CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
9. ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE.
Natural selection acts exclusively by the preservation and accumulation of
variations, which are beneficial under the organic and inorganic conditions
to which each creature is exposed at all periods of life. The ultimate
result is that each creature tends to become more and more improved in
relation to its conditions. This improvement inevitably leads to the
gradual advancement of the organisation of the greater number of living
beings throughout the world. But here we enter on a very intricate
subject, for naturalists have not defined to each other's satisfaction what
is meant by an advance in organisation. Among the vertebrata the degree of
intellect and an approach in structure to man clearly come into play. It
might be thought that the amount of change which the various parts and
organs pass through in their development from embryo to maturity would
suffice as a standard of comparison; but there are cases, as with certain
parasitic crustaceans, in which several parts of the structure become less
perfect, so that the mature animal cannot be called higher than its larva.
Von Baer's standard seems the most widely applicable and the best, namely,
the amount of differentiation of the parts of the same organic being, in
the adult state, as I should be inclined to add, and their specialisation
for different functions; or, as Milne Edwards would express it, the
completeness of the division of physiological labour. But we shall see how
obscure this subject is if we look, for instance, to fishes, among which
some naturalists rank those as highest which, like the sharks, approach
nearest to amphibians; while other naturalists rank the common bony or
teleostean fishes as the highest, inasmuch as they are most strictly fish-
like, and differ most from the other vertebrate classes. We see still more
plainly the obscurity of the subject by turning to plants, among 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.