THE CONQUEST OF THE DRY LAND 219 
the heap we can revive the fire by stirring it 
up to admit air. When we have added fresh 
fuel and cleared away the ashes it will burn as 
brightly as ever again. So with the winter 
sleeper. When the first warm breath of spring 
penetrates into the hedgehog’s hole, he gradu- 
ally awakens, stretches his stiffened limbs, and 
creeps forth with a new lease of life. His 
breathing quickens as his lungs fill with air, 
oxygen is carried to every part of his body, the 
heart beats more strongly and rapidly, and the 
now hungry hedgehog begins to search eagerly 
for the insects and worms on which he feeds. 
If we stir up our resting fire too suddenly and 
let in too much air at once the glow will die 
out, and no amount of fresh fuel will re-kindle 
the heap without fresh fire. Something simi- 
lar sometimes happens to the lighter sleepers 
if they have been roused by a spell of mild 
weather, and a hard frost sets in so suddenly 
that they are nipped by it before they have had 
time to settle down to sleep again. But, on the 
whole, hibernation is a very successful device 
for withstanding great changes of temperature. 
Another way of meeting the winter is by 
putting on a white dress. The ptarmigan, 
which is rather grouse-like in summer, with a 
