1912] Esterly: Copepoda of the San Diego Region 329 
two kinds of copepods so far as can be determined, have the same 
behavior, as is shown by the fact that the curves in figures 1—7 
are all different. While the amount of data is small in some eases, 
it shows that a species may be recognized by its behavior as well 
as by its structure. We do not know enough about the conditions 
under which these forms live to be able to state, as Hjort does 
in the passage mentioned, that each has its own area of distribu- 
tion, but we can say that the character of a species is shown by 
something else than its morphology. 
There is another point that should be mentioned here. If we 
start with the species as defined by structural characters, can it 
be shown that, on such a basis, the range of the form is limited 
by definite hydrographic conditions? In other words, is there 
any evidence that each species has its own area of distribution 
or not? 
There is some tendency to argue that if a certain form is 
found within a given area we may expect to find morphological 
characters that distinguish these forms from others. For example, 
Nordgaard (1905, p. 243) cites instances of the division of a species 
into two or more because of ‘‘more exact morphological investiga- 
tion’’ (the morphological investigation apparently being subse- 
quent to hydrographie studies) so that ‘‘instead of one species 
distributed over nearly every sea we now get three species with 
> 
a comparatively limited distribution.’’ The species referred to 
in the quotation is Calanus finmarchicus, and I have already men- 
tioned that there is some difference of opinion regarding the 
validity, on morphological grounds, of one of its derivatives. The 
question here, however, is whether or not within such an area 
as that under investigation by this station each species, or even 
a group of species, may be said to have a physical environment 
of its own. 
So far as the study of the copepods in the San Diego region 
has gone, there is no evidence that they form ‘‘hydrographic’ 
species. Calanus finmarchicus has been taken at the surface in 
’ 
as great numbers when the temperature is as low as it gets, as at 
the highest temperatures and those between the extremes. This 
species, like all the others, has a plurimum or center of migration 
for the day and also for the night, but there is no reason to 
