... Admitted
facts seem to show ... a general, but not a detailed
progression.... It is, however, by no means difficult to show
that a real progression in the scale of organisation is perfectly
consistent with all the appearances, and even with apparent
retrogression should such occur.
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.
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