462 IV. EXPLANATIONS OF THE 
the relations between Aggregates and Segregates, or the division of 
the former into the latter. The practicability of bringing these 
under the formula, which is mainly a question of time and 
familiarity, was unavoidably made the test for inclusion or ex- 
clusion, more than the recognizable distinctions between the 
plants themselves. Thus, Habenaria bifolia and chlorantha, 
Epilobium tetragonum and obscurum, Polygala vulgaris and 
depressa, Salia Smithiana and acuminata, Sparganium natans 
and minimum, with other such couplets, were before treated 
together as unit-species; so treated, not because there is now 
much difficulty in distinguishing the plants themselves, but 
because usually they have not been distinguished by recorders 
of localities. The result of that non-distinction being, that 
localities recorded under the first name of the two, long in use 
for both, cannot be safely assigned to either without re-examina- 
tion. If the plants now understood by those couplets of nares, 
had been well distinguished in books half a century ago, and 
habitually kept apart by subsequent authors and collectors, a 
stock of data would have been gradually accumulated, enabling 
us of the present time to trace their separate distribution suffi- 
ciently well. The great impediment to this satisfactory result 
has been raised by the very authors who did treat them apart; 
for, by adhering to the first of the two names for expressing the 
one half only, while in other works it expressed both halves or the 
whole, they stultified their own subdivision, and rendered the 
name meaningless through its uncertainty of application. 
With very few exceptions, all the segregates may be held native 
plants; and in this respect they differ from the plants placed in 
the other groups. In the following pages they can be dis- 
tinguished readily from almost all the rest, the non-indigenous 
plants, by their names being given in triplets; the name of the 
aggregate species being inserted between the generic name and 
the distinctive name of the segregate, as a brief and uniform 
manner of expressing the relation of one to the other. It is at 
the Reader's own choice to receive that distinctive name as varietal 
or as specific, in accordance with his own views in each separate 
