Moreover, the degree to
which the descendants of the same stock, varying in different
directions, may at length diverge is unknown. All we know is, that
varieties are themselves variable, and that very diverse forms have
been educed from one stock.
2. Species of the same genus are not distinguished from each other by
equal amounts of difference. There is diversity in this respect
analogous to that of the varieties of a polymorphous species, some of
them slight, others extreme. And in large genera the unequal
resemblance shows itself in the clustering of the species around
several types or central species, like satellites around their
respective planets. Obviously suggestive this of the hypothesis that
they were satellites, not thrown off by revolution, like the moons of
Jupiter, Saturn, and our own solitary moon, but gradually and
peacefully detached by divergent variation. That such closely related
species may be only varieties of higher grade, earlier origin, or
more favored evolution, is not a very violent supposition. Anyhow, it
was a supposition sure to be made.
3. The actual geographical distribution of species upon the earth's
surface tends to suggest the same notion.
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