MAYER: STAUROCEPHALUS GREGARICUS. ) 
unless the water has been quiet for some days.” ‘ When the con- 
ditions are favorable, they come forth soon after dark and swim rapidly 
about at the surface, sometimes in almost incredible numbers.” 
It would probably be advantageous to any species of worm already 
possessed of some such egg-laying habits as those of Nereis to have the 
duration of the egg-laying period restricted to as short a time as pos- 
sible, and also to have it occur in that part of the year most favorable 
for the safety and development of the larvee. With equal numbers of 
mature individuals of two species (a) and (6), if (a) possess a long 
ege-laying period and (4) a short one, there will be more individuals 
of (6) discharging sperm or ova at any given moment than there will 
be of (a), whose breeding season is longer. Consequently the eggs of 
(6) will be more certain of fertilization, other things being equal, than 
those of (2). For example, if V represent the total number of individuals 
of species (@), and also of species (0) ; and if Z’ represent the duration 
of the egg-laying period of species (a) and ¢ that of species (4): then 
in any definite unit of time there will be oT individuals of species (a) 
, ie | eo 
discharging sperm and ova, while at the same time re individuals of 
species (>) will be engaged in the same act. Consequently, if the areas 
of the breeding-grounds of the two species are equal, there will be 
eV. 7, sede : 
[oes times as many individuals of species (4) discharging sperm 
or ova at any moment, in a unit of area, than there are of species (a) 
engaged in the same act. Then in an area containing m individuals of 
Species (a) there are a individuals of species (>). Therefore the 
t 
Vn — 1 
times as far as in the case of species (2). Hence the spermatozoze of 
fmt 
species (a) will be obliged to travel " ¢ times as far as those of 
/m — 1 
species (6). We see, then, that a shortening of the egg-laying season 
causes a greater concentration of breeding individuals, and therefore 
shortens the average distance that the spermatozoa must travel in order 
to fertilize the ova; and as spermatozoa cannot survive for any great 
length of time, this is an advantage to the species. In this connection 
average distance apart of the individuals of species (a) is 
