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.
Grouping and naming are fixed here for one special purpose.
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.
This third possibility in philosophy does not enter at all into Lecky's grouping referred to above; in fact, it is very generally strange to older British thinking,3 t;csm.