I. The first men who peopled the centre of human appearance must at first have differed from each other only in individual features. At their beginning and during an indefinite lapse of time, mankind could only have been homogeneous, as is every animal and vegetable species which is restricted to an area of small extent. At the present time, we find mankind composed of numerous groups, which have peculiar characters, and constitute so many distinct races. How have these races originated? and how have they grown and multiplied? To give a definite reply to these questions, by going back from recent effects to first causes, is still impossible, and perhaps will always be so. Nevertheless, science may even now approach the general aspects of the problem. We are well acquainted with the circumstances under which varieties originate and races are formed among plants and animals: we have established in man the occurrence of a number of phenomena, which are in this respect identical, or very similar to those exhibited by the two inferior kingdoms. We are therefore clearly authorised to apply inferences drawn from them to ourselves, connecting particular with general facts. This study is instructive in many respects. Unfortunately, we cannot fully enter upon it here; we can only select some facts in the history of animals to justify our conclusions. II. The problem of the formation of human races presents two very distinct cases. Man at first was subject to the sole action of _natural modifying agents_. Under this influence _pure races_ were formed. When these races came in contact, they were _crossed_; this resulted in the formation of _mixed races_. Without being antagonistic to the natural forces, _crossing_ modifies them by its peculiar phenomena, and sometimes masks their manifestations. The two cases, therefore, require separate examination. We will begin with the first. III. Every organic species considered as a whole appears to be subjected to the action of two forces, one of which tends to maintain and the other to modify its characters. To what cause can this double action be referred? This is a question put by the greatest thinkers and the most eminent physiologists, from Aristotle and Hippocrates to Burdach and J. Müller.