We often see consumption affecting several members of the same family through several generations. The same is true of insanity. Gout is many times transmitted from father to son, but seldom to a third generation. Epilepsy, also, does not often extend to grandchildren, nor does cancer. Each of these diseases may come without inheritance. Then, we can sometimes, though not always, find at least a partial explanation of their origin otherwise.
Not all (if there be several) children in a family are likely to have the inheritable disease. Perhaps all may escape it; now and then it comes again in their children, having skipped a whole generation.
Children are not born with transmitted diseases; except syphilis, among those of real constitutional inheritance, and a few of the contagious affections. They are commonly affected with them about the time of life when their parents were so. Thus scrofulous disorders of the eyes, ears, skin, glands, and bones, are apt to show themselves in childhood; consumption of the lungs, in youth or early maturity ; gout near middle age; apoplexy, and. disease of the heart, from fifty to seventy years ; early deafness, or blindness, at various periods in different families.
Sometimes the inherited taint is modified in transmission. Thus the children of a gouty person may have, not regular gout, but neuralgia ; and the offspring of one who is insane may have inflammation of the brain, or convulsions*, etc. Children of intemperate parents are very likely to have some impairment of their nervous system, and often die in infancy.
Besides these special transmissions of tendencies to disease, there is a gradually degenerating influence in families, and even whole populations, from unhealthy living. It is most observed in large cities.