I. The question of the unity or multiplicity of the human species may be stated in the following terms: are the differences which distinguish the human groups characteristic of _species_ or of _race_? It is evident that the question depends entirely upon the two words species and race. It is then absolutely necessary to determine as accurately as possible the sense of each, and yet there are anthropologists, such as Knox, for instance, who declare that any discussion or investigation in connection with this subject is idle. There are others, like Dr. Nott, who would suppress the race and only establish various _categories of species_. In order to support their doctrines these authors ignore the work which has been carried on for nearly two centuries by the most illustrious naturalists, and the innumerable observations and experiments made by a vast number of eminent men upon plants and animals. In fact the theory of species and race has not been formed _à priori_, as it has been too often falsely asserted, but has been gradually acquired, and in a strictly scientific manner. II. The word _Species_ is one which exists in all languages which possess abstract terms. It represents, therefore, a general common idea. The idea is, in the first place, that of a great outward _resemblance_; but even in ordinary language that is not all. The idea of _filiation_ is connected, even in the most uncultivated minds, with that of resemblance. No peasant would hesitate to regard the children of the same parents as _belonging to the same species_ whatever real or apparent differences might distinguish them. Science has in reality done nothing more than define the idea of which the public had merely a vague consciousness, and it was not till very lately, and after a very curious oscillation, that she succeeded in doing so. In 1686, Jean Ray, in his _Historia Plantarum_, considered that those plants which had a common origin and could be reproduced by seed belonged to the same species, whatever their apparent differences might be. He only took filiation into account. Tournefort, on the contrary, who in 1700 was the first to make a clear statement of the question, termed the collection of plants a _species_ which were distinguished by some particular character. He relied only on resemblance.