It would seem, therefore, to be in order to show that science is now competent to deal with this question; not that she can give a final and conclusive answer, but that we can reach results which are probably in the main correct. We may grant very cheerfully that we can attain no demonstration; the most that we can claim for our results will be a high degree of probability. If our conclusions are very probably correct, we shall do well to act according to them; for all our actions in life are suited to meet the emergencies of a probable but uncertain course of events. We take for granted the probable truth of the theory of evolution as stated by Mr. Darwin, and that it applies to man as really as to any lower animal. At the same time it concerns our argument but little whether natural selection is "omnipotent" or of only secondary importance in evolution, as long as it is a real factor, or which theory of heredity or variation is the more probable. If man has been evolved from simple living substance protoplasm, by a process of evolution, it will some day be possible to write a history of that process. But have we yet sufficient knowledge to justify such an attempt? Before the history of any period can be written its events must have been accurately chronicled. Biological history can be written only when the successive stages of development and the attainments of each stage have been clearly perceived. In other words, the first prerequisite would seem to be a genealogical[A] tree of the animal kingdom. The means of tracing this genealogical tree are given in the first chapter, and the results in the second, third, and fourth chapters of this book. [Footnote A: See Phylogenetic Chart, p. 310.] Now, for some of the ancestral stages of man's development a very high degree of probability can be claimed. One of man's earliest ancestors was almost certainly a unicellular animal. A little later he very probably passed through a gastræa stage. He traversed fish, amphibian, and reptilian grades. The oviparous monotreme and the marsupial almost certainly represent lower mammalian ancestral stages. But what kind of fish, what species of amphibian, what form of reptiles most closely resembles the old ancestor? How did each of these ancestors look? I do not know. It looks as if our ancestral tree were entirely uncertain and we were left without any foundation for history or argument. But the history of the development of anatomical details, however important and desirable, is not the only history which can be written, nor is it essential. It would be interesting to know the size of brain, girth of chest, average stature, and the features of the ancient Greeks and Romans. But this is not the most important part of their history, nor is it essential. The great question is, What did they contribute to human progress?