They form, as it were, the base of the great
structure of animal life, on which the next higher forms rest; and
though in the course of ages they may undergo some changes, and
diversification of form and structure, in accordance with changed
conditions, their essential nature has probably remained the same from
the very dawn of life on the earth. The low aquatic diatomaceae and
confervae, together with the lowest fungi and lichens, occupy a similar
position in the vegetable kingdom, filling places in nature which would
be left vacant if only highly organised plants existed. There is,
therefore, no motive power to destroy or seriously to modify them; and
they have thus probably persisted, under slightly varying forms, through
all geological time.
_Extinction of Lower Types among the Higher Animals._
So soon; however, as we approach the higher and more fully developed
groups, we see indications of the often repeated extinction of lower by
higher forms. This is shown by the great gaps that separate the
mammalia, birds, reptiles, and fishes from each other; while the lowest
forms of each are always few in number and confined to limited areas.
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