CHAP. X.] 



THE EARTH'S AGE. 



205 



cainivora, insectivora, uugulata, and marsupials at a far earlier 

 period ; so that, on the lowest estimate, we must place the origin 

 of the mammalia very far back in Palseozoic times. Similar 

 evidence is afforded by reptiles, of which Professor Huxley says : 

 — If the very small differences which are observable between 

 the crocodiles of the older Secondary formations and those of the 

 present day furnish any sort of an approximation towards an 

 estimate of the average rate of change among reptiles, it is 

 almost appalling to reflect how far back in Palseozoic times 

 we must go before we can hope to arrive at that common stock 

 from which the crocodiles, lizards, Ornithoscelida, and Plesiosauria^ 

 w^hich had attained so great a development in the Triassic 

 epoch, must have been derived." Professor Ramsay has expressed 

 similar view^s, derived from a general study of the whole series 

 of geological formations and their contained fossils. He says, 

 speaking of the abundant, varied, and well-developed fauna of 

 the Cambrian period : ''In this earliest known varied life 

 we find no evidence of its hav^ing lived near the beginning of 

 the zoological series. In a broad sense, compared with what 

 must have gone before, both biologically and physically, all the 

 phenomena connected with this old period seem, to my mind, 

 to be of quite a recent description ; and the climates of seas and 

 lands were of the very same kind as those the world enjoys at 

 the present day." ^ 



These opinions, and the facts on which they are founded, are 

 so weighty, that we can hardly doubt that, if the time since the 

 Cambrian epoch is correctly estimated at 200 millions of years, 

 the date of the commencement of life on the earth cannot be 

 much less than 500 millions ; while it may not improbably have 

 been longer, because the reaction of the organism under changes 

 of the environment is believed to have been less active in 

 low and simple, than in high and complex forms of life, and 

 thus the processes of organic development may for countless 

 ages have been excessively slow. 



But according to the physicists, no such periods as are here 



1 ''On the Comparative Value of certain Geological Ages considered as 

 items of Geological Time." {Proceedings of the Royal Society, 1874, 

 p. 334.) 



