zi6 THE POPULAR SCIENCE MONTHLY, 



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 pecul- 

 iarities, and then they are always tending to come back. These heredi- 

 tary 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 ascertain- 

 able 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 dur- 

 ing the period of the building of the constitution that is, during ado- 

 lescence from thirteen to twenty-five are of the highest importance 

 in hastening and accentuating, or retarding and lessening, those nerv- 

 ous 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 dis- 

 ease. 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 



