tivity 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 suppose a long period of
somewhat similar action during the vast unknown interval at the
termination of the Palaeozoic 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.[B] 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 precise 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.

+--------------------------------------------------------------+
| [B] 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. |
+--------------------------------------------------------------+

I would also venture to suggest some reasons against the very nature of
the theory of Professor Forbes. Our knowledge 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 p