THE INTRODUCTION OF NEW SPECIES. 17 



been continually going on, and whenever any of the 

 higher groups have become nearly or quite extinct, 

 the lower forms which have better resisted the modi- 

 fied physical conditions have served as the antitypes 

 on which to found the new races. In this manner 

 alone, it is believed, can the representative groups 

 at successive periods, and the risings and fallings in 

 the scale of organization, be in every case explained. 



Objections to Forbes* Theory of Polarity. 



The hypothesis of polarity, recently put forward by 

 Professor Edward Forbes to account for the abun- 

 dance of generic forms at a very early period and at 

 present, while in the intermediate epochs there is 

 a gradual diminution and impoverishment, till the 

 minimum occurred at the confines of the Palaeozoic 

 and Secondary epochs, appears to us quite unne- 

 cessary, as the facts may be readily accounted for 

 on the principles already laid down. Between the 

 Palaeozoic and Neozoic periods of Professor Forbes, 

 there is scarcely a species in common, and the 

 greater part of the genera and families also dis- 

 appear to be replaced by new ones. It is almost 

 universally admitted that such a change in the 

 organic world must have occupied a vast period of 

 time. Of this interval we have no record ; pro- 

 bably because the whole area of the early formations 

 now exposed to our researches was elevated at the 

 end of the Palaeozoic period, and remained so through 

 the interval required for the organic changes which 



c 



