V. If from _animals_ and _plants_ we pass to man, we shall find in him, as in the two inferior kingdoms, groups distinguished by anatomical, physiological and psychological differences. In most cases the same organs and the same functions present analogous modifications. What reason can be alleged for the idea that, if _their nature_ is considered, these differences and modifications have a greater signification in man, and that they characterise _species_ and not _race?_ Clearly none; it would be reasoning against the laws of analogy. An argument based upon the variations presented by the manifestations of _morality_ and _religion_, would be a neglect of the fact that these faculties are the attributes of the human kingdom, that they are wanting in the other kingdoms, and are not in consequence susceptible of any comparison of this kind. In that which is exclusively human, man can only be compared with man. In conclusion, the facts of the variations and differences existing in man between _different groups_, are _of the same nature_ as those established between _different races_ of animals and plants. The nature of these phenomena cannot then be brought forward as an argument in favour of the theory that these groups are so many species. CHAPTER V. EXTENT OF VARIATIONS IN ANIMAL AND VEGETABLE RACES; APPLICATION TO MAN. I. The question to which this chapter is devoted is one of those which I shall treat most fully in this course. In fact, it has a special importance. Nearly all the polygenistic arguments are included in the following:—“The difference between the Negro and the White is too great for them to belong to the same species.” These types are the two extremes in the human series. Therefore, if it can be shown that between the two extremes, the limits of variation are almost always greater in plants and animals than in man, we shall have undermined the foundation of the whole polygenistic doctrine. Now, even if we leave plants out of the question, and there can be little doubt in respect to them; if we merely compare man and animals, organ for organ, function for function, we shall have no great difficulty in arriving at the conclusion, that this is really the case; so much so that we shall be led to ask the question, why the variability is less in man than in animals. The complete demonstration of this general fact would require more extended treatment than I am able to give. I shall, therefore, confine myself to citing some examples.