triking illustration of increasing
complexity.

In the lowest forms of animal life, we find general sensibility only,
and it is claimed that this exists in the lowest forms, without even the
presence of nerves. But as we rise higher in the scale, the special
organs of sense gradually become developed--one new sense after another
appears; but this is not the only line of increasing complexity. When an
organ of sense first appears, its function is of the simplest character;
and it is only when we reach the highest types of animal life that it
performs the greatest variety of offices peculiar thereto. That of touch
is, at first, but crude and simple, becoming delicate and complicated
only in the highest types. The sense of pain is a differentiated
function, possessed only in a slight degree by reptiles and fishes, and
probably not at all by animals still lower in the scale.

The eye-spots of star fishes and jelly fishes simply distinguish light
from darkness, much as we do with our eyes closed. There are many
degrees of development from this condition of the inferior organism to
that of the human eye, which distinguishes the nicest shades of color,
distance, form, and size of objects, and the play of passion on the
human countenance.

The same variety of function is acquired by the ear in its development
from its simplest to its most complex form. In the higher animals, the
organ of hearing is formed of three parts, an external, middle, and
internal portion; but in birds the external ear is wanting; in fishes
both the external and middle parts are wanting; in mollusks it is
reduced to a simple sack of microscopic dimensions, filled with a liquid
in which there are otolithes, or pebbly substances. Such an organ can
distinguish noises only; it can recognize nothing of the infinite
variety of articulations, notes, tones, melodies, harmonies of the human
voice and of musical instruments. There is even a great difference
between the disciplined, and therefore differentiated ear of a cultured
person, and the undisciplined, and therefore less differentiated ear of
a boor. Similar specializations of structure and function pertain to the
other senses; but we may pass them.

The digestive, circulatory, and respiratory systems, and all the other
systems of the animal structure, evince the same law.

The lowest form of the circulating fluid, as in sponges, is simply water
containing gases and organic particles; and this can scarc