130 SECRETS OF ANIMAL LIFE 
majority of active animals are going concerns 
between very narrow limits of temperature. A 
little too hot, or a little too cold, and the wheels 
won’t go round any more. We need not go into 
the question of the effect of cold on the synthesis of 
proteins, on which the continuance of vigorous life 
depends; and we confess our inability to explain 
the metabolism of deep-sea fishes, for instance, 
which live and thrive—at what pace we do not 
know—in the eternal winter of the great abysses 
where the temperature remains, year in year out, 
about the freezing-point of fresh water; but the 
average statement is safe that winter puts a brake 
on vital activity. Should the brake be put on too 
suddenly or too forcibly, the equipage of life may 
be capsized and broken, and all the king’s horses 
and all the king’s men will not be able to put it to 
rights. As many as two hundred dead birds have 
been gathered in one stackyard after a night of 
severe frost. The thermometer fell just a little bit 
too far—beyond the limit of a bird’s viability. 
So it is no small part of the biology of winter 
to inquire into the diverse ways in which living 
creatures have learned to meet, or are learning to 
meet, the time of cold and scarcity. The finest 
solution of all is that of the migratory birds, “in- 
telligent of the seasons,’ as Milton put it, who 
circumvent the winter by seeking lands that keep 
the sun. Very effective is that long result of time 
called warm-bloodedness, whereby birds and mam- 
mals are able, up to a certain limit, and in varied 
