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CORRESPONDING SOCIETIES. 327 
But when we are in good health, the height of the barometer, the intensity of 
light, and the dampness or dryness, purity or impurity, of the air, affect us even 
more than the actual temperature. We rely on the physiological mechanism 
by which the production and waste of heat respond to surrounding changes and 
cause the actual temperature of our tissues to remain almost constant. Very 
young mammals or birds, and all warm-blooded creatures when they are ill, have 
a feebler control of their own temperature, and the heat metabolism of the body 
has to be assisted by a more rigorous choice of environment, if the normal tem- 
perature is to be maintained. 
There is thus a marked contrast between warm-blooded and cold-blooded or- 
ganisms, which I may impress on you by asking you to compare an insectivorous 
bird and an insectivorous lizard. Both are swift and restless creatures which 
have to expend much energy in capturing their watchful and rapid prey. The 
alimentary canal and, so far as we know, the physiological processes of diges- 
tion are much alike in the two. The. euthermal temperature is nearly identical, 
although the bird is more strictly limited to about 100° Fahr. and the lizard has 
a wider range. But the bird uses the heat of the oxidation processes in its own 
tissues to produce the temperature it requires, and so maintains its activity in 
spite of surrounding changes. If it be provided with suitable food, it can endure 
the cold of winter and heat of summer, and can adapt itself to a great range of 
climate and locality. The lizard cannot abide by its own production of heat; it 
is limited by the surrounding conditions, and becomes torpid or dies when the 
external cold is too severe or the external heat too great. 
The evidence shows that warm-blooded mammals and birds are the descend- 
ants of cold-blooded reptiles. In the course of that evolution, the power of 
retaining and controlling the heat produced by oxidation in the tissues must 
have been acquired. No one can doubt the high utility of this power or its great 
advantage in the struggle for existence, as it widens the possible geographical 
range and increases the viability of its possessors. No one can doubt but that 
this kind of character would have come under the operation of natural selection. 
I desire to impress on your attention that the production of heat existed before 
it became useful. It was a waste product, an accident of the metabolism of the 
body, material ready for natural selection. 
An important part of the provision for heat-regulation in warm-blooded 
animals is the coat of fur or feathers that serves to ward off the inclemency of the 
weather, and to prevent waste of internal heat by radiation and conduction. 
When small birds are roosting in the open air at night, their sleek plumage 
becomes ruffled, each feather standing out at right angles to the surface of the 
skin, changing the smooth contours of the body into a globe of fluff. Exposure 
to cold similarly induces erection of the fur of most mammals, and the effect of 
cold on our own naked skins, that we call ‘ goose-flesh,’ is doubtless a surviving 
action of the mechanism that erected the hairs of our ancestors. Such devices 
control the loss of heat to a notable extent, but they are far from complete. If 
we had organs as sensitive to radiant heat as our eyes are sensitive to radiant 
light, the bodies of warm-blooded hirds and mammals would be perceptible to usin 
the dark, in inverse proportion to the efficiency of their heat-retaining covering. 
They would be perceptible in no vague fashion, but in what T may call some kind 
of contour, due to the different intensities of heat-discharge from differently 
protected parts of the body. There would be creatures radiating an even glow, 
flashing intermittently, banded, spotted, and irregularly surfaced. 
If enemies of warm-blooded animals were armed with such a power of appre- 
ciating differences in temperature, the mechanisms for preventing loss of heat by 
radiation would acquire a new selection-value. Such enemies cannot be said to 
exist, but there is a suggestion of the possibility in the behaviour of ectopara- 
sites. These seem to have some kind of directive heat-sense. Everyone who 
has had to handle the fresh bodies of dead warm-blooded animals must have 
noticed how quickly the lice and fleas migrate from the cooling corpse, and the 
frequency with which they find their way to the warm body of the anatomist 
seems to show that their wandering is not aimless. The concealment of heat- 
radiation does not seem to have had any importance in the evolution of animals, 
but if the necessity should come to pass, a character already exists which could 
be turned to advantage, 
