We shall see this by turning to the diagram: the letters, A to
L, may represent eleven Silurian genera, some of which have produced large
groups of modified descendants, with every link in each branch and
sub-branch still alive; and the links not greater than those between
existing varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could be
distinguished from their more immediate parents and descendants. Yet the
arrangement in the diagram would still hold good and would be natural; for,
on the principle of inheritance, all the forms descended, for instance from
A, would have something in common. In a tree we can distinguish this or
that branch, though at the actual fork the two unite and blend together.
We could not, as I have said, define the several groups; but we could pick
out types, or forms, representing most of the characters of each group,
whether large or small, and thus give a general idea of the value of the
differences between them. This is what we should be driven to, if we were
ever to succeed in collecting all the forms in any one class which have
lived throughout all time and space. Assuredly we shall never succeed in
making so perfect a collection: nevertheless, in certain classes, we are
tending toward this end; and Milne Edwards has lately insisted, in an able
paper, on the high importance of looking to types, whether or not we can
separate and define the groups to which such types belong.
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