i THE INTRODUCTION OF NEW SPECIES 13 



Again, each of these groups may not have become totally 

 extinct, but may have left a few species, the modified proto- 

 types of which have existed in each succeeding period, a faint 

 memorial of their former grandeur and luxuriance. Thus 

 every case of apparent retrogression may be in reality a pro- 

 gress, though an interrupted one : when some monarch of the 

 forest loses a limb, it may be replaced by a feeble and sickly 

 substitute. The foregoing remarks appear to apply to the 

 case of the Mollusca, which, at a very early period, had 

 reached a high organisation and a great development of forms 

 and species in the testaceous Cephalopoda. In each succeed- 

 ing age modified species and genera replaced the former ones 

 which had become extinct, and as we approach the present 

 era, but few and small representatives of the group remain, 

 while the Gasteropods and Bivalves have acquired an immense 

 preponderance. In the long series of changes the earth has 

 undergone, the process of peopling it with organic beings has 

 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 modified physical conditions 

 have served as the antitypes on which to found the new 

 races. In this manner alone, it is believed, can the represent- 

 ative groups at successive periods, and the risings and fallings 

 in the scale of organisation, be in every case explained. 



Objections to Forbes' Theory of Polarity 

 The hypothesis of polarity, recently put forward by Pro- 

 fessor Edward Forbes to account for the abundance of generic 

 forms at a very early period and at present, while in the in- 

 termediate epochs there is a gradual diminution and impover- 

 ishment, till the minimum occurred at the confines of the 

 Palaeozoic and Secondary epochs, appears to us quite unneces- 

 sary, 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 com- 

 mon, and the greater parts of the genera and families also 

 disappear, to be replaced by new ones. It is almost univer- 

 sally admitted that such a change in the organic world must 

 have occupied a vast period of time. Of this interval we 

 have no record ; probably because the whole area of the early 



