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 now existing low forms 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
organisation.

Nearly the same remarks are applicable, if we look to the different
grades of organisation within the same great group; for instance, in the
vertebrata, to the co-existence of mammals and fish--among mammalia, to
the co-existence of man and the ornithorhynchus--among fishes, to the
co-existence of the shark and the lancelet (Amphioxus), which latter
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
Muller, 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, co-exist in South
America in the same region with numerous monkeys, and probably interfere
little with each other. Although organisation, on the whole, may have
advanced and 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 necessarily lead to the extinction of those groups with which
they do not enter into close competition. In some cases, as we shall
hereafter see, lowly organised forms appear to have been preserved to
the present day, from inhabiting confined or peculiar stations, where
they have been subjected to less severe competition, and where their
scant