The question of the inheritance of deformities and diseases, and the causes of the germ-variations on which it depends, have been considered by Zeigler, whose conclusions, as quoted by Dr. Eimer (page 186), are too important to be omitted. The causes which Zeigler assigns for the origin of such germ-variations are of three kinds. These are: 1--Union of sexual nuclei which are not adapted for copulation; 2--Disturbance of the copulatory process itself; 3--Injurious influences which affect the sexual nuclei or the fertilized ovum at a time when separation of the sexual cells from the body cells has not yet occurred. "If the embryo is injuriously affected at a later period," says Zeigler, "either a malformation or a constitutional anomaly arises, which is not inherited, or only the sexual cells are injured, in which case the body-cells develop normally, and a disturbance shows itself only in the development of the next generation." The union of sexual nuclei not adapted for copulation appears, however, to be "the most frequent and most important cause of hereditary local malformations as well as of hereditary morbid tendencies, or of a defect in any system of the whole organism." If the nuclei are altogether unadapted to each other, sterility occurs, as in the sexual nuclei of distinct species. PSYCHICAL DISEASES.--Zeigler's conclusions are supported by reference to the enquiries of the distinguished psychiatrist, D. Von Krafft-Ebings, who has considered the heredity of psychical diseases, and in connection therewith mentions three "essential facts" which it is necessary to keep in view when dealing with that subject. The first of these facts is Atavism, by which "the bodily and mental organization and character can be transmitted from the first to the third generation, without any necessity that the second and intermediate one should exhibit the peculiarities of the first--thus the condition of the life and health of the grandparents are of interest for us." Secondly, "Only in rare cases is the actual disease transmitted in procreation (congenital insanity, hereditary syphilis), as a rule only the disposition thereto. Actual disease only occurs when accessory injurious influences produce an effect based upon that disposition. . . . We must, therefore, consider also the state of health of the relatives (uncles, cousins, aunts), and since here also the law of atavism holds good, the possible diseases of great-uncles and great-aunts." Thirdly, Dr. Von Krafft-Ebings says, "Only exceptionally does the same disease develop in ascendant as in descendant lines, in consequence of the transmission of morbid dispositions. On the contrary, there exists a remarkable variability in the forms of disease which may almost claim the value of a law (the law of polymorphism or transmutation)."