Now, when we say that the theory of evolution in the sense of Lamarck
and Darwin is an inductive law--in fact, the greatest of all
biological inductions--we rely, in the first place, on the facts of
paleontology. This science gives us some direct acquaintance with the
historical phenomena of the changes of species. From the situations in
which we find the fossils in the various strata of the earth we gather
confidently, in the first place, that the living population of the
earth has been gradually developed, as clearly as the earth's crust
itself; and that, in the second place, several different populations
have succeeded each other in the various geological periods. Modern
geology teaches that the formation of the earth has been gradual, and
unbroken by any violent revolutions. And when we compare together the
various kinds of animals and plants which succeed each other in the
history of our planet, we find, in the first place, a constant and
gradual increase in the number of species from the earliest times
until the present day; and, in the second place, we notice that the
forms in each great group of animals and plants also constantly
improve as the ages advance. Thus, of the vertebrates there are at
first only the lower fishes; then come the higher fishes, and later
the amphibia. Still later appear the three higher classes of
vertebrates--the reptiles, birds, and mammals, for the first time;
only the lowest and least perfect forms of the mammals are found at
first; and it is only at a very late period that placental mammals
appear, and man belongs to the latest and youngest branch of these.
Thus perfection of form increases as well as variety from the earliest
to the latest stage. That is a fact of the greatest importance. It can
only be explained by the theory of evolution, with which it is in
perfect harmony. If the different groups of plants and animals do
really descend from each other, we must expect to find this increase