298 EVOLUTIONAL GEOLOGY. 



with the distances which intervene between the heavenly ])odies; Imt 

 while some chose the distance of the nearest tixed star as their unit, 

 others were content to measure the years in terms of miles from 

 the sun. 



EVOLUTION OF ORGANISMS. 



The stratified rocks were eloquent of time, and not to the geolooist 

 alone; the}^ appealed with equal force to the biologist, Accepti no- 

 Darwin's explanation of the origin of species, the present rate at 

 which form flows to form seemed so slow as almost to amount to immu- 

 tability. How vast, then, must have been the period during which by 

 slow degrees and innumerable stages the protozoon was transformed 

 into the man! And if we turn to the stratified column, what do we 

 find^ ]Man, it is true, at the summit, the oldest fossiliferous rocks -U 

 miles lower down, and the fossils they contain already representing 

 most of the great classes of the Invertebrata, including Ci'ustacea and 

 Worms. Thus the evolution of the Vertebrata alone is known to have 

 occupied a period represented b}- a thickness of 'M miles of sediment. 

 How much greater, then, must have })een the inter\'al requii-ed for the 

 elaboration of the whole organic world! The human mind, dwelling 

 on such considerations as these, seems at times to have been affected 

 by a sur-excitation of the imagination, and a consequent paralysis of 

 the understanding, which led to a refusal to measure geological time 

 by 3^ears at all, or to reckon l)y anything less than '"eternities." 



GEOLOGIC PERIODS OF TIME. 



After the admirable address of your president last year it might ])e 

 thought needless for me to again enter into a consideration of thissul)- 

 ject; it has been said, however, that the question of geological time is 

 like the Djinn in Arabian tales, and will irrepressibly come up again for 

 discussion, however often it is disposed of. For my part, 1 do not 

 regard the question so despondingly, but rather hope that by perse- 

 vering effort we may succeed in discovering the talisman l)y which we 

 may compel the unwilling Djinn into our service. How immeasur- 

 able would be the advance of our science could we but bring the chief 

 events which it records into some relation with a standard of time ! 



Before proceeding to the discussion of estimates of time drawn 

 from a study of stratified rocks let us first consider those which have 

 been already suggested by other data. These are as follows: (1) Time 

 which has elapsed since the separation of the earth and moon, fifty-six 

 millions of years, minimum estimate by Prof. G. H. Darwin. (2) Since 

 the " consistentior status," twenty to forty millions (Lord Kelvin). 

 (3) Since the condensation of the oceans, eighty to ninety millions, 

 maximum estimate by Prof. J. Joly. 



It may be at once oliserved that these estimates, although independ- 

 ent, are all of the same order of magnitude and so far confirmatory of 



