Piper and Ficus, since the number of cosmopolitan or very widely distributed species is comparatively few, a geographical grouping is found specially convenient by those who are constantly receiving parcels of plants from known foreign sources.
This grouping is not always colour-producing, since diphenyl is colourless.
The grouping of reflex "units," and the paths wherein impulses travel and become associated, have been made out by the physiologist (Sherrington and others) working on the healthy animal, as well as by the record of disease; and not of spontaneous disease alone, for the artificial institution of morbid processes in animals has led to many of these discoveries, as in the method of A.
While very many coloured substances must obviously contain this grouping, yet in many cases it is necessary to assume a simple intermolecular change, while in others a more complex rearrangement of bonds is necessary.
The 15th century has the honour of composing the great commentary on the text of the Canon, grouping around it all that theory had imagined, and all that practice had observed.