300 ' Br, Wallich [Jan. 25, 



its bed, and its depth in any given locality ; the last of these items 

 necessarily involving the relative degrees of temperature, light, aeration, 

 and pressure, as compared with those to be met with near the surface. 

 Of these conditions climate exercises a very powerful influence ; for it is 

 found as we advance from the equator towards the poles that a gradual 

 diminution takes place, not only in the number of types met with, but 

 of the varieties ranged under those types. It has been maintained, that 

 in order to compensate for the diminution in the number of generic 

 forms, the number of individuals of each species is much augmented. 

 Although this law holds good as regards the higher orders, it can 

 hardly be said to do so in the case of the lower ; for the vast assemblages 

 of these lower forms met with on the surface of the sea in the tropics, 

 are in no wise less extensive than those met with in high latitudes. It 

 will be found that the lower the grade of being, the more equally 

 balanced will be its 'distribution, at the extremes of the globe ; inasmuch 

 as the greater range in depth commanded by these lower forms renders 

 them less amenable to conditions which are variable from being de- 

 pendent on atmospheric changes. 



The composition of the waters of the ocean is well known to become 

 much more equable at great depths ; and it therefore exercises a far 

 less marked influence on the presence of animal life than it does at 

 the surface. The same causes which equalize the temperature in so 

 remarkable a manner as the depth increases, are effective in equalizing 

 the relative proportions of the various ingredients that enter into the 

 composition of sea water, in all latitudes. For whilst the surface 

 stratum is subject to dilution with fresh water, from various sources, the 

 greater the depths the less subject can the waters be to this influence, 

 and the less can it operate in modifying the distribution of the organ- 

 isms that frequent them. 



Oxygen is essential to the presence of animal life — without it 

 animal life ceases. To air-breathing, as well as watfer-breathing 

 creatures a due supply of this gas is indispensable ; the function of 

 respiration, no matter whether performed by lungs, as in man and the 

 higher orders, or by a simple process of absorption and exudation 

 through the general surface of the body, as in some of the lower forms, 

 being, in every instance, essentially that process whereby oxygen is 

 received into the system in exchange for carbonic acid which is given 

 off. But although oxygen enters largely into the composition of both 

 atmospheric air and water, the supply of this element is not obtained, 

 in the case of creatures inhabiting the sea, under ordinary circumstances, 

 from its decomposition, but from a certain portion of atmospheric air 

 present in water in a state of solution. Most gases are absorbed by 

 water. Under pressure, the quantity absorbed is much increased, as 

 is seen in the familiar case of soda-water. It should be borne in 

 mind, however, when the fact is applied to the occurrence of animal 

 life at great depths in the sea, that in order to produce the absorption 

 of atmospheric air, its contact or mixing together at the surface by the 

 action of wind and wave is necessary, and the effect of this operation 



