_Proofs of Rapid Movements on Land._ Lightness of skeleton is a walking or running or flying adaptation, and not at all a swimming one; a swimming animal needs gravity in its skeleton, because sufficient buoyancy in the water is always afforded by the lungs and soft tissues of the body. The extraordinary lightness of these dinosaur vertebrae may therefore be put forward as proof of supreme fitness for the propulsion of an enormous frame during occasional incursions upon land[22]. There are additional facts which point to land progression, such as the point in the tail where the flexible structure suddenly becomes rigid, as shown in the diagram of vertebrae below; the component joints are so solid and flattened on the lower surface that they seem to demonstrate fitness to support partly the body in a tripodal position like that of a kangaroo. I have therefore hazarded the view that even some of these enormous dinosaurs were capable of raising themselves on their hind limbs, lightly resting on the middle portion of the tail. In such a position the animal would have been capable not only of browsing among the higher branches of trees, but of defending itself against the carnivorous dinosaurs by using its relatively short but heavy front limbs to ward off attacks. There are also indications of aquatic habits in some of the giant dinosaurs which render it probable that a considerable part of their life was led in the water. One of these indications is the backward position of the nostrils. Many, but not all, water-living mammals and reptiles have the nostrils on top of the head, in order to breathe more readily when the head is partly immersed. Another fact of note, although perhaps less conclusive, is the fitness of the tail for use while moving about in the water, if not in rapid swimming. The great tail, measuring from twenty-eight to thirty feet, was one of the most remarkable structures in these animals, and undoubtedly served a great variety of purposes, propelling while in the water, balancing and supporting and defending while on land. In _Diplodocus_ it was most perfectly developed from its muscular base to its delicate and whip-like tip, perhaps for all these functions. _The Three Kinds of Giant Dinosaurs._ It is very remarkable that three distinct kinds of these great dinosaurs lived at the same time in the same general region, as proved by the fact that their remains are freely commingled in the quarry. What were the differences in food and habits, in structure and in gait, which prevented that direct and active competition between like types in the struggle for existence which in the course of nature always leads to the extermination of one or the other type? In the last three years we have discovered very considerable differences of structure which make it appear that these animals, while of the same or nearly the same linear dimensions, did not enter into direct competition either for food or for territory.