A group does not reappear after it has once disappeared; or
its existence, as long as it lasts, is continuous. I am aware that
there are some apparent exceptions to this rule, but the exceptions
are surprisingly few, so few, that E. Forbes, Pictet, and Woodward
(though all strongly opposed to such views as I maintain) admit its
truth; and the rule strictly accords with my theory. For as all the
species of the same group have descended from some one species, it is
clear that as long as any species of the group have appeared in the
long succession of ages, so long must its members have continuously
existed, in order to have generated either new and modified or the
same old and unmodified forms. Species of the genus Lingula, for
instance, must have continuously existed by an unbroken succession of
generations, from the lowest Silurian stratum to the present day.
We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give
an explanation of this fact, which if true would have been fatal to my
views. But such cases are certainly exceptional; the general rule
being a gradual increase in number, till the group reaches its
maximum, and then, sooner or later, it gradually decreases. If the
number of the species of a genus, or the number of the genera of a
family, be represented by a vertical line of varying thickness,
crossing the successive geological formations in which the species are
found, the line will sometimes falsely appear to begin at its lower
end, not in a sharp point, but abruptly; it then gradually thickens
upwards, sometimes keeping for a space of equal thickness, and
ultimately thins out in the upper beds, marking the decrease and final
extinction of the species.
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