It will readily be seen how great the number of varieties in one species may be. There is, in fact, scarcely any either external or internal part of an animal or plant, which cannot be exaggerated, diminished or modified in a thousand ways, and each of these exaggerations, diminutions or modifications will characterise a fresh variety, with the one condition of its being sufficiently marked. IV. When the characters peculiar to a variety become _hereditary_, that is to say, when they are transmitted from generations to the descendants of the first modified individual, a _race_ is formed. For example, if a _thornless acacia_ ever reproduced by seed, trees resembling itself and enjoying the same power, then the Acacia spectabilis would cease to be a simple variety, and would have become a race. The race, then, will consist of:—“A number of individuals resembling each other, belonging to one species, having received and transmitting, by means of sexual generation, the characters of a primitive variety.” Thus the _Species_ is the point of departure; the _variety_ appears amongst the _individuals_ of which it is composed, and, when the characters of this variety become hereditary, a _race_ is formed. Such are the relations which, according to all naturalists, “from Cuvier to Lamarck himself,” as Isidore Geoffroy said, exist between these three terms. We have here a fundamental idea which we should never lose sight of in the study of the questions with which we are engaged. From neglect of it men of the highest distinction have failed to understand most significant facts. We see that the idea of _resemblance_, which is much curtailed in the _species_, reassumes in the _race_ an importance equal to that of _filiation_. We see also that the number of races which spring directly from one species may be equal to the number of varieties of the same species, and consequently very considerable. But this number has a tendency to increase still further to an indefinite extent. In fact, each of these _primary races_ is susceptible of fresh modifications, which may either extend no further than one individual, or become transmissible by means of generation. Thus _secondary_ and _tertiary varieties_ or _races_ come into existence. Our plants and domestic animals furnish innumerable examples of these facts. V. By reason of races originating in this manner from one another, and from their multiplication, they may assume differential characters which become more and more decided. But however numerous they may be, and whatever differences there may be between them, and however far they may seem to be removed from the primitive type, they nevertheless, still form part of the species from which the primitive races derived their origin.