et
that visits our system only once in five hundred years is controlled by
the same inflexible principle which causes the return of another comet
once in five years. The conditions requisite for a development more
perfect than usual,--that is, for the production of a new
species,--instead of a new individual of the same species, may be
fulfilled only at long intervals; but when they are fulfilled, the
result--the more perfect development--takes place as necessarily, as
much by the virtue of law, as the more ordinary phenomenon of the
propagation of one race. These conditions may be answered in the
successive stages of improvement, through which the earth and its
atmosphere pass, during the vast periods of time contemplated in
geology. In the era of the old red sand-stone, for instance, there were
no higher animals than fishes, because the atmosphere was highly charged
with carbonic acid, and could not support respiration by lungs. When the
air became purer, the gills were changed into the imperfect lungs of the
amphibious tribes, such as the huge saurians and the frogs. Deprive
these latter animals, in their lower stage, of all access to the light,
and they will not advance to their higher stage. Put a tadpole into a
perforated box, and sink it to the bottom of a river, and the animal
will never be perfected into a frog; he will grow to an enormous size,
but he will continue a tadpole.

We see, then, the process of an "organic creation by law," or by virtue
of the inherent qualities of inorganic matter. The ordinary chemical
affinities of different substances may draw them together into such
compounds as albumen and fibrin, which are the proximate principles of
organic tissues. The action of electricity, heat, light, or some other
mysterious imponderable agent, on these proximate principles, may
produce globules, or germinal vesicles. These germs, multiplying
themselves by fissiparous generation, will constitute a stock of animals
of a low type, such as a tribe of infusory animalcules. Then "this
simplest and most primitive type, under a law to which that of like
production is subordinate, gives birth to the type next above it, this
again produces the next higher, and so on to the very highest, the
stages of advance being in all cases very small,--namely, from one
species only to another; so that the phenomenon has always been of a
simple and modest character." Thus, the first reptile was born from a
fish, the first bi