On ttu Mectianical Conception of Nature. 655 



zation of an animal. In animals of a spherical 

 form the surface is quite sufficient for respiration, 

 so long as they are of microscopic size. But 

 such an organism cannot become enlarged at 

 pleasure, because the ratio of the surface to the 

 volume would become quite different. The surface 

 increases as the square, whilst the volume in- 

 creases as the cube, so that very soon the surface 

 of the more rapidly increasing bodily mass can 

 no longer suffice for respiration. 8 This sort of 

 limitation is in no way equivalent to that purely 

 external kind which, for instance, manifests itself 

 in such a manner as to prevent the indefinite 

 lengthening of the tail feathers of the Bird of 

 Paradise. In this case feathers that were too 

 long would hinder flight, and such individuals 

 would accordingly be eliminated by natural selec- 

 tion. The cause is in the former case purely 

 internal, depending upon the equilibrium of the 

 forces governing the organism. 



Von Hartmann is entirely in the right when he 

 asserts that variability is neither qualitatively nor 

 quantitatively unlimited. In both senses it is 

 limited (in direction as well as in amount) by the 

 physico-chemical forces acting in some contrary 

 way in each specific organism by the physical 



[This law has been beautifully applied by Herbert Spencer 

 in order to explain why, with an unlimited supply of food, an 

 organism does not indefinitely increase in size. " Principles of 

 Biology," vol. L p. 121 126. R. M.] 



