onsiderable numbers, of a species before unknown, by passing a voltaic
current through silicate of potash, and through nitrate of copper. The
existence of _entozoa_, or parasitic animals, found in the interior of
the bodies of other animals, and found nowhere else, is thought to
support the same doctrine. The question is, How came they there? Being
too large, either in their perfect form, or in the egg, to have passed
through the capillary blood-vessels, how came they within the body of
another animal,--itself but a few weeks or a few days old, or even in
the embryo stage,--unless they were created there without parentage of
their own species?

These facts and reasonings, it is true, only go to prove, that
animalcules, or beings of very small size, and low in the scale of
animated existence, can be produced in this way by the inherent
qualities of matter. No one will pretend, that a dog, a horse, or a man
can thus be created. How can we account for the existence of these
larger animals of a higher type, admitted to have been denizens of the
earth only since the latest geological epochs, and therefore of
comparatively recent origin? Here we come to another point in our
author's theory,--the transmutation of species, or the successive
_development_ of higher and higher orders of being out of the species
immediately below them, through the accidental or natural fulfilment of
certain conditions, in the course of a long period of years.

Natural history teaches us, that there is quite a regular gradation
among the several tribes of vegetables and animals; though we may not be
able to range all the species, as constantly advancing in a single line,
there is certainly the general appearance of a scale, beginning with the
most simple, and going on to the most complex forms. While the external
characteristics are very different, all are but variations of a single
plan, which exists as the basis of all, and is varied in each individual
only so as to accommodate it to the conditions under which the
individual is to live. The germ of a higher animal--a mammifer, for
instance--is the representative of a lower animal full-grown, like the
_volvox globator_; the latter remaining in this initial stage, as an
animalcule, through its whole existence; while the former is developed
out of it, by successive stages, into a quadruped, or even into a man.
Similar functions are performed in different animals by very different
organs, the gills