weight and shape with that of a big whale-bone whale! No extinct animal known approaches the existing whale in bulk and weight. He is 80 to 90 ft. long, and has no neck nor any length of tail. His outline is egg-like, narrower at the hinder end. He weighs 200 tons–forty times as much as a big elephant–and is perfectly supported without any strain on his structure by the water in which he floats. There is no such limit to his possible size as there is in the case of land-walking animals. But it seems probable that he too is limited in size by mechanical conditions of another kind. Probably he cannot exceed some 90 ft. in length and 200 tons of bulk on account of the relatively great increase of proportionate size and power in the heart required in order to propel the blood through such a vast mass of living tissue and keep him "going" as a warm-blooded mammal. The original pattern–the small dog-like ancestor of the whale–cannot be indefinitely expanded as an efficient working machine, though its limit of growth is not determined by the same mechanical causes as those which limit the bulk of the terrestrial quadruped. These considerations make it clear that we should compare as to "bigness" terrestrial animals with other terrestrial animals, and aquatic animals with aquatic ones. It seems probable that Diplodocus was an aquatic reptile, and never raised himself on to his four legs on dry land as the Carnegie skeleton at the Natural History Museum is doing. His legs and feet are quite unfitted to support his weight on a land surface; on land he would have rested on his belly, as a crocodile does, with much bent legs on each side. But submerged in 20 ft. depth of water, he could have trotted along, half-floating, with his feet touching the bottom and his head raised on its long neck to the surface, slowly sucking the floating vegetation into his moderate-sized mouth. (See drawing on p. 91.)