PRESIDENTIAL ADDRESS. 661 
matter of common knowledge, and as his experience increases he learns that 
he may sometimes neglect the identity even of these. If he asks the systema- 
tist to determine some type on which his attention is especially focussed, the 
physiologist only does this in order that he may be in a position to repeat all 
the conditions of an experiment required to verify or modify a conclusion. 
A passive attitude towards systematic study has thus been created in the mind 
of the physiologist ; this passivity has been intensified by the fact that the 
direct help which the physiologist can render to systematic study is limited. 
Physiological criteria are indeed directly applied for diagnostic purposes in 
one narrow field, where organography and anatomy are synonymous and 
inadequate. But if it be true that the diagnostic characters on which the 
bacteriologist relies belong to some non-corpuscular concomitant of his 
organism, this attempt to apply physiological characters to systematic ends 
has failed. In many cases physiological characters do influence taxonomic 
study. Differences in the alternation of generations, specialised habits con- 
nected with nutrition, peculiarities as regards response to stimulation, varia- 
tion in the matter of protective endowments, admit of application in syste- 
matic work, and are constantly so applied in the characterisation of every 
taxonomic grade. But the evidence as to these characters reaches the 
systematist through secondary channels, so that the help which physiology 
renders is indirect, and the passivity of the physiologist remains unaffected. 
This passivity has at last been shaken by the development of the study of 
plant distribution from a physiological standpoint. The practical value of 
this study has been affected by the employment of a terminology needlessly 
cumbrous for a subject that lends itself readily to simple statement, and by 
the neglect to explain the status, or verify the identity, of the units included 
in its plant associations. A reaction against the use of cryptic terms has now 
set in, and the physiological passivity which has led workers in this field to 
ignore systematic canons when identifying the units discussed shows signs of 
disappearing. The cecologist, it is true, must classify his units in accordance 
with characters that differ essentially from those on which reliance can be 
placed by the systematist. But the characters made use of must be possessed 
by his units, and the cecologist now realises that, in effecting his purpose, he is 
as immediately dependent on descriptive results as the economic worker or the 
geographical botanist, and that, if his work is to endure, his determinations 
must be as precise as those of the monographer, his limitations as uniform as 
those of the phytogeographer. The needs of the ccologist are, however, 
peculiar, and his units must be standardised accordingly. &cological units 
are not the groups of species, uniform as to relationship, which the geo- 
graphical botanist requires; nor are they the pragmatical ‘species’ of 
floristic and economic work. They are the states, now fewer, now more 
numerous, that these floristic ‘species’ assume in response to various 
influences ; and cecological associations can only be appreciated and explained 
when all such states have been accurately defined and uniformly delimited. 
In accomplishing this task the faculty for detecting differences is the first 
essential, and the physiologist has here provided a field of study wherein 
workers, whose tendency to nicety of discrimination unfits them for normal 
systematic study, may find ample scope for their peculiar talent, and accom- 
plish work of real and lasting value. 
We find, then, that the taxonomy of the wider and more general groups is 
now mainly based on phylogenetic study, and is largely scientific in character 
and application. The taxonomy of the narrower and more particular groups, 
based on organographic data supplemented by anatomical evidence, is often 
somewhat empirical in character, and is largely applied for technical pur- 
poses. Among the grades chiefly so applied, the ‘species’ is a matter of 
convenience, variously limited in response to special requirements, while the 
‘family’ isa matter of judgment, crystallising slowly into definite form as 
evidence accumulates. But the ‘genus’ is relatively stable, and, in con- 
