discovered substances capable of musical vibrations, or methods of impressing such vibrations. By substituting the bow for the plectrum or the fingers, they may have changed the Lyre into the Violin, Viola, Violoncello, and Bass. (It seems historically probable that the real origin of the Violin class was an instrument with one string played on by a bow.) By grouping together Pipes of various sizes they got the Panpipes; by substituting metal and enlarging the blowing apparatus they got the Organ. By beating on stretched parchment with the finger, they got the Tambourine and Tom-Tom; by doubling this and using a stick they got the Drum. By beating metal with metal they got the Cymbals; by beating wood they got the Castanets. 137. The application of this illustration is plain. Just as a wind-instrument is incapable of becoming a stringed instrument, so a Mollusc, with all its muscles unstriped, and its nervous system unsymmetrical, is incapable of becoming a Crustacean, with all its muscles striped and its nervous system symmetrical. Indeed there are probably few biologists of the present day who imagine the transmutation of one kind into the other to be possible; but many biologists assume that both may have been evolved from a common root. The point is beyond proof; yet I think there is a greater probability in the assumption that both were evolved from different roots. At any rate, one thing is certain; a divergence could only have been effected by a series of _substitutions_; and the question when and how these substitutions took place is unanswerable: one school believes them to have been creative fiats, the other school believes them to have been transmutations.