e of land was, with many oscillations, gaining on the
water, and there was much emigration to it from the over-populated seas.
When the fish went on land in the Devonian, it must have found a diet
(insects, etc.) there, and the insects must have been preceded by a
plant population. We have first, therefore, to consider the evolution of
the plant, and see how it increases in form and number until it covers
the earth with the luxuriant forests of the Carboniferous period.

The plant world, we saw, starts, like the animal world, with a great
kingdom of one-celled microscopic representatives, and the same
principles of development, to a great extent, shape it into a large
variety of forms. Armour-plating has a widespread influence among them.
The graceful Diatom is a morsel of plasm enclosed in a flinty box, often
with a very pretty arrangement of the pores and markings. The Desmid has
a coat of cellulose, and a less graceful coat of cellulose encloses the
Peridinean. 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. Others remained at the Alga-level, and grew in
great thickets along the sea bottoms, no doubt rivalling or surpassing
the giant sea-weeds, sometimes 400 feet lo