Fitness in the Living World 3 2 3 



Stimulus Continued Beneficial Response Continued 

 Unusual temperatures Acclimatization 

 Poisons or toxins Toleration or antitoxins 



Injury Regulation or regeneration 



Strong light, and especially light of short wave lengths 

 such as ultra-violet, is very injurious to protoplasm, and 

 when the skin of white persons is exposed to such light the 

 living cells suffer "sun-burn." But another result of such 

 exposure is that the skin becomes more deeply pigmented or 

 "tanned," and this screen of pigment serves to protect the 

 living cells from the injurious rays. 



Moderate friction and pressure on the skin, instead of 

 wearing it thin, leads to the thickening of the epidermis and 

 the formation of callosities by which the deeper lying parts 

 are protected. A similar result follows the application of 

 various chemicals to the skin. The epidermis of plants that 

 are exposed to salt-water spray becomes thickened, thus pro- 

 tecting the protoplasm from the injurious effects of the salt. 



It is a truism that in living things alone use strengthens 

 a part and disuse weakens it. The used muscle grows in 

 size and strength, and, within certain limits, it fits itself to 

 the task required of it, while the unused muscle grows small 

 and weak. A similar thing is true of glands, and even sense 

 organs or brains may be improved by use. 



Unusual kinds of food often lead to adaptive modifica- 

 tions of the digestive organs. Grain-eating birds have a 

 tough gizzard with a hard lining, but if they are fed on soft 

 foods the gizzard becomes soft and flabby. If animals which 

 live largely on meat are put upon a carbohydrate diet, or vice 

 versa, the character of the digestive fluids undergoes an 

 appropriate change. 



Remarkable also are the adaptations which many organ- 

 isms show to extremes of temperature and to dessication. By 



