he earth may have been almost as well peopled with
species of many genera, families, orders and classes, as at the present
day.

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.

If we take as the standar