38 
POPULAR SCIENCE REVIEW. 
It is easy to understand how an alteration of the conditions 
under which plants grow influences very materially the 
struggle we have been alluding to. A very slight change in 
climatal conditions — produced, for instance, by the growth of 
sheltering trees, or by the drainage of the soil — may be followed 
by the growth of quite a diflerent set of plants from those that 
occupied the ground previously. The altered conditions have 
been advantageous to the one and disadvantageous to the other 
set of plants. 
As an illustration of the complexity of the checks and rela- 
tions between organic beings struggling together, Darwin 
mentions the case of a barren heath which fell under his 
observation, part of which was left intact, while another por- 
tion had been enclosed and planted with Scotch fir. The 
change in the native vegetation of the planted part of the 
heath was most remarkable. ‘‘ IS'ot only the proportional 
numbers of the heath plants were wholly changed, but twelve 
species of plants, not counting grasses and carices, flourished in 
the plantations which could not be found on the heath.” 
This sort of change was pointedly referred to by Dureau de 
la Malle, who relates how, after the felling of tlie timber in 
forests of a particular district of France, broom, foxglove, 
heaths, birch-trees, and aspens sprung up, replacing the oaks, 
the beech, and the ash felled by the woodman. After thirty 
years the birch and poplars were felled in their turn. Still 
very few of the original possessors of the soil, the oaks, &c., 
made their appearance ; the ground was still occupied with 
young birch and poplar. It is not till after the third repeti- 
tion of the coppicing — after an interval of ninety years — that 
the oaks and beech reconquer their original position. They 
retain it for a time, and then the struggle begins again. 
Antiquarian researches also have proved that in the natural 
state of things, without any violent change in external con- 
ditions, the nature of forests becomes altered. The Hercynian 
forests, of which Caesar speaks, and which then consisted 
of deciduous-leaved trees, are now made up principally of 
conifers. A forest which, in the Middle Ages, was of beech, 
is now stocked with oak, and vice versa. Again, we have 
the evidence afforded by submerged forests and peat bogs, 
according to which certain plants, now extinct in particular 
localities, once flourished there. We are not alluding to plants 
that may have required a different climate from what they now 
experience, but to such cases as the silver fir, the Scotch fir. 
Firms Mughus, &c., which are found in this partially fossilised 
condition in spots where there is apparently nothing to pre- 
vent them from growing now, where in fact they do grow well 
when planted. 
