Thus, without quitting _morphological considerations_, which correspond to the _idea of resemblance_ contained in the definition of species, we are justified in concluding in favour of monogenism. To confirm this conclusion, however, we must turn our attention to other facts which correspond to the _idea of filiation_, and consider the teachings of _physiology_ concerning the phenomena of _generation_. CHAPTER VII. CROSSING OF RACES AND SPECIES IN THE ANIMAL AND VEGETABLE KINGDOMS.—MONGRELS AND HYBRIDS. I. Sexual unions in plants, as in animals, can take place between individuals of the _same species_ and the _same race_; further, between _different races_ of the _same species_, and, finally, between _different species_. In the two latter cases we have what is called a cross. This crossing itself is differently named according to whether it takes place between different races or different species. In the first case it produces a _mongrel_, in the second a _hybrid_. When the cross-unions are fertile the product of the union of mongrels is called a _mongrel_, the product of the union of hybrids a _hybrid_. If the difference of the relations existing between the _race_ and the _species_ has been properly understood, we ought to be inclined to admit that mongrels and hybrids would not present the same phenomena; experience and observation confirm this presentiment. We have, therefore, in this crossing a means of judging whether the human groups are only _races of a single species_, or rather _distinct species_. For this purpose it will be sufficient to study the phenomena which, in other organised and living beings, accompany the production of mongrels and hybrids, and then to compare with both the phenomena which characterise the crosses effected between human groups. If, in the latter case, the phenomena are those which characterise _hybridism_, we must conclude that the groups are specifically distinct, and admit the _multiplicity_ of human species. If, however, crosses between human groups, morphologically different, are accompanied by phenomena peculiar to the production of _mongrels_, we shall only be justified in considering these groups as races of one species; we must take our stand upon the doctrine of the _Specific Unity_ of all mankind. The question before us becomes then entirely a physiological one, and depends simply upon observation and experiment. For its solution we must again turn our attention to plants as well as to animals. It is in the phenomena of reproduction that the two kingdoms show the greatest resemblance. This is not a case of mere _analogy_, but almost of _identity_, and it is not the superior which lowers itself but the inferior which is raised. We might say that, ennobled by the importance of the function, the plant, as far as its reproductive system is concerned, becomes, for the time, animal.