Many of
these minute plants develop locomotion and a degree of
sensitiveness (Diatoms, Peridinea, Euglena, etc.). Some
(Bacteria) adopt animal diet, and rise in power of movement and
sensitiveness until it is impossible to make any satisfactory
distinction between them and animals. Then the social principle
enters. First we have loose associations of one-celled plants in
a common bed, then closer clusters or many-celled bodies. In some
cases (Volvox) the cluster, or the compound plant, is round and
moves briskly in the water, closely resembling an animal. In most
cases, the cells are connected in chains, and we begin to see the
vague outline of the larger plant.
When we had reached this stage in the development of animal life,
we found great difficulty in imagining how the chief lines of the
higher Invertebrates took their rise from the Archaean chaos of
early many-celled forms. We have an even greater difficulty here,
as plant remains are not preserved at all until the Devonian
period. We can only conclude, from the later facts, that these
primitive many-celled plants branched out in several different
directions. One section (at a quite unknown date) adopted an
organic diet, and became the Fungi; and a later co-operation, or
life-partnership, between a Fungus and a one-celled Alga led to
the Lichens.
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