§ C. Equivalence and Substitution
Substitution of equivalent feet.
17. The most important law of English prosody is that which permits and directs the interchange of certain of these feet with others, or, in technical language, the substitution of equivalent feet.
Its two laws.
18. This process of substitution is governed by two laws: one in a manner a priori, the other the result of experience only.
Confusion of base must be avoided.
19. Substitution must not take place in a batch of lines, or even (with rare exceptions) in a single line, to such an extent that the base of the metre can be mistaken.
(Of which the ear must judge.)
20. Even short of this result of confusion the ear must decide whether the substitution is allowable.
Certain substitutions are not eligible.
21. As a result of experience we find that the feet most suitable—if not alone suitable—as substitutes for the iamb—the commonest foot-staple—are the trochee, the anapæst, and the tribrach; that the dactyl substitutes well, if not too freely used, for the trochee.[27] These equivalences are reciprocal.