PREFACE. 
15 * 
found in this work, and to express a hope that these will be pointed 
out to me when discovered, so that they may be corrected in a future 
edition. Of the two principal sources of error in any work of the 
nature of a Systematic Flora, one is unavoidable, and that is the im- 
possibility of deciding, in many cases, as to what should be regarded 
a species and what a variety ; in my case this difficulty is greatly en- 
hanced by my having only dried specimens to examine. But this is 
not all ; for it is now admitted that one and the same species may be 
represented by two or more permanently distinct forms in one district, 
in other districts by but one of these forms, and in still other districts 
by forms which unite the characters of the most distinct forms of the first 
district ; and moreover, that these forms are usually permanent under 
cultivation. It hence follows that the several characters will have dif- 
ferent values in the estimation of the observers in each district, and 
that there must always be differences of opinion regarding the claims of 
such forms to take specific rank. The other great source of error is of 
more real importance, as it relates to facts and not at all to opinions ; 
it is, that in examining dried specimens, important and constant charac- 
ters are often overlooked, unimportant and transient ones exaggerated, 
and that errors accumulate in the successive process of examining so 
many organs, in applying technical terms to them, and in describing, 
transcribing, printing, and even in correcting the press. The num- 
ber of these errors is always great in works which, like the Phsenogamic 
Part of this Handbook, consist of descriptions of plants, two-thirds of 
which have been examined and described by one author alone, and 
it is to succeeding observers that I must look for their detection and 
correction. 
1 loyal Gardens , Keiv : 
June 30, 1864. 
