696 



REVIEW OF REVIEWS. 



September 1. 191S. 



by one atmosphere, and at a depth of 

 3000 fathoms the pressure on each 

 square inch of the body of an animal 

 amounts to three tons, whereas at the 

 surface of the waters it is about fifteen 

 pounds. So great is this pressure that 

 unless special precautions are taken the 

 glass of the thermometers which mea- 

 sure the bottom temperatures is crushed 

 to powder. 



THE RHYTHM OF THE OCEAN. 



The main distinctive fact about 

 marine life, particularly at low depths, 

 says Dr. Shipley, is rhythm. Quoting 

 Mr. Johnstone and his book, "Life in 

 the Sea," the reviewer says there is 

 rhythm in the ocean. 



There is a rhythm of a tide, a rhythm 

 which corresponds with the rise and fall 

 about twice every twenty- four hours, 

 and that is involved in a still bigger 

 fortnightly rhythm corresponding with 

 the full and the new moon ; for about 

 half-way between these two phases the 

 tide rises more slowly and to a lower 

 height ; and again, just as there is a 

 half-daily and a half-monthly rhythm, 

 so we have a half-yearl\' rhythm in the 

 vernal and autumnal equinoxes. So re- 

 gular are these rh\thms that the tide is 

 calculated years in advance, for all 

 parts of the world, and navigators rely 

 trustfully on these calculations, which 

 are not found wanting. 



This rhythmical change has impressed 

 itself upon many marine organisms. As 

 Mr. Johnstone reminds us, to keep 

 cockles healthy in aquaria under arti- 

 ficial conditions one must run the water 

 off the tank at least once a day so as to 

 simulate a low tide. Convoluta — a 

 small and lowly worm — which lives on 

 the sand and burrows beneath it when 

 the tide is ebbing off the beach, kept in a 

 laborator}' in vessels of sea-water, 

 periodically burrows under the artificial 

 sand at the bottom of the vessel when 

 the real tide is normally going out. The 

 phosphorescence of the surface organ- 

 isms which we have noticed above only 

 comes into play at best some time after 

 sunset. If these surface organisms, cap- 

 able of producing phosphorescence be 

 kept in an aquarium in a dark room, the 



same remains true. Although they are 

 exposed to no secular change of light 

 and darkness, they only show their 

 liehts at a time when the outside world 

 is dark. The same is true, as Gamble 

 and Keble have shown, with the chame- 

 leon-shrimp, which in the sea shows a 

 variety of protective colouring during 

 the daytime but at night becomes a 

 transparent blue. Hence it is obvious 

 that the tide has produced an effect 

 which is lasting on certain organisms 

 even when they have been removed from 

 their natural surroundings and from 

 the tidal influence for considerable 

 periods. 



WHEN THE SEA IS SALTEST. 



Then again we have a rhythmical 

 change of temperature, which is fairly 

 constant for given places in the sea. 

 About February and March the sea is at 

 its coldest, but it gradually warms up 

 until in August it attains its highest 

 normal temperature, in the north. In all 

 these rhythms there are many disturbing 

 features, such as the weather. But these 

 can fairly easily be discounted. Just as 

 we have an annual rise and fall of tem- 

 perature, so do we have a daily one, the 

 temperature being at its lowest about 

 sunrise, and gradual 1\' rising till about 

 the middle of the afternoon. And 

 again, there is a fortnightl)- rh\thm, in- 

 asmuch as near the hmd the sea is 

 warmer in the summer just after the 

 time of new or full moon, and colder at 

 the same periods during the winter. 



Other rh)thms might be printed out, 

 such as those dependent on the intensity 

 of sunlight, and on the degree of 

 salinity, which in turn dejiends to a very 

 large extent on the water circulation of 

 the sea. The pulsing-up of the Gulf 

 Stream is the direct result of this cir- 

 culation and affects not only the warmth 

 but the salinity of the waters on our 

 western shores. " The water is sal test 

 when the drift is strongest, in the months 

 of February to June, and is less salt 

 when the drift is weakest, in the months 

 of November to February." All these 

 features have a profound influence on 

 the life of the ocean ; and a remote in- 

 fluence on land animals whose ancestry 

 was marine. 



