_Heredity in General._--Heredity is a general phenomenon of natural life. The offspring resembles the parent to a greater or less degree. In the human species, also, children on the average resemble their own parents more closely than they resemble other persons, but the degree to which this resemblance is manifested is a variable one, and it remains an open question whether the concrete qualities and capabilities whereby the parents are distinguished from other persons are transmitted to their offspring by inheritance. It is possible that the qualities of the parents may not appear at all in the children; that they may appear somewhat modified, or in a very different form; that they may remain latent in one or more generations, to reappear in the grandchildren or great-grandchildren; or that the qualities of the parents may not appear to be present in the children at birth, but that as the latter grow up, these qualities may make their appearance at the same age at which they appeared in the parents. It is still in dispute whether qualities acquired by the parents are inherited by the offspring; nor is there general agreement as to what are the precise limits between inherited and acquired characters, respectively. This question as to the inheritance of acquired characters is one of profound importance; for if acquired characters are not inherited, racial improvement can be effected solely by means of the struggle for existence, and the continued elimination of the weaker elements of the species.[3] _Inheritance of Diseases._--The word “disease” is used here in the widest possible signification. Diseased parents as a rule procreate diseased children, or bring up diseased individuals. But the physical, mental, and moral defects of the parents may make their appearance in the children in a transmuted form. A disease in the parent, when transmitted by inheritance, may appear in the offspring as general weakness, either bodily, mental, or moral; or it may appear in the form of a predisposition to the particular disease; and conversely, that which in the parent is no more than predisposition to a disease, may appear in the offspring in the form of the actual disease. In concrete instances, it may be very difficult to determine whether persons are or are not diseased. Again, with respect to atavism and to the hereditary transmission of latent qualities, it is questionable whether, in cases in which the subject of investigation is not himself affected with disease, but his near relatives are so affected, we have reason to fear the hereditary transmission of harmful consequences to the offspring. It is especially with regard to the male sex that the question of the hereditary transmission of morbid qualities is so important, for, in marriage, it is the male partner who contributes the greater proportion of the diseases.