light, the composition of the air, the
distribution of water, oceanic and atmospheric currents, and other such
agencies remain unaltered; but if any one of these, or of a hundred
other incidents that might be mentioned, should suffer modification, in
an instant the fanciful doctrine of the immutability of species would be
brought to its true value. The organic world appears to be in repose,
because natural influences have reached an equilibrium. A marble may
remain for ever motionless upon a level table; but let the surface be a
little inclined, and the marble will quickly run off. What should we say
of him, who, contemplating it in its state of rest, asserted that it was
impossible for it ever to move?

[Sidenote: Orderly sequence of conditions is followed by orderly organic
changes.]

They who can see no difference between the race-horse and the Shetland
pony, the bantam and the Shanghai fowl, the greyhound and the poodle
dog, who altogether deny that impressions can be made on species, and
see in the long succession of extinct forms, the ancient existence of
which they must acknowledge, the evidences of a continuous and creative
intervention, forget that mundane effects observe definite sequences,
event following event in the necessity of the case, and thus
constituting a chain, each link of which hangs on a preceding, and holds
a succeeding one. Physical influences thus following one another, and
bearing to each other the inter-relation of cause and effect, stand in
their totality to the whole organic world as causes, it representing the
effect, and the order of succession existing among them is perpetuated
or embodied in it. Thus, in those ancient times to which we have
referred, the sunlight acting on the leaves of plants disturbed the
chemical constitution of the atmosphere, gave rise to the accumulation
of a more energetic element therein, diminished the mechanical pressure,
and changed the rate of evaporation from the sea, a series of events
following one another so necessarily that we foresee their order, and,
in their turn, making an impression on the vegetable and animal economy.
The natural influences, thus varying in an orderly way, controlled
botanical events, and made them change correspondingly. The orderly
procedure of the one must be imitated in the orderly procedure of the
other. And the same holds good in the animal kingdom; the recognized
variation in the material conditions is copied in the organ