BRITISH SPECIES OF THE GENUS MNIUM. T 
Tyre V.: Sometimes a character reaches its maximum in the leaves which 
are above the longest leaf.— Example : In Mn. punctatum the nerve is short 
and never reaches the summit of the leaf in the three first intervals ; in 
intervals 4—10 it grows longer, more and more approaching the summit and 
often (not always) reaching this. In the second leaf above the longest leaf, 
and further upwards, the nerve reaches the summit in all the specimens 
I have observed. 
Remark : The classification of the characters in five groups with reference 
to their gradation is an artificial one, as all possible transitions between the 
types are found to exist. A common feature of all the gradation curves is 
their ascending-descending form. All the observed differences may be 
brought under two principles :—1°, the variation of the position of the 
summit ; 2°, the variation of the rapidity of increase (or decrease) along the 
axis. In other words, the differences between the curves are quantitative *. 
§ 12.—From the examples given in the preceding paragraphs, we may 
conclude that the gradation of a particular series of measurements is a 
character in itself, independent of the absolute value of the feature under 
consideration. The gradation of a given character is more or less variable 
among the specimens of the same species (see § 8) and often very different 
from one species to another (see Table IX.). The gradation curve may be 
modified by the conditions of existence—we know at least one good example 
of such a modification T. 
The principle of gradation rules greatly in the morphology of plants, and 
also in a certain degree the structure of animals (see $ 23, note *, p. 33). 
A full discussion of this subject would go far beyond the limits of this 
paper. Therefore 1 want to close this part with five final remarks. 
Remark I.: The knowledge of the gradation of a certain number of 
characters enables us to make clear the variation of the leaves of the same 
stem.— Examples : In fig. 2 a given leaf (for instance, 25) is quite different 
from another given leaf (for instance, 17) with reference to the measurements 
of length and breadth. As each character has its own independent curve (see 
figs. 2 & 3), and as much diversity exists among the curves, the result is a 
practically unlimited number of combinations 1, the characters of each leaf 
depending on its position $. This form of variation seems to be of the same 
kind as the individual variation, but it is something quite different (see 82). 
* We may suppose that in the relation j—f (see note, $ 2, p. 8) certain variable factors 
intervene, the knowledge of which would perhaps enable us to explain the observed 
differences. 
+ The gradation of the number of marginal florets in Chrysanthemum carinatum, Schousb. 
i In Table IL the figures in each vertical column give the combination of the cor- 
responding leat. 
$ This is also the case with branches, buds, flowers, fruits, trichomes, hydathodes, etc. 
LINN. JOURN.-—BOTANY, VOL. XLIV. C 
