[Sidenote: Creations and extinctions by law.] What, then, is the manner of origin of this infinite succession of forms? It is often sufficient to see clearly a portion of a plan to be able to determine with some degree of certainty the general arrangement of the whole; it is often sufficient to know with precision a part of the life of an individual to guess with probable accuracy his action in some forthcoming event, of to determine the share he has borne in affairs that are past. It is enough to appreciate thoroughly the style of a master to ascertain without doubt the authenticity of an imputed picture. And so, in the affairs of the universe, it is enough to ascertain the manner of operation of a part in order to settle the manner of operation of the whole. When, therefore, 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.