Mr. Darwin frequently complains that our actual knowledge is incomplete. But instead of discovering in our lack of precise and extensive information a motive for caution, he appears to derive from it only greater daring. Doctrines based on the instability of species have often been combated by geologists and palæontologists. In reply to their objections Darwin devotes a whole chapter to shewing the imperfection of the geological record. "For my part," he concludes, "I look at the geological record as a history of the world imperfectly kept and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly-changing language, more or less different in the successive chapters, may represent the forms of life, which are entombed in our consecutive formations, and which falsely appear to have been abruptly introduced. On this view, the difficulties above discussed are greatly diminished, or even disappear." On my part [continues M. de Quatrefages] I will ask whether such a conclusion is the correct one. No doubt, Darwin is right in refusing to certain naturalists the right to dogmatize on the strength of uncompleted studies, or scanty and isolated observations. Is he therefore entitled to allege as proofs on his own behalf the very gaps of science, appealing to the lost volumes and leaves of Nature's chronicle? Clearly not. But the slightest reflection suffices to recognize that this appeal to the unknown, so frankly evidenced in the above passage, lies at the root of all argumentation analogous to that which I have tried to describe--that of Maillet, Lamarck, and Geoffroy,[216] as well as Darwin. Only the unknown, in sooth, can open the boundless region of speculation, where the possible replaces the actual, and where, despite the widest knowledge and the soundest intelligence, one comes as by a fatality to find a conclusive proof on one's own side, precisely in that of which we profess to know nothing. So again, speaking of a certain conclusion of Professor Haeckel's concerning the embryology of lemurs, which MM. Grandidier and Alphonse Edwards afterwards proved experimentally to be altogether erroneous, de Quatrefages writes:[217]