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The differences which these present are all of the same
_nature_ as those presented by the species of many large genera, but
much greater in _amount_; and they can all be explained by the action of
the same general laws and by the extinction of a larger or smaller
number of intermediate species. Whether the distinctions between the
higher groups termed Classes and Sub-kingdoms may be accounted for in
the same way is a much more difficult question. The differences which
separate the mammals, birds, reptiles, and fishes from each other,
though vast, yet seem of the same nature as those which distinguish a
mouse from an elephant or a swallow from a goose. But the vertebrate
animals, the mollusca, and the insects, are so radically distinct in
their whole organisation and in the very plan of their structure, that
objectors may not unreasonably doubt whether they can all have been
derived from a common ancestor by means of the very same laws as have
sufficed for the differentiation of the various species of birds or of
reptiles.

_The Change of Opinion effected by Darwin_.


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