of the Old World have five
molar teeth on each side of the jaw, narrow noses, tails usually short and
never prehensile, and fleshy protuberances for sitting; the apes of the New
World have six molar teeth, flat noses, and long prehensile tails. And on
the contrary, where closely related species are found on both parts of the
globe, they belong only to genera of which single species live or have
lived in the far North; as, for instance, the rein-deer, still common to
the Old and the New World in this very North which once formed a bridge
between the two halves of the earth. The same is true in regard to cattle,
the deer, the cat, the dog, the hare. Similar facts can also be shown of
other animal classes. The farther the different species of these genera
withdraw from the North Pole, the greater become the differences between
the species on the one half of the globe and the analogous species of the
other. Compare on this point K. E. von Baer's "Studien aus dem Gebiete der
Naturwissenchaften, ueber Darwin's Lehre," ("Studies from the Realm of
Natural Science upon Darwin's Teachings"), p. 356 f. If we add, further,
the before mentioned fact, that those genera which are exclusively peculiar
to one or the other continent, have their related predecessors in the
tertiary strata of these continents, {72} the hypothesis of a separate
origin for each single species, without genealogical connection with the
anatomically and physiologically related species, becomes neither more nor
less than a scientific impossibility.

Moreover, there are several facts of _comparative anatomy_ which have long
been the joy of all zooelogists and have rewarded the toilsome labors of
detailed investigations by a delightful view over the whole realm of the
organic world, but which find a scientific explanation only in the descent
theory. They are the _homology of the organs_, and to a certain degree also
the so-called _rudimentary organs_. By homology of organs we mean the fact
that within one and the same class-group of organisms all the organs, and
especially the organs in their most solid constituents, in the skeleton,
are built after one and the same fundamental plan, and therefore are even
in their most widely separated modifications varied after this one and the
same plan. This is especially true of the vertebrae and the limbs. This
homology goes so far within one class, particularly within the class of
mammalia, that, for instance, the hands and f