318 BIOLOGY 



no meaning from any attempt to find correlation between the 

 true mental processes and physical energy. 



Reproduction. The process of reproduction would seem to 

 be one which cannot possibly be explained as the result of 

 chemical and physical forces. Nowhere else in nature do we 

 find this property, and in this respect living organisms cannot 

 be compared to any other machine. Nevertheless, in its sim- 

 plest form reproduction also permits a partial explanation. 

 When a unicellular organism, like the Amoeba (Fig. 19), feeds 

 and grows, it increases in size. The increase in size is due to 

 the transformation of the chemical material of its food into a 

 material like that of the animal, and as these new materials 

 accumulate, the bulk of the animal becomes greater. As the 

 animal increases in bulk, it needs a larger supply of oxygen to 

 keep up its life processes, since all life processes require the 

 expenditure of oxygen, and the amount of oxygen needed is 

 dependent on the bulk of the animal that is to be supplied. 

 Now it is a principle of mathematics that the bulk of a solid 

 object increases as the cube of its dimensions, whereas its sur- 

 face increases only as its square. Since this Amoeba is obliged 

 to absorb all of its oxygen through the surface of its body, it 

 follows that the surface adapted for absorbing of oxygen in- 

 creases only as the square of its diameter, while its need for 

 oxygen increases as the cube. It is evident from this that in 

 time the surface will no longer be sufficient to absorb enough 

 oxygen for its increasing size. When this time comes the ani- 

 mal must either stop growing or devise some way of increasing 

 its absorptive surface. What happens is that the bit of living 

 jelly simply breaks in two. The result is that once more the 

 absorbing surface is large enough to accommodate a larger 

 bulk, and the animal again begins to grow. This explanation 

 of reproduction shows how the process may have been due 

 to overgrowth. Since all kinds of reproduction are forms of 

 division, it follows that if we can explain the simplest division 

 upon the basis of physical and chemical forces, we have at 



