56 FORESTRY OF NORWAY. 
plant is, and must be, as much dependent on them as upon 
the others.’ 
He goes on to say:—‘ The operation is a3 yet a com- 
plete mystery to us, and many of the at present wholly 
incomprehensible conditions in extension and distribution 
of species may sooner or later find sufficient explanation in 
these influences.’ Since this was written, the carefully 
made and well digested observations of Darwin have 
thrown no little light upon the subject, and introduced a 
method of study which is likely to throw yet more upon 
the subject in all its bearings. 
‘We find again,’ says he, ‘indications of the undoubted 
fact that the distribution of all plants is naturally regu- 
lated by law, but what laws we cannot evolve, and it looks 
as if it were wholly the result of caprice that particular 
plants are distributed widely over the globe, while others 
must be cribbed in the narrowest spot, as,eg., the Wulfenia 
found only on the Corinthian Alps; that particular 
families like the Compositae flourish abroad over the whole 
earth, while others like poppies and the palms only occur 
between very definite degrees of latitude on either side of 
the equator, the Protoceae only in the southern hemisphere, 
the cactus tribe only on the western half of the earth. 
‘Just as inexplicable is the mode of distribution of the 
families of plants. While the palms diminish in numbers 
from the equator into a higher latitude, the Compositae 
attain their highest development in the zones of mean 
temperature, their number of species diminishes from 
these in both directions equally towards the equator and 
towards the pole; while finally, the grasses increase con- 
staatly from the equator towards the pole. 
‘Having spoken of the increase of grasses as we proceed 
from the equator towards the poles, it is necessary that I 
should explain the mode of consideration according to 
which the distribution of the families is usually deter- 
mined. Of sedges, to take that family as an illustration, 
the species found in the flora of France amount to 134; 
the species in the flora of Lapland, on the contrary, only 
