It can breathe oxygen dissolved in water by its gills; it can also
breathe dry air by means of its swim-bladder, which has become a lung.
It is a _double-breather_, showing evolution in process. For seven
months of the year, the dry season, it can remain inert in the mud,
getting air through an open pipe to the surface. When water fills the
pools it can use its gills again. Mud-nests or mud encasements with the
lung-fish inside have often been brought to Britain and the fish when
liberated were quite lively.]
[Illustration: THE ARCHÆOPTERYX
(_After William Leche of Stockholm._)
A good restoration of the oldest known bird, Archæopteryx (Jurassic
Era). It was about the size of a crow; it had teeth on both jaws; it had
claws on the thumb and two fingers; and it had a long lizard-like tail.
But it had feathers, proving itself a true bird.]
[Illustration: WING OF A BIRD, SHOWING THE ARRANGEMENT OF THE FEATHERS
The longest feathers or primaries (PR) are borne by the two fingers (2
and 3), and their palm-bones (CMC); the second longest or secondaries
are borne by the ulna bone (U) of the fore-arm; there is a separate tuft
(AS) on the thumb (TH).]
The rarity of direct traces of life in the oldest rocks is partly due to
the fact that the primitive animals would be of delicate build, but it
must also be remembered that the ancient rocks have been profoundly and
repeatedly changed by pressure and heat, so that the traces which did
exist would be very liable to obliteration. And if it be asked what
right we have to suppose the presence of living creatures in the absence
or extreme rarity of fossils, we must point to great accumulations of
limestone which indicate the existence of calcareous algæ, and to
deposits of iron which probably indicate the activity of iron-forming
Bacteria. Ancient beds of graphite similarly suggest that green plants
flourished in these ancient days.
§ 3