214 SCIENCE OF THE STUDY OF TREES. PART If. 
The difficulty of being able to determine what is only a variety, and what 
may be ranked as a species, is ably pointed out by Dr. Lindley, in the fol- 
lowing passage : — “ The manner in which individuals agree in their external 
characters is the only guide which can be followed in the greater part of 
plants. We do not often possess the means of ascertaining what the effect of 
sowing the seed or mixing the pollen of individuals would be; and, conse- 
quently, this test, which is the only sure one, is, in practice, seldom capable 
of being applied. The determination of what is a species, and what a 
variety, becomes, therefore, wholly dependent npon external characters, the 
power of duly appreciating which, as indicative of specific difference, is only 
to be obtained by experience, and is, in all cases, to a certain degree, arbi- 
trary. It is probable that, in the beginning, species only were formed; and 
that they have, since the creation, sported into varieties, by which the limits 
of the species themselves have now become greatly confounded. For exam- 
ple, it may be supposed that a rose, or a few species of rose, were originally 
created. In the course of time these have produced endless varieties, some 
of which, depending for a long series of ages upon permanent peculiarities of 
soil or climate, have been in a manner fixed, acquiring a constitution and 
physiognomy of their own. Such supposed varieties have again intermixed 
with each other, producing other forms, and so the operation has proceeded. 
But, as it is impossible, at the present day, to determine which was the ori- 
ginal, or originals, from which all the roses of our own time have proceeded, 
or even whether they were produced in the manner I have assumed; and, as 
the forms into which they divide are so peculiar as to render a classification 
of them indispensable to accuracy of language; it has become necessary to 
give names to certain of those forms which are called species.” (Zdid. p. 366.) 
The secret of the great number of names of species which at present form 
the bulk of names in our catalogues is to be found in what follows from the 
_ same author: — “ Thus it seems that there are two sorts of species: the one, 
called natural species, determined by the definition given above; and the 
other, called botanical species, depending only upon the external characters of 
the plant. The former have been ascertained to a very limited extent: of 
the latter nearly the whole of systematic botany consists. In this sense a 
species may be defined to be ‘ an assemblage of individuals agreeing in all the 
essential characters of vegetation and fructification.’ ” (Jdid., p.366.) 
The difficulty of determining what is a species, and what is a variety, as 
far as concerns plants of culture, may here be considered as diminished; but, 
since it is acknowledged by Dr. Lindley, that nearly the whole of systematic 
botany consists of what are called “ botanical species, depending only upon 
the external characters of the plant,” the idea of determining, with any thing 
like absolute certainty, what is a species, at least a botanical species, and 
what is a variety, seems almost hopeless. The “ whole question,’ Dr. Lind- 
ley observes, “ lies with the word essential. What is an essential character 
ofa species? This will generally depend upon a proneness to vary, or to be 
constant in particular characters, so that one class of characters may be 
essential in one genus, another class in another genus; and these points can 
be only determined by experience. Thus, in the genus Dahlia, the form of 
the leaves is found to be subject to great variation ; the same species pro- 
ducing, from seed, individuals, the forms of whose leaves vary in a very strik- 
ing manner: the form of the leaves is, therefore, in Dahlia, not a specific 
character. In like manner, in Rosa, the number of prickles, the surface of 
the fruit, or the surface of the leaves, and their serratures, are found to be 
generally fluctuating characters, and cannot often be taken as essential to 
species. The determination of species is, therefore, in all respects, arbitrary, 
and must depend upon the discretion or experience of the botanist. It may, 
nevertheless, be remarked, that decided differences in the forms of leaves, in 
the figure of the stem, in the surface of the different parts, in the inflorescence, 
in the proportion of parts, or in the form of the sepals and petals, usually 
constitute good specific differences.” (bid., p. 366,367.) 
