1912] Esterly: Copepoda of the San Diego Region 331 
encounters, during vertical migrations, a range from 7° or less 
to 20° or more. The same sort of statement applies to salinity. 
At the present stage of our investigation (so far as the 
Copepoda are concerned) it seems that differences in temperature 
and salinity must be either more considerable or more abrupt 
than we have yet found, if we are to be able to speak of ‘‘copepod 
” 
provinees’’ in this region. On the other hand, changes in lght 
intensity are periodic as well as gradual, and it seems that those 
are the changes which influence the distribution of the copepods. 
We cannot claim that the temperature changes with the time of 
day (to any important extent, at least) and I feel more strongly 
than ever that light is the main determining factor in the vertical 
distribution of these organisms. It is only too plain that this 
is not the whole of the matter, but it is certain that, through 
the widespread occurrence of day and night oscillations, we have 
recognized a factor that is important. In view of our results 
at this station statements as to the depth limits of copepods 
should be based upon day or night hauls separately. That is, 
if a species is not found above 100 fathoms during an expedition 
when collecting at that level is done only during the day, it 
does not follow that the usual hydrographic conditions at or 
below that level are the optimum conditions for the species. The 
same form would probably be found at night above 100 fathoms 
where the hydrographie conditions are almost certain to be 
different from those met with during the day hauling. 
The foregoing discussion should be regarded as more in the 
line of suggestion than exposition. It is based entirely on the 
results of studying collections as embodied in this paper. and is 
not intended as a criticism of the views of others. That would 
be warranted only when our collections are much more numerous 
and especially when we know the sub-surface temperature and 
salinity conditions under which a large series of hauls with the 
Kofoid horizontal closing net were made. 
Our results hardly warrant more than the mention of their 
apparent bearing on the question of coincident distribution of 
related species. The hauls, especially with the Kofoid net, in 
which ‘‘couplets’’ of species occurred are so few that I can cite 
only a few instances (which are notorious for proving anything). 
