42 THE STETTGGLE FOE EXISTENCE. 



organic constituents taken up from it, have enriched it directly wita 

 the carbon of the deconiposing plants-parts and indirectly with a fresh 

 supply of nitrogen compounds from the atmosphere. Forest soils (uu 

 less indeed the dead leaves and other disjecta are swept away and 

 removed as soon as they fall) are never wanting in the mineral consti- 

 tuents of plant food, but nothing save actual special manuring can 

 restore to them what fires deprive them of. Besides consuming all 

 organic matter in the upper layers of the soil and in its'dead vegeta- 

 ble covering, forest fires also bake its surface into a hard crust and 

 not seldom even glaze the surface of siHcious stones and rocks, 

 thereby delaying very considerably their disintegration under 

 weather influences. It is thus evident that species that can thrive, 

 or at least maintain -a vigorous growth, in poor soils are favoured 

 by the occurrence of fires^in their struggle with their more exact- 

 ing neighbours. See also conclusion of second paragraph imder 

 ^^ Floods" on next page. 



(xxii) Exacting p.ature of the species in respect of moisture in 

 the soil. — It is obvious that this circumstance can affect only such 

 species as bring out their new flush of leaves or. are in leaf 

 during the season of forest fires. The extent to which individuals 

 of such'species will be influenced by fires sweeping through the 

 prop will depend on the relative mass of their roots spreading with- 

 in the layers of soil affected by the heat of the fire. 



(xxiii) Size and shape ^nd easy germination of their seed. — 

 Whatever low growth exists on the ground at the beginning of the 

 rains, checks, in proportion to its closeness, the washing away 

 of the seeds that are shed by the trees overhead. The more fre- 

 quent and severe forest fires are, the sparse^ M'ill be this growth, 

 and hence the greater will be the advantage possessed by species 

 producing seeds, which, being small or flat, are at once caught in 

 crevices in the ground or do not easily roll or slip away, or which, 

 germinating readily, anchor themselves forthwith; and this advan- 

 tage will be the greater, the more sloping aud smoother the surface 

 of the ground is. 



(e) Floods. — All the remarks made under this head in the 

 First Case have a more extended appHcation here than in the 

 Second Case, since the difference between plant and plant in the 

 present Case n^ay be practicaUy unhmited in every respect except 

 age. It will be perceived that abihty to produce suckers increases 

 in a very remarkable degree the resistance any given species can 

 offer to the destructive pflFects of floods. Whether floods bend 

 J)reak or strain the aerial or subterranean portions of a tree, or 



