80 THE STEUGSIiE TOE EXISTENCE. 



canopy, which there are no overtopped trees to present fill up before 

 the soil and crop deteriorates from exposure to injurious climatic in- 

 fluences. Moreover in mixed crops of different-aged individuals the 

 surrounding young, and, therefore, presumably healthy, growth 

 serves, if not as a complete check, at least as a temporary break, to 

 the spread of many insect pests. As regards the ravages of deer, 

 cattle and other herbivorous quadrupeds, it is a well established fact 

 that individuals small enough to be within their reach are never- 

 theless, in most cases, very effectually protected, if they stand in 

 the midst of a taller growth of other species not touched by those 

 animals. 



(b) Attacks of parasitic and epiphytic plants. — The remarks 

 made in the preceding paragraph with regard to the attacks of in- 

 sects apply with still greater force here in the case of tree-killing 

 fungi. Beyond this there is nothing to add to what has been said 

 imder the present subhead under the Third Case. 



(c) Damage caused by climatic influences. — Against frost, drought, 

 and sunstroke the shelter of older trees of hardy species is often 

 indispensable, and is always useful, for the permanence of other 

 species that are delicate in their youth. In the torrid valleys and 

 plains of Rajputana, nearly every nim tree springs up out of an 

 Euphorbia bush; and, similarly in the Dehra Dun, nearly every tea 

 bush has one or more tun plants growing up through it, so that a 

 tea garden left to itself would soon degenerate into a pure tun grove. 

 In the Changa Manga Plantation mulberry has possessed itself of 

 the ground under the original sissu. In creating a new forest 

 where there has been none recently, it is often found necessary to 

 raise at first a crop of a hardy species, under the shelter of which 

 more valuable, but less hardy, species are afterwards introduced. 

 Hence difference of ages in a mixed crop is a great help to delicate 

 species in the struggle for existence, and is in most cases an indis- 

 pensable condition for their presence in sufficient numbers, and often 

 even for their permanence in the crop. 



(d) Conflagrations. — A crop consisting of trees of various species 

 and different ages forms, as a rule, a denser growth and leaf-canopy 

 than 'any other, and is hence exposed to less risk from fire. So 

 that here also a difference of ages is a great help to weak species, 

 which must otherwise suffer more severely from fire. Indeed it is 

 not difficult to imagine the case of a species, well endowed in every 

 respect to struggle with its neighbours except in the sole matter of 

 resistance to the injurious effects of fire, being kept down in, and 

 even almost banished from, a crop subject to annual or frequent 



