The relative or absolute sterility characterizing first crosses and hybrids depends upon a biological law which might be called the “Law of Similarity.” The degree of sterility, in either case,[1640] runs, at least to a certain extent, parallel with the general affinity of the forms that are united. Thus, most animal hybrids are produced by individuals belonging to the same genus, whilst species belonging to distinct genera can rarely, and those belonging to distinct families perhaps never, be crossed.[1641] The parallelism, however, is not complete, for a multitude of closely allied species will not unite, or unite only with great difficulty, though other species, widely different from each other, can be crossed with facility. Hence Mr. Darwin infers that the difficulty or facility in crossing “apparently depends exclusively on the sexual constitution of the species which are crossed, or on their sexual elective affinity, _i.e._, the ‘Wahlverwandtschaft’ of Gärtner.” But as species rarely, or never, become modified in one character, without being at the same time modified in many, and as systematic affinity includes all visible resemblances and dissimilarities, any difference in sexual constitution between two species would naturally stand in more or less close relation with their systematic position.[1642] With regard to the instinct in question, man follows the general rule in the animal kingdom. Our notions of morality are closely connected with the instinctive feelings engraved in our nature; and bestiality is commonly looked upon as one of the most heinous crimes of which man can make himself guilty. Several passages both in ancient[1643] and modern writers[1644] prove the occasional occurrence of this crime, but always under circumstances analogous to those under which single birds sometimes form connections against nature,[1645] _i.e._, either because of isolation, or on account of vitiated instincts.[1646] * * * * *