216 SCIENCE OF THE STUDY OF TREES. PART Il. 
ginal species ; because a tendency to this has been found to take place both in 
plants and animals. 
Varieties. — A variety differs from a race, in not being susceptible of propa- 
gation by seed, at least with any thing like constancy and certainty. For ex- 
ample, the jargonelle pear may be continued by seed ; but a jargonelle pear 
with variegated foliage could not be so propagated with certainty. We allow, 
however, that, if a great many seeds taken from the fruit of a jargonelle pear 
with variegated leaves were sown, some of the plants so raised would pro- 
bably show variegation in their foliage. The same thing, we think, would take 
place in the case of sowing the seed of variegated hollies, or of fastigiate or 
pendulous-branched plants, but to what extent it is impossible to say. It 
certainly would not take place to such an extent as to confound varieties with 
races, or to render it desirable to propagate varieties in this way; and, conse- 
quently, varieties are always propagated bysome modification of division, such 
as by cuttings, layers, grafting, &c. 
Variations differ from varieties in not being transmittible by any mode of 
propagation. They are always produced by local circumstances operating on 
the individual; and the moment these circumstances are changed, the varia- 
tion disappears. For example, plants grown in the dark will have their leaves 
white; other plants with hairy leaves, when grown in water, will have their 
leaves smooth; and the hydrangea, grown in a certain description of soil, will 
have its flowers blue: but, remove the plant with white leaves to the light, 
and place the plant grown in water in dry soil, and the hydrangea in common 
soil, and it will be found that the leaves of the first will become green, and 
those of the second hairy, and that the flowers of the hydrangea will resume 
their natural pinkish hue. 
Mules and Hybrids. — Some confusion exists as to the use of these terms, 
when applied to plants. The term mule, we think, ought to be limited to such 
hybrids as are raised between different aboriginal species, and which it is be- 
lieved are not susceptible of propagation by seed: such, for example, as the 
Pyrus spuria, which seems to be a hybrid between Pyrus Sérbus or A‘ria and 
Pyrus Chamzméspilus. The term hybrid,on the other hand, we think, ought 
to be limited to the produce, by cross fecundation, of different races and varie- 
ties of the same species. Every one knows that this is one of the most im- , 
portant elements of culture, having given rise to the most valuable garden 
flowers, table fruits, culinary vegetables, and agricultural plants. 
Botanical Species. — It will be seen, from the preceding remarks, that we 
follow De Candolle in denominating what Dr. Lindley and other British bo- 
tanists distinguish as botanical species, races, or varieties. 
It is not to be supposed, however, that we undervalue botanical species, or 
that we either deny the distinctness of many that exist, or the propriety of 
having different names for them, and keeping them distinct. On the con- 
trary, to compare plants with men, we consider aboriginal species as mere 
savages, and botanical species, or, according to De Candolle’s classification, 
races and varieties, as civilised beings. What, then, it may be asked, is our 
object in endeavouring to show that many of our botanical species are only 
varieties ? We have two objects in view ; and both, we think, are very im- 
portant ones. In the first place, by confounding varieties or garden or bota- 
nical species with aboriginal ones, a beginner, ignorant of the extent to which 
this is done, cannot make a judicious selection; and while, in the case Fraxi- 
nus, for instance (of which there are, in reality, as we think, only three spe- 
cies known, exclusive of O’rnus), if he wished to select, perhaps, three sorts, 
he might, instead of selecting the three really distinct species, which would 
give him a complete idea of the genus, fix on three of the varieties of /. ame- 
ricana or of F. excélsior, which would only give him an idea of one species. 
In the second place, we wish to prevent beginners, in the study or cultivation 
of trees, from puzzling themselves unnecessarily to make out the minute dif- 
ferences which distinguish what are called botanical species; believing, as we 
do, that it is impossible to make out many of these from the specific charac- 
