There is still one more question to ask. Are men all alike, differing merely in the _conditions_ under which they live (education, resources, government), and is evolution produced solely by changes in these _conditions_? Or are there groups of men with _hereditary differences_, born with tendencies to different activities and with aptitudes leading to different evolutions, so that evolution may be the product, in part at least, of the increase, the diminution, and the displacement of these groups? Taking the extreme cases, the white, black, and yellow races of mankind, the differences in aptitude are obvious; no black people has ever developed a civilisation. It is thus probable that smaller hereditary differences may have had their share in the determination of events. If so, historical evolution would be partly produced by physiological and anthropological causes. But history provides us with no sure means of determining the action of these hereditary differences between men; it goes no further than the conditions of their existence. The last question of history remains insoluble by historical methods. CHAPTER V EXPOSITION We have still to study a question whose practical interest is obvious: What are the forms in which historical works present themselves? These forms are, in fact, very numerous. Some of them are antiquated; not all are legitimate; the best have their drawbacks. We should ask, therefore, not only what are the forms in which historical works appear, but also which of these represent truly rational types of exposition. By "historical works" we mean here all those which are intended to communicate results obtained by the labour of historical construction, whatever may be the nature, the extent, and the bearing of these results. The critical elaboration of documents, which is treated of in Book II., and which is preparatory to historical construction, is naturally excluded. Historians may differ, and up to the present have differed, on several essential points. They have not always had, nor have they all now, the same conception of the end aimed at by historical work; hence arise differences in the nature of the facts chosen, the manner of dividing the subject, that is, of co-ordinating the facts, the manner of presenting them, the manner of proving them. This would be the place to indicate how "the mode of writing history" has evolved from the beginning. But as the history of the modes of writing history has not yet been written well,[216] we shall here content ourselves with some very general remarks on the period prior to the second half of the nineteenth century, confining ourselves to what is strictly necessary for the understanding of the present situation.