484 DISTRIBUTION OF LIFE IN THE OCEAN 



sible. A sponge, living along the coast, if sunk to a great 

 depth would show little diminution in size owing to its 

 watery consistency, its tissues accordingly would suffer 

 no collapse, and if food were available it probably would 

 live. At all events, several species of widely related 

 animals dredged from depths ranging from one to two 

 miles have been kept alive for days and even weeks. 



On the other hand it frequently happens that compara- 

 tively large amounts of gases are dissolved in the tissues 

 of deep sea animals. When such individuals are cap- 

 tured in nets and drawn to the surface this gas expands 

 with the reduced pressure, and as it escapes from the 

 tissues literally tears the body into shreds. 



Light. — White light is the combined product of 

 vibrations or waves in the ether of different lengths. 

 Those of relatively great length produce the sensation 

 of red when they fall upon the retina of the eye, and 

 shorter vibrations produce orange, still shorter yellow, 

 and so through green, blue, to violet with the shortest 

 length of all. When the sun's rays enter the ocean 

 the depth of their penetration depends upon their 

 wave length. No red reaches a greater depth than 300 

 feet. No green passes the 1500 foot level and at the 

 depth of a mile there is no sunlight whatever. Four- 

 fifths of the ocean floor, therefore, is in absolute dark- 

 ness so far as the sun is concerned. Nevertheless, there 

 is a strong probability that a feeble light may be present, 

 as we shall see presently. 



These physical features, salinity, temperature, press- 

 ure and light, are very evidently not peculiar to the 

 ocean, but play an important part in the distribution of 

 land and freshwater animals as well. Their relative im- 

 portance, however, differs in the two cases. A few other 

 distribution factors will be noted in some of the succeed- 

 ing paragraphs. 



Distribution of Shore Plants. — Practically all of 

 the attached seaweeds or marine algsD grow at depths less 



