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.[A] [Footnote A: Weismann, Essays, p. 286.] 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.