vision of physiological labor. But
we shall see how obscure this subject is if we look, for instance, to
fishes, amongst which some naturalists rank those as highest which, like
the sharks, approach nearest to amphibians; whilst other naturalists
rank the common bony or teleostean fishes as the highest, inasmuch as
they are most strictly fishlike and differ most from the other
vertebrate classes. We see still more plainly the obscurity of the
subject by turning to plants, amongst which the standard of intellect
is, of course, quite excluded; and here some botanists rank those plants
as highest which have every organ, as sepals, petals, stamens, and
pistils, fully developed in each flower; whereas other botanists,
probably with more truth, look at the plants which have their several
organs much modified and reduced in number as the highest.

If we take as the standard of high organization the amount of
differentiation and specialization 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 toward this
standard; for all physiologists admit that the specialization 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 toward specialization 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 organization.

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 everywhere supplanted and exterminated the lower? 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