It is not through inheritance only that diseases may be transmitted from parents to children; the same result may follow from the fact that parents and children live in such close association, or because children are brought up by their parents. The existence of morbid qualities or conditions in the parents may lead in the offspring, not only to the inheritance of disease, but to other disastrous results. Morbid conditions in either parent, besides being transmitted to the offspring by inheritance, may be communicated by the husband to the wife, or by the wife to the husband, either in the act of sexual intercourse, or through the close association of married life. Parents suffering from disease cannot bring up their children properly. The treatment of their own illness may be very costly, and may involve the expenditure of much time and pains, and these things work adversely to the interest of the children. Sickly parents whose children are likewise sickly are apt to endeavour to make up for the deficient quality of their offspring by an increase in their number, whereby matters are made considerably worse. Those who enter into marriage when already ill are apt subsequently to reproach themselves upon their conduct towards their sexual partners; this is likely to react unfavourably upon the illness, and to disturb the married life, to the disadvantage of the children. Sickly parents die sooner than healthy ones, whereby the children are prematurely orphaned, and are exposed to the dangers of poverty. The state of engagement to marry (with consequent ungratified sexual excitement up to the time of marriage), sexual intercourse, pregnancy, and childbirth, may all exercise an unfavourable influence upon the diseased organism, may favour or accelerate the course of the disease, and may even lead to its fatal issue. From the children’s point of view, all these things are extremely undesirable. Thus, there are certain persons to whom marriage is permissible, but who should on no account procreate children--that is to say, such a married pair may enjoy sexual congress, but must not fail to use efficient means for the prevention of conception. _Individual Diseases._--(_a_) Of all diseases transmissible by inheritance, mental disorders pass most readily from parents to offspring, and undergo the least alteration as they pass. In the etiology of mental disorders, hereditary transmission plays an important part.