d of high organisation, the amount of
differentiation and specialisation of the several organs in each being
when adult (and this will include the advancement of the brain for
intellectual purposes), natural selection clearly leads towards this
standard: for all physiologists admit that the specialisation of organs,
inasmuch as in this state they perform their functions better, is
an advantage to each being; and hence the accumulation of variations
tending towards specialisation is within the scope of natural selection.
On the other hand, we can see, bearing in mind that all organic
beings are striving to increase at a high ratio and to seize on every
unoccupied or less well occupied place in the economy of nature, that
it is quite possible for natural selection gradually to fit a being to
a situation in which several organs would be superfluous or useless: in
such cases there would be retrogression in the scale of organisation.
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