122 PRINCIPLES OF CLASSIFICATION 
arrangement for practical use, but now superseded by the 
natural system. We must always work with the aggregate of 
characters of every species ; this is easily understood after 
what we have said above. Very often, indeed, a single 
character or combination of characters enables us to recog- 
nise the approximate relationship of a species very easily, 
but its exact relationship can only be determined by a study 
of all its characters. 
The larger the aggregate of characters we can use the 
more certain the results. There is good reason to believe 
that polyphyly (p. 29) is extremely common in the evolution 
of plants ; if we use one character only as a diagnostic we 
shall be certain often to go astray in this respect (cf. remarks 
on adaptation, p. 25), and even it is probable that at times 
several distinct characters may be correlated and accompany 
one another, but if we use large aggregates of characters we 
shall usually be comparatively safe. 
Careful and extensive comparison of characters is then the 
method mainly in use for determining the relationships 
and phylogeny of plants. The remarks on p. 27 should be 
carefully read in this connection. A character which is 
nearly uniform and common to a large number of species is 
regarded as of high importance, it being, upon the theory of 
descent, one which has been handed down unaltered from 
very ancient times. It may thus be used as a diagnostic 
character of a large group, but it must always be remembered 
that unless it is usually accompanied by other widely spread 
characters, we may be dealing with a case of polyphyly. 
To take a concrete instance, Angiosperms are divided into 
two groups mainly upon the possession of one or two coty- 
ledons, but these characters are usually accompanied by 
others of almost equal persistence, such as the “ scattered ” 
vascular bundles of Monocotyledons. Characters of more 
inconstant nature, and common to fewer species mark smaller 
groups, and so on. But there must always be a considerable 
individual equation in the definition of groups of plants, 
because one individual will regard one character, another 
another, as of more importance in doubtful cases. 
Now consider the kind of characters used. “The less 
any part of the organism is concerned with special habits the 
