VII. FORMULA EXPLAINED. 63 
profess to show the outside extension of the species treated in their 
Floras ;— a good practice in itself, lately adopted by several 
botanists, but in this country almost exclusively in the ‘ Hand- 
book’ by Mr. Bentham. 
The Natives, Denizens, Colonists, as explained on pages 60—~ 
61, are treated under the formula. Aliens and Casuals will be 
subsequently treated in a general commentary ; as also several 
Segregate species, the distribution of which is yet too imperfectly 
ascertained, to allow of the formula being adhered to for them, 
taken apart from each other. Their localities having been 
recorded usually under the name formerly applied to them col- 
lectively as an unit, it remains at present too often impossible to 
distinguish which one of two or more segregates was observed in 
the places indicated; fresh examination and corrected records 
being now required, in order to remove the uncertainty. It is by 
no means always the separated or new-named segregate, whose 
distribution is the more difficult to show; being even more fre- 
quently the remnant of the original aggregate species, which is 
thus uncertain. The localities ascertained and recorded for the 
recently distinguished segregate, will often be found only incom- 
plete or insufficient, not uncertain or erroneous. Those on record 
for the older aggregate must usually be uncertain between the 
two, and may be assigned erroneously to either. By way of 
example, it would be easier now to trace the distribution of the 
segregate species H’pilobiwn obscurum, than that of the remnant 
Epilobium tetragonum, from which the former was first separated 
-many years ago. We know exactly what is intended by the 
former of these names, supposing it correctly applied by a botanist 
of the present age ; but we do not know what was intended under 
the latter name in records of past ages, or in present records by 
botanists who still use the aggregate name of tetragonum, unless 
accompanied by some explanation that it intends the segregate 
remnant only, and not either or both of the two segregates 
formerly included under the one name. It is this difficulty which 
prevents the distribution of Epilobium obscurum from being shown 
