20 ON THE LAW WHICH HAS REGULATED 



to any causes but those 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 



