474 ZOOLOGY 



rivers ; the most famous is the Gulf Stream. The 

 surface conditions are very different from those below, 

 and the coasts are unlike deep waters. The zone along 

 the coasts, known as the littoral zone, is narrow, some- 

 times very narrow. It may be defined as that area 

 in which a considerable amount of light penetrates to 

 the bottom. There is the region between the highest 

 and lowest tide marks, and a variable extension 

 beyond the level of the lowest tide, until we reach 

 really deep water. Here the large red and brown sea- 

 weeds grow in abundance, and animals are often 

 brightly colored. Here, in the tropics, are the coral 

 reefs. Organisms belonging properly to the littoral 

 zone are confined to singularly narrow limits, as if in a 

 gigantic river. Most of the long coast lines extend 

 north and south, and consequently the littoral fauna 

 is blocked in its migrations north and south by climatic 

 changes. These changes may be quite abrupt, owing 

 to the meeting of warm and cold currents ; thus the 

 faunas north and south of Cape Cod and : on the 

 Pacific coast, of Point Concepcion are markedly dif- 

 ferent. This confinement to a relatively narrow area 

 makes the species of marine animals more local in their 

 distribution than we might at first expect. In many 

 cases, however, the animals have free-swimming early 

 stages, which are carried out to sea by the currents, 

 and may reach remote islands or other continents. Of 

 such larvae, setting out for the unknown, nearly all 

 perish, but a few survive and establish their kind on 

 other shores. Nature, as the poet said, is careless of 

 the single life, but careful of the race. 



The plank- 4- The floating organisms of the open sea, or even 

 of the surface layers of a lake, are known collectively 

 as the plankton. Properly speaking, the plankton in- 



