There is no warrant for supposing that the flying reptiles or
Pterodactyls gave rise to birds, for the two groups are on different
lines, and the structure of the wings is entirely different. Thus the
long-fingered Pterodactyl wing was a parachute wing, while the secret of
the bird's wing has its centre in the feathers. It is highly probable
that birds evolved from certain Dinosaurs which had become bipeds, and
it is possible that they were for a time swift runners that took "flying
jumps" along the ground. Thereafter, perhaps, came a period of arboreal
apprenticeship during which there was much gliding from tree to tree
before true flight was achieved. It is an interesting fact that the
problem of flight has been solved four times among animals--by insects,
by Pterodactyls, by birds, and by bats; and that the four solutions are
on entirely different lines.
In the _Cretaceous_ period the outstanding events included the waning of
giant reptiles, the modernising of the flowering plants, and the
multiplication of small mammals. Some of the Permian reptiles, such as
the dog-toothed Cynodonts, were extraordinarily mammal-like, and it was
probably from among them that definite mammals emerged in the Triassic.
Comparatively little is known of the early Triassic mammals save that
their back-teeth were marked by numerous tubercles on the crown, but
they were gaining strength in the late Triassic when small arboreal
insectivores, not very distant from the modern tree-shrews (_Tupaia_),
began to branch out in many directions indicative of the great divisions
of modern mammals, such as the clawed mammals, hoofed mammals, and the
race of monkeys or Primates. In the Upper Cretaceous there was an
exuberant "radiation" of mammals, adaptive to the conquest of all sorts
of haunts, and this was vigorously continued in Tertiary times.
There is no difficulty in the fact that the earliest remains of definite
mammals in the Triassic precede the first-known bird in the Jurassic.
For although we usually rank mammals as higher than birds (being mammals
ourselves, how could we do otherwise?), there are many ways in which
birds are pre-eminent, e.g. in skeleton, musculature, integumentary
structures, and respiratory system. The fact is that birds and mammals
are on two quite different tacks of evolution, not related to one
another, save in having a common ancestry in extinct reptiles. Moreover,
there is no reason to believe that the Jurassic _Archæopteryx_ was the
first bird in any sense except that it is the first of which we have any
record. In any case it is safe to say that birds came to their own
before mammals did.
Looking backwards, we may perhaps sum up what is most essential in the
Mesozoic era in Professor Schuchert's sentence: "The Mesozoic is the Age
of Reptiles, and yet the little mammals and the toothed birds are
storing up intelligence and strength to replace the reptiles when the
cycads and conifers shall give way to the higher flowering plants."
§ 2