THE EVIDENCE OF THE NATURALIST 5 



. . . If the geological clock is wrong, all the naturalist 

 will have to do is to modify his notions of the rapidity 

 of change accordingly.' This contention is obviously 

 true as regards the time which has elapsed since the 

 earliest fossiliferous rocks were laid down. For the 

 duration of the three great periods we must look to the 

 geologist ; but the question as to whether the whole of 

 organic evolution is comprised within these limits, or, if 

 not, what proportion of it is so contained, is a question 

 for the naturalist. The naturalist alone can tell the 

 geologist whether his estimate is sufficient, or whether it 

 must be multiplied by a small or by some unknown but 

 certainly high figure, in order to account for the evolu- 

 tion of the earliest forms of life known in the rocks. 

 This, I submit, is a most important contribution to the 

 discussion. 



Before proceeding further it is right to point out that 

 obviously these arguments will have no weight with 

 those who do not believe that evolution is a reality. But 

 although the causes of evolution are greatly debated, it 

 may be assumed that there is no perceptible difference of 

 opinion as to evolution itself, and this common ground 

 will bear the weight of all the zoological arguments 

 I shall advance to-day. 



It will be of interest to consider first how the matter 

 presented itself to naturalists before the beginning of this 

 controversy on the age of the habitable earth. I will 

 content myself with quotations from three great writers 

 on biological problems — men of extremely different types 

 of mind, who yet agreed in their conclusions on this 

 subject. 



In the original edition of the Origin of Species (1859), 

 Darwin, arguing from the presence of trilobites, Nautilus, 

 Lingula, &c., in the earliest fossiliferous rocks, came to 

 the following conclusion (pp. 306, 307) : ' Consequently, 

 if my theory be true, it is indisputable that before the 

 lowest Silurian stratum was deposited long periods 

 elapsed, as long as, or probably far longer than, the 

 whole interval from the Silurian age to the present day ; 



