i THE INTRODUCTION OF NEW SPECIES 15 



of great activity and violent convulsions, and it is in the 

 formation immediately succeeding this that the poverty of 

 forms of life is most apparent. We have then only to sup- 

 pose a long period of somewhat similar action during the vast 

 unknown interval at the termination of the Palaeozoic period, 

 and then a decreasing violence or rapidity through the 

 Secondary period, to allow for the gradual repopulation of 

 the earth with varied forms, and the whole of the facts are 

 explained. 1 We thus have a clue to the increase of the forms 

 of life during certain periods, and their decrease during others, 

 without recourse 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 know- 

 ledge of the organic world during any geological epoch is 

 necessarily very imperfect. 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 living, as 

 the whole human race who have lived and died upon the 



1 Professor Ramsay has since shown that a glacial epoch probably occurred 

 at the time of the Permian formation, which will more satisfactorily account 

 for the comparative poverty of species. 



