X 
IOÏ2 Zoic Maxima, or Periods of [ December, 
this has an important influence in securing a great or small 
representation of the marine animals in modern seas is un- 
questioned, though it is always, of course, a variable function of 
the more or less rapid-and safe methods nature uses for their 
multiplication. Dr. Brooks says :! “ The most critical time in the 
life of the American oyster is undoubtedly the time when the 
egg is discharged into the water to be fertilized, for the chance 
that each egg which floats out into the ocean to shift for itself 
will immediately meet with a male cell, is very slight, and it is 
essential that the egg should be fertilized very quickly, for the 
unfertilized egg is destroyed by the sea water in a very short 
time.” 
Tryon has suggested that the swimming species of Cephalo- 
poda may experience some difficulty in effecting sexual union,” 
and the observations of Steenstrup upon the many different ways 
adopted in this group of Mollusca for fertilization, justify the in- 
ference that under unfavorable circumstances individuals of the 
same group may not encounter each other, and the chances for 
the fruitage of the same genera be diminished in exact ratio to 
the opposite plan pursued by its congeners for their fecundation. 
With some of the gastropods sexual union is effected directly, 
and no danger is incurred from the exigencies of the unprotected 
female ovum searching for the spermatic vesicles in the water. 
This establishes a safeguard which favors the multiplication of 
those prosobranchiates which possess it, but in other groupe 
(Trochus, Scutibranchs, Cyclobranchs) the male elements are dis- 
charged into the water, and are then taken into the uterus. This 
introduces a risk which must increase or lessen according to the 
presence or absence of predatory fishes who devour the spat, the 
favorable stillness of the water or its temperature, or chemical 
condition, which if abnormal would destroy the germs. 
? Bull. U. S. National Mus., No. 27, p. 210. 
anual of Conchology, Vol. 1, p. 42 
stun ‘Soc. Nat, Hist., Vol. XVIII, p. 284. 
