A SHORT HISTORY OF THE WORLD BY H. G. WELLS
7.THE FIRST BIRDS AND THE FIRST MAMMALS
In a few paragraphs a picture of the lush vegetation and swarming reptiles of that first great summer of life, the Mesozoic period, has been sketched. But while the Dinosaurs lorded it over the hot selvas and marshy plains and the Pterodactyls filled the forests with their fluttering's and possibly with shrieks and croakings as they pursued the humming insect life of the still flowerless shrubs and trees, some less conspicuous and less abundant forms upon the margins of this abounding life were acquiring certain powers and learning certain lessons of endurance, that were to be of the utmost value to their race when at last the smiling generosity of sun and earth began to fade.
A group of tribes and genera of hopping reptiles, small creatures of
the dinosaur type, seem to have been pushed by competition and the
pursuit of their enemies towards the alternatives of extinction or
adaptation to colder conditions in the higher hills or by the sea.
Among these distressed tribes there was developed a new type of
scale—scales that were elongated into quill-like forms and that
presently branched into the crude beginnings of feathers. These
quill-like scales layover one another and formed a heat-retaining
covering more efficient than any reptilian covering that had hitherto
existed. So they permitted an invasion of colder regions that were
otherwise uninhabited. Perhaps simultaneously with these changes there
arose in these creatures a greater solicitude for their eggs. Most
reptiles are apparently quite careless about their eggs, which are left
for sun and season to hatch. But some of the varieties upon this new
branch of the tree of life were acquiring a habit of guarding their
eggs and keeping them warm with the warmth of their bodies.
With these adaptations to cold other internal modifications were going
on that made these creatures, the primitive birds, warm-blooded and
independent of basking. The very earliest birds seem to have been
seabirds living upon fish, and their fore limbs were not wings but
paddles rather after the penguin type. That peculiarly primitive bird,
the New Zealand Ki-Wi, has feathers of a very simple sort, and neither
flies nor appears to be descended from flying ancestors. In the
development of the birds, feathers came before wings. But once the
feather was developed the possibility of making a light spread of
feathers led inevitably to the wing. We know of the fossil remains of
one bird at least which had reptilian teeth in its jaw and a long
reptilian tail, but which also had a true bird’s wing and which
certainly flew and held its own among the pterodactyls of the Mesozoic
time. Nevertheless birds were neither varied nor abundant in Mesozoic
times. If a man could go back to typical Mesozoic country, he might
walk for days and never see or hear such a thing as a bird, though he
would see a great abundance of pterodactyls and insects among the
fronds and reeds.
And another thing he would probably never see, and that would be any
sign of a mammal. Probably the first mammals were in existence
millions of years before the first thing one could call a bird, but
they were altogether too small and obscure and remote for attention.
The earliest mammals, like the earliest birds, were creatures driven by
competition and pursuit into a life of hardship and adaptation to cold.
With them also the scale became quill-like, and was developed into a
heat-retaining covering; and they too underwent modifications, similar
in kind though different in detail, to become warm-blooded and
independent of basking. Instead of feathers they developed hairs, and
instead of guarding and incubating their eggs they kept them warm and
safe by retaining them inside their bodies until they were almost
mature. Most of them became altogether vivaparous and brought their
young into the world alive. And even after their young were born they
tended to maintain a protective and nutritive association with them.
Most but not all mammals to-day have mammæ and suckle their young. Two
mammals still live which lay eggs and which have not proper mammæ,
though they nourish their young by a nutritive secretion of the under
skin; these are the duck-billed platypus and the echidna. The echidna
lays leathery eggs and then puts them into a pouch under its belly, and
so carries them about warm and safe until they hatch.
But just as a visitor to the Mesozoic world might have searched for
days and weeks before finding a bird, so, unless he knew exactly where
to go and look, he might have searched in vain for any traces of a
mammal. Both birds and mammals would have seemed very eccentric and
secondary and unimportant creatures in Mesozoic times.
The Age of Reptiles lasted, it is now guessed, eighty million years.
Had any quasi-human intelligence been watching the world through that
inconceivable length of time, how safe and eternal the sunshine and
abundance must have seemed, how assured the wallowing prosperity of the
dinosaurs and the flapping abundance of the flying lizards! And then
the mysterious rhythms and accumulating forces of the universe began to
turn against that quasi-eternal stability. That run of luck for life
was running out. Age by age, myriad of years after myriad of years,
with halts no doubt and retrogressions, came a change towards hardship
and extreme conditions, came great alterations of level and great
redistributions of mountain and sea. We find one thing in the Record
of the Rocks during the decadence of the long Mesozoic age of
prosperity that is very significant of steadily sustained changes of
condition, and that is a violent fluctuation of living forms and the
appearance of new and strange species. Under the gathering threat of
extinction the older orders and genera are displaying their utmost
capacity for variation and adaptation. The Ammonites for example in
these last pages of the Mesozoic chapter exhibit a multitude of
fantastic forms. Under settled conditions there is no encouragement
for novelties; they do not develop, they are suppressed; what is best
adapted is already there. Under novel conditions it is the ordinary
type that suffers, and the novelty that may have a better chance to
survive and establish itself....
There comes a break in the Record of the Rocks that may represent
several million years. There is a veil here still, over even the
outline of the history of life. When it lifts again, the Age of
Reptiles is at an end; the Dinosaurs, the Plesiosaurs and Ichthyosaurs,
the Pterodactyls, the innumerable genera and species of Ammonite have
all gone absolutely. In all their stupendous variety they have died
out and left no descendants. The cold has killed them. All their
final variations were insufficient; they had never hit upon survival
conditions. The world had passed through a phase of extreme conditions
beyond their powers of endurance, a slow and complete massacre of
Mesozoic life has occurred, and we find now a new scene, a new and
hardier flora, and a new and hardier fauna in possession of the world.
It is still a bleak and impoverished scene with which this new volume
of the book of life begins. The cycads and tropical conifers have given
place very largely to trees that shed their leaves to avoid destruction
by the snows of winter and to flowering plants and shrubs, and where
there was formerly a profusion of reptiles, an increasing variety of
birds and mammals is entering into their inheritance.