212 ISLAND LIFE 



malian type occur perfectly developed in the Eocene for- 

 mation. What countless ages back must we then go for 

 the origin of these groups, the whales from some ancestral 

 carnivorous animal, and the bats from the insectivora ! And 

 even then we have to seek for the common origin of car- 

 nivora, insectivora, ungulata, and marsupials at a far earlier 

 period ; so that, on the lowest estimate, Ave must jDlace 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 

 Ave can hope to arrive at that common stock from Avhich 

 the crocodiles, lizards, Ornitlioscclida, and Pksiosauo'ia, 

 Avhich 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 Avhole series of sfeolog^ical formations and their con- 

 tained fossils. He says, speaking of the abundant, varied, 

 and Avell-developed fauna of the Cambrian period : " In 

 this earliest knoAvn I'aried life AA^e find no evidence of its 

 havino' lived near the bes^innins; of the zooloo-ical series. 

 In a broad sense, compared AAdth Avhat must have gone 

 before, both biologically and 23hysically, all the phenomena 

 connected Avith this old period seem, to my mind, to be of 

 quite a recent description; and the climates of seas and 

 lands Avere of the very same kind as those the Avorld enjoys 

 at the present day." ^ 



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

 are so Aveighty, that Ave 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 ; Avhile it 

 may not imjDrobably have been longer, because the reaction of 



^ " On the Comparative Value of certain Geological Ages considered as 

 items of Geological Time." {^Proceedings of the lluyal Society ^ 1874, p. 

 334.) 



