HOW PLANTS EAT. ^ 47 



through the action of Variation and Natural Se- 

 lection. For holly grows chiefly in rough and 

 wild spots, where all the green leaves are liable 

 to be eaten by herbivorous animals. If, there- 

 fore, any plant showed the slightest tendency 

 towards prickliness or thorniness, it would be 

 more likely to survive than its unprotected neigh- 

 bours. And indeed, as a matter of fact, you will 

 soon see that almost all the bushes and shrubs 

 which frequent commons, such as gorse, butcher's 

 broom, hawthorn, blackthorn, and heather, are 

 more or less spiny, though in most of these cases 

 it is the branches, not the leaves, that form the 

 defensive element. Holly, however, wastes no 

 unnecessary material on defensive spikes, for 

 though the lower leaves, within reach of the cat- 

 tle and donkeys, are very prickly indeed, you will 

 find, if you look, that the upper ones, above six 

 or eight feet from the ground, are smooth-edged 

 and harmless. These upper leaves stand in no 

 practical danger of being eaten, and the holly 

 therefore takes care to throw away no valuable 

 material in protecting them from a wholly imagi- 

 nary assailant. 



Often, too, in these prickly plants we can 

 trace some memorial of their earlier history. 

 Gorse, for example, is a peaflower by family, a 

 member of the great group of "papilionaceous," 

 or butterfly-blossomed, plants, which includes the 

 pea, the bean, the laburnum, the clover, and many 

 other familiar trees, shrubs, and climbers. It is 

 descended more immediately from a special set 

 of trefoil-leaved peaflowers, like the clovers and 

 lucernes ; but owing to its chosen home on open 

 uplands, almost all its upper leaves have been 

 transformed for purposes of defence into sharp, 



