The _Carboniferous_ period was marked by a mild moist climate and a
luxuriant vegetation in the swampy low grounds. It was a much less
strenuous time than the Devonian period; it was like a very long summer.
There were no trees of the type we see now, but there were forests of
club-mosses and horsetails which grew to a gigantic size compared with
their pigmy representatives of to-day. In these forests the
jointed-footed invaders of the dry land ran riot in the form of
centipedes, spiders, scorpions, and insects, and on these the primeval
Amphibians fed. The appearance of insects made possible a new linkage of
far-reaching importance, namely, the cross-fertilisation of flowering
plants by their insect visitors, and from this time onwards it may be
said that flowers and their visitors have evolved hand in hand.
Cross-fertilisation is much surer by insects than by the wind, and
cross-fertilisation is more advantageous than self-fertilisation because
it promotes both fertility and plasticity. It was probably in this
period that _coloured_ flowers--attractive to insect-visitors--began to
justify themselves as beauty became useful, and began to relieve the
monotonous green of the horsetail and club-moss forests, which covered
great tracts of the earth for millions of years. In the Carboniferous
forests there were also land-snails, representing one of the minor
invasions of the dry land, tending on the whole to check vegetation.
They, too, were probably preyed upon by the Amphibians, some of which
attained a large size. Each age has had its giants, and those of the
Carboniferous were Amphibians called Labyrinthodonts, some of which were
almost as big as donkeys. It need hardly be said that it was in this
period that most of the Coal-measures were laid down by the immense
accumulation of the spores and debris of the club-moss forests. Ages
afterwards, it was given to man to tap this great source of
energy--traceable back to the sunshine of millions of years ago. Even
then it was true that no plant or animal lives or dies to itself!
The Acquisitions of Amphibians.
As Amphibians had their Golden Age in the Carboniferous period we may
fitly use this opportunity of indicating the advances in evolution which
the emergence of Amphibians implied. (1) In the first place the passage
from water to dry land was the beginning of a higher and more promiseful
life, taxed no doubt by increased difficulties. The natural question
rises why animals should have migrated from water to dry land at all
when great difficulties were involved in the transition. The answers
must be: (_a_) that local drying up of water-basins or elevations of the
land surface often made the old haunts untenable; (_b_) that there may
have been great congestion and competition in the old quarters; and
(_c_) that there has been an undeniable endeavour after well-being
throughout the history of animal life. In the same way with mankind,
migrations were prompted by the setting in of prolonged drought, by
over-population, and by the spirit of adventure. (2) In Amphibians for
the first time the non-digitate paired fins of fishes were replaced by
limbs with fingers and toes. This implied an advantageous power of
grasping, of holding firm, of putting food into the mouth, of feeling
things in three dimensions. (3) We cannot be positive in regard to the
soft parts of the ancient Amphibians known only as fossils, but if they
were in a general way like the frogs and toads, newts and salamanders of
the present day, we may say that they made among other acquisitions the
following: true ventral lungs, a three-chambered heart, a movable
tongue, a drum to the ear, and lids to the eyes. It is very interesting
to find that though the tongue of the tadpole has some muscle-fibres in
it, they are not strong enough to effect movement, recalling the tongue
of fishes, which has not any muscles at all. Gradually, as the tadpole
becomes a frog, the muscle-fibres grow in strength, and make it possible
for the full-grown creature to shoot out its tongue upon insects. This
is probably a recapitulation of what was accomplished in the course of
millennia in the history of the Amphibian race. (4) Another acquisition
made by Amphibians was a voice, due, as in ourselves, to the rapid
passage of air over taut membranes (vocal cords) stretched in the
larynx. It is an interesting fact that for millions of years there was
upon the earth no sound of life at all, only the noise of wind and wave,
thunder and avalanche. Apart from the instrumental music of some
insects, perhaps beginning in the Carboniferous, the first vital sounds
were due to Amphibians, and theirs certainly was the first voice--surely
one of the great steps in organic evolution.
[Illustration: _Photo: British Museum (Natural History)._