180 BULLETIN OF THE 
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 
mollusk, since it has been shown by Brooks and Ryder 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 
