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 fed and overlooked by the mother. In birds we
have a still higher development of the same marked parental tendency;
only three or four eggs are laid each year, as a rule, and on these
eggs the mother sits, while both parents feed the callow nestlings
till such time as they are able to take care of themselves and pick
up their own living. Among mammals, which stand undoubtedly at the
head of created nature, the lower types, like mice and rabbits, have
frequent broods of many young at a time; but the more advanced groups,
such as the horses, cows, deer, and elephants, have usually one foal