52 PHYSIOGRAPHIC GEOLOGY. 



exliibitiou of perfect system under law. If the earth has its barren ice-tields 

 about the poles, and its deserts no less barren toward the equator, they are 

 not accidents in the making, but results involved in the scheme from its 

 very foundation. 



V. GEOGRAPHICAL DISTRIBUTION OF PLANTS AND ANIMALS. 



The geographical distribution of plants and animals is dependent on both 

 physical and biological conditions. 



1. Temperature has universal influence. Species are usually confined 

 within narrow temperature limits. They differ therefore in the different 

 zones from the equator to the poles, some having a range of only a few 

 degrees, and others of half a hemisphere. They differ also with the height 

 on passing from the sea level to the limit of life (the limit of perpetual 

 snow) about the summits of the highest mountains, or even higher, as 

 regards Microbes or Bacteria, the lowest of cryptogamous plants, the only 

 kinds having the range of the world. They also differ as we descend in 

 the ocean. 



2. Light is another universal cause. Some species need for successful 

 growth and reproduction the direct rays of the sun ; others are confined to 

 shady places, dark places, and very dark places, like caves ; some to the surface 

 waters of the ocean, because of the light that penetrates them, and others 

 to dark depths. A lawn will have a rich surface of grass in the sunshine, 

 and become full of weeds under the shade of a tree, because the weeds 

 flourish in the shade, while the grass dwindles and becomes crowded out ; 

 and in such a case fertilizers may help only the weeds instead of the grass. 



3. Difference in jrressure. — This cause also is universal in its action, but 

 very feeble in its effects. The atmospheric pressure near the earth's surface 

 diminishes about one pound per square inch for each 1900 feet of ascent, or, 

 approximately, three pounds for 6000 feet. In the ocean, the pressure in- 

 creases at the rate of about one pound per square inch for each 2-2 feet of 

 descent, or 2750 pounds for 6000 feet or 1000 fathoms, and 11,000 pounds for 

 24,000 feet. 



But marine species readily become adapted to all pressures, as the outside 

 water penetrates them. Twenty-six fishes are known to have a range of 5400 

 feet, and some macrural Crustaceans a range of more than 12,000 feet. The 

 Shrimp, Sergestes mollis, for example, ranges from 2238 to 17,694 feet. 



But after a sudden change, or when brought to the surface in a dredge, 

 a fish presents "a. most disreputable appearance," the swimming bladder 

 protruding from its mouth, the eyes forced from their sockets, and the scales 

 fallen off (A. Agassiz). 



4. Differences in moisture and dryness of climate are great sources of lim- 

 itation in the range of species. Differences in soil have wide influence ; for 

 a soil must contain the materials essential to a plant's growth before it will 



