Whether organisation on the whole has actually
advanced from the remotest geological periods to the present day will be
more conveniently discussed in our chapter on Geological Succession.
But it may be objected that if all organic beings thus tend to rise in the
scale, how is it that throughout the world a multitude of the lowest forms
still exist; and how is it that in each great class some forms are far more
highly developed than others? Why have not the more highly developed forms
every where supplanted and exterminated the lower? Lamarck, who believed
in an innate and inevitable tendency towards perfection in all organic
beings, seems to have felt this difficulty so strongly that he was led to
suppose that new and simple forms are continually being produced by
spontaneous generation. Science has not as yet proved the truth of this
belief, whatever the future may reveal. On our theory the continued
existence of lowly organisms offers no difficulty; for natural selection,
or the survival of the fittest, does not necessarily include progressive
development--it only takes advantage of such variations as arise and are
beneficial to each creature under its complex relations of life. And it
may be asked what advantage, as far as we can see, would it be to an
infusorian animalcule--to an intestinal worm--or even to an earth-worm, to
be highly organised. If it were no advantage, these forms would be left,
by natural selection, unimproved or but little improved, and might remain
for indefinite ages in their present lowly condition.
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