As to competitors and aggressive enemies, there were snakes and crocodiles; but, confining our attention to carnivorous mammals, the time seems to have been favourable to the enterprise of a new hunter. By the middle of the Oligocene, the ancient Creodonts (primitive flesh-eaters which had flourished in the Eocene) were nearly extinct, represented in the deposits by their last surviving family, the Hyœnodonts. Ancestors of the modern carnivores, such as may be called by anticipation dogs and cats, derived (according to Prof. Scott) from the Creodont Family of the Miacidæ, were becoming numerous, but for the most part were still of small size. Apparently, the primitive dogs and their allies must, for some time, have been more formidable adversaries than the primitive cats, especially if we suppose them to have already begun to hunt in pack; and this is not improbable, both on account of their structure and because several distinct varieties and even genera, now extant, have that habit—such as wolf, jackal, dingo, dhole, Cape hunting-dog, etc. In the Upper Oligocene of North America, occurs a dog as big as a large modern wolf, and in Europe the bear-like dog, Amphicyon, of about the same size, but said to have been clumsy and slow-moving. There were several other dog-like species; they continue in the Miocene, and some of them increase in bulk; but true modern dogs or wolves (_Canis_) do not appear before the Pliocene. Then, too, first occur true bears (_Ursus_); hyænas in the Upper Miocene. “Cats” belong to two sub-families: (i) the true felines, our modern species and their ancestors; and (ii) the machærodonts, or sabre-toothed cats. The latter first appear in North America in the Lower Oligocene; the former in Europe in the Middle Oligocene. The sabre-tooths are so called from their thin, curved upper canines; which were so long (3 to 6 inches) that it is not easy to understand how they could open their mouths wide enough to bite with them. That they were effective in some way is proved by the fact that machærodonts, first appearing in the Lower Oligocene, increased in numbers and diversity of species for ages, and some of them in bulk. In North America, in the Upper Oligocene, one species was as large as a jaguar, and some of the biggest and most terrifying were contemporary with Man, and only became extinct in the Pleistocene. Their limbs were relatively shorter and thicker than those of the _Felinæ_. These, the true cats, at first progressed more slowly than the _Machærodontidæ_; but in the Siwalik deposits (Pliocene) there occur, along with machærodonts, forms resembling the leopard and the lynx, with others as large as tigers. The largest of all this group seems to have been the cave-lion, perhaps a large variety of the common African lion, which also lived with Man in Europe in the Pleistocene. These were serious competitors in the hunting-life of Lycopithecus and of primitive Man; and the effect of such competition in exterminating inferior forms is shown by the fate of the carnivorous marsupials of South America (allied to _Thylacinus_), which were the predatory fauna of that region, until in the Pliocene, North and South America having become united by continuous land, cats and dogs came in from the northern continent and put an end to them; and also by the fate of the creodonts, which in the Oligocene seem everywhere to have been exterminated by the new carnivores. In both cases the beaten competitors were very inferior in the size and complexity of their brains; and if Man has succeeded in the struggle for life against the same foes, in spite of his inferior bodily adaptation, it is probably due to his very superior brains. This may also be the reason why modern Man (_Homo sapiens!_), wandering everywhere over the world, has everywhere exterminated such experiments in human nature as Pithecanthropus, Eoanthropus, and Neanderthalensis; as others are soon to follow them into the Hades of extinct species.