42 
Paul jaccard. 
total number common to four localities, 3 to 4% common to six 
localites, and 1 to 2% common to eight localities. Three species 
only, Gentiana excisa, Homogyne alpina and Nigritella angusti/olia, 
or less than 1%. have been collected in each of the ten localities. 
And meanwhile, what naturalist does not carry away from the 
Alps the impression that the majority of the species of the alpine 
meadows and swards are found everywhere ? Close observation, 
supplemented by careful statistical analysis, is necessary to con¬ 
vince us of the infinitely various floristic composition of the alpine 
meadow, and that different stations, in spite of physiognomic 
uniformity, are in reality covered by very different plant associa¬ 
tions. The fact is that besides the obvious ecological variations, 
such as those of the degree of humidity, of slope, of exposure, and 
of the chemical nature of the soil—variations which betray them¬ 
selves in the plant covering by a comparatively small number of 
formations (meadows, swards, rocks, screes, bogs, etc.)—there 
exist many slighter variations of habitat, more difficult to appre¬ 
ciate, which, in a station apparently uniform, create an infinite 
diversity. The substratum especially, even the most homogenous 
from the geological standpoint, may present in its chemical com¬ 
position, in its structure, in its compactness, and in the thermal 
and hygroscopic properties which depend upon these characters, 
numerous differences which have their effect on floristic compo¬ 
sition. 
In this respect nothing is more instructive than the flora of 
the summits of the southern Jura, of which the uppermost zone, 
formed entirely of Kimmeridgian and Sequanian limestones, with 
some outcrops of Argovian, possesses a remarkable petrographical 
uniformity. All the summits are of about the same altitude, 
between 1671 and 1723 metres: they are all situated on the 
southern ridge of the Jura and present the greatest similarity in 
regard to topographical conditions, especially in the direction, 
force and frequency of the winds. Everywhere we have the same 
turf-covered ridges, the same escarpments of white limetone, the 
same dryness, accentuated by the same winds. And everywhere, 
apart from some scattered Alpine and Mediterranean types, there 
is a depressing floristic uniformity. 
Yet in spite of this extraordinary apparent uniformity, any two 
localities of approximately a hectare (say 2 acres), and each 
separated by from two to ten kilometres, possess in common only 
40 to 50% of the species collected on the two together. 
