for instance, to ascertain the place of any animal, or of a chemical process for that of a mineral substance. It would certainly be better to adopt as much as possible such exterior and visible characteristics as every traveller is competent to observe, to ascertain and to relate. But with this objection, lying but in a small degree, Linnæus' method was received, understood, and conventionally settled among the learned, and was even getting into common use. To disturb it then was unfortunate. The new system attempted in botany, by Jussieu, in mineralogy, by Haüy, are subjects of the same regret, and so also the no-system of Buffon, the great advocate of individualism in opposition to classification. He would carry us back to the days and to the confusion of Aristotle and Pliny, give up the improvements of twenty centuries, and co-operate with the neologists in rendering the science of one generation useless to the next by perpetual changes of its language. In botany, Wildenow and Persoon have incorporated into Linnæus the new discovered plants. I do not know whether any one has rendered us the same service as to his natural history. It would be a very acceptable one. The materials furnished by Humboldt, and those from New Holland particularly, require to be digested into the Catholic system. Among these, the Ornithorhyncus mentioned by you, is an amusing example of the anomalies by which nature sports with our schemes of classification. Although without mammæ, naturalists are obliged to place it in the class of mammiferæ; and Blumenbach, particularly, arranges it in his order of Palmipeds and toothless genus, with the walrus and manatie. In Linnæus' system it might be inserted as a new genus between the anteater and manis, in the order of Bruta. It seems, in truth, to have stronger relations with that class than any other in the construction of the heart, its red and warm blood, hairy integuments, in being quadruped and viviparous, and may we not say, in its _tout ensemble_, which Buffon makes his sole principle of arrangement? The mandible, as you observe, would draw it towards the birds, were not this characteristic overbalanced by the weightier ones before mentioned. That of the Cloaca is equivocal, because although a character of birds, yet some mammalia, as the beaver and sloth, have the rectum and urinary passage terminating at a common opening. Its ribs also, by their number and structure, are nearer those of the bird than of the mammalia. It is possible that further opportunities of examination may discover the mammæ. Those of the Opossum are asserted, by the Chevalier d'Aboville, from his own observations on that animal, made while here with the French army, to be not discoverable until pregnancy, and to disappear as soon as the young are weaned. The Duckbill has many additional particularities which liken it to other genera, and some entirely peculiar. Its description and history needs yet further information.