Fitness in the Living World 309 



fitness of the organs of touch and taste and smell, and the 

 complex fitnesses and coadaptations of the many parts of the 

 ear and eye. Many instruments of human invention are 

 more sensitive to particular kinds of stimuli than some of the 

 sense organs. For example, a thermometer is more sensitive 

 to temperature changes than our heat and cold organs, the 

 photographic plate is more sensitive to light than the retina, 

 the microphone more sensitive to vibration than the ear; 

 but when one considers the range and variety of stimuli to 

 which higher animals are sensitive, there is no doubt that 

 their sense organs are much more efficient than any non- 

 living mechanism. 



Finally, consider the durability of the living organism and 

 its power of self-regulation and self-repair as compared with 

 any other machine. Not for one moment between birth and 

 death does the living engine stop. The heart and respira- 

 tory muscles cannot rest for a minute at a time during the 

 whole course of life. Engines have been built that would run 

 for a month or two without stopping for repairs, but the 

 heart may continue to beat without interruption every second 

 for a hundred years, pumping during this time not less than 

 sixty million gallons of blood. The regulatory power of an 

 organism is incomparably more varied and perfect than in 

 any other mechanism; not only do all the complex processes 

 of life occur, under normal conditions, in the best possible 

 sequence and to the most favorable extent, but when as the 

 result of abnormal conditions these processes are disturbed, 

 the living machine has a wholly unparalleled capacity of 

 regulation and restoration. When the living machine under- 

 goes wear, injury, or loses parts, it is able to a surprising ex- 

 tent to repair itself and to restore or compensate for lost 

 parts. What other kind of machine has a regulatory power 

 that is comparable with that of a living thing? 



