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.
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