204 



ISLAND LIFE. 



[PAKT I. 



edition of his Frinciides of Geology (omitted in later editions), 

 by which he arrived at 240 millions of years as having probably 

 elapsed since the Cambrian period — a very moderate estimate 

 in the opinion of most geologists. This calculation was founded 

 on the rate of modification of the species of mollusca ; but 

 much more recently Professor Haughton has arrived at nearly 

 similar figures from a consideration of the rate of formation 

 of rocks and their known maximum thickness, whence he 

 deduces a maximum of 200 millions of years for the whole 

 duration of geological time, as indicated by the series of 

 stratified formations.^ But in the opinion of all our first natu- 

 ralists and geologists, the period occupied in the formation of 

 the known stratified rocks only represents a portion, and perhaps 

 a small portion, of geological time. In the last edition of the 

 Origin of Species (p. 286), Mr. Darwin says : — " Consequently, if 

 the theory be true, it is indisputable that before the lowest Cam- 

 brian stratum was deposited long periods elapsed, as long as, or 

 probably far longer than, the v,^hole interval from the Cambrian 

 age to the present day ; and that during these vast periods the 

 world swarmed with living creatures." Professor Huxley, in his 

 anniversary address to the Geological Society in 1870, adduced 

 a number of special cases showing that, on the theory of de- 

 velopment, almost all the higher forms of life must have 

 existed during the Palaeozoic period. Thus, from the fact that 

 almost the whole of the Tertiary period has been required to 

 convert the ancestral Orohippus into the true horse, he believes 

 that, in order to have time for the much greater change of the 

 ancestral Ungulata into the two great odd-toed and even-toed 

 divisions (of which change there is no trace even among the 

 earliest Eocene mammals), we should require a large portion, 

 if not the whole, of the Mesozoic or Secondary period. Another 

 case is furnished by the bats and whales, both of which strange 

 modifications of the mammalian type occur perfectly developed 

 in the Eocene formation. 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 

 1 Nature, Vol. XVIII. (July, 1878), p. 2G8. 



