tae eg S 2 hry ot hd be ea ng ee 
4 
1886.] Numerical variations in Animals. 1015 
tributed it to the variable temperature in the latter locality. Rate 
of growth as it is rapid or slow, continuous or periodic, will affect 
the numerical display of a species. Thus Pecten irradians grows 
very quickly, stops in winter, beginning again when the scallops 
are one year old, and on the whole this irregularity may be re- 
garded as tending to diminish numbers. Power of locomotion 
again assists its possessors to escape from unfavorable surround- 
ings or from enemies. Swarming or migratory habits, as with 
lobsters, affect the numerical proportion of the species at a given 
point in certain seasons, but probably has little influence on the 
fertility or abundance of individuals. Those animals, as crusta- 
ceans, which cast their integuments, are exposed to accidents 
during their exposed period, and should they then be subjected 
to especially destructive influences would suffer great numerical 
depletion. 
III. Character of habitat in relation to bottom, temperature, 
depth, isolation, salinity and supply of food—The overwhelming 
importance of these very variable factors upon the numerical ex- 
hibit of a species is most evident, and has, from many points of 
view, been emphasized by naturalists. Thus the nature of the 
bottom exercises a predisposing selective influence upon mollus- 
cous distribution. It is well known that oysters are killed in the 
mud, that the round clam affects sandy and muddy shores, the 
edible muscle flourishes in a variety of positions and surround- 
ings, that the Purpura loves rocky headlands, and so on indefi- 
nitely, Thc temperature of the water exercises an accelerating 
or retarding influence upon the growth and spawning of both 
shells and crustaceans according to their nature in this respect, 
and the increase or decrease of heat. In the matter of living at 
different depths, animals vary extremely, and the sudden settling 
of a shore or even rapid secular change would tend to destroy 
the classes of shallow-water loving organisms. Isolation permits 
close interbreeding, subjects the species to more uniform condi- 
tions, and if it diminishes the volume of water seriously modifies 
-the size, as shown with Lymnza by Semper and Hilger, while of 
course it induces peculiarities of local development. Salinity 
varies in sea water and enclosed areas of the ocean, and distinctly 
modifies the abundance of animal life. The character of the 
— Supply of food and its abundance is an obvious element of great 
importance in the production of sea animals. 
