ZOOLOGY. 345 



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

 but of the varieties ranged under those types. It has been main- 

 tained 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 vari- 

 able from being dependent 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. 



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

 animal life ceases. To creatures inhabiting air, or water, a due 

 supply of this gas is indispensable. 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 inhab- 

 iting 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 can only extend to a limited 

 depth, unless, as has been assumed by some of our highest authorities, 

 the lower strata of sea-water, being subject to increased pressure, be- 

 come capable of holding in solution a greater quantity of oxygen, 

 and, by robbing the superincumbent strata of that which they con- 

 tain, gradually become saturated with it. Should this view be cor- 

 rect, there must be a point at which the maximum amount of oxygen 

 which sea-water can absorb is permanently present in it. But, inas- 

 much as the vegetable cell, simple though it be in structure, can 

 eliminate carbon from the medium in which it lives, it is not unrea- 

 sonable to assume that the lowest forms of animal life, even where no 

 specialized organs are traceable, may, in like manner, be able to elim- 

 inate oxygen directly from the water around them. 



The temperature of the sea is materially influenced by the climatic 

 conditions of different latitudes, and, of course, exercises a powerful 

 effect both on the distribution and abundance of the higher orders of 

 living beings present in its waters. But this influence is not so man- 

 ifest in the lower orders ; for, at greater depths, the variability of the 

 temperature is reduced within very narrow limits in all latitudes. 

 Now, the higher orders of oceanic creatures inhabit only the surface 

 waters, never sinking down to extreme depths. In the case of some 

 of the lower forms, on the other hand, a very extended bathymetrical 



