These morbid manifestations of heredity occur in certain categories, either local as to organs or structures, or affecting the body as a whole. These categories Moreau (de Tours) lucidly sums up as: First, absence of conception; second, retardation of conception; third, imperfect conception; fourth, incomplete products (monstrosities); fifth, products whose mental, moral, and physical constitution is imperfect; sixth, products specially exposed to nervous disorders in order of frequency as follows--epilepsy, imbecility or idiocy, deaf-mutism, insanity, cerebral paralysis, and other cerebral disorders; seventh, lymphatic products; eighth, products which die in infancy in a greater proportion than sound infants under like conditions; ninth, products which, although they escape the stress of infancy, are less adapted than others to resist disease and death. The explanation of these morbid manifestations lies in the very foundations of embryology. Bearing in mind the principles of individuation pointed out by Spencer, it is easily understood how reversal of this principle would produce greater and greater destruction of the complex functions, resultant on increased reproductive power of cells (whose environment is not suited to such reproductions) and thus lead to such a struggle for existence as to produce sterility (from interdestruction of the ovum cells, or the cells forming the spermatozoon). This condition is further increased by the operation of two biologic principles. The first relates to the cells or organs forming an organism. The second, as Von Baer has shown, deals with the relation of the organs to each other. Vertebrate embryos of a common type, at their origin, assume successively a number of common forms before definitely differentiating. Dareste points out that supernumerary organs do exist in these common forms at one phase of embryonic life. This community of embryonic types and this last fact explain repetition of teratologic types or monstrosities in vertebrates. This community of origin, moreover, indicates that a higher vertebrate embryo contains in essence the organs and potentialities of lower vertebrates, and that under the influence of heredity or accidental defect an organ belonging to another species may develop, or an organ constant in a species may be lacking in an individual, without the necessity of explaining the immediate effects by distant atavism. Some anomalies found among degenerates recall types less elevated than man, and very distant from him, even his possible Lemurian precursor.