THE DARWINIAN THEORY 51 



again vary in number in relation to the extent of the forest-land, and 

 this is determined by the number of ruminants which browse on the 

 young growth of the woods (Darwin). 



That forests can actually be totally destroyed by ruminants is 

 proved by the case of the island of St. Helena among others. On its 

 discovery the island was covered with thick wood, but in the course 

 of 300 years it was transformed into a bare rock by goats and pigs, 

 which devoured the young growth so completely that trees which 

 were felled or which died were not replaced. 



This point is vividly illustrated by Darwin's observation of a 

 wide heath on which stood only a few groups of old pine-trees. The 

 mere fencing in of a portion of the heath sufficed to call forth a thick 

 growth of young seedling pines within the enclosure, and an examina- 

 tion of the open part of the heath revealed that the grazing cattle had 

 eaten up all the young pine-trees which sprang from seed, and that 

 again and again. In one small space thirty-two little trees stood con- 

 cealed in the grass, and several of these showed as many as twenty- 

 six yearly rings. 



How definitely the number of individuals in different species 

 living on the same area mutually limit and thereby regulate each 

 other, Darwin sought to illustrate also by the case of the primitive 

 forest, where the numerous species of plants occur, not mixed together 

 irregularly, but in a definite proportion. We can find examples of the 

 same kind wherever the plant-growth of a district has been left to 

 itself. If we walk along the banks of our little river, the Dreisam, we 

 see a wild confusion of the most diverse trees, shrubs and herbaceous 

 plants. But, even though it cannot be demonstrated, we may be 

 certain that these are represented in definite numerical proportions, 

 dependent on the natural qualities and requirements of each species, 

 on the number of their seeds and the facilities for their distribution, 

 on the favourable or unfavourable season at which they ripen, and on 

 their varying capacity for taking root in the worst ground, and 

 springing quickly up, &c. They limit each other mutually, so that 

 the whole flora of the river-bank will be made up of one per cent, of 

 this species, one per cent, of that, and, it may be, five per cent, of a 

 third, and the same combination will repeat itself in the same 

 proportions on the banks of other rivers of our country in as far as 

 the external conditions are the same. The same must be true of the 

 fauna of such a plant- thicket; the animal species also limit one another 

 mutually, and thereby regulate the number of individuals, which 

 becomes relatively stable over any area on which the conditions remain 

 the same. That is to say, a 'normal number' is attained and persists. 



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