Besides those polygenists who are influenced by prejudices almost or entirely unscientific, there are sincere and disinterested men of science who believe in the multiplicity of human origins. Foremost among the latter are _medical_ men, who are accustomed to the study of the _individual_ and who only possess a slight familiarity with the study of the _species_. Then again there are palæontologists, who from the nature of their work are compelled only to take into account morphological resemblances and differences, without even turning their attention to facts of reproduction or of _filiation_. Finally, there are entomologists, conchologists, etc., who, exclusively interested in the distinction of innumerable species by purely external characters, are entirely ignorant of physiological phenomena, and judge living beings as they would fossils. On the other hand, monogenism reckons among its partisans nearly all those naturalists who have turned their attention to the phenomena of life, and among them some of the most illustrious. In spite of the difference of their doctrines, Buffon and Linnæus, Cuvier and Lamarck, Blainville and the two Geoffroys, Müller the physiologist and Humboldt agree upon this point. Apart from any influence which the name of these great men might exercise, it is clear that I share their opinion. I have on different occasions explained the purely scientific reasons for my convictions. I shall now endeavour to sum them up in as few words as possible. II. Let us first establish the importance of the question. It escapes many minds and I have heard a doubt expressed upon it by men who have enthusiastically followed anthropological studies. It is, however, easily proved. If the human groups have appeared with all their distinctive characters in the isolated condition, and in the various localities where geography teaches us to seek them; if we can trace them up to stocks originally distinct, thus constituting so many _special species_, then the study of them is one of the most simple, presenting no more difficulty than that of animal or vegetable species. There would be nothing singular in the diversity of the groups. It would be sufficient to examine and describe them one after the other, merely determining the degree of _affinity_ between them. At most we should have to fix their limits and to discover the influence which groups geographically brought in contact had been able to exercise upon each other.