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The causes which have led to this degeneration will be
discussed in a future chapter; but the facts are undisputed, and they
show us that although variation and the struggle for existence may lead,
on the whole, to a continued advance of organisation; yet they also lead
in many cases to a retrogression, when such retrogression may aid in the
preservation of any form under new conditions. They also lead to the
persistence, with slight modifications, of numerous lowly organised
forms which are suited to places which higher forms could not fully
occupy, or to conditions under which they could not exist. Such are the
ocean depths, the soil of the earth, the mud of rivers, deep caverns,
subterranean waters, etc.; and it is in such places as these, as well as
in some oceanic islands which competing higher forms have not been able
to reach, that we find many curious relics of an earlier world, which,
in the free air and sunlight and in the great continents, have long
since been driven out or exterminated by higher types.

_Summary of the first Five Chapters.


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