The Canon of Variations, again, deals not with sudden irruptions of a cause, force or agent, but with some increase or decrease of an agent already present, and a corresponding increase or decrease of some other phenomenon—­say an increase of tax and a rise of price.  But there are cases in which the energy of a cause is not immediately discharged and dissipated.  Whilst a tax of 6_d._ per lb. on tea raises the price per lb. by about 6_d._, however long it lasts, the continuous application of friction to a body may gradually raise its temperature to the point of combustion; because heat is received faster than it is radiated, and therefore accumulates.  Such cases are treated by Mill under the title of ‘progressive effects’ (Logic:  B. III., c. 15):  he gives as an example of it the acceleration of falling bodies.  The storage of effects is a fact of the utmost importance in all departments of nature, and is especially interesting in Biology and Sociology, where it is met with as heredity, experience, tradition.  Evolution of species of plants and animals would (so far as we know) be impossible, if the changes (however caused) that adapt some individuals better than others to the conditions of life were not inherited by, and accumulated in, their posterity.  The eyes in the peacock’s tail are supposed to have reached their present perfection gradually, through various stages that may be illustrated by the ocelli in the wings of the Argus pheasant and other genera of Phasianidae.  Similarly the progress of societies would be impossible without tradition, whereby the improvements made in any generation may be passed on to the next, and the experience of mankind may be gradually accumulated in various forms of culture.  The earliest remains of culture are flint implements and weapons; in which we can trace the effect of tradition in the lives of our remote forefathers, as they slowly through thousands of years learnt to improve the chipping of flints, until the first rudely shaped lumps gave place to works of unmistakable design, and these to the beautiful weapons contemporary with the Bronze Age.

The Method of Gradations, the arranging of any phenomena to be studied in series, according to the degree in which some character is exhibited, is, perhaps, the most definite device in the Art of Discovery. (Bain:  Induction, c. 6, and App.  II.) If the causes are unknown it is likely to suggest hypotheses:  and if the causes are partly known, variation in the character of the series is likely to indicate a corresponding variation of the conditions.

Sec. 5.  THE CANON OF RESIDUES.

Subduct from any phenomenon such part as previous inductions have shown to be the effect of certain antecedents, and the residue of the phenomenon is the effect of the remaining antecedents.

The phenomenon is here assumed to be an effect:  a similar Canon may be framed for residuary causes.

This also is not a fresh method, but a special case of the method of Difference.  For if we suppose the phenomenon to be p q r, and the antecedent to be A B C, and that we already know B and C to have (either severally or together) the consequents q r, in which their efficacy is exhausted; we may regard