Horns of the earliest known
fossil deer have only two prongs; in the rocks above are remains of deer
with additional prongs, and point after point is added as the ancient
history of deer is traced upwards through the rocks to modern species. We
know that the life-history of a modern species of animals reviews the
ancestral record of the species, and what happens during the development
of deer can be directly compared with the fossil series. It is a matter of
common knowledge that the year-old stag has simple spikes as horns, and
that these are shed to be replaced the following year by larger forked
horns. Every year the horns are lost and new ones grow out, and become
more and more elaborately branched as time goes on, thus giving a series
of developmental stages that faithfully repeats the general order of
fossil horns. Even Agassiz, who was a believer in special creation and an
opponent of evolution, was constrained to point out many other instances,
mainly among the invertebrata, where there was a like correspondence
between the ontogeny of existing species and their phylogenetic history as
revealed by the fossil remains of their ancestors.
* * * * *
In the last place, we must give more than a passing consideration to some
of the extinct types of animals that occupy the position of "links"
between groups now widely separated by their divergence in evolution from
the same ancestors.
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