Page:Popular Science Monthly Volume 85.djvu/321

Rh dwarfs; myopia in which the eyeball is elongated; glaucoma or swelling of the eyeball; coloboma, or open suture of the iris; otosclerosis, or thickened tympanic membrane, causing "hardness of hearing"; some forms of deaf-mutism, due to certain defects of the inner ear; and many other characters too numerous to mention here. On the other hand many abnormal or monstrous conditions are due to abnormal environment and are not inherited.

The question of the inheritance of diseases may be briefly considered here. If a disease is due to some defect in the hereditary constitution, it is inherited; otherwise, according to our definition of heredity, it is not. Of course no disease develops without extrinsic causes but when one individual takes a disease while another under the same conditions does not, the differential cause may be an inherited one, or it may be due to differences in the previous conditions of life. There is no doubt that certain diseases run in families and have the appearance of being inherited, but in this case as in many others it is extremely difficult in the absence of experiments to distinguish between effects due to intrinsic causes and those due to extrinsic ones. Where the specific cause of a disease is some microorganism the individual must have been infected at some time or other, almost invariably after birth. In few instances, is the oosperm itself infected, and even when it is this is not, strictly speaking, a case of inheritance, but rather one of early infection. Pearson has found that there is a marked correlation (represented by the number.55 when complete correlation is 1) between the tuberculous parents and tuberculous children, but there is very little evidence that the child is ever infected before birth. What is inherited in this case is probably slight resistance to the tubercle bacillus. There is evidence that almost all adult persons have been infected at one time or another by this bacillus, but it has not developed far in all of them because some have superior powers of resistance. Such greater or smaller resistance, stronger or weaker build, is inherited, and while diminished resistance is not the direct cause of tuberculosis it is a predisposing cause. The same is probably true of many other diseases, the immediate causes of which are extrinsic, while only the more remote, or predisposing causes, are hereditary.

(c) Physiological peculiarities are inherited as well as morphological ones; indeed function and structure are only two aspects of one and the same thing, namely, organization. For all morphological characters there are functional correlatives, for functional characters morphological expressions, and if the one is inherited so is the other. But there are certain characters in which the physiological aspect is more striking than the morphological one. For example, longevity is a physiological character which is undoubtedly dependent upon many causes, but in the case of species which differ greatly in length of life there can be little