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although it may be an approximation to the truth, it cannot be regarded as a complete and satisfactory explanation.
The object of this work is to present a new hypothesis which will be seen to bear a close resemblance to the one which has been advocated by Darwin, although careful examination will show that it is in reality very different. I hope to show that it is not open to the objections which are urged against the pangenesis hypothesis, while it contains all the features which give value to the latter.
A NEW THEORY OF HEREDITY.
The objection to the hypothesis of pangenesis would be almost entirely removed if it could be simplified—Statement of a new theory—Heredity is due to the properties of the egg—Each new character has been impressed upon the egg by the transmission of gemmules—Tendency to form gemmules is due to the direct action of external conditions—The ovum is the conservative element—The male cell is the progressive element— This theory has features of resemblance to most of the hypotheses which have been noticed—It fills most of Mivart's conditions also—It is not necessary to assume that the ovum is as complicated as the adult—There are many race characters which are not congenital—There are many congenital characters which are not hereditary—Direct action of external conditions—Our theory stands midway between Darwin's theory of natural selection and Lamarckianism.
If the hypothesis of pangenesis could be so remodelled as to demand the transmission of only a few gemmules from the various parts of the body to the reproductive elements, instead of the countless numbers which are demanded by the hypothesis in its original form, we should escape many of the objections which have been urged against it.
If it can be shown that these few gemmules are not necessarily present at all times and in all parts of the body, but only occasionally and in certain regions, we shall escape the difficulty presented by Galton's experiments, and the presumption in favor of the hypothesis will be greatly increased.
If the theory of heredity, in its new form, agrees with all that we know of the functions of the two sexual elements and if, besides furnishing an explanation of all the phenomena which are accounted for by other hypotheses, it embraces new classes of facts as well, the presumption in its favor becomes still greater.
Finally, if it leads to the discovery of new and unexpected relations between phenomena, and to the establishment of laws which group and interpret phenomena between which no connection had previously been recognized, its value must be acknowledged.
I venture, then, to advance a new theory of heredity, which, briefly stated, is as follows:
The union of two sexual elements gives variability. Conjugation is the primitive form of sexual reproduction. Here the functions of the two elements are alike, and the union of parts derived from the bodies of two parents simply insures variability in the offspring.
In all multicellular organisms the ovum and the male cell have gradually become specialized in different directions.
The ovum is a cell which has gradually acquired a complicated organization, and which contains material particles of some kind to correspond to each of the hereditary characteristics of the species.
The ovum, like other cells, is able to reproduce its like, and it not only gives rise during its development to the divergent cells of the organism, but also to cells^ like itself. ••^^^ \
The ovarian ova of the offspring are these1',.' >tv: or their direct unmodified descenda
Each cell of the body / ^^morphologica\ '^-uee,' an independent individual »*• .Nthe power to grow, to give rise by divisio/ > , .iv "} / cells, and to throw off minute germs. IX,' y^olution of the species it has by natural selection acquired distinctive properties or functions, which are adapted to the conditions under which it is placed. So long as these conditions remain unchanged it performs its proper function as a part of the body; but when, through a change in its environment, its function is disturbed and its conditions of life become unfavorable, it throws off small particles which are the germs or "gemmules" of this particular cell.
These germs may be carried to all parts of the body. They may penetrate to an ovarian ovum or to a bud, but the male cell has gradually acquired, as its especial and distinctive function, a peculiar power to gather and store up germs.
When the ovum is fertilized each germ or "gemmule" unites with, conjugates with, or impregnates, that particle of the ovum which is destined to give rise in the offspring to the cell which corresponds to the one which produced the germ or gemmule; or else it unites with a closely related particle, destined to give rise to a closely related cell.
When this cell becomes developed in the body of the offspring it will be a hybrid, and it will therefore tend to vary.
As the ovarian ova of the offspring share by direct inheritance all the properties of the fertilized ovum, the organisms to which they ultimately give rise will tend
to varv in the same way.
1 which has thus varied will continue to throw ova by direct inheritance, the characteristic will be established as an hereditary race characteristic, and will be perpetuated and transmitted, by the selected individuals and their descendants, without gem&ules.
According to this view, the origin of a new variation is neither purely fortuitous nor due to the direct and definite modifying influence of changed conditions. A change in the environment of a cell causes it to throw off genimules, and thus to transmit to descendants a tendency to vary in the part which is affected by the change.
The occurrence of a variation is due to the direct action of external conditions, but its precise character is not. My view of the cause of variation is thus seen to be midway between that accepted by Darwin and that advocated by Semper and other Lamarkians.
Many naturalists have given reasons for believing that the transmutation of species is not always gradual, but that a form which has ^ong persisted without change may suddenly vary greatly, and thus give rise to a stronglymarked race of descendants. Mivart has discussed this subject at considerable length, and he quotes Professor Huxley's opinion that "we greatly suspect that Nature does make considerable jumps in the way of variation now and then, and that these saltations give rise to some of the gaps which appear to exist in the series of known forms;" and Dall has proposed the term saltatory evolution for abrupt change of this kind. According to the theory here advanced, variation must tend to accumulate or culminate, and one variation must cause others; for when any particular cell changes, the harmonious adjustment between it and adjacent or related cells will be disturbed, and all the cells which are thus affected will tend to throw off gemmules, and thus to induce variability in the same cells of succeeding generations. Then, too, a