8 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



make and for living animal mechanisms. Great 

 difficulties attend the testing of this question in the 

 animal organism, owing to that complexity which 

 arises from the interaction of the numerous processes 

 that go on simultaneously and side by side. As the 

 result of this complexity we cannot test the law of 

 conservation as we can test it in simple machines, 

 for we have to make various disturbing allowances 

 for interferences of many kinds. Nevertheless it 

 is safe to regard it as a practical certainty that the 

 doctrine of conservation holds good for living ani- 

 mals. The human machine is superior to all engines 

 in the extent to which it utilizes the power stored in its 

 sources of energy. This superiority depends largely 

 on the better absorption of heat generated by com- 

 bustion. Owing to the conditions of combustion 

 in the living body, this absorption of heat approaches 

 the ideal level; owing to the crude conditions of 

 absorption in the artificial machine, there is an 

 enormous loss of energy which human ingenuity 

 has never been able in practice to correct. Doubt- 

 less an artificial engine could be so constructed as to 

 overcome this fault, but it would be a scientific 

 curiosity and not a practical source of energy. 



A beautiful example of the mechanical principle 

 which subserves a highly refined function of the body 

 is to be seen in the relations of the crystalline lens 

 of the eye to the formation of visual images on the 

 retina. The physical and mathematical laws gov- 

 erning the formation of these images were admirably 



