n] BIOLOGICAL FOUNDATIONS 53 



with raw materials 1 , and these materials, instead of 

 being plastered on to the outside, can and do pass in 

 to the interior, and only there are worked up into 

 those combinations of brick and architect, the mole- 

 cules of protoplasm. Thus, though the absorption of 

 raw materials must of necessity take place at the 

 surface, the actual formation of new living matter, or 

 in other words assimilation and growth, goes on only 

 in the interior. 



As a further result of its partially fluid nature, 

 protoplasm is subject to the laws of surface-tension, 

 and a mass of it will therefore tend to become 

 spherical. But in a sphere, as in any other solid body 

 of fixed shape, surface increases with the square, bulk 

 with the cube of the diameter. When we say that 

 one ball is three times as big as another, we usually 

 mean that its diameter is three times as long, for- 

 getting, or leaving implied, that in surface it is nine 

 times, in cubic content twenty-seven times as big. 

 With our balls of living substance, this disproportion 

 between increase of bulk and increase of surface 

 brings difficulties. 



Every molecule in the inner parts of the sphere 

 must have oxygen and food if the whole is to go on 

 living. As the organism grows, that is to say as its 



1 Though, as in commerce, one organism's manufactured article is 

 another's raw material ; take as an example the quadruple chain of 

 nitrogen-fixing bacterium, clover, ox, and man. 



