The legitimacy of this conclusion is incontestable. Now, I repeat, that this conclusion is sufficient to destroy the very foundation of the polygenistic theory. In reality this theory rests entirely upon _morphological_ considerations. Its partisans, struck only by the material differences presented by the human groups, have thought it impossible to account for them, except by the admission of the existence of several species. By showing that facts of this nature can be equally well interpreted under the hypothesis of the Unity of the Species, monogenism and polygenism are, so to speak, placed on an equal footing. CHAPTER VI. INTERCROSSING AND FUSION OF CHARACTERS IN ANIMAL RACES; APPLICATION TO MAN. Without even quitting the ground of morphology, it will be easy to prove which theory is most probably the correct one. We know that naturalists consider that all individuals which pass from one to another by invisible shades belong to the same species, however different the extremes may be. All great museums contain examples of this fact. The grounds for this conclusion are much stronger when there exists an _intercrossing of characters_. This intercrossing exists when a very decided and apparently exclusive character reappears in one or several individuals differing widely in other respects, and undoubtedly belonging to distinct groups. It is a case of intercrossing again, when the same character varies in such a manner as to lead, if considered apart, to the division of a natural group, and to the separation of the fractions into very different groups. Now there is no animal species which presents these essentially morphological characters in a higher degree than man. When the human groups are studied in some detail, the difficulty does not consist in finding resemblances, but in clearly defining the differences. The more carefully they are considered, the more they disappear and become obliterated. We then understand the accounts given by most trustworthy travellers, such as d’Abbadie, of countries where the Negro and the White live side by side. In their extremes these two types are certainly very distinct. But in Abyssinia, for example, where they have long lived in contact, and intermingled, the Negro is no longer characterised by either colour, features, or hair, but simply by the exaggerated protuberance of the heel. This character in its turn, however, loses its value on the Eastern coast of Africa, where whole Negro tribes have the heel formed like ours.