CHAPTER X

THE EARTH'S AGE, AND THE RATE OF DEVELOPMENT OF ANIMALS AND PLANTS

Various Estimates of Geological Time—Denudation and Deposition of Strata as a Measure of Time—How to Estimate the Thickness of the Sedimentary Rocks—How to Estimate the Average Rate of Deposition of the Sedimentary Rocks—The Rate of Geological Change Probably greater in very Remote Times—Value of the Preceding Estimate of Geological Time—Organic Modification Dependent on Change of Conditions—Geographical Mutations as a Motive Power in bringing about Organic Changes—Climatal Revolutions as an Agent in Producing Organic Changes—Present Condition of the Earth one of Exceptional Stability as Regards Climate—Date of last Glacial Epoch and its Bearing on the Measurement of Geological Time—Concluding Remarks.

The subjects discussed in the last three chapters introduce us to a difficulty which has hitherto been considered a very formidable one—that the maximum age of the habitable earth, as deduced from physical considerations, does not afford sufficient time either for the geological or the organic changes of which we have evidence. Geologists continually dwell on the slowness of the processes of upheaval and subsidence, of denudation of the earth's surface, and of the formation of new strata; while on the theory of development, as expounded by Mr. Darwin, the variation and modification of organic forms is also a very slow process, and has usually been considered to require an

even longer series of ages than might satisfy the requirements of physical geology alone.

As an indication of the periods usually contemplated by geologists, we may refer to Sir Charles Lyell's calculation in the tenth edition of his Principles of Geology (omitted in later editions), by which he arrived at 240 millions of years as having probably elapsed since the Cambrian period—a very moderate estimate in the opinion of most geologists. This calculation was founded on the rate of modification of the species of mollusca; but much more recently Professor Haughton has arrived at nearly similar figures from a consideration of the rate of formation of rocks and their known maximum thickness, whence he deduces a maximum of 200 millions of years for the whole duration of geological time, as indicated by the series of stratified formations.[[82]] But in the opinion of all our first naturalists and geologists, the period occupied in the formation of the known stratified rocks only represents a portion, and perhaps a small portion, of geological time. In the sixth edition of the Origin of Species (p. 286), Mr. Darwin says: "Consequently, if the theory be true, it is indisputable that before the lowest Cambrian stratum was deposited long periods elapsed, as long as, or probably far longer than, the whole interval from the Cambrian age to the present day; and that during these vast periods the world swarmed with living creatures." Professor Huxley, in his anniversary address to the Geological Society in 1870, adduced a number of special cases showing that, on the theory of development, almost all the higher forms of life must have existed during the Palæozoic period. Thus, from the fact that almost the whole of the Tertiary period has been required to convert the ancestral Orohippus into the true horse, he believes that, in order to have time for the much greater change of the ancestral Ungulata into the two great odd-toed and even-toed divisions (of which change there is no trace even among the earliest Eocene mammals), we should require a large portion, if not the whole, of the Mesozoic or Secondary period. Another case is furnished by the bats and whales, both of which strange modifications of the

mammalian type occur perfectly developed in the Eocene formation. What countless ages back must we then go for the origin of these groups, the whales from some ancestral carnivorous animal, and the bats from the insectivora! And even then we have to seek for the common origin of carnivora, insectivora, ungulata, and marsupials at a far earlier period; so that, on the lowest estimate, we must place the origin of the mammalia very far back in Palæozoic times. Similar evidence is afforded by reptiles, of which Professor Huxley says: "If the very small differences which are observable between the crocodiles of the older Secondary formations and those of the present day furnish any sort of an approximation towards an estimate of the average rate of change among reptiles, it is almost appalling to reflect how far back in Palæozoic times we must go before we can hope to arrive at that common stock from which the crocodiles, lizards, Ornithoscelida, and Plesiosauria, which had attained so great a development in the Triassic epoch, must have been derived." Professor Ramsay has expressed similar views, derived from a general study of the whole series of geological formations and their contained fossils. He says, speaking of the abundant, varied, and well-developed fauna of the Cambrian period: "In this earliest known varied life we find no evidence of its having lived near the beginning of the zoological series. In a broad sense, compared with what must have gone before, both biologically and physically, all the phenomena connected with this old period seem, to my mind, to be of quite a recent description; and the climates of seas and lands were of the very same kind as those the world enjoys at the present day."[[83]]

These opinions, and the facts on which they are founded, are so weighty, that we can hardly doubt that, if the time since the Cambrian epoch is correctly estimated at 200 millions of years, the date of the commencement of life on the earth cannot be much less than 500 millions; while it may not improbably have been longer, because the reaction of

the organism under changes of the environment is believed to have been less active in low and simple, than in high and complex forms of life, and thus the processes of organic development may for countless ages have been excessively slow.