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First, so to speak, the corps separate to preside over the formation of different body regions. Then the different divisions, brigades, and regiments, composing each next higher unit, separate, being detailed to form ever smaller portions of the body. The process of changing germ-plasm into somatoplasm is one of disintegration. The germ-plasm contains representatives of the whole army; a somatic cell only representatives of one special arm of a special training. Germ-plasm in the egg is like Humpty-Dumpty on the wall; somatoplasm, like Humpty-Dumpty after his great fall.
I use these rude illustrations to make clear one point: Germ-plasm can easily change into somatoplasm, but somatoplasm once formed can never be reconverted into germ-plasm, any more than the fallen hero of the nursery rhyme could ever be restored.
The germ-plasm is, according to Weismann, a very peculiar, complex, stable substance, continuous from generation to generation since the first appearance of life on the globe. It is in the body of the parent, but scarcely of it. Its relation to the body is like that of a plant to the soil or of a parasite to its host. It receives from the body practically only transport and nourishment. It is like a self-perpetuating, close corporation; and the somatoplasm has no means of either controlling it or of gaining representation in it.
Says Weismann*: "The germ-cells are contained in the organism, and the external influences which affect them are intimately connected with the state of the organism in which they lie hid. If it be well nourished, the germ-cells will have abundant nutriment; and, conversely, if it be weak and sickly, the germ-cells will * Essays upon Heredity, p. 105.
be arrested in their growth. It is even possible that the effects of these influences may be more specialized; that is to say, they may act only upon certain parts of the germ-cells. But this is indeed very different from believing that the changes of the organism which result from external stimuli can be transmitted to the germ-cells and will redevelop in the next generation at the same time as that at which they arose in the parent, and in the same part of the organism."
But if the germ-plasm has this constitution and relation to the rest of the body, how is any variation possible? Different individuals of any species have slightly different congenital tendencies. Hence in the act of fertilization two germ-plasms of slightly different structure and tendency are mingled. The mingling of the two produces a germ-plasm and individual differing from both of the parents. Thus, according to Weismann's earlier view, the origin of variation was to be sought in sexual reproduction through the mingling of slightly different germ-plasms.
But how did these two germ-plasms come to be different? How was the variation started? To explain this Weismann went back to the unicellular protozoa. These animals are undoubtedly influenced by environment and vary under its stimuli. Here the variations were stamped upon the germ-plasm, and the commingling of these variously stamped germ-plasms has resulted in all the variations of higher animals.
Of late Weismann has modified and greately improved this portion of his theory. He now accepts the view that external influences may act upon the germ-plasm not only in protozoa but also in all higher animals. Variation is thus due to the action or stimu
lus of external influences, supplemented by sexual reproduction.
But the very constitution of the germ-plasm and its relation to the body absolutely forbids the transmission of acquired somatic characteristics and of the special effects of use and disuse. Muscular activity promotes general health, and might thus conduce to better-nourished germ-cells and to more vigorous and therefore athletic descendants. The exercise of the muscles might possibly cause such a condition of the blood that the portion of the germ-plasm representing the muscular system of the next generation might be especially nourished or stimulated. Thus an athletic parent might produce more athletic children.
But let us imagine twin brothers of equal muscular development. One from childhood on exercises the lower half of his body; the other, the upper. Both take the same amount of exercise, and have perhaps equal muscular development, but located in different halves of the body. Now it is hard to conceive that it can make any difference in the nourishing or stimulating influence of the blood, whether the muscular activity resides in one half of the body or the other. The children might be exactly alike.
One man drives the pen, a second plays the piano, and a third wields a light hammer. All three use different muscles of the hand and arm. How can this use of special muscles stamp itself upon the germ-cells in such a way that the offspring will have these special muscles enlarged? Granting that external influences of environment and bodily condition may effect the germ-cells; granting even that some of the most general effects of use and disuse might be transmitted,
what warrant have we for believing that the special acquired characteristic can be transmitted? Weismann answers, None at all. The somatoplasm can only in the most general way affect the self-perpetuating, close corporation of the germ-plasm.*
There is thus, according to Weismann, nothing to direct variation to certain organs, or to guide and combine the variations of these organs along certain lines, except natural selection. To a certain extent variation may be limited by the very structure of the animal. But within these limits there are wide ranges where one variation is apparently just as likely to occur as another.
Within these wide limits variation appears to be fortuitous. Natural selection must wait until the individuals appear in which these variations occur already correlated, and then seize upon these individuals. It is apparently the only guiding, directing force. Linear variation, that is, a variation advancing continuously along one or very few straight lines, would appear to be impossible.
In Nägeli's theory initial tendency is overwhelmingly dominant; in Weismann's, natural selection is almighty.
Weismann's followers have received the name of Neo-Darwinians. The so-called Neo - Lamarckian school believes in the transmissibility of acquired characteristics, and of at least particular effects of use and disuse. The one theory is neither more nor less Darwinian than the other. For while Darwin emphasized natural selection, he accepted to a certain extent the transmission of special effects of use and disuse.
* Weismann, Essays, p. 286.
A special theory of heredity, pangenesis, has been accepted by many of the Neo-Lamarckian school. The theory of pangenesis, as propounded by Mr. Darwin, may be very briefly stated as follows: The cells in all parts of the body are continually throwing off germinal particles, or "gemmules." These become scattered through the body, grow, and multiply by division. On account of mutual attraction they unite in the reproductive glands to form eggs or spermatozoa. The germ-cells are thus the bearers of heredity because they contain samples, so to speak, of all the organs of the body.
In heredity, according to Weismann's theory, the egg is the centre of control, the continuous germ-plasm the source of all transmitted changes; according to Darwin's theory, the body is the source, and the egg is derived in great part at least from it. If you put to the two the time-honored question, Which is first, the owl or the egg? Weismann would announce, with emphasis, The egg; Darwin would say, The owl. One proposition is the converse of the other, and most facts accord almost equally well with both theories.
In any family, devoted for generations to literary or artistic pursuits, the children show, as a rule, an aptitude for such pursuits not manifested by those of other families. According to the Neo-Lamarckian view, this inherited aptitude is to a certain extent the result of the constant exercise of these faculties through a series of generations. The active efforts and voluntary disposition of the parents have given an increased predisposition to the child. "Quite the reverse," says Weismann, "the increase of an organ in the course of generations does not depend upon the