PRELIMINARY REMARKS. vii 
on the other, there is always room for difference of opinion even 
between those who admit these three classes. 
The Editor proposes to recognize swb-species, and under this 
designation to direct attention to those plants which have less 
strongly-marked differences between them than are found between 
generally received species, but which are, nevertheless, too 
constant in their characters to be considered merely varieties. 
Such plants have recently attracted much notice from many 
continental and a few of our own Botanists; and though their 
labours have sometimes been stigmatized as species-making, we 
are indebted to them for a much more accurate knowledge of 
plants than we previously possessed. 
The term variety is applied by the Editor to forms which are, 
or are supposed to be, confined to individuals, and which may 
revert to the original type in a single or a few generations, while 
a sub-species transmits its peculiarities for an indefinite period. 
In most cases, the permanence of character, which ought to be 
ascertained by observation, is only inferred, from the difference 
which exists between some two forms being considered as great 
as between others which are generally admitted to be permanent 
and distinct ; and thus many mistakes no doubt occur respecting 
varieties and sub-species which better observation and long- 
continued cultivation may in time correct. 
A state is even less permanent than a variety, for it may be 
removed in the same individual by altering the external circum- 
stances,—such as soil, climate, place of growth, &c. 
The foregoing is a brief explanation of the way in which these 
terms are applied in the present work, without which, misunder- 
standing might arise. 
After the enumeration of the localities for each species, there 
will be found a line containing a general indication of the dis- 
tribution, duration, and time of flowering. In this line the names 
England, Scotland, Ireland, denote that the species has occurred 
