DISTRIBUTION OF ANIMALS 503 



variations involved in the existence of wet and dry seasons. Higher 

 up is a zone which possesses a warm temperate chmate, with summer and 

 winter, but involving at no season a cessation of life activity. At a 

 still higher level there is a cool, temperate climate with such a lowering 

 of temperature in the winter as necessitates dormancy on the part of a 

 number of living forms. The upper limit of this zone is the limit of tree 

 growth. Above the tree line is a frigid zone, extending to the limit of 

 all vegetation, and still above this an arctic zone. Thus vertical life 

 zones occur on the mountains corresponding to the horizontal life zones 

 at sea level; and just as seasonal changes occur in temperate regions far- 

 ther north, so will they occur in temperate zones on the mountain in the 

 tropics. Vertical migrations will also occur. In many cases particular 

 species found on the upper parts of mountains are also found farther 

 north where a similar climate exists. Such a fact is usually explained by 

 assuming that in the past has occurred such a climatic change, affecting 

 a widely distributed species, that the species has become extinct except 

 on higher mountains southward and in areas farther north with a cor- 

 responding climate. Thus in both cases it has persisted in a region where 

 the climate is suitable. 



575. Oceanic Distribution. — The conditions at any particular loca- 

 tion in the ocean are much more uniform than are those in fresh water or 

 on land. This stability shows itself especially in temperature, salinity, 

 and in the gaseous content of the water. At points widely separated in 

 the ocean, however, there are marked differences in temperature and, 

 to a certain extent, in the other factors mentioned, which affect the dis- 

 tribution of life. Another factor concerned in marine distribution is 

 the existence of ocean currents, by means of which many species are 

 dispersed. Vertical distribution in the ocean is affected by a variety of 

 conditions, including pressure, which increases by 14 pounds to the square 

 inch for every increase of 10 meters in depth. At a depth of 3660 meters, 

 or about 12,000 feet, this is over 2}2 tons to each square inch of area. 

 This great pressure does not affect animals living at great depths, since 

 the pressure within their bodies is equal to that which is without, but it 

 limits vertical movements. Light decreases by absorption and ceases to 

 have any effect upon life at a depth of about 900 meters (3,000 feet). 

 It is stated that all of the heat due to the rays of the sun is lost below 

 about 275 meters (900 feet). In the Atlantic Ocean it has been found 

 that with a surface temperature of 20°C. (68°F.) the temperature at 500 

 fathoms is about 3°C. (39°F.) and that at 1000 fathoms it is little less, 

 decreasing very slowly to the bottom, where it is about freezing. The 

 highest sea temperature known is in the Persian Gulf, where there is a 

 surface temperature of 35°C. (95°F.), while in the polar seas surface 

 temperatures of approximately — 3.3°C. (26°F.) have been recorded. 

 In the depths of the sea animals live under a condition of high pressure, 



