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 Paleozoic 
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 
Palzozoic 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 becausesthe whole area of the early formations 
now exposed to our researches was elevated at the 
end of the Palzozoic period, and remained so through 
the interval required for the organic changes which 
c 
