104 THE FAUNA OF THE DEEP SEA 



inhospitable habitats, and that only when, as it were, 

 every door is closed, when no island, continent, or 

 cape can afford it a free scope for life in shallow 

 water, does it become a true deep-sea species. 



Steps taken towards the darkness in one period 

 may be retrieved in the next. The competing species 

 may itself have become extinct or have moved to 

 another locality. Organs may have become modified 

 or a new source of food supply tapped which enable 

 them to return once more to shallower waters. No 

 wonder that the steps in the progress, or rather retreat, 

 to the abyss have been the work of a time that can be 

 counted only by geological periods ; and no wonder 

 then at the remark made by many deep-sea natu- 

 ralists that the abysmal fauna becomes poorer the 

 farther it is from shallow water. 



The group of the Asteroidea, or star-fishes, con- 

 tributes largely to the fauna of the abyss. 



During the voyage of the ' Challenger ' no fewer 

 than 109 different species were found in depths of 

 over 500 fathoms, and in some localities a very large 

 number of star-fish were taken in one haul of the 

 dredge. 



Nevertheless, there are not many abysmal genera 

 that differ to any remarkable degree from the littoral 



