Jäger's hypothesis is one of crolution, pure and simple, for the egg is, at no stage of its growth, unspecialized, and it does not require the assimilation of “ soul-stuff” in order to develop into an organism.
We must conclude, then, that however satisfactory and accordant with observed fact the hypothesis of epigenesis seems to be at first sight, more careful analysis shows that it is in no sense a true explanation of the phenomenon of development.
The analogy between the evolution of the species from an unicellular ancestor, and the development of the individual from an unicellular egg, is simply an analogy, for the cause of the first phenomenon, the selection of congenital variations, is wanting in the second case, and there is nothing to take its place if it is truc that an egg is really, like a rhizopod, an unspecialized cell.
Haeckel's statement that heredity is memory, however true it may be, cannot be accepted as an explanation, for we have no knowledge of the existence of memory apart from organization, and we cannot conceive that an ovum can retain the memory of the past history of its species, unless it possesses a corresponding organization.
Jäger's view that the embryonic ovum ix unspecial- S ized, and that its specialization is gradually assimilated during the development of the organism which contains it, fails to account for the phenomena of reversion, and to account for reversion he is compelled to assume that the egg is organized from the time of its origin in the developing egg of the preceding generation. · In each case we are driven to the same conclusion, that the epigenesis hypothesis is inadequate; and we are forced to accept some form of the evolution hypothesis.
This necessity has not escaped the notice of some of our most acute thinkers. Huxley, for example, says
(Encyc. Brit., Art. Evolution), “ Harvey's definition of a germ as 'matter potentially alive, and having within itself the tendency to assume a definite living form,' appears to meet all the requirements of modern science. For notwithstanding it might be justly questioned whether a germ is not merely potentially but rather actually alive, though its vital manifestations are reduced to a minimum, the term potential may fairly be used in a sense broad enough to escape the objection. And the qualification of potential has the advantage of reminding us that the great characteristic of the germ is not so much what it is, but what it may under suitable conditions become. “From this point of view the process, which in its superficial aspects is epigenesis, appears in essence to be evolution, ... and development is merely the expansion of a potential organism or organic preformation according to fixed laws.”
HISTORY OF THE THEORY OF HEREDITY
Some form of the evolution hypothesis a logical necessity
Darwin's pangenesis hypothesis— This is an evolution hypothesis, since all the characteristics of the adult are supposed to be latent in the germ-Miscellaneous objections to it-These objections do not show that it conflicts with fact-Difficulty in imagining detailed working is no reason for rejecting it -Galton's experimental disproof-There are many reasons for believing that the sexual elements have different functions - The evidence from parthenogenesis-Polar-cell hypothesis -The evidence from hybrids, from variation, and from structures confined to one sex—The pangenesis hypothesis recognizes no such difference in the functions of the reproductive elements—We must therefore distrust its absolute correctness -Summary of last two chapters.
Some Form of the Evolution Hypothesis a Logical
Necessity. Most of the hypotheses which have been proposed, of late years, to account for the phenomena of heredity, are like the two we have quoted, epigenesis hypothesis, for they are attempts to show that the ovum is in reality, as well as in form, an unspecialized cell. Analysis shows, however, that they all rest ultimately upon the assumption that this is not true, but that the ovum really contains, in some form or other, actually or potentially, the future organism, with all its hereditary characteristics.
We know that eggs which are to all appearances essentially alike, may, when artificially removed from the ova
ries and artificially fertilized, and when kept under exactly the same conditions, develop into widely different organisms, and as like things canrot, under like conditions, give rise to different results, we are forced to conclude that these eggs are not essentially alike, but that each contains within itself in some form the organism to which it is to give rise—that individual development is, in some sense, the unfolding of a germ which already exists in the egg. There is no escape from this conclusion, at least there is none which can be accepted by the scientific student, and we see that logical thinkers like Prof. Huxley are driven to conclude that the process which in its superficial aspects is epigenesis, appears in essence to be evolution.
Darwin's Hypothesis of Pangenesis. In contrast to the views already quoted we have the well-known pangenesis hypothesis of Darwin, an hypothesis which is thoroughly one of evolution, since Darwin believes that the whole organization of the species is present not only in the egg but in the male cell also; that each of these not only contains the complete organization of the parent, but an indefinite series of similar organizations, inherited from a long line of ancestors. It is true that Darwin does not believe that each of these ancestors is represented in the ovum and in the male cell by a minute but perfect animal, like those imagined by Bonnet, but he imagines what is essentially the same thing, that each of the cells of each parent, and every cell of each ancestor for a long and practically an unlimited series of generations, is represented in each ovum and each male cell by a germ capable of producing that particular cell with all its distinctive characteristics.
Darwin's original statement (Variation, chaps. xxvii.