at the same
life-giving force is concerned in the production of specific and generic
forms.

[Illustration: 185. Female Stylops.]

Who can explain the origin of the sexes? What is the cause that
determines that one individual in a brood of Stylops, for example (Fig.
184, male; Fig. 185, grub-like female in the body of its host), shall be
but a grub, living as a parasite in the body of its host, while its
fellow shall be winged and as free in its actions as the most highly
organized insect? It is no less mysterious, because it daily occurs
before our eyes. So perhaps none the less mysterious, and no more
discordant with known natural laws may the law that governs the origin
of species seem to those who come after us. Certainly the present
attempts to discover that law, however fatuitous they may seem to many,
are neither illogical, nor, judging by the impetus already given to
biology, or the science of life, labor altogether spent in vain. The
theory of evolution is a powerful tool, when judiciously used, that must
eventually wrest many a secret from the grasp of nature.

But whether true or unproved, the theory of evolution in some shape has
actually been adopted by the large proportion of naturalists, who find
it indispensable in their researches, and it will be used until found
inadequate to explain facts. Notwithstanding the present distrust, and
even fear, with which it is received by many, we doubt not but that in
comparatively few years all will acknowledge that the theory of
evolution will be to biology what the nebular hypothesis is to geology,
or the atomic theory is to chemistry. While the evolution theory is as
yet imperfect, and many objections, some seemingly insuperable, can be
raised against it, it should be borne in mind that the nebular
hypothesis is still comparatively crude and unsatisfactory, though
indispensable as a working theory to the geologist; and in chemistry,
though the atomic theory may not be satisfactorily demonstrated to some
minds until an atom is actually brought to sight, it is yet invaluable
in research.

Many short sighted persons complain that such a theory sets in the
back-ground the idea of a personal Creator; but minds no less devout,
and perhaps a trifle more thoughtful, see the hand of a Creator not less
in the evolution of plants and animals from preexistent forms, through
natural laws, than in the evolution of a summer's shower, through the
laws discovered by the meteor