288 ELEMENTARY BIOLOGY 



a chemical reaction, we find that the effect of the temperature 

 is to produce a definite kind of organic behavior on the part of 

 certain portions of the animal or on the part of the animal as a 

 whole. This behavior is of a continuous kind, not momentary. 



334. Light and development. Light influences the rate of growth 

 and the direction of growth in plants, and it influences the formation 

 of pigments in animals as well as in plants. But we know very little 

 about the influence of light upon the development of organisms, apart 

 from the influence upon growth. So far as studies have been made, 

 most organisms develop as well in darkness as in light, although all 

 protoplasm is sensitive to extreme light, which is fatal to bacteria of 

 many kinds and to other living beings that cannot protect themselves 

 adequately against the light rays. It is for this reason that the higher 

 organisms, which have opaque, or pigmented, exteriors, generally thrive 

 better in well-lighted places than in total darkness. Horses and cattle 

 kept in dark stables are exposed to more danger from bacteria ; human 

 beings that are kept in dark dwellings, workrooms, or underground 

 cellars or mines are exposed to more danger from bacteria. 



335. Food and development. The amount of food received 

 by a plant or an animal at a given time for example, early 

 in youth has a greater effect than the amount received at 

 another time. If a calf has been underfed, he will have a 

 comparatively large head, long legs, and large joints. No amount 

 of overfeeding later in life will even up the development. The 

 same is true of children. Underfeeding or unsuitable feed- 

 ing in infancy will not only stunt the growth, but will cause a 

 disproportionate growth which can never be made up later. 



Experiments have shown that the character of the food will 

 in many cases influence the development of an organism in a 

 more direct way. Thus, the intestines of tadpoles fed on 

 vegetable matter, compared with the intestines of tadpoles fed 

 on more concentrated food, were found to be considerably 

 longer. This may be a case of stimulating the growth of a 

 special organ by overexercising it. In general, we should 

 expect a full development of all the parts of an organism only 



