We can determine species only by the comparison of individuals. If all these agree in all their characters except those appertaining to sex, age, and other conditions of the individual merely, we say that they belong to the same species. If all species were invariable to this extent, there could be no practical difficulty, except that of obtaining specimens for comparison. But in the case of very many species there are minor differences, not sufficient to establish specific diversity, but to suggest its possibility; and in such cases there is often great liability to error. In cases of this kind we have principally two criteria: first, the nature and amount of the differences; secondly, their shading gradually into each other, or the contrary. Under the first of these we inquire--Are they no greater in amount than those which may be observed in individuals of the same parentage? Are they no greater than those which occur in other species of similar structure or habits? Do they occur in points known in other species to be readily variable, or in points that usually remain unchanged? Are none of them constant in the one supposed species, and constantly absent in the other? Under the second we ask--Are the individuals presenting these differences connected together by others showing a series of gradations uniting the extremes by minute degrees of difference? If we can answer these questions--or such of them as we have the means of answering--in the affirmative, we have no hesitation in referring all to the same species. If obliged to answer all or many in the negative, we must at least hesitate in the identification; and if the material is abundant, and the distinguishing characters clear and well defined, we conclude that there is a specific difference. Species determined in this way must possess certain general properties in common: 1. Their individuals must fall within a certain range of uniform characters, wider or narrower in the case of different species. 2. The intervals between species must be distinctly marked, and not slurred over by intermediate gradations. 3. The specific characters must be invariably transmitted from generation to generation, so that they remain equally distinct in their limits if traced backward or forward in time, in so far as our observation may extend. 4. Within the limits of the species there is more or less liability to variation; and this, though perhaps developed by external circumstances, is really inherent in the species, and must necessarily form a part of its proper description.