In the Palæozoic period, as we have said, life had not spread beyond the swampy river valleys and the borders of sea lagoons and the like; but in the Mesozoic, life was growing ever more accustomed to the thinner medium of the air, was sweeping boldly up over the plains and towards the hillsides. It is well for the student of human history and the human future to note that. If a disembodied intelligence with no knowledge of the future had come to earth and studied life during the early Palæozoic age, he might very reasonably have concluded that life was absolutely confined to the water, and that it could never spread over the land. It found a way. In the Later Palæozoic Period that visitant might have been equally sure that life could not go beyond the edge of a swamp. The Mesozoic Period would still have found him setting bounds to life far more limited than the bounds that are set to-day. And so to-day, though we mark how life and man are still limited to five miles of air and a depth of perhaps a mile or so of sea, we must not conclude from that present limitation that life, through man, may not presently spread out and up and down to a range of living as yet inconceivable. [Illustration: Some Late Mesozoic Reptiles] The earliest known reptiles were beasts with great bellies and not very powerful legs, very like their kindred amphibia, wallowing as the crocodile wallows to this day; but in the Mesozoic they soon began to stand up and go stoutly on all fours, and several great sections of them began to balance themselves on tail and hind legs, rather as the kangaroos do now, in order to release the fore limbs for grasping food. The bones of one notable division of reptiles which retained a quadrupedal habit, a division of which many remains have been found in South African and Russian Early Mesozoic deposits, display a number of characters which approach those of the mammalian skeleton, and because of this resemblance to the mammals (beasts) this division is called the _Theriomorpha_ (beastlike). Another division was the crocodile branch, and another developed towards the tortoises and turtles. The _Plesiosaurs_ and _Ichthyosaurs_ were two groups which have left no living representatives; they were huge reptiles returning to a whale-like life in the sea. _Pliosaurus_, one of the largest plesiosaurs, measured thirty feet from snout to tail tip--of which half was neck. The _Mosasaurs_ were a third group of great porpoise-like marine lizards. But the largest and most diversified group of these Mesozoic reptiles was the group we have spoken of as kangaroo-like, the _Dinosaurs_, many of which attained enormous proportions. In bigness these greater _Dinosaurs_ have never been exceeded, although the sea can still show in the whales creatures as great. Some of these, and the largest among them, were herbivorous animals; they browsed on the rushy vegetation and among the ferns and bushes, or they stood up and grasped