In very many cases it may be said that actual extermination has not taken place, but that in the course of evolution one species has passed into another; species may have been lost, but the race, or phylum endures, just as in the growth of a tree, the twigs and branches of the sapling disappear, while the tree, as a whole, grows onward and upward. This is what we see in the horse, which is the living representative of an unbroken line reaching back to the little Eocene Hyracothere. So in a general way it may be said that much of what at the first glance we might term extinction is really the replacement of one set of animals by another better adapted to surrounding conditions. Again, there are many cases of animals, and particularly of large animals, so peculiar in their make up, so very obviously adapted to their own special surroundings that it requires little imagination to see that it would have been a difficult matter for them to have responded to even a slight change in the world about them. Such great and necessarily sluggish brutes as Brontosaurus and Diplodocus, with their tons of flesh, small heads, and feeble teeth, were obviously reared in easy circumstances, and unfitted to succeed in any strenuous struggle for existence. Stegosaurus, with his bizarre array of plates and spines, and huge-headed Triceratops, had evidently carried specialization to an extreme, while in turn the carnivorous forms must have required an abundant supply of slow and easily captured prey. Coming down to a more recent epoch, when the big Titanotheres flourished, it is easy to see from a glance at their large, simple teeth that these beasts needed an ample provision of coarse vegetation, and as they seem never to have spread far beyond their birthplace, climatic change, modifying even a comparatively limited area, would suffice to sweep them out of existence. To use the epitaph proposed by Professor Marsh for the tombstone of one of the Dinosaurs, many a beast might say, "I, and my race perished of over specialization." To revert to the horse it will be remembered that this very fate is believed to have overtaken those almost horses the European Hippotheres; they reached a point where no further progress was possible, and fell by the wayside. There is, however, still another class of cases where species, families, orders, even, seem to have passed out of existence without sufficient cause. Those great marine reptiles, the Ichthyosaurs, of Europe, the Plesiosaurs and Mosasaurs, of our own continent, seem to have been just as well adapted to an aquatic life as the whales, and even better than the seals, and we can see no reason why Columbus should not have found these creatures still disporting themselves in the Gulf of Mexico. The best we can do is to fall back on an unknown "law of progress," and say that the trend of life is toward the replacement of large, lower animals by those smaller and intellectually higher.