194 CLASS-BOOK OF BOTANY. 
It may here be pointed out that the sub-classes and their 
subdivisions are not always very natural, so that even in the 
best schemes of classification yet devised some more or legs 
artificial groups are always to be met with. This is parti- 
cularly the case with the Incomplete, whiclr has hitherto 
served as the waste-paper-basket of the systematic botanist. 
Whenever he came on a group of plants which, either by direct 
descent or by degeneration, have their organs in a condition 
of very imperfect development, he placed them in this sub- 
class, which thus has come to have no definite characters at 
all. Its members only agree in this, that they have either no 
corolla or no perianth at ail; but these, as we know, are non- 
essential organs, readily liable to modification (vide pp. 75 and 
110), and therefore, from a classificatory point of view, they 
ought to be treated as of little value. But until we know a 
great deal more about the relationships of plants than we do 
at present, something like the above classification must be 
generally adopted. 
In any natural scheme of classification the most convenient 
starting-point is that assemblage of individuals which we call 
a species. But this is a term very difficult to define. It may 
be said to include all those forms ‘which have the most 
essential properties in common, are descended from one 
another, and produce fruitful descendants.” In practice it is 
often very difficult to apply any rule for limiting the characters 
of species, and nearly every botanist who attempts to classify 
plants exercises his own judgment to a considerable extent. 
. Thus, hardly any two botanists in or out of New Zealand 
could be got to agree as to the number of species of Veronica 
which grow wild in this colony, and the same remark applies 
to many other groups of plants—e.g., Hpilobium. 
Next in importance to the species—indeed, in some respects 
ot greater importance—comes the genus, which consists of a 
number of species agreeing in most points of structure, though 
they may differ a good deal in detail of external form, &c. 
But, while the definition of a genus is as vague as that of a 
Species, and also depends to some extent on the ideas of 
individual botanists, yet there is on the whole greater una- 
nimity as to generic limits than exists as to specific. 
The reasons of this want of fixity in our definitions of these 
terms are not far to seek. We already know that all plants 
are liable to variation (though we need not inquire here into 
the causes of this), and that certain variations become more 
or less permanent, and we say that those individuals exhibit- 
ing such constitute a variety; but it is quite impossible to 
draw any hard-and-fast line between varieties and species, 
and to say that: certain characteristics are only of varietal 
and others of specific importance. The modifications of struc- 
