Throughout these immense periods of time there are gaps which we cannot yet fill up. No one can yet say, for instance, when the last of the mammoths disappeared, and the first of their near relations, the Indian and African elephants, took their place. These are the missing parts of “the puzzle of life” which you may perhaps one of these days find when you come to study the subject, and when you have learned all that is known at present. But you may be sure of this, that throughout all time there has been _progress_, the lower forms of animal life have been followed by more perfect forms as the Earth grew older. It is true the lower forms of life have not all died out. These imperfect animals have run through all the ages—the chalk builder of the Cretaceous age lives in the ocean now—and there are many other simple animals which lived in Old Red Sandstone times, and are not extinct yet, but wherever a superior kind of animal has passed away another more perfect has taken its place. This will be seen at once if we compare the “Reptile Age” with the Tertiary. The great ichthyosaurus, plesiosaurus, and pterodactyl are gone, but now we have the more perfect crocodiles and birds. The mammoth is gone, but we have the elephant. There are no giant mosses or towering tree ferns, but our forest trees are more perfect and more varied. The plants which formed the coal forests and once clothed the Earth with beauty have dwindled away to the lowly forms which we must stoop to examine in swamps, and these humble plants are all the surviving relatives of their once noble family. The lordly oaks and elms, stronger, and even more lovely in the sweet drapery of their foliage, and much better fitted for our use, have succeeded all those soft-stemmed plants which grew so fast and were the best possible kind for forming coal. When you are able to study what is called comparative anatomy you will see how wonderful the _plan_ of creation is, and how beautifully it has been worked out by its great Designer. You will see in the bones of the reptiles of the oolite rocks a prophecy as it were of the birds and animals which were to come. What could be more prophetic of animals with the power of perfect flight than the leather-winged pterodactyl, half lizard and half bird? In some of these animals you will see bones only half formed, and useless to that creature, which were brought to perfection in later times, and became the most important part of the body. It is very difficult for me to make all this plain to you, but if you are really interested in it you will go to a museum where the fossils are collected, and then I am very much mistaken if you do not find a new and strange world opened to you. FOOTNOTES: [7] Numerous specimens in Case No. 7, Room V. [8] Specimens of fossil fishes from various rocks in Wall-case No. 1, Room II. [9] See examples in the large Wall-cases in Rooms I., II., and III., North Gallery.