Physical phenomena we all admit follow one another in unbroken succession, each derived from a preceding, and giving origin to a succeeding. We call this the law of causation, and say that it is axiomatic. We might call it a law of derivation. So also organic _forms_ follow one another in continuous chain, each derived from a preceding and giving origin to a succeeding. We call this a law of derivation. We might call it a _law of causation_, and say that it too is axiomatic. The origins of new phenomena are often obscure, even inexplicable, but we never think to doubt that they have a natural cause; for so to doubt is to doubt the validity of reason, and the rational constitution of Nature. So also the origins of new organic _forms_ may be obscure or even inexplicable, but we ought not on that account to doubt that they had a natural cause, and came by a natural process; for so to doubt is also to doubt the validity of reason, and the rational constitution of organic Nature. The law of evolution is naught else than the scientific or, indeed, the rational mode of thinking about the origin of things in every department of Nature. In a word, it is naught else than the law of necessary causation applied to _forms_ instead of phenomena. Evolution, therefore, is no longer a school of thought. The words _evolutionism_ and _evolutionist_ ought not any longer to be used, any more than _gravitationism_ and _gravitationist_; for the law of evolution is as certain as the law of gravitation. Nay, it is far more certain. The nexus between _successive events in time_ (causation) is far more certain than the nexus between _coexistent objects in space_ (gravitation). The former _is a necessary truth_, the latter is usually classed as a contingent truth. I have used and may continue to use the term evolutionist, but if so it is only in deference to the views of many intelligent persons, who do not yet see the certainty of the law. CHAPTER II. SPECIAL PROOFS OF EVOLUTION. _Introductory._