ANIMAL LIFE 179 



light. It is probable that the animals giving 

 out phosphorescent light are not evenly dis- 

 tributed throughout the deep sea, but are 

 more or less localised and may be more abun- 

 dant in some localities than in others, while 

 certain regions may be utterly devoid of light, 

 but this does not account for the great varia- 

 tions in the development of the eyes in deep- 

 sea forms, in some of which the eyes are very 

 large while in others the eyes are small or 

 altogether wanting. Sometimes in the same 

 haul of the trawl creatures with large eyes are 

 brought up along with others that are totally 

 blind. 



It has been stated that the action of the 

 digestive enzymes is retarded at low tem- 

 peratures, and this action must therefore be 

 very slow in the deep sea, so that the food 

 requirements of deep-sea animals are less 

 than those of the more active animals living 

 in the warmer waters of the ocean. 



The colonising of the deep sea seems to 

 have been effected by successive migi'ations 

 from the shallower reaches of the ocean, 

 especially from the region of the mud-line 

 (where it is supposed that the simplest forms 

 of life originally appeared in pre-Cambrian 

 times), but apparently more frequently from 

 cold regions than from warm regions. If 

 there were once a nearly universal warm 



