Now the income of the animal is proportional to its
surface, its expense to its mass, and activity. And the ratio of
surface to mass is most favorable in the smallest animals.[A] Hence,
smaller animals, as a rule, increase faster than larger ones; and
this is only one illustration of the fact that great size in an
animal is anything but an unmixed advantage to its possessor. But
muscles and nerves are the most expensive systems; here most of the
food is burned up. Hence energetic animals have a small balance
remaining. Now the turbellarian is small and sluggish, with a fair
digestive system. With a great amount of nutriment at its disposal
the reproductive system came rapidly to a high development, and
relatively to other organs stands higher than it almost ever will
again.
[Footnote A: Cf. p. 35.]
It is only fair to state that good authorities hold that so
primitive an animal could not originally have had so highly
developed a system, and that this characteristic must be acquired,
not ancestral.
That certain portions of it may be later developments may be not
only possible but probable. But anyone who has carefully studied the
different groups of worms, will, I think, readily grant that in the
stage of these flat worms reproduction was the dominant function,
which had most nearly attained its possible height of development.
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