CHAP. X THE EARTH'S AGE 211 



even longer series of ages than might satisfy tlic require- 

 ments of physical geology alone. 



As an indication of the periods usually contemplated by 

 geologists, we may refer to Sir Charles Lyell's calculation 

 in the tenth edition of his Principles 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 geolo- 

 gists. This calculation was founded on the rate of modi- 

 fication 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 naturalists 

 and geologists, the period occupied in the formation of the 

 known stratified rocks only represents a portion, and per- 

 haps a small portion, of geological time. In the sixth edition 

 of the Origin of Species (p. 286), Mr. Darwin says : " Con- 

 sequently, if the theory be true, it is indisputable that 

 before the lowest Cambrian stratum was deposited long 

 periods elapsed, as long as, or probably far longer than, the 

 whole 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 num- 

 ber of special cases showing that, on the theory of develop- 

 ment, 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 Second- 

 ary period. Another case is furnished by the bats and 

 whales, both of which strange modifications of the mam- 

 1 Nature, Vol. XVIII. (July, 1878), p. 268. 



p2 



