But why should the generalized comprehensive forms stand at the
bottom rather than the top of the systematic arrangement of their
classes? Why should the system of classification coincide with the
order of geologic occurrence, and this with the series of embryonic
stages? Above all, why should the embryos of bird and perch form
their tails by such a roundabout method? Why should the embryo of
the bird have the tail of a lizard? No one could give any
satisfactory explanation, although the facts were undoubted.
Mr. Darwin's theory was the one impulse needed to crystallize these
disconnected facts into one comprehensible whole. The connecting
link was everywhere common descent, difference was due to the
continual variation and divergence of their ancestors. The
classification, which all were seeking, was really the ancestral
tree of the animal kingdom. Forms more generalized should be placed
lower down on the ancestral tree, and must have had an earlier
geological occurrence because they represented more nearly the
ancestors of the higher. But this explains also the facts of
embryonic development.
According to Mr. Darwin's theory all the species of higher animals
have developed from unicellular ancestors. It had long been known
that all higher forms start in life as single cells, egg and
spermatozoon.
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