Page:Popular Science Monthly Volume 24.djvu/228

216 wrong, as a forty-shilling watch. Nothing is more certain than that every man and woman is like their progenitors in the main. It takes generations for new conditions of life to eradicate hereditary peculiarities, and then they are always tending to come back. These hereditary peculiarities in youth are mostly not seen as actualities that can be pointed out and proved to exist by any outward signs. They exist as potentialities only, and come out as actual measurable and ascertainable facts at certain ages, or under certain conditions. A young man who inherits gout strongly may for the first five-and-twenty years of his life be absolutely free from any trace of the disease. Yet we are warranted in inferring that something is there which must be taken into account in the diet and conditions of life, if we wish to contract and eradicate the tendency. Many nervous diseases and conditions are the most hereditary of all, and we have good reason to think that, in those subject to them, the conditions of life, and the treatment to which the brain and the rest of the nervous system are subjected during the period of the building of the constitution that is, during adolescence from thirteen to twenty-five—are of the highest importance in hastening and accentuating, or retarding and lessening, those nervous peculiarities. The problems of the hereditary transmission of qualities and tendencies to disease are some of the most wonderful in nature, and they are as yet by no means clearly elucidated. Many of them, as yet, can not be brought under any law. In our present state of physiological knowledge, it is, for instance, a quite inconceivable thing what takes place when we have two generations of perfectly healthy persons intervening between an insane great-grandmother and an insane great-grandchild. The grandparent and the parent carried something in their constitutions which was never appreciable to us at all. Yet it was there just as certainly as if it had broken out as a disease. It is one of the future problems of physiology and medicine to deduce the exact laws of heredity in living beings, and to counteract the evil hereditary tendencies through conditions of life. To do the latter we shall undoubtedly have to begin early in life, and we shall have to control the education especially, and make it conformable to Nature's indications, laws, and conditions.

Another law of living beings to be kept in mind is this: There is a certain general energy in the organism which may be used in many directions, and may take different forms, such as for growth, nutrition, muscular force, thinking, feeling, or acquiring knowledge, according as it is called out or needed. But its total amount is strictly limited, and if it is used to do one thing, then it is not available for another. If you use the force of your steam-engine for generating electricity, you can not have it for sawing your wood. If you have the vital energy doing the work of building the bones and muscles and brain during the year that a girl grows two inches in height, and gains a stone in weight, you can not have it that year for the acquisition of