In the great assemblage of annulose animals, the two highest
classes, the insects and spider tribe, exhibit a wonderful persistency
of type. The cockroaches of the carboniferous epoch are exceedingly
similar to those which now run about our coal-cellars; and its locusts,
termites and dragon-flies are closely allied to the members of the same
groups which now chirrup about our fields, undermine our houses, or
sail with swift grace about the banks of our sedgy pools. And, in like
manner, the palaeozoic scorpions can only be distinguished by the eye
of a naturalist from the modern ones.
Finally, with respect to the 'Vertebrata', the same law holds good:
certain types, such as those of the ganoid and placoid fishes, having
persisted from the palaeozoic epoch to the present time without a
greater amount of deviation from the normal standard than that which is
seen within the limits of the group as it now exists. Even among the
'Reptilia'--the class which exhibits the largest proportion of entirely
extinct forms of any one type,--that of the 'Crocodilia', has persisted
from at least the commencement of the Mesozoic epoch up to the present
time with so much constancy, that the amount of change which it exhibits
may fairly, in relation to the time which has elapsed, be called
insignificant. And the imperfect knowledge we have of the ancient
mammalian population of our earth leads to the belief that certain of
its types, such as that of the 'Marsupialia', have persisted with
correspondingly little change through a similar range of time.
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