THE NATURAL HISTORY OF THE SEA. 391 



considerable depths. In 1891 Russell found them on the 

 floor of the Mediterranean itself fifteen kilometres from 

 land at a depth of 500-600 fathoms. Their vertical dis- 

 tribution in the ocean may be summed up as scarce in the 

 surface layers, increasing downward to 100 fathoms, occur- 

 ring regularly to 200 fathoms, but steadily diminishing from 

 100 to the depths between 400-600, beyond which they 

 have not been traced with certainty. It appears, moreover, 

 fairly certain that the destructive action of sunlight is the 

 cause of their scarcity at the surface, that they increase with 

 the darkness, and reaching an optimum of general condi- 

 tions of life at 100 fathoms, decrease again with the lowering 

 of the temperature and the increase of pressure. Bacteria 

 at 500-600 fathoms must undergo a pressure of over 100 

 atmospheres, and though it is known from experiment that 

 they survive considerable pressures yet the vast pressures 

 of the deep sea may be expected to reckon as a factor in 

 determining their range in depth. Temperature also must 

 play an even more important part. In the Mediterranean, 

 where Russell's observations were made, the bottom tem- 

 perature is constantly 1 3° C. all the year round, below 

 about 100 fathoms, while above that it varies with the 

 seasons. In the ocean the bottom temperature at great 

 depths is always so low that no development of bacteria 

 could be expected. The bottom of the Atlantic is either 

 extraordinarily poor in bacteria or wholly free from them, 

 the observations not being conclusive. In fact below 400 

 to 600 fathoms there is no certain record. These are the 

 depths to which one would assign in fact the graveyard of 

 the dead sinking surface plankton. That sunlight is, as 

 every known fact would indicate, the cause of the scarcity 

 in the surface layers is proved further by the fact that the 

 strong light of summer periodically reduces the quantity in 

 the upper waters. 



Marine bacteria are moreover of peculiar sorts, though 

 morphologically they resemble their land allies in adopting 

 the well-known globular, rod-like and spiral forms. The 

 luminous forms are confined to the coastal waters for the 

 most part where they are found regularly on fishes, etc., 



