ely be spoken
of as circulating, for it is merely drawn in and then expelled. A little
higher in the scale naturalists find a 'chylaqueous fluid,' which
oscillates in the general cavity of the sack-like animal. The true blood
is another step in development; and even this organized fluid changes
its character as the scale advances. Most animals have no heart; and
when the organ does first appear, it is but a simple, rudimentary
structure, very unlike the complex machine which plays at the centre of
circulation in the higher types.

Though fishes breathe through their gills, receiving all the oxygen they
require from the small amount of air in the water, the swimming bladder
is in them the rudimentary lung--a very simple structure, indeed, when
compared with the more complex arrangement for respiration in the higher
animals.

Some animals of gelatinous, and therefore flexible structure, perform
digestion by folding their bodies over the food, and pressing the
nutritious matter out of it: they extemporize a stomach for the
occasion. And even in some of their higher types, in such as have a
permanent mouth and stomach, the digestive process is simply a squeezing
out of the elements of nutrition. The digestive apparatus, from being a
simple sack in the polype and similar organisms, becomes, by a
continuous unfolding, the complicated structure which we find in the
higher animals, with various organs effecting various parts of the
digestive change, and even different parts of the same organ having
specialized functions to perform.

The most complex animal proceeds originally from a simple cell; and 'at
the two extremes we may contemplate the single germinal membrane of the
ovum, which is discharging contemporaneously every function--digesting,
absorbing, respiring, etc.; and the complete organic apparatus of man,
the stomach, the lungs, the skin, the kidneys, and the liver--mechanisms
set apart each for the discharge of a special duty, yet each having
arisen, as we know positively from watching the order of their
development, from that simple germinal membrane.' (Draper.) This is what
one physiologist says of the ovum which is being developed into a
complex being. Here is what another says of animals at the lower end of
the zoological scale: 'The simplest organisms breathe, exhale, secrete,
absorb, and reproduce, by their envelopes alone.' (Lewes.) Here we
perceive the resemblance between the ovum of the higher animals and