The lecture to-day was upon the fishes of South America. “I will give you this morning a slight sketch of the characteristic fishes in South America, as compared with those of the Old World and North America. Though I do not know how the fishes are distributed in the regions to which we are going, and it is just upon the investigation of this point that I want your help, I know their character as distinguished from those of other continents. We must remember that the most important aim of all our studies in this direction will be the solution of the question whether any given fauna is distinct and has originated where it now exists. To this end I shall make you acquainted with the Brazilian animals so far as I can in the short time we have before beginning our active operations, in order that you may be prepared to detect the law of their geographical distribution. I shall speak to-day more especially of the fresh-water fishes. “In the northern hemisphere there is a remarkable group of fishes known as the Sturgeons. They are chiefly found in the waters flowing into the Polar seas, as the Mackenzie River on our own continent, the Lena and Yenissei in the Old World, and in all the rivers and lakes of the temperate zone, communicating with the Atlantic Ocean. They occur in smaller numbers in most tributaries of the Mediterranean, but are common in the Volga and Danube, as well as in the Mississippi, in some of the rivers on our northern Atlantic and Pacific coasts, and in China. This family has no representatives in Africa, Southern Asia, Australia, or South America, but there is a group corresponding in a certain way to it in South America,—that of the Goniodonts. Though some ichthyologists place them widely apart in their classifications, there is, on the whole, a striking resemblance between the Sturgeons and Goniodonts. Groups of this kind, reproducing certain features common to both, but differing by special structural modifications, are called ‘representative types.’ This name applies more especially to such groups when they are distributed over different parts of the world. To naturalists the comparison of one of these types with another is very interesting, as touching upon the question of origin of species. To those who believe that animals are derived from one another the alternative here presented is very clear: either one of these groups grew out of the other, or else they both had common ancestors which were neither Sturgeons nor Goniodonts, but combined the features of both and gave birth to each.