(1.) Is it possible, according to the known laws of actual nature, or is it probable, on any analogies of nature, that the vast series of fish, from those of the Ludlow rock and the Old Red Sandstone to those of our actual seas, lakes, and rivers, are derived from one common original low type, in the way of development and by propagation or natural breeding? I should say, NO. But my knowledge is feeble and at second-hand. Yours is strong and from the fountain-head. (2.) Is the organic type of fish higher now than it was during the carboniferous period, when the Sauroids so much abounded? If the progressive theory of Geoffroy be true, in his sense, each class of animals ought to be progressive in its organic type. It appears to me that this is not true. Pray tell me your own views on this point. (3.) There are "ODD FISH" (as we say in jest) in the Old Red Sandstone. Do these so graduate into crustaceans as to form anything like such an organic link that one could, by generation, come naturally from the other? I should say, NO, being instructed by your labors. Again, allowing this, for the sake of argument, are there not much higher types of fish which are contemporaneous with the lower types (if, indeed, they be lower), and do not these nobler fish of the Old Red Sandstone stultify the hypothesis of natural generative development? (4.) Will you give me, in a few general words, your views of the scale occupied by the fish of the Old Red, considered as a natural group? Are they so rudimentary as to look like abortions or creatures derived from some inferior class, which have not yet by development reached the higher type of fish? Again, I should say, NO; but I long for an answer from a great authority like yours. I am most anxious for a good general conception of the fish of the Old Red, with reference to some intelligible scale. (5.) Lastly, is there the shadow of ground for supposing that by any natural generative development the Ichthyosaurians and other kindred forms of reptile have come from Sauroid, or any other type of fish? I believe you will say, NO. At any rate, the facts of geology lend no support to such a view, for the nobler forms of Reptile appear in strata below those in which the Ichthyosaurians, etc., are first seen. But I must not trouble you with more questions. Professor Whewell is now Master of Trinity College. We shall all rejoice to see you. Ever, my dear Professor, your most faithful and most grateful friend, A. SEDGWICK. FROM LOUIS AGASSIZ TO A. SEDGWICK. NEUCHATEL, June, 1845.