Fitness in the Living World 35 



temperature never exceeds 110 F. and is usually much less 

 than this, while it reaches 5000 F. in a gas engine; the rate 

 of contraction may vary from movements so slow as to be 



r w 



scarcely visible to a rapidity of about 24,000 contractions 

 a minute, as in the beating of a mosquito's wings ; the extent 

 of movement may vary from a scarcely perceptible shorten- 

 ing to one one-hundredth part or less of the maximum ex- 

 pansion, as for example, in certain worms and in the tenta- 

 cles of some coelenterates. 



Consider the variety, complexity, and efficiency of the 

 means of locomotion in animals. Among the marvels of 

 nature Solomon enumerates "The way of the serpent on the 

 rock and the way of the eagle in the air," but the more 

 usual forms of locomotion, such as running, jumping, climb- 

 ing, digging, sailing, wading, and swimming show fitness 

 and efficiency that are equally marvelous. Consider the 

 manner in which unusual speed has been attained in the 

 horse, giraffe, and antelope by the lengthening of legs and 

 digits, the elevation of the animal upon the ends of the 

 middle digits and the loss of the lateral ones. Consider the 

 remarkable contrivances of the kangaroo, the jack-rabbit, 

 the grasshopper, the flying squirrel for leaping; of the sloth, 

 squirrel, and woodpecker for climbing; of the earthworm, 

 mole cricket, and mole for burrowing. Among aquatic 

 animals almost every means of propulsion which man has 

 devised was discovered ages before by lower animals: the 

 Portuguese man-of-war and the paper nautilus spread their 

 purple sails to the breeze and "sail the uncharted main" ; 

 the jelly-fish and squid use hydraulic motors; fish, seals, 

 and whales employ oars and sculls. Finally, consider the 

 wonderful adaptations by means of which animals travel 

 the highways of the air: the spider which spins a thread 

 that floats out on the breeze and then, clinging to this gos- 



