556 INTERRELATIONSHIPS OF ORGANISMS 



changes in the relative amounts of daylight and darkness in each 24 

 hours. 



Other factors of the physical environment. Some of the ways in 

 which temperature, humidity, and light modify the environment and 

 influence the behavior and distribution of organisms have been briefly 

 described. With these for examples, some of the other important factors 

 may be even more briefly mentioned. All these (except gravity and 

 perhaps contact) exhibit a wide range of intensities, for which organisms 

 have various limits of toleration and optima and to which many organisms 

 respond by positive or negative tropisms. 



Pressure of the medium is of most importance to the organisms that 

 live in the sea or in deep lakes, where some species are limited to the 

 surface waters and others to various levels in the greater depths. Some 

 of the whales appear to show the widest limits of toleration to pressure, 

 and range from the surface to depths of several hundred fathoms. There 

 is little evidence that air pressure, as such, is very important in setting 

 limits of toleration for terrestrial animals, but tropistic behaviors toward 

 varying intensities of pressure are shown by a number of forms. Reactions 

 to pressure of the medium are termed barotropisms. 



Currents in the medium are most important for aquatic organisms, 

 although a considerable number of insects show reactions to air currents. 

 Nearly all fish automatically head into a current, changing their positions 

 promptly with each change in its direction. This is easily demonstrated 

 by placing a number of small fish in a pan of water and then stirring the 

 water so that it whirls. If the pan contains both stream and pond fishes, 

 the former usually show a much more delicate and precise reaction. Reac- 

 tions to the current in the medium are termed rheotropisms — positive if 

 the organism faces or moves against the current; negative if it avoids or 

 attempts to escape from it. 



Various chemicals that are dissolved or diffused in the medium are 

 important factors, both because reactions (chemotropisms) on the part of 

 organisms are produced, and because many of these substances may 

 exist in concentrations above or below the limits of toleration of some or 

 all organisms. Among the very important substances in solution in 

 water are oxygen, carbon dioxide, various calcium salts, and hydrogen 

 and hydroxyl ions. Less generally important, but of particular interest 

 from the standpoint of conservation, are the vast quantities of sewage 

 and of poisonous mine and factory wastes that, through thousands of 

 miles of the streams of the United States, have come to exceed the limits 

 of toleration of nearly all forms of life. 



Contact. Numerous animals exhibit a positive reaction to contact 

 with some comparatively stable surface. This reaction (positive thigmo- 

 tropism) tends to keep a considerable number of invertebrates and perhaps 



