, it was perceived how the disappearance of vanishing forms
from the surface of the globe is accomplished--that it is not by a
sudden and grand providential intervention--that there is no visible
putting forth of the Omnipotent hand, but slowly and silently, yet
surely, the ordinary laws of Nature are permitted to take their
course--that heat, and cold, and want of food, and dryness, and
moisture, in the end, as if by an irresistible destiny, accomplish the
event, it seemed to indicate that, as regards the introduction of
new-comers, a suitableness of external conditions had called them forth,
as an unsuitableness could end them. Changes in the constitution of the
air or its pressure, in the composition of the sea or its depth, in the
brilliancy of light or the amount of heat, in the inorganic material of
a medium, will modify old forms into new ones, or compel their
extinction. Birth and death go hand in hand; creation and extinction are
inseparable. The variation of organic form is continuous; it depends
upon an orderly succession of material events; appearances and
eliminations are managed upon a common principle; they stand connected
with the irresistible course of great mundane changes. It was impossible
that geologists could reach any other conclusion than that such
phenomena are not the issue of direct providential interventions, but of
physical influences. The procession of organic life is not a motley
march; it follows the procession of physical events; and, since it is
impossible to re-establish a sameness of physical conditions that have
once come to an end, or reproduce the order in which they have occurred,
it of necessity follows that no organic form can reappear after it has
once died out--once dead, it is clean gone for ever.

[Sidenote: Interstitial molecular creations.] In the course of the life
of individual man, the parts that constitute his system are undergoing
momentary changes; those of to-day are not the same as those of
yesterday, and they will be replaced by others to-morrow. There have
been, and are every instant, interstitial deaths of all the constituent
particles, and an unceasing removal of those that have performed their
duty. In the stead of departing portions, new ones have been introduced,
interstitial births and organizations perpetually taking place. In
physiology it became no longer a question that all this proceeds in a
determinate way under the operation of principles that are fix