_Age of the earth as determined by the date of the glacial epoch._—Professor A. Winchell, by a most careful examination of the probable relative lengths of geological periods, arrived at the conclusion that the time which elapsed since the beginning of the _glacial epoch_ is to the time which has elapsed since the solidification of the earth’s surface as 1 to 250.[55] According to the eccentricity theory of the cause of the glacial epoch, that epoch began 240,000 years ago; consequently this makes the time since solidification took place 60,000,000 years, a period which agrees roughly with that deduced from denudation, and is so far presumptive evidence of the truth of that theory of the cause of the glacial cold. [Footnote 55: _World Life_, p. 369.] _Testimony of Biology_.—The time required for the variation and modification of organic forms has, Mr. Alfred R. Wallace states, been generally considered to require an even longer series of ages than might satisfy the demands of physical geology. This is a point, however, on which I am not qualified to venture an opinion. I shall simply refer to the views held by our highest authorities on the subject. Referring to Professor Huxley’s anniversary address to the Geological Society in 1870, where he shows that almost all the higher forms of life must have existed during the Palæozoic period, Mr. Wallace says: “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, Professor Huxley, 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.”[56] “If the very small differences,” says Professor Huxley,[57] “which are observable between the _Crocodilia_ of the older Mesozoic formations and those of the present day furnish any sort of approximation towards an estimate of the average rate of change among the _Sauropsida_, 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 _Crocodilia_, _Lacertilia_, _Ornithoscelida_, and _Plesiosauria_, which had attained so great a development in the Triassic epoch, must have been derived.