by respiration, yet you would not have us believe that
this is electricity, or that there is any reason to suppose that
electricity has a peculiar and special share in producing the functions
of life.

_The Unknown_.--I wish to guard you against the adoption of any
hypothesis on this recondite and abstruse subject. But however difficult
it may be to define the exact nature of respiration, yet the effect of it
and its connexions with the functions of the body are sufficiently
striking. By the action of air on the blood it is fitted for the
purposes of life, and from the moment that animation is marked by
sensation or volition, this function is performed, the punctum saliens in
the ovum seems to receive as it were the breath of life in the influence
of air. In the economy of the reproduction of the species of animals,
one of the most important circumstances is the aeration of the ovum, and
when this is not performed, from the blood of the mother as in the
mammalia by the placenta, there is a system for aerating as in the
oviparous reptiles or fishes, which enables the air freely to pass
through the receptacles in which the eggs are deposited, or the egg
itself is aerated out of the body through its coats or shell, and when
air is excluded, incubation or artificial heat has no effect. Fishes
which deposit their eggs in water that contains only a limited portion of
air, make combinations which would seem almost the result of scientific
knowledge or reason, though depending upon a more unerring principle,
their instinct for preserving their offspring. Those fishes that spawn
in spring or the beginning of summer and winch inhabit deep and still
waters, as the carp, bream, pike, tench, &c., deposit their eggs upon
aquatic vegetables, which by the influence of the solar light constantly
preserve the water in a state of aeration. The trout, salmon, hucho, and
others of the Salmo genus, which spawn in the beginning or end of winter,
and which inhabit rivers fed by cold and rapid streams which descend from
the mountains, deposit their eggs in shallows on heaps of gravel, as near
as possible to the source of the stream where the water is fully combined
with air; and to accomplish this purpose they travel for hundreds of
miles against the current, and leap over cataracts and dams: thus the
Salmo salar ascends by the Rhone and the Aar to the glaciers of
Switzerland, the hucho by the Danube, the Isar, and the Save, passing
thro