variety
of shapes. Continued observation will reveal the fact that these little
creatures multiply; and a more careful scrutiny will enable him to see
_how_ they increase. Each divides into two equal parts so nearly alike
that they cannot be distinguished apart. In this case the process of
generation is simply the production of two similar individuals from
one.

A small quantity of slime taken from the surface of a stone near the
bottom of an old well, or on the seaside, when placed under the
microscope, will sometimes be found to contain large numbers of small,
round, living bodies. Careful watching will show that they also
multiply by division; but before the division occurs, two cells unite
to form one by a process called _conjugation_. Then, by the division
of this cell, instead of only two cells, a large number of small cells
are formed, each of which may be considered as a bud formed upon the
body of the parent cell and then separated from it to become by growth
an individual like its parent, and, like it, to produce its kind. In
this case, we have new individuals formed by the union of two
individuals which are to all appearance entirely similar in every
particular.

Sex.--Rising higher in the scale of being, we find that, with rare
exceptions, reproduction is the result of the union of two dissimilar
elements. These elements do not, in higher organisms, as in lower forms
of life, constitute the individuals, but are produced by them; and being
unlike, they are produced by special organs, each adapted to the
formation of one kind of elements. The two classes of organs usually
exist in separate individuals, thus giving rise to distinctions of
_sex_; an individual possessing organs which form one kind of elements
being called a male, and one possessing organs for the formation of
the other kind of elements, a female. The sexual differences between
individuals of the same species are not, however, confined to the sexual
organs. In most classes of plants and animals, other sexual differences
are very great. In some of the lower orders of animals, and in many
species of plants, the male and female individuals are so much unlike
that for a long time after they were well known, no sexual relation
was discovered.

Hermaphrodism.--An individual possessing both male and female organs
of reproduction is called an _hermaphrodite_. Such a combination is
very rare among higher animals; but it is by no means uncommon among