parts, which offends either by excess or defects. In a warm climate, there will be in excess what will be deficient in a cold climate, therefore, when we join together animals of those opposite climates, we must expect the produce to be complete; and as the most perfect work in Nature is that which has the fewest defects, and the most perfect forms, those that have the fewest deformities, so the produce of two animals, whose defects exactly counterbalance each other, will be the most perfect production of its species: they counterbalance one another the better, in proportion to the distance between the countries the animals matched together were bred in; the compound that results therefrom is the more perfect, the more opposite the excesses or defects of the constitution of the male are to the defects or excesses of the temperament of the female. Thus the breed is always improved by matching the mares with foreign horses, and they will always be more beautiful in proportion as the climates in which the horse and mare were bred are the more distant, and, on the contrary, the produce will be much debased by suffering horses of the same race to breed together; for they infallibly degenerate in a very little time. The climate and food have not so much influence on the human species as on animals; and the reason is plain: man can defend himself better than any other animal from the intemperance of the climate; he is lodged and clothed suitably to the seasons; in his food also there is more variety, and consequently it cannot influence all individuals in the same manner. The defects or excesses which arise from these two causes, and which are so constantly and so sensibly felt in animals, are much less conspicuous in men. Besides, as there have been frequent migrations, as nations are mixed, and great numbers travel and are dispersed every where, it is no wonder that the human race should appear less subject to the influence of climate, and that there should be men strong, well-made, and even ingenious in all countries. Nevertheless, we may believe, from experience much further back than memory can trace, that men formerly knew the misfortunes which resulted from alliances with the same blood; since in the most uncivilized nations, it has rarely been permitted for the brother to marry the sister. This custom, which among Christians is a divine law, and which is practised by other people from political views, is perhaps grounded on this observation. Policy is never extended in so general and absolute a manner, unless supported by physical principles: but if men once discovered by experience that their race degenerated, when intercourse was admitted between children of the same family, they would soon have looked upon alliances with other families as a law of nature, and agreed in not suffering a mixture of blood among their children. In short, from analogy it may be presumed, that in most