Horizontal Distribution — Swarms. 
367 
einmal an einer Stelle einige tausend Menschen zusammenstrb- 
men, so wird dadurch die Bewohnung noch nicht ungleichmassig. " 
(Hensen, ’95, p. 172.). 
I have placed at the head of this section Hensen’s words 
which seem to me to express with great clearness and 
wisdom the general truth regarding the still disputed question 
of the uniformity of the distribution of plankton animals. On no 
question relating to the plankton are opinions so widely at vari¬ 
ance, yet no question is more fundamental to the value of numer¬ 
ical work in investigation. For example, Wesenberg-Lund says 
(’96, p. 153) that plankton animals occur “saa godt som altid i 
Svaerme.” On the other hand Apstein says: (’96, p. 64) “Es 
ist bis jetzt nicht ein einziger wohl verbuergter Schwarm beo- 
bachtet worden. ” Thus in the same year opinions diametrically 
opposed are expressed, each based upon investigation. Under 
these circumstances the result of my work extending over two 
and a half years, including some 400 catches, each of which con¬ 
tained from 3 to 12 species, may contribute something to the 
discussion. 
It is not easy to define what is meant by a “swarm. ” No 
student of the plankton expects to find the plants and ani¬ 
mals distributed with absolute uniformity, and it is impossible 
to state the degree of variation in distribution which will en¬ 
title us to say that the species in question occurs in swarms. I 
agree with Apstein (’96, p. 53) that two- to fourfold variations 
are not to be counted as swarms. Apstein computes the actual dis¬ 
tance of individuals of Diaptomus when the numbers are about 
198,000 and 540,000 per square meter, and finds in the first case 
the average distance would be 2.2 mm. and in the second 1.36 
mm. He rightly states that such a difference in distance does 
not justify the name of swarm. Most will agree, I think, that 
a ten-fold difference in numbers will justify the statement that 
such species occur in swarms. Certainly animals whose number 
differ to that extent are very irregularly distributed, and if 
they were found in large numbers in compact areas, and the 
space between these areas was thinly populated, it would not 
be unfair to say that the species appears in swarms. 
In general, there is no evidence of swarms in my observations* 
