Fitness 6i 



this nervous tissue or to special sense organs, but it has been 

 possible to find that all plants are irritable to a great variety 

 of disturbances or stimulations. For example, by turning 

 an ordinary house plant, as it stands near the window, half 

 way around every two or three days, we can find that it is 

 sensitive to the direction from which the light strikes it. In 

 such a case, the plant manifests its irritability by changing 

 the direction of its growth. By means of similar experi- 

 ments we can show that the plant is sensitive to the direction 

 of gravity, to various chemical substances, to electrical stimu- 

 lation, to water, and to mere contact — the same as our- 

 selves but not to the same degree. On the other hand, 

 sensitiveness to light varies a great deal among different or- 

 ganisms so that many plants respond to an amount of light 

 which our eyes cannot observe, and many insects are sensitive 

 to ranges of the spectrum that our eyes cannot perceive — 

 just as dogs are sensitive to odors which most human beings 

 cannot distinguish or even detect. 



Chemical Response 



A striking example of a reaction to outward stimulation 

 in a form other than contraction was brought out in the 

 studies of Metchnikoff , Behring, Ehrlich, and x)ther. When 

 a foreign substance is introduced into the body of a warm- 

 blooded animal a special kind of chemical change is brought 

 about. It is possible to show that the new substance formed 

 is present in the blood. It can also be shown that the new 

 substance is specifically related to the chemical nature of the 

 stimulating foreign substances. It is upon this principle 

 that anti-toxins have been developed for the combating of 

 diphtheria and other diseases. 



Fitness 



We know of course that irritability is not confined to 

 living things. Many of our high explosives are set off by a 



