GENERAL CAUSES OF STERILITY.
They do not consist in definite defects of both of the married pair; the man ait well as the woman is capable of procreation, but the influence of external powers hinders the fertility of the marriage union. These powers cannot be passed over in speaking of the causes of sterility, but must be consid ered for what they are worth as hindrances to conception.
influence of Teniperature and Climate.—We have already seen how greatly the fertility of plants and animals may be diminished or even abso lutely terminated, by a change of locality, particularly when there is a coincident transplantation to a markedly higher or lower temperature. Do the same powers act in the same way upon the human species? That temperature and climate influence the sexual functions, may be proven by our experiences in respect to menstruation. There can be no doubt but that the latter appears much earlier in hot than in cold climates. Whether other deviations in this function oecur under the influence of different celestial forces is not known with certainty. Fertility in the different zones does not appear to be influenced by the action of heat or cold. Practically it is important whether a change of climate has any influence on the fertility of individuals. There are too few facts known about this. Even English physicians, who, on account of the settlement of their own people in the most distant lands, are in a better condition to answer this question, cannot furnish us with any trustworthy conclusions. This question must, therefore, be considered as open.
Marriage of Blood-relations.—We have before drawn attention to the evil consequences of inbreeding in animals, the first signs of deterioration heing a diminished fruitfulness. Does the same thing occur when the intermarriage of blood-relations is practised by human beings ? What effect does it have on fecundity, and what results does it entail upon the succeeding generations? As regards the first we must not forget that marriage does not take place between parents and children, or brothers and sisters, though in breeding as close as this is common among animals. Nor is infecundity a consequence thereof among them. Recorded cases of incest in human
beings have been followed by conception. Many evils have been attributed to the intermarriage of blood-relations, one step further removed, such as that of first cousins, of uncle or aunt with niece or nephew; it may be a question whether fecundity is lessened thereby, but I have certainly never observed absolute sterility in consequence.
Among others A. H. Huth hats especially combatted the idea that such marriages entail sterility and other evil consequenees. lie is in error, however, in believing that statistics afford us no reliable information upon the subject; Ma.ntegazza's collection certainly shows the harmless ness of such connections. Tie found only eight to nine cases of sterility in 312 cases of marriage between blood-relations, but he forgets that the same proportion of ordinary marriages are sterile, and holds that these figures militate against such connections. G. Darwin found 14 sterile among 97 marriages between first cousins, and 33 sterile among 217 mar riages between non-related persons; about the same percentage. Thus marriage between blood-relations does not seem to interfere with concep tion.
It is, however, somewhat different as regards the progeny of these unions. Stress has been laid upon the fact that some ancient races have died out from close intermarriage. We have no proof of it, but the fre quency with which prominent families, whose genealogies are well known, become extinct, would seem to render the question worthy of considera tion.
Continuous intermarriage is common among the aristocracy of our towns, and the country gentry; it is the rule in certain isolated commu nities, and in some societies the members of which are closely bound to gether by economic or religious ties. A. Voisin denies that this.leads to either physical or mental depreciation; and as a rule the children of such marriages do not ag-ain intermarry with those near of kin. Thus, as with animals, the evils of inbreeding are soon neutralized by cross-breeding. G. Darwin has proved statistically the harmlessness of marriage between cousins, and has denionstnited the fertility of such unions.