20 ON THE LAW WHICH HAS REGULATED 
to any causes but these we know to have existed, 
and to effects fairly deducible from them. The pre- 
cise manner in which the geological changes of the 
early formations were effected is so extremely 
obscure, that when we can explain important facts 
by a retardation at one time and an acceleration at 
another of a process which we know from its nature 
and from observation to have been unequal,—a cause 
so simple may surely be preferred to one so obscure 
and hypothetical as polarity. 
I would also venture to suggest some reasons 
against the very nature of the theory of Professor 
Forbes. Our knowledge of the organic world 
during any geological epoch is necessarily very im- 
perfect. Looking at the vast numbers of species 
and groups that have been discovered by geologists, 
this may be doubted; but we should compare their 
numbers not merely with those that now exist upon 
the earth, but with a far larger amount. We have 
no reason for believing that the number of species 
on the earth at any former period was much less 
than at present; at all events the aquatic portion, 
with which geologists have most acquaintance, was 
probably often as great or greater. Now we know 
that there have been many complete changes of 
species; new sets of organisms have many times been 
introduced in place of old ones which have become 
extinct, so that the total amount which have existed 
on the earth from the earliest geological period must 
have borne about the same proportion to those now 
