Using once more the analogy of a branching tree to illustrate the
natural arrangement of species and their successive creation, he clearly
shows how "apparent retrogression may be in reality a progress, though
an interrupted one"; as "when some monarch of the forest loses a limb,
it may be replaced by a feeble and sickly substitute." As an instance he
mentions the Mollusca, which at an early period had reached a high state
of development of forms and species, while in each succeeding 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 had undergone, the process of peopling it with organic beings
had been continually going on, and whenever any of the higher groups had
become nearly or quite extinct, the lower forms which better resisted
the modified physical conditions served as the antetype on which to
found new races. In this manner alone, it was believed, could the
representative groups of successive periods, and the risings and
fallings in the scale of organisations, be in every case explained.

Again, attending to a recent article by Prof. Forbes, he points out
certain inaccuracies and how they may be proved to be so; and continues:

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 the 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 earth to the
population at the present time.... Records of vast geological
periods are entirely buried beneath the ocean ... beyond our
reach. Most of the gaps in the geological series may thus be
filled up, and vast numbers of unknown and unimaginable animals
which might help to elucidate the affinities of the numerous
isolated groups which are a perpetual puzzle to the z