THE BIRD AND THE MAMMAL
These two types of organisms were the bird and the mammal. Both
existed in the Jurassic, and the mammals at least had many
representatives in the Triassic. In other words, they existed,
with all their higher organisation, during several million years
without attaining power. The mammals remained, during at least
3,000,000 years, a small and obscure caste, immensely
overshadowed by the small-brained reptiles. The birds, while
making more progress, apparently, than the mammals, were far
outnumbered by the flying reptiles until the last part of the
Mesozoic. Then there was another momentous turn of the wheel of
fate, and they emerged from their obscurity to assume the
lordship of the globe.
In earlier years, when some serious hesitation was felt by many
to accept the new doctrine of evolution, a grave difficulty was
found in the circumstance that new types--not merely new species
and new genera, but new orders and even sub-classes--appeared in
the geological record quite suddenly. Was it not a singular
coincidence that in ALL cases the intermediate organisms between
one type and another should have wholly escaped preservation? The
difficulty was generally due to an imperfect acquaintance with
the conditions of the problem. The fossil population of a period
is only that fraction of its living population which happened to
be buried in a certain kind of deposit under water of a certain
depth. We shall read later of insects being preserved in resin
(amber), and we have animals (and even bacteria) preserved in
trees from the Coal-forests. Generally speaking, however, the
earth has buried only a very minute fraction of its
land-population. Moreover, only a fraction of the earth's
cemeteries have yet been opened. When we further reflect that the
new type of organism, when it first appears, is a small and local
group, we see what the chances are of our finding specimens of it
in a few scattered pages of a very fragmentary record of the
earth's life. We shall see that we have discovered only about ten
skeletons or fragments of skeletons of the men who lived on the
earth before the Neolithic period; a stretch of some hundreds of
thousands of years, recorded in the upper strata of the earth.
Whatever serious difficulty there ever was in this scantiness of
intermediate types is amply met by the fact that every fresh
decade of search in the geological tombs brings some to light. We
have seen many instances of this-- the seed-bearing ferns and
flower-bearing cycads, for example, found in the last decade--and
will see others. But one of the most remarkable cases of the kind
now claims our attention. The bird was probably evolved in the
late Triassic or early Jurassic. It appears in abundance, divided
into several genera, in the Chalk period. Luckily, two
bird-skeletons have been found in the intermediate period, the
Jurassic, and they are of the intermediate type, between the
reptile and the bird, which the theory of evolution would
suggest. But for the fortunate accident of these two birds being
embedded in an ancient Bavarian mud-layer, which happened to be
opened, for commercial purposes, in the second half of the
nineteenth century, critics of evolution--if there still were any
in the world of science--might be repeating to-day that the
transition from the reptile to the bird was unthinkable in theory
and unproven in fact.
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