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 



