Trusting that you may help me in this way to bring together in Cambridge a more complete collection, illustrative of the natural history of mankind than exists thus far anywhere,* (* All the ethnographical collections of the Museum of Comparative Zoology have now been transferred to the Peabody Museum, where they more properly belong.) I remain, ever truly your friend and brother, LOUIS AGASSIZ. The following letter to Mr. Ticknor is in the same spirit as previous ones to Mr. Haldeman and others, concerning the distribution of fishes in America. It is given at the risk of some repetition, because it illustrates Agassiz's favorite idea that a key to the original combination of faunae in any given system of fresh waters, might be reached through a closer study than has yet been possible of the geographical or local circumscription of their inhabitants. TO MR. GEORGE TICKNOR. NAHANT, October 24, 1863. MY DEAR SIR, Among the schemes which I have devised for the improvement of the Museum, there is one for the realization of which I appeal to your aid and sympathy. Thus far the natural productions of the rivers and lakes of the world have not been compared with one another, except what I have done in comparing the fishes of the Danube with those of the Rhine and of the Rhone, and those of the great Canadian lakes with those of the Swiss lakes. I now propose to resume this subject on the most extensive scale, since I see that it has the most direct bearing upon the transmutation theory. . .First let me submit to you my plan. Rivers and lakes are isolated by the land and sea from one another. The question is, then, how they came to be peopled with inhabitants differing both from those on land and those in the sea, and how does it come that every hydrographic basin has its own inhabitants more or less different from those of any other basin? Take the Ganges, the Nile, and the Amazons. There is not a living being in the one alike to any one in the others, etc. Now to advance the investigation to the point where it may tell with reference to the scientific doctrines at present under discussion, it is essential to know the facts in detail, with reference to every fresh-water basin on earth. I have already taken means to obtain the tenants of all the rivers of Brazil, and partly of Russia, and I hope you may be able to put me in the way of getting those of Spain, if not of some other country beside. The plan I propose for that country would be worthy of the Doctors of Salamanca in her brightest days. If this alone were carried out, it would be, I believe, sufficient to settle the whole question.