Conclusion
We must, however, beware of simply transferring to the Mesozoic
world the kinds of Monotremes and Marsupials which we know in
nature to-day. In some of the excellent "restorations" of
Mesozoic life which are found in recent illustrated literature
the early mammal is represented with an external appearance like
that of the Duckbill. This is an error, as the Duckbill has been
greatly modified in its extremities and mouth-parts by its
aquatic and burrowing habits. As we have no complete skeletons of
these early mammals we must abstain from picturing their external
appearance. It is enough that the living Monotreme and Marsupial
so finely illustrate the transition from a reptilian to a
mammalian form. There may have been types more primitive than the
Duckbill, and others between the Duckbill and the Marsupial. It
seems clear, at least, that two main branches, the Monotremes and
Marsupials, arose from the primitive mammalian root. Whether
either of these became in turn the parent of the higher mammals
we will inquire later. We must first consider the fresh series of
terrestrial disturbances which, like some gigantic sieve, weeded
out the grosser types of organisms, and cleared the earth for a
rapid and remarkable expansion of these primitive birds and
mammals.
We have attended only to a few prominent characters in tracing
the line of evolution, but it will be understood that an advance
in many organs of the body is implied in these changes. In the
lower mammals the diaphragm, or complete partition between the
organs of the breast and those of the abdomen, is developed. It
is not a sudden and mysterious growth, and its development in the
embryo to-day corresponds to the suggestion of its development
which the zoologist gathers from the animal series. The ear also
is now fully developed. How far the fish has a sense of hearing
is not yet fully determined, but the amphibian certainly has an
organ for the perception of waves of sound. Parts of the
discarded gill-arches are gradually transformed into the three
bones of the mammal's internal ear; just as other parts are
converted into mouth cartilages, and as--it is believed--one of
the gill clefts is converted into the Eustachian tube. In the
Monotreme and Marsupial the ear-hole begins to be covered with a
shell of cartilage; we have the beginning of the external ear.
The jaws, which are first developed in the fish, now articulate
more perfectly with the skull. Fat-glands appear in the skin, and
it is probably from a group of these that the milk-glands are
developed. The origin of the hairs is somewhat obscure. They are
not thought to be, like the bird's feathers, modifications of the
reptile's scales, but to have been evolved from other structures
in the skin, possibly under the protection of the scales.
My purpose is, however, rather to indicate the general causes of
the onward advance of life than to study organs in detail--a vast
subject--or construct pedigrees. We therefore pass on to consider
the next great stride that is taken by the advancing life of the
earth. Millions of years of genial climate and rich vegetation
have filled the earth with a prolific and enormously varied
population. Over this population the hand of natural selection is
outstretched, as it were, and we are about to witness another
gigantic removal of older types of life and promotion of those
which contain the germs of further advance. As we have already
explained, natural selection is by no means inactive during these
intervening periods of warmth. We have seen the ammonites and
reptiles, and even the birds and mammals, evolve into hundreds of
species during the Jurassic period. The constant evolution of
more effective types of carnivores and their spread into new
regions, the continuous changes in the distribution of land and
water, the struggle for food in a growing population, and a dozen
other causes, are ever at work. But the great and comprehensive
changes in the face of the earth which close the eras of the
geologist seem to give a deeper and quicker stimulus to its
population and result in periods of especially rapid evolution.
Such a change now closes the Mesozoic Era, and inaugurates the
age of flowering plants, of birds, and of mammals.
|
