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 
Paleozoic 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 Palwozoic 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 
