On this
principle of inheritance with modification we can understand how it is that
sections of genera, whole genera, and even families, are confined to the
same areas, as is so commonly and notoriously the case.
There is no evidence, as was remarked in the last chapter, of the existence
of any law of necessary development. As the variability of each species is
an independent property, and will be taken advantage of by natural
selection, only so far as it profits each individual in its complex
struggle for life, so the amount of modification in different species will
be no uniform quantity. If a number of species, after having long competed
with each other in their old home, were to migrate in a body into a new and
afterwards isolated country, they would be little liable to modification;
for neither migration nor isolation in themselves effect anything. These
principles come into play only by bringing organisms into new relations
with each other and in a lesser degree with the surrounding physical
conditions. As we have seen in the last chapter that some forms have
retained nearly the same character from an enormously remote geological
period, so certain species have migrated over vast spaces, and have not
become greatly or at all modified.
According to these views, it is obvious that the several species of the
same genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they are descended from
the same progenitor.
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