CHAP. X.] 
THE EARTH’S AGE. 
205 
carnivora, insectivora, ungulata, 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 Palaeozoic 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 Palaeozoic times 
we must go before we can hope to arrive at that common stock 
from which the crocodiles, lizards, Ornithoscelida, and Plesiosauria , 
which had attained so great a development in the Triassic 
epoch, must have been derived.” Professor Ramsay has expressed 
similar views, 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 having 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.” 1 
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.) 
