Under a scientific point of view, and as leading to further investigation,
but little advantage is gained by believing that new forms are suddenly
developed in an inexplicable manner from old and widely different forms,
over the old belief in the creation of species from the dust of the earth.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we consider, by so much the arguments in favour of
community of descent become fewer in number and less in force. But some
arguments of the greatest weight extend very far. All the members of whole
classes are connected together by a chain of affinities, and all can be
classed on the same principle, in groups subordinate to groups. Fossil
remains sometimes tend to fill up very wide intervals between existing
orders.
Organs in a rudimentary condition plainly show that an early progenitor had
the organ in a fully developed condition, and this in some cases implies an
enormous amount of modification in the descendants. Throughout whole
classes various structures are formed on the same pattern, and at a very
early age the embryos closely resemble each other. Therefore I cannot
doubt that the theory of descent with modification embraces all the members
of the same great class or kingdom. I believe that animals are descended
from at most only four or five progenitors, and plants from an equal or
lesser number.
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