P.S. My love to Auguste. To-day (Sunday) I went again to see M. Humboldt about Auguste's* (* Concerning a business undertaking in Mexico.) plan, but did not find him. Then follow several pages, addressed to his father, in answer to the request contained in one of his last letters that Louis would tell him as much as he thinks he can understand of his work. There is something touching in this little lesson given by the son to the father, as showing with what delight Louis responded to the least touch of parental affection respecting his favorite studies, so long looked upon at home with a certain doubt and suspicion. The whole letter is not given here, as it is simply an elementary treatise on geology; but the close is not without interest as relating to the special investigations on which he was now employed. "The aim of our researches upon fossil animals is to ascertain what beings have lived at each one of these (geological) epochs of creation, and to trace their characters and their relations with those now living; in one word, to make them live again in our thought. It is especially the fishes that I try to restore for the eyes of the curious, by showing them which ones have lived in each epoch, what were their forms, and, if possible, by drawing some conclusions as to their probable modes of life. You will better understand the difficulty of my work when I tell you that in many species I have only a single tooth, a scale, a spine, as my guide in the reconstruction of all these characters, although sometimes we are fortunate enough to find species with the fins and the skeletons complete. . . "I ask pardon if I have tired you with my long talk, but you know how pleasant it is to ramble on about what interests us, and the pleasure of being questioned by you upon subjects of this kind has been such a rare one for me, that I have wished to present the matter in its full light, that you may understand the zeal and the enthusiasm which such researches can excite."