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and no chance was left of remoulding it into a hand. They clung
obstinately to one important character of their reptilian ancestry—
the shelled egg, whereas their mammalian rivals came to specialize in
the internal nourishment of the young inside the mother's body; and
by this the birds debarred themselves from ever being born into the
world at such an advanced state of development as is possible to man
and other higher mammals. But in one thing at least they went
further than any mammal; they not only developed a constant
temperature, but kept it constant at a greater height. Birds and
mammals arc unique among living things in having evolved the self-
regulating central-heating system that we call" warm blood,15 a system
which is of the utmost importance, since it enables their activities of
body and mind to continue on a more or less constant level instead of
being slowed down by cold, speeded up by heat, as is the ease with all
other kinds of animals, and makes it possible for them to laugh at
extremes of temperature which send insects or reptiles into the; sleep
of hibernation or aestivation. But birds have pushed the invention
to its limits: they live at temperatures which would be the extremes
of fever for us.
It is this extremely high temperature, 105 degrees or over, com-
bined with the agility that comes of flight, which gives birds their
fascinating quality of seeming always so intensely alive. But being
intensely alive does not necessarily, as we know from human examples,
mean being intensely intelligent. And iu fact, in respect of their
minds just as much as their bodies, birds have developed along other
lines than mammals. Mammals have gradually perfected intelligence
and the capacity for learning by experience, until this line has cul*
minatcd in that conscious reason and in that deliberate reliance upon
the accumulated experience of previous generations, which are unique
properties of the human species. And with the gradual rise of in-
telligence, the power and fixity of the instincts has diminished. Birds,
on the other hand, have kept instinct as the mainstay of their be-
haviour; they possess, like all other backboned animals, some in-
telligence and some power of profiting by experience, but these arc
subordinate, used merely to polish up the outfit of instincts which is
provided by heredity without having to be paid for in terms of
experience. Indeed, the anatomist could tell you as much by looking
at the brains of bird and mammal, even if he had never studied the
way the creatures behave. For whereas in mammals we can trace a
steady increase in the size and elaboration of the cerebral hemispheres,
the front part of the brain which we know to be the seat of intelligence
and learning, this region is never highly developed in any bird, but