M. Ribot, who, as we have seen, endorses the popular belief as to the possibility of the fetus being affected, during uterine existence, through the organism of the mother, reduces all the obscure causes of deviation from heredity to two classes. Of these, the first is the disproportion of effects to causes, already mentioned; and the second is the transformation of heredity. As to the first of these causes, he lays it down as a general truth that "the more complicated the mechanism, the greater the disproportion between accidental causes and their effects." He supports this conclusion by reference to Geoffroy Saint-Hilaire's researches on the production of monsters, and he affirms that the disproportion between cause and effect cannot be foreseen by measuring, but is known only by experience, as "psychological laws are analogous now to mechanical and now to chemical laws," so that it is impossible to proceed by deduction from causes to effects. (Page 207.)
Birthmarks Rare.—And yet the very fact that cases of birthmarks are comparatively rare, proves the greatly preponderating influence of heredity over the constitution of the offspring, modified by the disposition of the parents at the time of procreation. Professor Cope has some explanatory remarks on that subject which deserve quotation. He says—after referring to the hypothesis that growth-force may be, through the motive force of the animal, directed to any locality, whether the commencement of an executive organ has begun or not—that "A difficulty in the way of this hypothesis is the frequently unyielding character of the structure of adult animals, and the difficulty of bringing sufficient pressure to bear on them without destroying life. But, in fact, the modifications must, in most instances, take place during the period of growth. It is well known that the mental characteristics of the father are transmitted through the spermatozoid, and that, therefore, the molecular movements which produce the mechanism of such mental characters must exist in the spermatozoid. But the material of the spermatozoid is combined with that of the ovum, and the embryo is compounded of the animal contents of both bodies. In a wonderful way the embryo develops into a being which resembles one or both parents in minute details. This result is evidently determined by the molecular and dynamic character of the original reproductive cells which necessarily communicate their properties to the embryo which is produced by their subdivisions." Professor Cope goes on to say, "Richard Hering has identified this property of the original cells with the faculty of memory. This is a brilliant thought, and, under restriction, probably correct. The sensations of persons who have suffered amputation show that their sensorium maintained a picture or map of the body so far as regards the location of all its sensitive regions. This simulcrum is invested with consciousness whenever the proper stimulus is applied, and the character of the stimulus is fixed by it. This picture probably resides in many of the cells, both sensory and motor, and it probably does so in the few cells of simple and low forms of life. The spermatozoid is such a cell, and, how or why we know not, also contains such an arrangement of its contents, and contains and communicates such a type of force. It is probable that in the brain-cell this is the condition of memory of locality. If, now, an intense and long-continued pressure of stimulus produces an unconscious picture of some organ of the body in the mind, there is reason to suppose that the energies communicated to the embryo by the spermatozoid and ovum will partake of the memory thus created. The only reason why the oft-repeated stories of birthmarks are so often untrue, is because the effect of temporary impressions on the mother is not strong enough to counterbalance the molecular structure established by impressions often repeated throughout much larger periods of time."[71:A]
Why Children Resemble Parents.—That children reproduce the general and physical and mental characteristics of their parents in combination is unquestionable truth, although the particular mode in which they are communicated is yet undetermined, notwithstanding the fact mentioned by Professor Cope that they are somehow conveyed by the microscopic sperm and germ in the union of which the new being has its beginning. Thus every individual must possess the general characteristics of the primitive human family from which through a vast number of ancestors he has descended. And yet at every stage of descent the organism may have obtained fresh characters, or at least have undergone some modification. As remarked by Dr. G. H. Th. Eimer, "Every character which must have been formed through the activity of the organism is an acquired character. All characters, therefore, which have been developed by exertion are acquired, and these characters are inherited from generation to generation. The same holds for all organs atrophied through disease—the degree of atrophy is acquired and inherited. In the first class we see especially the action of direct adaptation; in the second, the results of the cessation of the action. A third class of acquired characters is to be traced simply to the immediate action of the environment on the organism, and, originally, at the commencement of their appearance, all characters must have belonged to this class."[72:A] We have here a general argument in opposition to the theory propounded by Professor Weismann, that acquired characters are not transmissible. Elsewhere (page 382) Dr. Eimer observes: "Phyletic growth, or the evolution of the organic world ever into higher and more complex forms, or at least into forms of different structure, is, as I have said, merely the sum of the processes of growth of the ancestors—together with the result of external influences on the forms during their development and their existence. This additional modification which the individuals as such undergo is—together with the influence of crossing—the very cause of the constantly progressing evolution. All that the members of a series of individuals directly connected by descent acquire constitutes together the material for the formation of a new species."
Life's Experiences Affecting Child.—Unless characteristics acquired by an individual, that is, the modifications of the organism due to his own life experiences, are capable of being handed down to his offspring, it is difficult to see how any progress could be made in the development of the race. Weismann's declaration that acquired characters are not transmissible was a surprise to the scientific world when first made, but it has been accepted by many Darwinians. His conclusion is dependent on his doctrine of heredity, which differs from that propounded by Darwin, but is by no means new; as its leading ideas, as pointed out by Professor G. J. Romanes,[73:A] are largely a reproduction of those of Mr. Francis Galton, whose work on heredity attracted much attention when first published. The views of Darwin, Galton and Weismann on that subject have been compared by Professor Romanes, who explains the distinction between them. He says (page 133), after referring to the supposed continuity of the germ-plasm, common to the theories of Galton and Weismann, but not required by that of Darwin, "The three theories may be ranked thus—The particulate elements of heredity all proceed centripetally from somatic-cells to germ-cells (gemmules): the inheritance of acquired characters is therefore habitual.
"These particulate elements proceed for the most part, though not exclusively, from germ-cells to somatic-cells (stirp): the inheritance of acquired characters is therefore but occasional.
"The elements in question proceed exclusively in the centrifugal direction last mentioned (germ-plasm): the inheritance of acquired characters is therefore impossible."
The first of these theories is that of Darwin, and the last that of Weismann, whose notion of the continuity of germ-plasm supposes that no part of an organism generates any of the formative material which goes to make up its offspring. This material is regarded in much the same light as the sperm which the male parent confides to the keeping of the female, according to the notion of the ancient world above referred to. For, as Romanes states (page 26): "In each generation a small portion of this substance [germ-plasm] is told off to develop a new body to lodge and nourish the ever-growing and never-dying germ-plasm—this new body, therefore, resembling its so-called parent body simply because it has been developed from one and the same mass of formative material; and, lastly, that this formative material, or germ-plasm, has been continuous through all generations of successively perishing bodies, which therefore stand to it in much the same relation as annual shoots to a perennial stem: the shoots resemble one another simply because they are all grown from one and the same stock."
Although Professor Weismann denies that acquired characters, that is, individual peculiarities arising as the result of personal experience, are transmitted, he admits that congenital characters, that is, peculiarities with which an individual is born, are transmitted to offspring. As congenital characters must, originally, have been individual, it is not easy at first sight to perceive Weismann's real meaning. It is necessary, therefore, to enter more particularly into a consideration of his theory, which he regards as in general accord with Darwin's theory of pangenesis. Darwin supposes that all the cells of the body continually give off great numbers of gemmules, which are conveyed by the blood and deposited in the germ-cells of the organism. These cells are thus endowed with the power of developing a new organism of the same kind, each gemmule reproducing the cell from which it was derived. These ultimate vital units are called by Weismann biophors, but he supposes them not to be the ultimate "bearers of vitality." They are said to be arranged in groups to which the term determinants is applied, and these groups are combined so as to form ancestral ids or germ-plasms. Each determinant, which is made up of perfectly definite numbers and combinations of biophors, is the primary constituent of a particular cell, or of a group of cells, such as a blood corpuscle. The determinants thus "control the cell by breaking up into biophors, which migrate into the cell body through the nuclear membrane, multiply there, arrange themselves according to the forces within them, and determine the histological structure of the cell," impressing upon it its inherited specific character. The structure of the cell, and of every subsequent stage, exists therefore potentially in the inherited structure of the id, and the determination of its character "depends on the biophors which the corresponding determinant contains, and which it transmits to the cell."
Germ-plasm.—While Weismann regarded germ-plasm as absolutely stable, the only mode by which congenital variation could be brought about was that of amphimixis, or intermingling of individuals in the process of generation. As modified, however, by his latest work, "The Germ-plasm, a Theory of Heredity," published in 1892, his theory now allows the plasm to be capable of modification, and he ascribes that variation to the direct effects of external influences on the biophors and determinants of the germ-plasm. The instability of this substance is so slight, however, that congenital variations cannot be acted on and perpetuated by natural selection, and the influence of amphimixis is thus required for the purpose. Mr. Herbert Spencer, however, in criticising Weismann's theory, declares that "functionally produced modifications of structure are transmissible," and he refers in support of his contention to the remarkable effect of arrested nutrition on the structure and habits of wasps and bees. It especially affects the reproductive organs, and hence there is no occasion to call in the aid of amphimixis to perpetuate the variations produced, its office being the blending of the elements on which the characteristics of offspring depend.
If it be asked how modifications are actually transmitted, we may say that it can be only by an affection of the germ-cell. This probably takes place by deviations in the structure of what Weismann calls determinants, or of groups of determinants, through rearrangement of their primary units. The modification would be preceded, however, by a corresponding change in the nerve centers concerned in the use or disuse of the organs affected. Mr. Spencer shows that under certain conditions changes take place in the conduct of certain insects, and that "the maternal activities and instincts undergo analogous changes,"[77:A] facts which point to a loss of nervous energy and to an intimate connection between the nervous system and the reproductive function. Use or disuse first increases or diminishes the activity of certain nerve centers, and this leads to a modification of the corresponding germ-cells. If so, the determinants, instead of being first affected, as proposed by Weismann, and thus determining the variations, are in reality modified as the result of the functional changes, and are thus capable of transmitting these changes to succeeding generations.
