196 FISH-CULTURAL ASSOCIATION. 
kept in a similar pouch under the tail, as in the male sea-horse 
(Hippocampus). It seems as if the number of eggs was diminished 
just in proportion to the amount of care taken—usually by the 
male—of the embryos. This, of course, indicates that in the case 
of eggs which are not protected in the way mentioned, millions 
of surplus ova are destroyed in the struggle for existence, where- 
as with the species which protect their ova, the struggle for ex- 
istence at the commencement of development must be much less 
severe. 
There are other points to be noticed. Some species have very 
small ova. Such are usually hatched in a condition in which 
the little fish is much feebler than in those cases in which the 
ova are large, and in which the young fish leave the egg in.a 
much more vigorous conditon—in a condition, in short, in which 
they are able to contend with the environment more effectually. 
That is an important fact to be considered. Again, there are 
some species which leave the egg with the throat perforated, and 
other forms which do not. In the case of the shad, for instance, 
the young fish cannot swallow at the time of hatching, but in 
other forms the young can swallow as soon as they leave the egg 
membrane. There are still other causes which would affect the 
percentage of survivals, such as changes in their habitat pro- 
duced by man, or the pollution of a river by substances which 
sink into its ooze, and so vitiate the water and thus render incu- 
bation on the bottom impossible. 
There are also forms in which there are protective contriv- 
ances developed on the eggs themselves. We are all familiar 
with gelatinous strings that we find in stagnant ponds and 
which enclose the eggs of the toad, for example. Most of the 
various kinds of frogs have a different kind of spawn, adhering 
together in masses instead of in strings. Certain fish-ova, again, 
have long thread-like appendages, by which they are suspended 
on weeds and grass, so that the currents of sea-water can pass 
backward and forward among them, erating them and prevent- 
ing them from being smothered. This isthe case witha number 
of marine, and some few fresh-water forms of fishes. There are 
cases where mimicry doubtless plays a part in preventing the 
capture of young fish, as in the case of the young of the stickle- 
