2 
THE VOYAGE OF H.M.S. CHALLENGER. 
expeditions hitherto made, the remaining Holothurids met with in the great depths are 
comparatively few both in species and individuals, and unmistakably show the closest 
relation to the present shallow-water fauna ; so that while the Elasipoda have retired 
towards the abysses an infinitely long time ago, the latter have emigrated only at a com- 
paratively much later period. The Cucumarice and several other present shallow-water 
forms that have descended as far down as 2000 fathoms are veritable Cucumarice, &c., 
that, in spite of their highly altered mode of life, have not yet had time to acquire any 
noteworthy deviations from the typical character of the genera in question. Such an 
emigration, which can be traced in the case of divers groups of marine animals, can 
only be efiected extremely slowly, and undoubtedly takes place from competition 
with the shore fauna. We may suppose that the livelier struggle for existence which 
must arise in the littoral region or its vicinity, where an infinite number of various 
forms fight for subsistence, compels such forms, as lack sufficient power of resistance, 
to this so-called migration, and to retire to somewhat deeper and more peaceful regions, 
partly in their fully-developed state, partly in their later and more “ conscious ” 
embryonic stages, when, being independent of current -and wind, they begin to crawl 
about. The others, from being unable to seek these places of refuge, succumb in 
that “ Bellum omnium inter omnes ” which is going on especially at or near the 
shore. But in proportion as the remaining shore fauna increases and extends 
downwards, even these formerly peaceful places become* the field of a livelier com- 
petition in every respect, thereby necessitating a continual migration towards the greater 
depths. 
The faculty hereby gained by the animals of enduring an ever-increasing depth, and 
of accommodating themselves t6 its conditions, being transmitted by inheritance and 
accumulated from generation to generation, must become a matter of great importance. 
If we take, for instance, two such forms as are with certainty known to belong to the 
same species, the one living at a great depth, let us say 1000 to 2000 fathoms, the other, 
on the contrary, belonging to shallow water, e.g., Brissopsis, it appears to me impossible 
that the larvae of the form accustomed to live near the shore should be able to attain 
their full development and to settle in the great depth ; nor is the contrary case any 
more imaginable. The faculty of thriving and existing under such extremely peculiar 
conditions as are offered by the deep sea cannot be acquired at once. Indeed, we see 
every day examples of animals obliged to change climate and to alter their mode of life ; 
the parents sustain the new conditions only with difficulty, or even perish by their 
influence, but their immediate progeny possesses greater power of resistance, and their 
later descendants do so to yet a higher degree, until the animal becomes finally 
acclimatised. The larva of a marine invertebrate or the full-grown animal itself, 
accustomed to live at a pretty moderate depth, for instance 50 to 100 fathoms, can only 
with difficulty, and during a short time, endure to live in aa aquarium, however con- 
