E 'coin 't ion of the Earth. 101 



the Eozoon Canadense* and Lingula of the Potsdam sand- 

 stone, on the borders of the Azoic period — the former, as 

 claimed by Professor Dawson, possibly over the usually re- 

 cognized line. How much below the Cambrian rocks life orig- 

 inally appeared, we do not know. Evidences, if they ever 

 existed, have been destroyed by igneous agencies or other- 

 wise. As we follow the scale of geological succession along 

 up from the era in which these forms appeared, there are 

 many " missing links " it is true, absent from our stone vol- 

 umes, but many exceedingly suggestive links which are not 

 missing. The pedigree of the horse constitutes one such 

 indication, most interesting and instructive. Some of the 

 later reptiles, also, have many of the characteristics of birds 

 and mammals. The earliest birds were strangely reptilian 

 in their characters. Some of them have teeth like reptiles 

 and mammals. The evolution of life from the fish to the 

 reptilian stage is indicated by the peculiarities of the re- 

 mains of fish and reptiles belonging to the Carboniferous 

 •era. An outline of the order of zoological succession, as 

 determined by the remains found in the rocks through the 

 different geological periods, is suggestive of the complete- 

 ness of the phylogenetic proof of the doctrine of organic 

 evolution. 



Duration of Geological Periods. That an enormous 

 length of time must have been occupied by these processes 

 of geological evolution is so evident as to demand no argu- 

 ment. A definite estimate of this time, however, is exceed- 

 ingly difficult. It may be said, in general, that the tenden- 

 cy at the present day is to diminish somewhat the enormous 

 periods of duration which have been assumed by modern 

 geologists as necessary for the production of the observed 

 geological changes. The time necessary for the Falls of 

 Niagara to wear their way back from Lake Ontario to their 

 present position has been variously estimated as 36,000, 

 90,000, 100,000, even 200,000 years. Sir Charles Lyell es- 

 timated that they were wearing their way on an average of 

 about a foot a year. But a very recent estimate reduces 

 the time to 7,000 years. f Take even the last figures, which 



* Geologists are not all agreed that the Eozoon of Professor Dawson is of or- 

 ganic origin. 



t Niagara Falls must have originated since the most recent Glacial period, in 

 ■a comparatively modern era, as the drift deposits everywhere underlie the river- 

 deposits between the Falls and Lake Ontario. 



