Some light may be thrown on this deviation from the original type by considering the forces that are concerned in the hereditary act. The statement that like produces like is the expression of an obvious law. But we must bear in mind that the law is only so far observed as is necessary to maintain the characters of the species. Within that range there is every possible variety, and for a very obvious reason. Every individual represents immediately two others, and, indirectly, an indefinite number. This is done by uniting in himself qualities and features drawn from each parent, without any obvious principle or law of selection and combination. One parent may be, apparently, more fully represented than the other; the defects of the parent may be transmitted, rather than the excellences; the tendencies to health and strength may be outnumbered and overborne by the tendencies to disease. No individual, of course, can receive, entirely and completely, the features and attributes of both parents, for that would be a sort of practical absurdity; but in the process of selecting and combining, Nature exhibits the same inexhaustible variety that appears in all her operations. Even in the offspring of the same parents, however numerous, uniformity in this respect is seldom so obvious as diversity. This cerebral deterioration is subject to the same laws of descent as other traits, with a few exceptions without much bearing on the present question. We might as reasonably expect to see the nose or the eyes, the figure or the motions of either parent transmitted with the exactest likeness to all the offspring, as to suppose that an hereditary disease must necessarily be transmitted fully formed, with all the incidents and conditions which it possessed in the parent. And yet, in the case of mental disease, the current philosophy can recognize the evidence of transmission in no shape less demonstrative than delusion or raving. Contrary to all analogy, and contrary to all fact, it supposes that the hereditary affection must appear in the offspring in precisely the same degree of intensity which it had in the parent. If the son is stricken down with raving mania, like his father before him, then the relation of cause and effect is obvious enough; but if, on the contrary, the former exhibits only extraordinary outbreaks of passion, remarkable inequalities of spirit and disposition, irrelevant and inappropriate conduct, strange and unaccountable impulses, nothing of this kind is charged practically to the parental infirmity.