Benjamin Rush[8] (greatly influenced by the Erasmus Darwin school) remarks that through hereditary sameness of organisation of the nerves, brain and blood vessels, the predisposition to insanity pervades whole families and renders them liable to this disease from a transient and feeble operation of its causes. Insanity when hereditary is excited by more feeble causes than in persons in whom this predisposition has been acquired. It generally attacks the descendants in those stages of life in which it has appeared in the ancestors. Children born previously to the attack of madness in their parents are less liable to inherit it than those who are born after it. Children born of parents who are in the decline of life are more predisposed to insanity than children born under contrary circumstances. A predisposition to certain diseases, seated in parts contiguous to the seat of insanity, often descends from parents to their children. Thus it occurs in a son whose father or mother has been afflicted only with hysteria or habitual headaches. The reverse likewise takes place. There are families in which insanity has existed where the disease has spared the mind in the posterity, but appeared in great strength and eccentricity of the memory and of the passions, or in great perversion of their moral faculties. Sometimes it passes by all the faculties of the mind, and appears only in the nervous system of persons descended from deranged parents; again, madness occurs in children whose parents were remarkable only for eccentricity of mind. Among the diseases that attack the children of the insane, but did not exist in their ancestors, are consumption and epilepsy. Similar studies were later published by Pinel, Tissot, Chiarrurgi, Stedman, Parkman, Brigham, Prichard, Esquirol, Jacobi, and other American, English, French, Italian, and German alienists. Based upon the data thus obtained, and upon the general principles thus outlined, then appeared--nearly at the same time as the like epoch-making work (on another phase of evolution), Darwin's _Origin of Species_--Morel's _Treatise on Human Degeneracy_, wherein the principle of natural selection was shown to involve the recognition of the physical conditions that constitute degeneracy, and, necessarily, to exclude primeval perfection. Morel's definition of degeneration as a marked departure from the original type tending more or less rapidly to the extinction of it, forms the basis of commonly accepted definitions.