geological record on the other,
all difficulty is removed from the kind of evidence to which I have
adverted; and that we are justified in believing that all such cases
are examples of what I have designated negative or indifferent
evidence--that is to say, they in no way directly advance the hypothesis
of evolution, but they are not to be regarded as obstacles in the way of
our belief in that doctrine.

I now pass on to the consideration of those cases which, for reasons
which I will point out to you by and by, are not to be regarded as
demonstrative of the truth of evolution, but which are such as must
exist if evolution be true, and which therefore are, upon the whole,
evidence in favour of the doctrine. If the doctrine of evolution be
true, it follows, that, however diverse the different groups of animals
and of plants may be, they must all, at one time or other, have been
connected by gradational forms; so that, from the highest animals,
whatever they may be, down to the lowest speck of protoplasmic matter in
which life can be manifested, a series of gradations, leading from
one end of the series to the other, either exists or has existed.
Undoubtedly that is a necessary postulate of the doctrine of evolution.
But when we look upon living Nature as it is, we find a totally
different state of things. We find that animals and plants fall into
groups, the different members of which are pretty closely allied
together, but which are separated by definite, larger or smaller,
breaks, from other groups. In other words, no intermediate forms which
bridge over these gaps or intervals are, at present, to be met with.

To illustrate what I mean: Let me call your attention to those
vertebrate animals which are most familiar to you, such as mammals,
birds, and reptiles. At the present day, these groups of animals are
perfectly well-defined from one another. We know of no animal now living
which, in any sense, is intermediate between the mammal and the bird, or
between the bird and the reptile; but, on the contrary, there are many
very distinct anatomical peculiarities, well-defined marks, by which the
mammal is separated from the bird, and the bird from the reptile. The
distinctions are obvious and striking if you compare the definitions of
these great groups as they now exist.

The same may be said of many of the subordinate groups, or orders, into
which these great classes are divided. At the present time, for example,
ther