XIV Introduction. 
genus as Rosa or Lilium, and the application of this plan will 
be clearly seen. Perhaps nothing is more artificial, in a sense, 
than the so-called natural system of botanists. Scarcely 
two botanists agree as to what should constitute an order, 
a genus, or a species. These differences of opinion often 
appear greater than what they really are, for they all resolve 
themselves into the question of the value to be attached to 
certain characters. The cultivation of plants and compari- 
sons of the same species from different parts of the world, 
have taught us that variability, more or less rapid or wide 
according to conditions and circumstances, is a prominent 
feature of most species under observation. What the limits of 
this variability are, nobody has yet determined, and some de- 
clare it to be illimitable. But this is not the place to discuss 
the stability of species; suffice it to say that for all practical 
purposes there is little difficulty. With the horticulturist it 
becomes a question whether a certain plant, whatever rank we 
may assign to it, be worthy of cultivation, either for its use or 
beauty. And this point decided, there is little to prevent him 
from ascertaining whether it will be better to propagate it 
direct from seed, or by some non-sexual means, as from cuttings, 
grafting, etc. Of course the method adopted will depend upon 
the easiest way of transmitting it pure. 
There are no general rules by which botanists are guided in 
defining species. In some groups of plants, certain organs 
appear to be so constant in their form, number, hairiness, etc., 
as to characterise species, whilst the same set of organs in 
another group of plants vary so much as to be of no use in 
distinguishing species, and consequently the botanist has re- 
course to a different set of organs, affording more permanent 
and reliable characters. This, coupled with the variation of 
plants under diverse conditions, will explain the difficulties 
experienced in determining species from written descriptions. 
Indeed, it may safely be averred that the most accomplished 
botanists often fail, after careful study, to identify a plant with 
its description, even when that description is as perfect as it is 
possible to make it from half-a-dozen or more specimens ; and 
it ig usually considered necessary to compare the new specimen 
with the original in the case of little known species. We 
