524 THE EVOLUTION OF LIFE. 



ticular substance, by its presence in the soil, gives new quali 

 ties to some of the tissues; causing greater rigidity or flexi^ 

 bility, and so affecting the general aspect. Here then we 

 have changes towards modified sets of functions and struc 

 tures, in equilibrium with modified sets of external forces. 



But now let us turn to other classes of organs possessed by 

 plants organs which are not at once affected in their actions 

 by variations of incident forces. Take first the organs of 

 defence. Many plants are shielded against animals that 

 would else devour them, by formidable thorns; and others, 

 like the nettle, by stinging hairs. These must be counted 

 among the appliances by which equilibrium is maintained 

 between the actions in the organism and the actions in its 

 environment; seeing that were these defences absent, the 

 destruction by herbivorous animals would be so much in-, 

 creased, that the number of young plants annually produced 

 would not suffice, as it now does, to balance the mortality, 

 and the species would disappear. But these defensive 

 appliances, though they aid in maintaining the balance 

 between inner and outer actions, cannot have been directly 

 called forth by the outer actions which they serve to neutra 

 lize; for these outer actions do not continuously affect the 

 functions of the plant even in a general way, still less in the 

 special way required. Suppose a species of nettle bare of 

 poison-hairs, to be habitually eaten by some mammal in 

 truding on its habitat. The actions of this mammal would 

 have no direct tendency to develop poison-hairs in the 

 plant; since the individuals devoured could not bequeath 

 changes of structure, even were the actions of a kind to pro 

 duce fit ones ; and since the individuals which perpetuated 

 themselves would be those on which the new incident force 

 had not fallen. Organs of another class, similarly cir 



cumstanced, are those of reproduction. Like the organs of 

 defence these are not, during the life of the individual plant, 

 variably exercised by variable external actions ; and there* 

 fore do not fulfil those conditions under which structural 



