Both are extinct in the sense that each has completely
disappeared and that nothing like either is to be found in the world
to-day. But whereas all the individual titanotheres finally died out,
leaving no descendants, a number of the three-toed horses did leave
descendants, and these descendants, constantly changing as the ages
went by, finally developed into the highly specialized one-toed
horses, asses, and zebras of to-day.
The analogy between the facts thus indicated and certain facts in the
development of human societies is striking. A further analogy is
supplied by a very curious tendency often visible in cases of intense
and extreme specialization. When an animal form becomes highly
specialized, the type at first, because of its specialization,
triumphs over its allied rivals and its enemies, and attains a great
development; until in many cases the specialization becomes so
extreme that from some cause unknown to us, or at which we merely
guess, it disappears. The new species which mark a new era commonly
come from the less specialized types, the less distinctive, dominant,
and striking types, of the preceding era.
When dealing with the changes, cataclysmic or gradual, which divide
one period of palaeontological history from another, we can sometimes
assign causes, and again we cannot even guess at them.
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