Although the weather is fine, the motion of the ship continues to be so great that those of us who have not what are popularly called “sea-legs,” have much ado to keep our balance. For my own part, I am beginning to feel a personal animosity to “the trades.” I had imagined them to be soft, genial breezes wafting us gently southward; instead of which they blow dead ahead all the time, and give us no rest night or day. And yet we are very unreasonable to grumble; for never were greater comforts and conveniences provided for voyagers on the great deep than are to be found on this magnificent ship. The state-rooms large and commodious, parlor and dining-hall well ventilated, cool, and cheerful, the decks long and broad enough to give a chance for extensive “constitutionals” to everybody who can stand upright for two minutes together, the attendance punctual and admirable in every respect; in short, nothing is left to be desired except a little more stable footing. _April 10th._—A rough sea to-day, notwithstanding which we had our lecture as usual, though I must say, that, owing to the lurching of the ship, the lecturer pitched about more than was consistent with the dignity of science. Mr. Agassiz returned to the subject of embryology, urging upon his assistants the importance of collecting materials for this object as a means of obtaining an insight into the deeper relations between animals. “Heretofore classification has been arbitrary, inasmuch as it has rested mainly upon the interpretation given to structural differences by various observers, who did not measure the character and value of these differences by any natural standard. I believe that we have a more certain guide in these matters than opinion or the individual estimate of any observer, however keen his insight into structural differences. The true principle of classification exists in Nature herself, and we have only to decipher it. If this conviction be correct, the next question is, How can we make this principle a practical one in our laboratories, an active stimulus in our investigations? Is it susceptible of positive demonstration in material facts? Is there any method to be adopted as a correct guide, if we set aside the idea of originating systems of classification of our own, and seek only to read that already written in nature? I answer, Yes. The standard is to be found in the changes animals undergo from their first formation in the egg to their adult condition.