56 ON MAGNITUDE [ch. 



and explained. The thickness of each film lies on the very limit of 

 microscopic vision, and the least change or irregularity in this 

 minute dimension would throw the whole display of colour out of 

 gear. No phenomenon of organic magnitude is more striking than 

 this constancy of size; none more remarkable than that these fine 

 lamellae should have their tenuity so sharply defined, so uniform 

 in feather after feather, so identical in all the individuals of a species, 

 so constant from one generation to another. 



A simpler phenomenon, and one which is visible throughout the 

 whole field of morphology, is the tendency (referable doubtless in 

 each case to some definite physical cause) for mere bodily surface 

 to keep pace with volume, through some alteration of its form. The 

 development of villi on the lining of the intestine (which increase 

 its surface much as we enlarge the effective surface of a bath-towel), 

 the various valvular folds of the intestinal lining, including the 

 remarkable "spiral valve" of the shark's gut, the lobulation of the 

 kidney in large animals*, the vast increase of respiratory surface in 

 the air-sacs and alveoli of the lung, the development of gills in the 

 larger Crustacea and worms though the general surface of the body 

 suffices for respiration in the smaller species — all these and many 

 more are cases in which a more or less constant ratio tends to be 

 maintained between mass and surface, which ratio would have been 

 more and more departed from with increasing size, had it not been 

 for such alteration of surface-form f. A leafy wood, a grassy sward, 

 a piece of sponge, a reef of coral, are all instances of a hke pheno- 

 menon. In fact, a deal of evolution is involved in keeping due 

 balance between surface and mass as growth goes on. 



In the case of very small animals, and of individual cells, the 

 principle becomes especially important, in consequence of the 

 molecular forces whose resultant action is limited to the superficial 

 layer. In the cases just mentioned, action is facilitated by increase 

 of surface : diffusion, for instance, of nutrient liquids or respiratory 

 gases is rendered more rapid by the greater area of surface; but 



* Cf. R. Anthony, C.R. clxix, p. 1174, 1919, etc. Cf. also A. Putter, Studien 

 uber physiologische Ahnlichkeit, Pfluger's Archiv, clxviii, pp. 209-246, 1917. 



t For various calculations of the increase of surface due to histological and 

 anatomical subdivision, see E. Babak, Ueber die Oberflachenentwickelung bei 

 Organismen, Biol. Centralbl. xxx, pp. 225-239, 257-267, 1910. 



