1912] Ritter: The Marine Biological Station of San Diego 195 
occur in the area, would give a very imperfect picture of the 
population as it actually exists. Thus of the nearly 79,000 
specimens collected during the five-year period, 1904-1909, 
covered by his report, about 51,600 were of one species, Sagitta 
bipunctata. Over 10,000 of the remaining 17,400 belonged to one 
other species, S. enflata, and nearly half of the balance to another, 
S. serratodentata. One species, S. draco, was represented by a 
single specimen. 
Thus it appears that much the same rule prevails respecting 
abundance of different kinds in this group of oceanic organisms 
as that with which we are familiar in many groups of land plants 
and animals. By far the larger part of all the grass in almost 
any naturally grass-grown region will be of one species, and so 
with forested regions. Most of the trees belong to one or to a 
very few species, though several species may be represented in 
comparatively small numbers. 
So far as the evidence goes, it indicates that none of these 
species lives chiefly on the surface of the sea; that all make 
bi-daily excursions, more or less extensive, up and down: that 
each has, within rather wide limits, its own most favored stratum 
or “‘center of migration’’ as Mr. Michaels terms it; and that this 
center and the movements from it depend upon several factors. 
Thus S. bipunctata occurs most abundantly on the surface twice 
each day, namely within an hour after sunrise, and within an 
hour after sunset. From fifteen to twenty fathoms seems to be 
the most favorable depth for this species. The evidence tends to 
show that its movements are influenced by light, by temperature, 
and probably by density of the water. Since these three are the 
only environmental factors considered in the investigation, it is 
not known whether others are operative. Contrary to what might 
have been expected, the data do not reveal correlation between 
abundance and different seasons of the year. 
The group in which the next best headway has been made in 
finding how abundant the different kinds are, and how and where 
the creatures pass their time, is the copepods or ‘‘oar-footed”’ 
crustaceans. Dr. Esterly has published two preliminary papers 
on this phase of his work, and has a third, much more extensive, 
nearly ready for the press. 
