588 SUMMARY OF CURRENT RESEARCHES RELATING TO 



unrecognized fact that a fair proportion (say 20 per cent, in the 

 present case) have a vertical range which extends from the true 

 littoral region (less than 50 fathoms) to the depths of the abyssal 

 region (250 to 2000 fathoms), unlimited by temperatures actually 

 encountered. 



(2) Of the species with great vertical range (from less than 100 

 to more than 500 fathoms), the smallest part (10 per cent.) are of 

 groups which have been regarded as belonging to or characteristic of 

 the shores of cold or boreal areas. The nest larger part (20 per 

 cent.) belong to groujjs hitherto considered characteristic of shoaler 

 warm or tropical waters, while more than 60 per cent, belong to 

 groups not especially characteristic of the littorale of either region. 



(3) Of the species found in the abyssal fauna without regard to 

 their vertical range above it, 10 per cent, may be termed boreal, 

 13 per cent, tropical, and more than 75 per cent, uncharacteristic forms. 



(4) Since the tropical forms belong to the same groups as those 

 characteristic of the local littoral mollusc fauna, it is eminently 

 probable that the abyssal regions have local faunae proper to their 

 various portions, and that a universal exclusive abyssal fauna, so far 

 as molluscs are concerned, does not exist. This must be qualified by 

 the admission of the existence in the abysses (as well as on the 

 littorale) of ubiquitous species-forms ; which, however, do not form a 

 universal abyssal fauna any more than Mytilus edulis, Saxicava rugosa, 

 and Poronia rubra, form a universal littoral fauna. The local nature 

 of diiferent portions of the abyssal fauna is also confirmed by the 

 distinctness of the ' Challenger ' molluscs from those of the ' Blake,' 

 but a very small number appearing identical as far as a cursory 

 examination could determine. 



There can be no doubt that the uniformity of generally low 

 temperatures (and consequently of food) affords special facilities for 

 the wide distribution of boreal forms through the abyssal region. 

 But where adjacent shores can (by washing and sinking) afford a 

 diiferent or greater variety of food without too excessive temperatures, 

 local abyssal faunae will probably always be developed, and with 

 characteristics assimilated to those of the littoral fauna of the same 

 part of the earth's surface. The present collection shows conclusively 

 that a difference in pressure of some 120 atmospheres and in tempera- 

 ture of 41 • 5° has been sustained by different individuals of the same 

 species without perceptible change in the external appearance of their 

 hard parts or shells, 



(5) The specific characters of many of the strictly abyssal species 

 appear to exhibit a very remarkable degree of variation within 

 supposed specific limits, although it would seem as if the conditions 

 under which they live must be remarkably uniform. This would 

 indicate that the tendency to variation is less dependent upon change 

 in the existing environment than has generally been assumed. 



The total number of littoral species recorded by Adams and 

 D'Orbigny, throwing out those groups, like the Pteropods, not germane 

 to the inquiry, is 580, as compared with 461 collected by the ' Blake.' 

 The number of genera represented by the former is about 110, while 



