In like manner the knowledge of the past history of animal life is the result of explorations by trained scholars into the records of the past. We have remains of ancient life in the rocks, and also traces of past conditions in the developing stages of animals. These are all more ancient than the inscriptions left by the hand of man upon his tombs, his temples, and his columns, but nevertheless full of meaning if we can only understand them. This historical method of investigation applied to the organic world has brought new and unexpected views regarding the antiquity of life. The Diversity of Living Forms.--Sooner or later the question of the derivation of the animals and plants is bound to come to the mind of the observer of nature. There exist at present more than a million different kinds of animals. The waters, the earth, the air teem with life. The fishes of the sea are almost innumerable, and in a single order of the insect-world, the beetles, more than 50,000 species are known and described. In addition to living animals, there is entombed in the rocks a great multitude of fossil forms which lived centuries ago, and many of which have become entirely extinct. How shall this great diversity of life be accounted for? Has the great variety of forms existed unchanged from the days of their creation to the present? Or have they, perchance, undergone modifications so that one original form, or at least a few original types, may have through transformations merged into different kinds? This is not merely an idle question, insoluble from the very nature of the case; for the present races of animals have a lineage reaching far into the past, and the question of fixity of form as against alteration of type is a historical question, to be answered by getting evidence as to their line of descent. Are Species Fixed in Nature?--The aspect of the matter which presses first upon our attention is this: Are the species (or different kinds of animals and plants) fixed, and, within narrow limits, permanent, as Linnæus supposed? Have they preserved their identity through all time, or have they undergone changes? This is the heart of the question of organic evolution. If observation shows species to be constant at the present time, and also to have been continuous so far as we can trace their parentage, we must conclude that they have not been formed by evolution; but if we find evidence of their transmutation into other species, then there has been evolution.