II] OF THE FORCE OF GRAVITY 51 



magnitude is true of all the rhythmic actions of the body, though 

 for reasons not always easy to explain. The elephant's heart beats 

 slower than ours*, the dog's quicker; the rabbit's goes pit-a-pat; 

 the mouse's and the sparrow's are too quick to count. But the very 

 "rate of Hving" (measured by the consumed and COg produced) 

 slows down as size increases; and a rat lives so much faster than 

 a man that the years of its life are three, instead of threescore and 

 ten. 



From all the foregoing discussion we learn that, as Crookes once 

 upon a time remarked I, the forms as well as the actions of our 

 bodies are entirely conditioned (save for certain exceptions in the 

 case of aquatic animals) by the strength of gravity upon this globe; 

 or, as Sir Charles Bell had put it some sixty years before, the very 

 animals which move upon the surface of the earth are proportioned 

 to its magnitude. Were the force of gravity to be doubled our 

 bipedal form would be a failure, and the majority of terrestrial 

 animals would resemble short-legged saurians, or else serpents. 

 Birds and insects would suffer Ukewise, though with some com- 

 pensation in the increased density of the air. On the other hand, 

 if gravity were halved, we should get a lighter, slenderer, more active 

 type, needing less energy, less heat, less heart, less lungs, less blood. 

 Gravity not only controls the actions but also influences the forms 

 of all save the least of organisms. The tree under its burden of 

 leaves or fruit has changed its every curve and outline since its 

 boughs were bare, and a mantle of snow will alter its configuration 

 again. Sagging wrinkles, hanging breasts and many another sign 

 of age are part of gravitation's slow relentless handiwork. 



There are other physical factors besides gravity which help to 

 limit the size to which an animal may grow and to define the con- 

 ditions under which it may Hve. The small insects skating on a 

 pool have their movements controlled and their freedom hmited by 

 the surface-tension between water and air, and the measure of that 

 tension determines the magnitude which they may attain. A man 

 coming wet from his bath carries a few ounces of water, and is 

 perhaps 1 per cent, heavier than before; but a wet fly weighs twice 

 as much as a dry one, and becomes a helpless thing. * A small 



* Say 28 to 30 beats to the minute. 



t Proc. Psychical Soc. xn, p. 338-355, 1^97. 



