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 earthworm--to be highly organized. If it were no
advantage, these forms would be left, by natural selection, unimproved
or but little improved, and might remain for indefinite ages in their
present lowly condition. And geology tells us that some of the lowest
forms, as the infusoria and rhizopods, have remained for an enormous
period in nearly their present state. But to suppose that most of the
many low forms now existing have not in the least advanced since the
first dawn of life would be extremely rash; for every naturalist who has
dissected some of the beings now ranked as very low in the scale must
have been struck with their really wondrous and beautiful organization.

Nearly the same remarks are applicable if we look to the different
grades of organization within the same great group; for instance, in the
vertebrata to the coexistence of mammals and fish; amongst mammalia to
the coexistence of man and the ornithorhynchus; amongst fishes to the
coexistence of the shark and the lancelet (Amphioxus), which later fish
in the extreme simplicity of its structure approaches the invertebrate
classes. But mammals and fish hardly come into competition with each
other; the advancement of the whole class of mammals, or of certain
members in this class, to the highest grade would not lead to their
taking the place of fishes. Physiologists believe that the brain must be
bathed by warm blood to be highly active, and this requires aerial
respiration; so that warm-blooded mammals when inhabiting the water lie
under a disadvantage in having to come continually to the surface to
breathe. With fishes, members of the shark family would not tend to
supplant the lancelet; for the lancelet, as I hear from Fritz Mueller,
has as sole companion and competitor on the barren sandy shore of South
Brazil an anomalous annelid. The three lowest orders of mammals, namely,
marsupials, edentata, and rodents, coexist in South America in the same
region with numerous monkeys, and probably interfere little with each
other.

Although organization, on the whole, may have advanced and may be still
advancing throughout the world, yet the scale will always present many
degrees of perfection; for the high advancement of certain whole
classes, or of certain members of each class, does not at all
necessari