Reptiles, then, the most instructive group we can select with reference to the structural relations of the type as it now exists, and their history in past times, will be the alligator. We must therefore neglect no opportunity of collecting their eggs in as large numbers as possible. “There are other animals in Brazil, low in their class to be sure, but yet very important to study embryologically, on account of their relation to extinct types. These are the sloths and armadillos,—animals of insignificant size in our days, but anciently represented in gigantic proportions. The Megatherium, the Mylodon, the Megalonyx, were some of these immense Mammalia. I believe that the embryonic changes of the sloths and armadillos will explain the structural relations of those huge Edentata and their connection with the present ones. South America teems with the fossil bones of these animals, which indeed penetrated into the northern half of the hemisphere as high up as Georgia and Kentucky, where their remains have been found. The living representatives of the family are also numerous in South America, and we should make it one of our chief objects to get specimens of all ages and examine them from their earliest phases upward. We must, above all, try not to be led away from the more important aims of our study by the diversity of objects. I have known many young naturalists to miss the highest success by trying to cover too much ground,—by becoming collectors rather than investigators. Bitten by the mania for amassing a great number and variety of species, such a man never returns to the general consideration of more comprehensive features. We must try to set before ourselves certain important questions, and give ourselves resolutely to the investigation of these points, even though we should sacrifice less important things more readily reached. “Another type full of interest, from an embryological point of view, will be the Monkeys. Since some of our scientific colleagues look upon them as our ancestors, it is important that we should collect as many facts as possible concerning their growth. Of course it would be better if we could make the investigation in the land of the Orangs, Gorillas, and Chimpanzees,—the highest monkeys and the nearest to man in their development. Still even the process of growth in the South American monkey will be very instructive. Give a mathematician the initial elements of a series, and he will work out the whole; and so I believe when the laws of embryological development are better understood, naturalists will have a key to the limits of these cycles of growth, and be able to appoint them their natural boundaries even from partial data.