Now it is a most remarkable circumstance that, viewed on the great
scale, living beings have differed so little throughout all geologic
time that there is no sub-kingdom and no class wholly extinct or
without living representatives.
If we descend to the smaller groups, we find that the number of orders
of plants is about two hundred; and I have it on the best authority
that not one of these is exclusively fossil; so that there is
absolutely not a single extinct ordinal type of vegetable life; and it
is not until we descend to the next group, or the families, that we
find types which are wholly extinct. The number of orders of animals,
on the other hand, may be reckoned at a hundred and twenty, or
thereabouts, and of these, eight or nine have no living representatives.
The proportion of extinct ordinal types of animals to the existing
types, therefore, does not exceed seven per cent.--a marvellously small
proportion when we consider the vastness of geologic time.
Another class of considerations--of a different kind, it is true, but
tending in the same direction--seems to have been overlooked. Not only
is it true that the general plan of construction of animals and plants
has been the same in all recorded time as at present, but there are
particular kinds of animals and plants which have existed throughout
vast epochs, sometimes through the whole range of recorded time, with
very little change.
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