In the genus Lingula, for instance, the species which
have successively appeared at all ages must have been connected by an
unbroken series of generations, from the lowest Silurian stratum to the
present day.
We have seen in the last chapter that whole groups of species sometimes
falsely appear to have been abruptly developed; and I have attempted to
give an explanation of this fact, which if true would be fatal to my views.
But such cases are certainly exceptional; the general rule being a gradual
increase in number, until the group reaches its maximum, and then, sooner
or later, a gradual decrease. If the number of the species included within
a genus, or the number of the genera within a family, be represented by a
vertical line of varying thickness, ascending through 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, often keeping of equal
thickness for a space, and ultimately thins out in the upper beds, marking
the decrease and final extinction of the species. This gradual increase in
number of the species of a group is strictly conformable with the theory;
for the species of the same genus, and the genera of the same family, can
increase only slowly and progressively; the process of modification and the
production of a number of allied forms necessarily being a slow and gradual
process, one species first giving rise to two or three varieties, these
being slowly converted into species, which in their turn produce by equally
slow steps other varieties and species, and so on, like the branching of a
great tree from a single stem, till the group becomes large.
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