Pleistocene mammals and the middle Tertiary flora were contemporary with each other? For nearly half a century geologists have never had the courage to face this problem fairly and squarely, with all preconceived prejudices about uniformity cast aside. Is it possible that all the plants and animals of the Tertiaries and the Pleistocene may have really lived together in the same world after all? But the trouble would then be that, with this much conceded, the whole "phylogenic series" would tumble with it, and become only the taxonomic or classification series of that ancient world with which these fossil men were acquainted. To appropriate the words of one who has done much to clear the ground for a common-sense study of geology, I know of nothing against such an idea save "the almost pathetic devotion of a large school of thinkers to the religion founded by Hutton, whose high priest was Lyell, and which in essence is based on _a priori_ arguments like those which dominated Mediaeval scholasticism and made it so barren."[36] Baron Cuvier's work in the line of comparative osteology has never been surpassed, perhaps never equalled since, and he is said to have been "the greatest naturalist and comparative anatomist of that, or perhaps of any time." (LeConte, "Evol. and Rel. Thought," pp. 33, 34); and yet he maintained till the last that all those which we now call the Pleistocene mammals were distinct species from the modern ones; and it is only of recent years and with extreme reluctance that many of them have been admitted to be identical with the ones now living. All of which tends to show how unreliable are those assertions commonly found in the text-books about all the species of the so-called "older" rocks being extinct. It is only with hesitation that such specific distinctions are surrendered even to-day, though during the last few decades a steady progress has been made in bringing the palaeontology of the higher vertebrates into line with our increased knowledge of zoology, thus breaking down many of the specific distinctions which have long been maintained between the fossil and the living forms. Even the mammoth has been found to have so many characters identical with the modern elephant of India, and such a complete gradation exists between the two types, that Flower and Lydekker acknowledge the transition from one to the other is "almost imperceptible," and express a doubt whether they "can be specifically distinguished" from one another.[37] But the extreme reluctance with which anything like a confession of this fact leaks out in our modern literature can be readily understood when we try the hopeless task of splicing the environment of the modern form with that of the ancient on any basis of uniformity. Zittel gives us a peep behind the scenes which helps us to appreciate the value of a percentage of extinct species as a test of the age of a rock deposit.