INTRODUCTION. 11 
much cultivated in the Northern United States,—the CaLcro.aria, or 
slipper-shaped flower ; of which an immense number of varieties, differ- 
ing widely in the shape, as well as the color of the flower, are now 
known, almost every horticultural exhibition having a new one; and the 
beautiful South American Amarytuiis has a like tendency, of which 
the gardener has taken similar advantage. 
4. Hence in discriminating what are real species from what are simply 
varieties, the Botanist is treading on very insecure ground, until he has 
ascertained, for every species, its tendency to run into varieties of form, 
whether spontaneous or induced by change of external conditions. His 
greatest difficulty arises from those cases, in which have arisen what are 
termed permanent varieties, which reproduce themselves with the same — 
regularity as do real species. An instance of this in the animal kingdom 
is that of the different races of men, which are respectively distinguished 
by marked peculiarities, that are regularly repeated through each gener- 
ation; so that many naturalists have been inclined to regard them as 
really distinct species. There is, however, good evidence (independently 
of the Mosaic History,) to prove that they have all descended from a 
common stock. Precisely the same is the case in regard to Plants, many _ 
races of which even in the Northern United States, are still under discus- 
sion amongst Botanists ; some maintaining that they are distinct species, 
and others that they are but varieties. Thus of the Willow, seventy- 
one species have been stated by one authority to exist, whilst another 
reduces them to twenty-nine. The genus Rubus or common Bramble, 
has been thought to contain twenty-one species, which are probably re- 
ducible to six or eight. These details are here introduced for the 
purpose of putting the young Botanist on his guard against the tendency 
to multiply species, which is now sadly prevalent among many superficial _ 
writers, and which is still further encouraged by gardeners, who give 
new 8 c names to such varieties as is those just alluded to, and even to 
hybrids between these. 
COMBINATION OF SPECIES INTO GENERA. 
‘5. When the Botanist has satisfied himself regarding the species which 
he has collected, his next step is to combine those amongst which he finds 
the greatest resemblance, into genera. Now in this process he must not 
be altogether influenced by similarity in their general external aspect, 
for this will often conceal great differences in their most important organs. 
There are certain parts which furnish essential characters, without 
similarity in which it would be wrong to associate species, however alike _ 
in other respects, in the same genus; and, on the other hand, there are _ 
parts so susceptible of variation, that the differences between them must 
be very striking indeed, to warrant the plants being arranged under _ 
