1918 | Packard: Molluscan Fauna from San Francisco Bay 227 
processes for successive generations, there should be a definite relation- 
ship between its structures and certain factors in its optimum habitat. 
Such codrdinations as these ought to be revealed through a detailed 
study of the distribution of the animal. But such adjustments of the 
organism to habitat might vary according to what has been termed 
the plasticity of species toward changes, and, in addition, to other 
causes, according to the length of time in which the species has been 
subjected to the present conditions. Not all of the species listed in 
this paper are specifically of the same age, as is indicated by their 
palaeontologic histories. Therefore it is improbable that all of them, 
or eyen a majority of them, have become specialized in the same de- 
gree. Thus all would not be so nearly adjusted to a particular habitat 
that their spatial distribution would be a function of any one type 
of environment. It is quite unlikely, then, that the distribution pat- 
terns, such as shown on plates 42 to 59, of all of the species could be 
interpreted with the same facility. On the other hand, it is not sur- 
prising to note that certain species have become, for some reason, so 
specialized that they now appear to be distributed according to one 
or more factors of their environment. Of course, certain other hy- 
potheses bearing upon speciation and dispersal would not all yield 
these conclusions. 
The separation of a single factor from the diversified environment 
and the determination of its effects upon an organism is attended with 
difficulties even to the biologist who can bring his subjects into the 
laboratory under controlled conditions. The student of marine Mol- 
lusea seldom controls the environments with which he deals. His 
conclusions as to the importance of a factor are obtained by indirect 
methods. They involve the elimination of certain important factors 
and the studying of the effects of those that remain. 
Some of these factors governing distribution are: the depth of the 
water; temperature; salinity; character of the bottom; the food sup- 
ply, and other biological factors, including organisms which may not 
be beneficial to the species under consideration. 
INFLUENCE OF DEPTH 
The influence of depth upon a marine organism is a result of the 
interaction of several factors, including pressure, temperature, light, 
access to food supply, and gaseous content of the water. The pressure 
at the deepest part of the Golden Gate exceeds that of ten atmospheres. 
It is improbable that this interferes greatly with the molluscan life, 
