II THE STRUGGLE FOR EXISTENCE 21 



Blangsted, strikingly illustrates our subject.'' The chief com- 

 batants are the beech and the birch, the former being every- 

 where successful in its invasions. Forests composed wholly 

 of birch are now only found in sterile, sandy tracts ; every- 

 where else the trees are mixed, and wherever the soil is 

 favourable the beech rapidly drives out the birch. The latter 

 loses its branches at the touch of the beech, and devotes all 

 its strength to the upper part where it towers above the beech. 

 It may live long in this way, l)ut it succumbs ultimately in 

 the fight — of old age if of nothing else, for the life of the 

 birch in Denmark is shorter than that of the beech. The 

 writer believes that light (or rather shade) is the cause of the 

 superiority of the latter, for it has a greater development of 

 its branches than the birch, Avhich is more 02:)en and thus 

 allows the rays of the sun to pass through to the soil below, 

 while the tufted, bushy top of the beech preserves a deep 

 shade at its base. Hardly any young plants can grow under 

 the beech excei:>t its own shoots ; and while the beech can 

 flourish under the shade of the birch, the latter dies im- 

 mediately under the beech. The birch has only been saved 

 from total extermination by the facts that it had possession of 

 the Danish forests long before the beech ever reached the 

 country, and that certain districts are unfavourable to the 

 growth of the latter. But wherever the soil has been enriched 

 by the decomposition of the leaves of the birch the battle 

 begins. The birch still flourishes on the borders of lakes and 

 other marshy places, where its enemy cannot exist. In the 

 same way, in the forests of Zeelancl, the fir forests are dis- 

 appearing before the beech. Left to themselves, the firs are 

 soon displaced by the beech. The struggle between the latter 

 and the oak is longer and more stubborn, for the branches and 

 foliage of the oak are thicker, and offer much resistance to the 

 passage of light. The oak, also, has greater longevity ; but, 

 sooner or later, it too succumbs, because it cannot develop 

 in the shadow of the beech. The earliest forests of Denmark 

 were mainly composed of aspens, with which the birch Avas 

 apparently associated ; gradually the soil was raised, and the 

 climate grew milder ; then the fir came and formed large 

 forests. This tree ruled for centuries, and then ceded the 



^ See Nature, vol. xxxi. p. G3. 



