ich are
classed by biologists under the heads of Embryology and of
Palaeontology. Embryology proves that every higher form of individual
life becomes what it is by a process of gradual differentiation from
an extremely low form; palaeontology proves, in some cases, and renders
probable in all, that the oldest types of a group are the lowest; and
that they have been followed by a gradual succession of more and more
differentiated forms. It is simply a fact, that evolution of the
individual animal and plant is taking place, as a natural process, in
millions and millions of cases every day; it is a fact, that the
species which have succeeded one another in the past, do, in many
cases, present just those morphological relations, which they must
possess, if they had proceeded, one from the other, by an analogous
process of evolution.

The alternative presented, therefore, is: either the forms of one and
the same type--say, _e.g._, that of the Horse tribe[13]--arose
successively, but independently of one another, at intervals, during
myriads of years; or, the later forms are modified descendants of the
earlier. And the latter supposition is so vastly more probable than
the former, that rational men will adopt it, unless satisfactory
evidence to the contrary can be produced. The objection sometimes put
forward, that no one yet professes to have seen one species pass into
another, comes oddly from those who believe that mankind are all
descended from Adam. Has any one then yet seen the production of
negroes from a white stock, or _vice versa_? Moreover, is it
absolutely necessary to have watched every step of the progress of a
planet, to be justified in concluding that it really does go round the
sun? If so, astronomy is in a bad way.

I do not, for a moment, presume to suggest that some one, far better
acquainted than I am with astronomy and physics; or that a master of
the new chemistry, with its extraordinary revelations; or that a
student of the development of human society, of language, and of
religions, may not find a sufficient foundation for the doctrine of
evolution in these several regions. On the contrary, I rejoice to see
that scientific investigation, in all directions, is tending to the
same result. And it may well be, that it is only my long occupation
with biological matters that leads me to feel safer among them than
anywhere else. Be that as it may, I take my stand on the facts of
embryology and of palaeont