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 Paleozoic 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.t 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, 
