II] OF SCALES OF MAGNITUDE 67 



until to creep upon wall or ceiling becomes as easy as to walk upon 

 the ground. Fishes, by evading gravity, increase their range of 

 magnitude both above and below that of terrestrial animals. Smaller 

 than all these, passing out of our range of vision and going down to 

 the least dimensions of hving things, are protozoa, rotifers, spores, 

 pollen-grains* and bacteria. All save the largest of these float 

 rather than swim; they are buoyed up by air or water, and fall 

 (as Stokes's law explains) with exceeding slowness. 



There is a certain narrow range of magnitudes where (as we have 

 partly said) gravity and surface tension become comparable forces, 

 nicely balanced with one another. Here a population of small 

 plants and animals not only dwell in the surface waters but are 

 bound to the surface film itself — the whirligig beetles and pond- 

 skaters, the larvae of gnat and mosquito, the duckweeds (Lemna), 

 the tiny Wolffia, and Azolla; even in mid-ocean, one small insect 

 (Halobates) retains this singular habitat. It would be a long story 

 to tell the various ways in which surface-tension is thus taken full 

 advantage of. Gravitation not only Hmits the magnitude but 

 controls the form of things. With the help of gravity the quadruped 

 has its back and its belly, and its limbs upon the ground ; its freedom 

 of motion in a plane perpendicular to gravitational force ; its sense 

 of fore-and-aft, its head and tail, its bilateral symmetry. Gravitation 

 influences both our bodies and our minds. We owe to it our sense 

 of the vertical, our knowledge of up-and-down; our conception of 

 the horizontal plane on which we stand, and our discovery of two 

 axes therein, related to the vertical as to one another; it was gravity 

 which taught us to think of three-dimensional space. Our archi- 

 tecture is controlled by gravity, but gravity has less influence over 

 the architecture of the bee ; a bee might be excused, might even be 

 commended, if it referred space to four dimensions instead of three ! t 

 The plant has its root and its stem", but about this vertical or 



* Pollen-grains, like protozoa, have a considerable range of magnitude. The 

 largest, such as those of the pumpkin, are about 200/x in diameter; these have to 

 be carried by insects, for they are above the level of Stokes's law, and no longer 

 float upon the air. The smallest pollen-grains, such as those of the forget-me-not, 

 are about 4^ /x in diameter (Wodehouse). 



f Corresponding, that is to say, to the four axes which, meeting in a pfoint, make 

 co-equal angles (the so-called tetrahedral angles) one with another, as do the basal 

 angles of the honeycomb. (See below, chap, vu.) 



