204 
ISLAND LIFE. 
[part I. 
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 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. 1 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 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 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 t-he common origin of 
O 
1 Nature , Vol. XVIII. (July, 1878), p. 2G8. 
