LENGTH OF LIFE 251 



ground freezes to a depth of several feet, and the plants 

 and most of the animals that live at or near the surface 

 are completely frozen for several months. In this cate- 

 gory are trees, shrubs, and herbaceous plants, worms, 

 snails, insects and lower forms of life. Also in the shallow 

 ponds and streams of Alaska there is a species of fish 

 (Dallia) that every winter is frozen solid. Individuals 

 caught in November have been kept in this condition 

 until the following March, and upon being thawed out 

 appeared none the worse for the experience. There is 

 some evidence that bear animalcules, a low species of 

 animal, found frozen in glacial ice, may have existed in 

 this imprisoned condition for many, possibly scores, of 

 years. It is conceivable though not probable that some 

 of the activities of the body may be carried on in these 

 frozen organisms, but there is no visible sign of life 

 and the death limit is certainly close at hand. 



Length of Life. — According to an old saying, " a 

 wren lives three years, a dog lives three times as long as 

 a wren, a horse lives three times as long as a dog, and a 

 man lives three times as long as a horse." Although 

 these figures are somewhat inaccurate when the average 

 length of life is considered, they call attention to the fact 

 that the life of every species of animal and plant is 

 limited. A worker honey-bee lives six weeks, and be- 

 tween this limit and the hundred-year existence of the 

 elephant and eagle other species can be found with almost 

 any intermediate period of activity. 



The explanations offered by biologists to account for 

 these facts have been of the most varied character, and 

 yet none at the present time meets with general approval. 

 Some scientists have asserted that size determines the 

 age limit ; that large animals live longer than small ones. 

 This is undoubtedly true within limits. An elephant is 

 twenty-five years of age before it attains full size, and a 

 series can be arranged as follows: elephant, 100 years; 

 horse, 40 years; blackbird, 18 years; mouse 6 years; 



