406 W. H. DALL 
400 species might be expected. With a range between 60° and 70° 
(warm temperate) we should find about 500 species, and in the tropical 
zone (70° to 80° F.) not less than 600 species; and in specially fa- 
vorable localities of the tropics nearly twice as many. 
Learning from the characteristic genera what zone of temperature 
a given fauna may have belonged to, we can with confidence predict 
approximately the number of species which it will prove to contain 
when fully explored. Of course in a single locality where the char- 
acteristic situs is exclusively mud, or rock, or fine sand, only a certain 
proportion of the total fauna will be represented, but these minor 
groups are not entitled to the designation of a fauna as used in this 
paper. 
Relations of temperature to the jauna.—In considering the relations 
of temperature of the water to the fauna, account must be taken of the 
vertical differences. ‘The temperature of the water at the surface 
differs materially from that at the bottom in most regions, where the 
depth is over a few fathoms. Arctic or Antarctic species may extend 
in cold depths of ocean for thousands of miles; while, in the warm 
superficial strata above them and inshore from them, a totally different 
assembly lives and thrives. It is easy, in the case of widely diffused 
northern species, when deep water dredgings have revealed the distri- 
bution, to observe in the tables the boreal forms descending with the 
temperatures to deeper and deeper water as they approach the tropics. 
That this is so generally true is satisfactory evidence that the factor 
of pressure, being equalized by thorough permeation of the organism, 
is less effective in limiting distribution than most other factors. It 
seems incredible that the large eggs of abyssal mollusks can go through 
the processes of development under a pressure of tons to the square 
inch; since there must be a limit somewhere to the permeability of 
tissues. Still it is evident that they do. 
Why temperature should be so important in limiting distribution 
is a question which may be answered in several ways. Brooks has 
shown that, while the embryonic oysters (Ostrea virginica) are swim- 
ming at the surface of the sea, an entire brood may be destroyed to the 
last individual, by a fall in temperature of a few degrees, due toa cold 
rain. While it is not improbable that oysters from the northern part 
of the range of the species, say Nova Scotia, may have in the embry- 
