This led to so powerful a development of the
mammals that they succeeded in overthrowing the reptiles.
There are several serious difficulties in the way of this theory.
The first and most decisive is that the great reptiles have
practically disappeared before the mammals come on the scene.
Only in one series of beds (Puerco) in America, representing an
early period of the Tertiary Era, do we find any association of
their remains; and even there it is not clear that they were
contemporary. Over the earth generally the geological record
shows the great reptiles dying from some invisible scourge long
before any mammal capable of doing them any harm appears; even if
we suppose that the mammal mainly attacked the eggs and the
young. We may very well believe that more powerful mammals than
the primitive Mesozoic specimens were already developed in some
part of the earth--say, Africa--and that the rise of the land
gave them a bridge across the Mediterranean to Europe. Probably
this happened; but the important point is that the reptiles were
already almost extinct. The difficulty is even greater when we
reflect that it is precisely the most powerful reptiles
(Deinosaurs) and least accessible reptiles (Pterosaurs,
Ichthyosaurs, etc.
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