friction of the tides, the influence of melting polar ice, etc., speculations that have by no means taken their place among the established principles of physical astronomy. But Sir William Thomson concludes that a limit is to be put to the time during which life can have existed upon the earth. Yet it must not be supposed from this that his chronology at all approximates to that of Archbishop Usher. He assumes, and draws his arguments from, the nebular hypothesis, and, instead of starving the geologists in their allowance of time, it must be confessed that he deals with them very liberally. He says he believes that "the existing state of things on the earth, life on the earth—all geological history showing continuity of life—must be limited within some such period of time as one hundred million years." "Some such period of time!" This is sufficiently vague, and raises the query, "Does it mean that the time may have been two, three, or four hundred million years?" Prof. Thomson himself puts this interpretation upon it when he says elsewhere of the high surface-temperature which made life impossible: "We must still admit some limit, such as fifty million years, one hundred million years, or two or three hundred million years ago. Beyond that we cannot go." And, again, he expresses the opinion that the sun has not really illuminated the earth for a period of five hundred million years.
But are the geologists so very badly cramped by these limitations—even assuming them to be established? The total thickness of stratified rocks containing traces of life may be taken, on the best geological authority, as one hundred thousand feet, or nearly twenty miles. The deposit of one hundred thousand feet of stratified rock, in one hundred million years, implies that the deposit has taken place at the rate of about one-eighty-third (183) of an inch per year. If the "some such period" was double that time, then a hundred and sixty years would be allowed for the accumulation of an inch of sedementary rock; or, if three hundred million years are taken, the rate of stratified growth would be one-two-hundred-and-forty-ninth (1249) of an inch annually. This is a very moderate pace, and certainly affords little ground of complaint on the part of the biologist that he is pinched for time by the geologist and physicist. Prof. Huxley, therefore, had not the slightest reason for admitting that his theory was not "in agreement" with what physical astronomy teaches us of the "ways of Nature."
Continuing the same line of thought, Dr. Deems quotes our remark that "it is a demonstrated fact that life has existed on the globe through periods so vast as to be incalculable," and asks: "Where, when, and how, was this ever 'demonstrated?' Has it not been shown that within a period not 'incalculable' life could not have existed on this globe?" We certainly did not mean that the resources of arithmetic are insufficient to express the time during which life has existed upon earth, but we did mean that the periods are so vast and obscure as not to be brought within definite estimate or "calculation." And of this the whole science of geology affords the demonstration. If the rocks have been formed in succession, as geology has proved, and twenty miles of strata have been piled over the earliest-appearing forms of life, then the time since living creatures came has been indefinitely vast, and that the periods are not amenable to anything like "calculation" or trustworthy estimate is shown by the way the subject is dealt with in our most authoritative geological works. Where uncertainty enters largely, positive calculation is excluded, and accordingly we find that when the ablest geologists approach this subject they either abstain from any attempt at calcidation, or they refuse to deal with it, in terms of years and talk of eras, epochs, and cycles. Prof. Dana speaks
