180 BULLETIN OF TflE 



shall find in shallow water of the appropriate temperature the species 

 in question. Drawing a line from Hatteras to Madeira, and consider- 

 ing the species dredged from the Atlantic Ocean north of this line, by 

 all expeditions up to 1883, in water more than one thousand fathoms 

 deep, we find that more than forty-two per cent of all the species of 

 mollusks are found somewhere or other living in water less than one 

 hundred fathoms deep. If we knew the littoral fauna of the tropics 

 better, it is probable that the percentage would be much increased. A 

 similar result has followed the study of the Blake collections, though 

 the exact figures are not ready to be given. 



if, on the other hand, we consider the larger groups, such as genera 

 or families of mollusks, we shall find that the percentage of those pecu- 

 liar to the archibenthal and abyssal regions is extremely small, though 

 future researches are likely to enlarge it. We must regard the species 

 which have extended their range so far beyond their littoral area of 

 origin as having taken advantage of the uniform conditions of food and 

 temperature offered by the deep sea. In this connection, it should 

 be observed that the temperature limits of many species are more 

 sharply defined on the side of cold than on that of heat. The differ- 

 ence between 45° and 40° F. may absolutely check the distribution of 

 a species which would find no inconvenience in a rise of temperature 

 from 45° to 80°. It is probable that this is connected with the de- 

 velopment of the young, rather than the resisting powers of the adult 

 tnollusk, since it has been shown by Brooks and Eyder that a fall of 

 a very few degrees in temperature of the water was fatal to all the 

 floating embryos of the American oyster. A much greater rise would 

 probably only have hastened the development of the embryos. 



It is quite within the limits of probability that archibenthal species 

 might rise to the littoral zone in some far distant locality, and by a 

 change in the direction or temperature of an ocean current all the in- 

 tervening deep-water individuals might perish, leaving two widely sepa- 

 rated colonies of the same littoral species. The weight of probability, 

 however, is greatly in favor of the continuous uniformity of the deep 

 sea as compared with the shores, and it is probable that they are ma- 

 terially modified only by physical changes of great importance, such 

 as raised the Isthmus of Panama above the sea. 



On the steep slopes above referred to, the currents bring a great 

 variety and amount of material, which sinks to the bottom and furnishes 

 food or protection to the creatures which live there. Often the most 

 diverse elements enter into the accumulations. In one haul made by 



