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As we have seen, however, that every part of the organism
appears to be varying independently, at the same time, though to
different amounts, there seems no reason to believe that the necessity
for two or more coincident variations would prevent the required change
from taking place.

_The Continued Existence of Low Forms of Life._
Since species are continually undergoing modifications giving them some
superiority over other species or enabling them to occupy fresh places
in nature, it may be asked--Why do any low forms continue to exist? Why
have they not long since been improved and developed into higher forms?
The answer, probably, is, that these low forms occupy places in nature
which cannot be filled by higher forms, and that they have few or no
competitors; they therefore continue to exist. Thus, earthworms are
adapted to their mode of life better than they would be if more highly
organised. So, in the ocean, the minute foraminifera and infusoria, and
the larger sponges and corals, occupy places which more highly developed
creatures could not fill.


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