hopped about inquiringly on the dry land, under the shadow of the ancient tree-ferns and club-mosses, they were the only terrestrial vertebrates then existing, and they had the field (or, rather, the forest) all to themselves.  For a while, therefore, like all dominant races for the time being, they blossomed forth at their ease into relatively gigantic forms.  Frogs as big as donkeys, and efts as long as crocodiles, luxuriated to their hearts’ content in the marshy lowlands, and lorded it freely over the small creatures which they found in undisturbed possession of the Carboniferous isles.  But as ages passed away, and new improvements were slowly invented and patented by survival of the fittest in the offices of nature, their own more advanced and developed descendants, the reptiles and mammals, got the upper hand with them, and soon lived them down in the struggle for life, so that this essentially intermediate form is now almost entirely restricted to its one adapted seat, the pools and ditches that dry up in summer.

The reptiles, again, are a class in which the biggest modern forms are simply nowhere beside the gigantic extinct species.  First appearing on the earth at the very close of the vast primary periods—­in the Permian age—­they attained in secondary times the most colossal proportions, and have certainly never since been exceeded in size by any later forms of life in whatever direction.  But one must remember that during the heyday of the great saurians, there were as yet no birds and no mammals.  The place now filled in the ocean by the whales and grampuses, as well as the place now filled in the great continents by the elephants, the rhinoceroses, the hippopotami, and the other big quadrupeds, was then filled exclusively by huge reptiles, of the sort rendered familiar to us all by the restored effigies on the little island in the Crystal Palace grounds.  Every dog has his day, and the reptiles had their day in the secondary period.  The forms into which they developed were certainly every whit as large as any ever seen on the surface of this planet, but not, as I have already shown, appreciably larger than those of the biggest cetaceans known to science in our own time.

During the very period, however, when enaliosaurians and pterodactyls were playing such pranks before high heaven as might have made contemporary angels weep, if they took any notice of saurian morality, a small race of unobserved little prowlers was growing up in the dense shades of the neighbouring forests which was destined at last to oust the huge reptiles from their empire over earth, and to become in the fulness of time the exclusively dominant type of the whole planet.  In the trias we get the first remains of mammalian life in the shape of tiny rat-like animals, marsupial in type, and closely related to the banded ant-eaters of New South Wales at the present day.  Throughout the long lapse of the secondary ages, across the lias, the oolite, the wealden, and the chalk,