In order to proceed methodically in this investigation, our first step must be to examine minutely, whether the fishes of Lake Superior are the same as those of other lakes, in this or any other country; and, if not, how they differ. To satisfy ourselves in this respect, we shall successively examine all the families of fishes, which have representatives in those great fresh-water seas. (_Agassiz on Lake Superior_, p. 246.) Professor Agassiz, after admirable histories of the fishes of Lake Superior, concludes with the following excellent observations:--[3] Footnote 3: "Lake Superior," p. 373. III.--_General Observations; all Fresh-water Fishes of North America different from those of Europe--Lake Superior and the Lakes north of it constitute a distinct Zoological District--These Fishes have been created where they now live--Deductions from this fact._ Such a critical revision of the fishes of Lake Superior, and the other great Canadian lakes, was the first necessary step in the investigation I am tracing, in order to ascertain the natural primitive relations between them and the region which they inhabit. Before drawing the conclusions which follow directly from these facts, I should introduce a similar list of the fishes living in similar latitudes, or under similar circumstances, in other parts of the world; and more particularly of the species of Northern Europe. But such a list, to be of any use, should be throughout based upon a critical comparative investigation of all the species of that continent, which would lead to too great a digression. The comparison of the fresh-water fishes of Europe, which correspond to those of North America, has been carried so far, that I feel justified in assuming, what is really the fact, that all the species of North America, without a single exception, differ from those of Europe, if we limit ourselves strictly to fishes which are exclusively the inhabitants of fresh water. I am well aware that the salmon which runs up the rivers of Northern and Central Europe, also occurs on the eastern shores of the northern part of North America, and runs up the rivers emptying into the Atlantic. But this fish is one of the marine arctic fishes, which migrates with many others, annually further south, and which migratory species is common to both continents. Those species, however, which never leave the fresh waters, are, without exception, different on the two continents. Again, on each of the continents, they differ in various latitudes; some, however, taking a wider range than others in their natural geographical distribution.