18 THE STORY OF THE BIRDS. 
energy she gave it a better means through its cover- 
ing of hooking on to the air. Because of its struggle 
upward she gave the bird in its soft apparel a new fin 
for a new element. 
Nature shows her heat-preserving tendencies in 
the bird by another change which was radical. The 
fishes and reptiles had various external pores, as also 
have some mammals. Through them they moistened 
the skin with various secretions for various purposes, 
perhaps for lubrication in the fishes. To a quiet, lazy 
land creature, as a tree frog, Nature could even give 
oxygen by dampening its surface; and others could 
thus breathe through the skin solely while beneath 
the water. But in a bird which moved so rapidly, 
any surface moisture would tend to cool it too much 
and to weaken it. Besides, it would dampen the 
plumage—another thing not desirable. So the pores 
of the skins of birds were largely suppressed, and the 
dire possibility of cleaving the air at a hundred miles 
an hour in a high state of perspiration was prevented. 
It is possible that this, too, was the result of flight by 
a long line of fitting survivals; for chilling the sur- 
face closes the pores, as we well know when we “take 
cold.” 
Scales in no way tended toward heat preservation. 
In fact, they were rather fitted to dissipate and radiate 
it. Perhaps this may have been one of their purposes. 
The primary mission of cold blood was adaptation, 
without shock, of the creature to a constantly cold or 
watery environment. But a bird invaded a realm of 
various degrees of temperature. The one best clothed 
