They have no visible
organisation, though, naturally, they must have some kind of
structure below the range of the microscope. Their life consists
in the absorption of food-particles, at any point of their
surface, and in dividing into two living microbes, instead of
dying, when their bulk increases. A very lowly branch of the
Bacteria (Nitrobacteria) sometimes dispute their claim to the
lowest position in the hierarchy of living nature, but there is
reason to suspect that these Bacteria may have degenerated from a
higher level.
Here we have a convenient starting-point for the story of life,
and may now trace the general lines of upward development. The
first great principle to be recognised is the early division of
these primitive organisms into two great classes, the moving and
the stationary. The clue to this important divergence is found in
diet. With exceptions on both sides, we find that the non-moving
microbes generally feed on inorganic matter, which they convert
into plasm; the moving microbes generally feed on ready-made
plasm--on the living non-movers, on each other, or on particles
of dead organic matter. Now, inorganic food is generally diffused
in the waters, so that the vegetal feeders have no incentive to
develop mobility.
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