DISTRIBUTION IN SPACE. 163 



of the rivDROinA by referring every species to some one or more of the bathymctrical zones of 

 Forbes ; and for all that regards the tidal shores of our own latitudes, down to a depth of about 

 one hundred fathoms, the zones of depth, as laid down by Forbes, will be found convenient. 



The recent deep-sea explorations, however, have shown how entirely Forbes's views must be 

 modified in all that regards the deeper regions of the sea. His zero of submarine life has not yet 

 been found, and has probably no existence, for there is no distance from the surface to which the 

 sounding line or dredge has reached — even though the enormous depth of more than two thousand 

 four hundred fathoms has been explored by them — where living beings are not now known to 

 dwell. 



It is plain, too, that exactly equivalent regions are not necessarily demonstrable in all seas, 

 and that zones situated at the same depth from the surface in different seas may present physical 

 characters so very different as to determine in each an entirely different fauna and flora. 



If this difference could be discovered from known data ; if, as in the case of the zones of 

 altitude in the distribution of terrestrial life, we could take the latitude as a factor, which, with 

 the distance from the sea level, would express the conditions which determine the peopling of 

 each zone, we should find the marine zones as full of interest and significance for the laws of dis- 

 tribution as we know to be the case with the zones of altitude. In the case of the sea, however, 

 so many disturbing operations come into play that general assertions can scarcely be ven- 

 tured on. 



The higher regions of the sea, as may be expected, participate largely on the effects of super- 

 ficial currents, and in local cliriiatic and tidal influences. We are not, however, to suppose that 

 the deeper regions are withdrawn from distiu'bing influences ; we now know that the distribution 

 of heat in the deeper sea regions is very irregular, being in many cases under the influence of local 

 conditions which cannot be determined by any a priori reasoning ; such, for instance, as the sub- 

 marine currents, which have determined the deep cold area demonstrated by the "Lightning" 

 and "Porcupine" explorers in the North Atlantic. 



It is such facts as these which greatly take away from the value of definite bathymctrical 

 zones as a form in which the submarine distribution of animals may be expressed. When applied 

 to locally limited faunas they are very convenient ; but in comparing the faunas of seas far apart, 

 we must be careful not to give them too much value, or regard as equivalent what have really 

 little relation with one another. 



Indeed, the recent investigations in the animal Kfe of the deep sea have only shown how very 

 far we yet are from a knowledge of the laws of the bathymctrical distribution of animals. A 

 number of facts of great interest have been accumulated, but these, so far from being in favour of 

 a definite distribution in depth of marine animals, only extend the known range of forms to wiiich 

 our earlier imperfect knowledge had assigned a much more confined and definite limit. 



There can be no doubt, however, that while many animal species have been now proved to 

 extend through almost all depths hitherto explored, there are some whose bathymctrical distribu- 

 tion is more limited, and every explorer of marine life knows that there are both shallow water 

 and deep water forms. Some species, indeed, may be allocated to very definite zones, and when 

 we assign to each zone the species which have hitherto been found in it, determining the few 

 which have not yet been met with beyond its limits, and indicate moreover certain relations 

 between the animal and vegetable life of the sea, we shall have said almost as much as our 

 knowledge of the distribution of animals in depth will justify. 



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