78 THE POPULAR SCIENCE MONTHLY. 



their structure just alluded to, and which enables them to bear their 

 part bravely in the conflict. 



It is easy to understand how an alteration of the conditions under 

 which plants grow influences veiy materially the struggle we have 

 been alluding to. A very slight change in climatal conditions pro- 

 duced, for instance, by the growth of sheltering trees, or by the drain- 

 age of the soil may be followed by the growth of quite a different 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 relations be- 

 tween 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 portion had been enclosed and planted with 

 Scotch fir. The change in the native vegetation of the planted part 

 of the heath was most remarkable. " Not only the proportional num- 

 bers 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 the timber in forests of a 

 particular district of France, broom, foxglove, heaths, birch-trees, and 

 aspens sprang 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, etc., made their appearance : the ground was still occupied with 

 young birch and poplar. It is not till after the third repetition 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 conditions, the na- 

 ture 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, Pinus 3fi(ffhus, etc., which 

 are found in this partially-fossilized condition in spots where there is 

 apparently nothing to prevent them from growing now, where, in fact, 

 they do grow well when planted. 



Foresters in all countries are perfectly well aware of these facts, 

 and botanists watch with interest the appearance of a different vegeta- 



