| Natural
→acts exclusively
by the preservation and accumulation of variations, which
beneficial under the organic and inorganic conditions
to which each creature
→is
→exposed at all periods of life. The ultimate result
that each creature
to become more and more improved in relation to
This improvement
→OMIT
inevitably
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
satisfaction what is meant by an advance in organisation. Amongst 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
in their development from the 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
standard seems the most widely applicable and the best, namely, the amount of differentiation of the
parts
the
→same organic being, in the adult state
as I should be inclined to
→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
subject
is if we look, for instance, to
amongst which some naturalists rank those as highest which, like the sharks, approach nearest to
whilst 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,
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,
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