at were unknown before the discovery of this wonderful aid to human
vision,--whole tribes of living organisms, each of which, though
insignificant in size, possesses organs as perfect and as useful to
it in its sphere as do animals of greater magnitude. Under a powerful
magnifying glass, a drop of water from a stagnant pool is found to be
peopled with curious animated forms; slime from a damp rock, or a speck
of green scum from the surface of a pond, presents a museum of living
wonders. Through this instrument the student of nature learns that life
in its lowest form is represented by a mere atom of living matter, an
insignificant speck of trembling jelly, transparent and structureless,
having no organs of locomotion, yet able to move in any direction; no
nerves or organs of sense, yet possessing a high degree of sensibility;
no mouth, teeth, nor organs of digestion, yet capable of taking food,
growing, developing, producing other individuals like itself, becoming
aged, infirm, and dying,--such is the life history of a living creature
at the lower extreme of the scale of animated being. As we rise higher
in the scale, we find similar little atoms of life associated together
in a single individual, each doing its proper share of the work
necessary to maintain the life of the individual as a whole, yet
retaining at the same time its own individual life.

As we ascend to still higher forms, we find this association of minute
living creatures resulting in the production of forms of increasing
complicity. As the structure of the individual becomes more complex
and its functions more varied, the greater is the number of separate,
yet associated, organisms to do the work.

In man, at the very summit of the scale of animate existence, we find
the most delicate and wonderfully intricate living mechanism of all.
In him, as in lower, intermediate forms of life, the life of the
individual is but a summary of the lives of all numberless minute
organisms of which his body is composed. The individual life is but
the aggregate life of all the millions of distinct individuals which
are associated together in the human organism.

Animals and Vegetables.--The first classification of living creatures
separates them into two great kingdoms, animals and vegetables.
Although it is very easy to define the general characteristics of each
of these classes, it is impossible to fix upon any single peculiarity
which will be applicable in every ca