1386. ] Numerical variations in Animals. 1013 
to maintain the most complete- generative activity, though this 
very contraction of size might diminish vitality, disarrange the 
seminal function and result in sterility. 
The orders of Cladocera, Copepoda and Ostracoda will, in this 
respect, be favorably placed in comparison with other crustacean 
groups, as in these tribes fertilization of the female lasts during 
her lifetime, or at least a season, and in some cases the young 
females are born fertilized from the impregnated mother. Thus 
favorable conditions with these groups gain a predominating in-' 
fluence, as they fortify and assist an already preéxistent advan- 
tageous arrangement for security of fecundation. 
Amongst fishes advantage will be given to those whose spawn- 
ing season is most extended, as with cod, with which, according 
to the observations of Professor Sars, it extends over nine con- 
secutive months, “a period éxceeding that required by any other 
Species of which we have any knowledge.”! These are not only 
more likely, with equal vitality, to produce a larger number of in- 
dividuals, but they are absolutely favored, by the greater extension 
of time, to escape variable inimical circumstances, which latter, 
being limited in duration, might, if coincident with the shorter 
period of other fishes, greatly impair the prospects of successful 
fecundation. The longer period of the cod renders its partial or 
entire escape from such disasters more probable. The spawning 
season for an individual of shad, salmon, or white fish is only a 
few days. But the likelihood of impregnation seems to be dimin- 
ished on spawning grounds where strong currents are found, or 
during storms, as imense numbers of the eggs are driven on 
the shores, or are so diffused and distributed as not to meet the 
milt-of the male, and as the egg of the cod quickly loses its 
Vitality, great numbers perish. . Under very favorable circum- 
Stances such conditions for impregnation might prevail as would 
result in an enormous excess of individuals produced, whilst an 
Opposite state of affairs would reduce the production to a mini- 
mum. Again, an insufficient supply of males would greatly mod- 
ify the results of fecundation, as the extraordinary fertility in 
eggs of the female amongst fish necessitates the presence of sev- 
eral males to accomplish their fertilization. Eggs of fish which 
are of such a gravity as not to rise to the surface, unlike those of 
the cod and mackerel, come less in contact with the destructive 
R Bull. U. S. Fish Commission, Part v. 
