ne ever came to maturity,
there would still be a hundred thousand codfish in the sea this year
for every pair that swam in it last year: and these would increase to
a hundred thousand times that number next year; and so on, till in
four or five years' time the whole sea would be but one solid mass of
closely-packed cod-banks. We can see for ourselves that nothing of the
sort actually occurs--practically speaking, there are about the same
number of cod one year as another. In spite of this enormous
birth-rate, therefore, the cod population is not increasing--it is at
a standstill. What does that imply? Why, that taking one brood and one
year with another, only a pair of cod, roughly speaking, survive to
maturity out of each eight or nine million eggs. The mother cod lays
its millions, in order that two may arrive at the period of spawning.
All the rest get devoured as eggs, or snapped up as young fry, or else
die of starvation, or are otherwise unaccounted for. It seems to us a
wasteful way of replenishing the earth: but it is nature's way; we can
only bow respectfully to her final decision.

Frogs and other amphibians stand higher in the scale of life than
fish; they have acquired legs in place of fins, and lungs instead of
gills; they can hop about on shore with perfect freedom. Now, frogs
still produce a great deal of spawn, as every one knows: but the eggs
in each brood are numbered in their case by hundreds, or at most by a
thousand or two, not by millions as with many fishes. The spawn
hatches out as a rule in ponds, and we have all seen the little black
tadpoles crowding the edges of the water in such innumerable masses
that one would suppose the frogs to be developed from them must cover
the length and breadth of England. Yet what becomes of them all?
Hundreds are destroyed in the early tadpole stage--eaten up or
starved, or crowded out for want of air and space and water: a few
alone survive or develop four legs, and absorb their tails and hop on
shore as tiny froglings. Even then the massacre of the innocents
continues. Only a tithe of those which succeeded in quitting their
native pond ever return to it full grown, to spawn in due time, and
become the parents of further generations.

Lizards and other reptiles make an obvious advance on the frog type;
they lay relatively few eggs, but they begin to care for their young.
The family is not here abandoned at birth, as among frogs, but is
frequently tended and f