330 University of California Publications in Zoology  [Vou.9 
claim that a given temperature or salinity of the region most 
frequented during the day is any more of an optimum (so far as 
those physical factors are concerned) than is the temperature of 
the region of most abundance during the night. In other words, 
a species like Calanus finmarchicus, in its extensive diurnal 
migrations probably passes back and forth through as many 
different physical conditions as would be encountered in a hori- 
zontal journey of many miles at a given level. 
As our knowledge of the temperature and salinity conditions 
of the waters of this region inereases, it may be that we shall 
find that there are regions for certain species with bounds that - 
are definite, within which the species may be said to be isolated. 
It seems to me, however, that the body of facts presented here 
concerning vertical migrations is no small indication that the 
boundaries are not temperature or salinity lines, even if it shall 
be found that boundaries of some kind actually exist. 
It is true that maps have been made showing the distribution 
of certain species of copepods. Damas (1905) has done this for 
the larvae and adults of Calanus finmarchicus and Farran (1910) 
for a number of species. Both these investigators have dealt 
with region which are affected by great ocean currents, where 
there are, accordingly, barriers due to abrupt changes in tem- 
perature. It is possible, therefore, to delimit the regions occupied 
by given species more or less sharply. The San Diego region, 
however, seems not to present such conditions, and, as far as we 
know, changes in temperature and salinity are gradual. Six 
species of copepods were never taken at the surface, but there 
is no evidence that they avoid the surface because of the tem- 
peratures or salinity there. We know that each of these species 
appears to execute diurnal migrations, and in so doing it is 
presumable that they encounter changes of temperature and 
salinity. We do not know how great the changes are. It must 
be admitted that there are temperatures or salinities which would 
act as barriers, but so far as our investigations have gone they 
do not seem to oecur in this region. The probable reason that 
temperature has no apparent effect in such a case as that of 
Calanus is that a variation of from 16°-20° at the surface is 
too small to cause an appreciable reaction when an organism 
