Genera and species are, at the present time, for a large part
artificial, or stated more correctly, conventional groups. Every
systematist is free to delimit them in a wider or in a narrower sense,
according to his judgment. The greater authorities have as a rule
preferred larger genera, others of late have elevated innumerable
subgenera to the rank of genera. This would work no real harm, if
unfortunately, the names of the plants had not to be changed each time,
according to current ideas concerning genera. Quite the same inconstancy
is observed with species. In the Handbook of the British Flora, Bentham
and Hooker describe the forms of brambles under 5 species, while
Babington in his Manual of British Botany makes 45 species out of the
same material. So also in other cases. For instance, the willows which
have 13 species in one and 31 species in the other of these manuals, and
the hawkweeds for which the figures are 7 and 32 [37] respectively.
Other authors have made still greater numbers of species in the same
groups.

It is very difficult to estimate systematic differences on the ground of
comparative studies alone. All sorts of variability occur, and no
individual or small group of specimens can really be considered as a
reliable representative of the supposed type. Many original diagnoses of
new species have been founded on divergent specimens and of course, the
type can afterwards neither be derived from this individual, nor from
the diagnosis given.

This chaotic state of things has brought some botanists to the
conviction that even in systematic studies only direct experimental
evidence can be relied upon. This conception has induced them to test
the constancy of species and varieties, and to admit as real units only