12 



REPRODUCTION 



Figure 2. Pleurococcus, a common one- 

 celled plant, as seen under the microscope 

 — Coulter s Plant Life and Plant Uses. 



by the volume of the 

 same sphere, for the 

 whole volume must be 

 fed. Now as this sphere 

 is nourished and grows, 

 both the surface and 

 the volume increase, 

 but not at the same 

 rate. The surface in- 

 creases as the square of 

 the radius; but the vol- 

 ume increases as the 

 cube of the radius. For 

 example, if by growing 

 the radius were trebled, 

 the food-taking sur- 

 face would increase only nine times; whereas the mass 

 that is to be fed would be increased twenty-seven timtes. 

 This shows that the living thing must keep up a right 

 adjustment between the internal need and the external 

 supply. 



It is important that the student should not fall into an 

 error here. When we ask why an organism does thus 

 and so, we may think of either of two things. We are 

 likely to think of the gain or advantage that will come as 

 the result of doing it and assign this as a reason. Now 

 an advantage lies ahead, always; and events to come can't 

 act as causes, unless the organism is intelligent and can 

 be thought of as looking ahead. We can see in the case 

 above that division would be advantageous. It would 

 increase the surface without changing the volume, and 

 allow more nutrition and more growth. When we ask 

 why, we really mean the cause of the thing. The 

 advantage cannot be the cause. The cause must come 

 before the event. 



Come back to our example. Not all organisms are as 

 simple as the illustration. Yet it is true of all organisms 



