In animals as in plants, universal, free and indefinite fertility, whether between themselves or between all the races of the same species, is one of the characters of mongrels. Atavism attests the physiological bond which unites all mongrels. III. In hybrids we shall meet with some very different phenomena. Let us first, with M. Godron, establish the fact that in the vegetable hybrid the physiological equilibrium is destroyed in favour of the organs conducive to the life of the individual, and at the expense of those conducive to the life of the species. The stalk and leaves are always developed in an exaggerated manner relatively to the flowers. The most common animal hybrid, the mule, is an entirely similar case, being invariably stronger, more robust, more hardy than its parents, but sterile. This sterility is not absolute, however, among all hybrids of the first generation. It generally affects the male organs in an entirely special manner. Koelreuter, to whom we should always refer when treating of plants, states that the anthers scarcely ever enclose veritable pollen, but merely irregular granulations. It was not quite so unusual to find ovules in good condition in the ovary. Guided by these observations, Koelreuter artificially fertilised hybrid flowers with pollen from the male species, and thus obtained a _vegetable quadroon_. By continuing this process he soon brought back again to the original male type the descendants of the first hybrid, which regained all their generative faculties, but at the same time lost all trace of the female type. These experiments have been repeated and varied, but always with the same result. In a small number of hybrids of the first generation the elements which characterise the two sexes remained capable of reproduction. Nevertheless the fertility is always immensely reduced. From his hybrids of the datura, M. Naudin only obtained five or six fertile seeds from each plant. All the others had completely failed, or were without an embryo. The capsules themselves were only half the normal size. If two of these first hybrids are united they produce hybrids of the second generation. In most cases, however, the latter are either sterile, or present the phenomenon of a _spontaneous return_ to one or the other of the parent types, or to both. M. Naudin crossed the _large-leaved primrose_ with the _primula officinalis_, and obtained an intermediate hybrid between the two species, having seven fertile seeds. When these were sown they produced three primroses of the male species, three of the female, and a single hybrid plant which was perfectly barren.