6i8 



THE POPULAR SCIENCE MONTHLY. 



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 neb- 

 ular 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.' 1 " 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 interpreta- 

 tion 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. Be- 

 yond 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 as- 

 suming them to be established? The 

 total thickness of stratified rocks con- 

 taining traces of life may be taken, on 

 the best geological authority, as one 

 hundred thousand feet, or nearly twen- 

 ty 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 (^ s ) 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 (-^fy) 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 ex- 

 isted 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 ' incal- 

 culable ' life could not have existed on 

 this globe ? " We certainly did not mean 

 that the resources of arithmetic are in- 

 sufficient 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 with- 

 in definite estimate or " calculation." 

 And of this the whole science of geol- 

 ogy 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 ear- 

 liest-appearing forms of life, then the 

 time since living creatures came has 

 been indefinitely vast, and that the pe- 

 riods are not amenable to anything like 

 " calculation " or trustworthy estimate 

 is shown by the way the subject is dealt 

 with in our most authoritative geologi- 

 cal 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 



