THE LOCAL DISTRIBUTION OF PLANTS. 683 



whether by the advent of man or from whatever cause, and this equi- 

 librium is immediately disturbed. The chained forces are set free ; a 

 general swarming begins; some individuals are destroyed, others are 

 liberated ; each pushes its advantage to the utmost, and all move for- 

 ward in the direction of least resistance, till at length they again 

 mutually neutralize each other, and again come, under new conditions 

 and modified forms, into the former state of quiescence. 



The most frequent and prominent cause of these disturbances of 

 the natural fixity of vegetation is the influence of man. The results 

 of this influence may be said to be the products of agriculture, horti- 

 culture, and floriculture, on the one hand, and, on the other, weeds. 

 But there may be many other causes of disturbance besides that pro- 

 duced by man, such as the appearance of new animals, geological 

 revolutions, or climatal and meteorological vicissitudes. Anything 

 which destroys the stability which the perpetually-operating vegetal 

 forces impose upon the plants of any region is certain to reveal a 

 latent vitality, which, when liberated, proves itself capable of profiting 

 by conditions far different from, and superior to, those under which it 

 is originally found. The willow, the alder, the elm, and the sycamore, 

 hug the banks of streams because baffled and beaten back at every 

 attempt to invade the drier ground. The wild-columbine and the 

 saxifrage are driven into their rocky fastnesses by more powerful 

 rivals for the rich forest loams. The thistle and the chamomile flour- 

 ish in lawns and commons because their human foes are less formi- 

 dable than the enemies of the plain. The fruit-trees, the cereals, and 

 the roses, reach those wonderful heights of development under man's 

 care, because he not only proves their friend, but wards off all their 

 enemies. And just here it should be remarked that the alleged ten- 

 dency of cultivated plants to relapse, when neglected, into their ori- 

 ginal state, upon which Prof. Agassiz laid so much stress as an unan- 

 swerable argument against transmutation, becomes, under the law of 

 mutual repulsion, the necessary result of remanding them to their old 

 conditions. As man's care and protection were necessary to enable 

 them to advance, so, when these are withdrawn, they must be expected 

 to again yield to hostile forces, and fall back to the level of their 

 original state. It is not the special adaptation of a plant for the spot 

 on which it grows, so much as the hostile attitude of other plants 

 around it, which restricts and determines its range. The elements 

 which decide where plants shall grow, are to be found in vegetation 

 itself, and not in inorganic conditions. The power of self-adaptation 

 which they possess is sufficient to habituate almost any species to 

 almost any inorganic conditions. Each species, therefore, keeps 

 within its own restricted limits, not because it cannot live in other 

 soils, but because prior occupants forbid it to come. 



The law of adaptation may therefore be reduced to this : that 

 every plant possesses the power of self-adaptation to such a degree 



