60 THE STRUGGLE JFOE EXISTENCE. 



the duration of sunlight, and, by reason of the clearness and rarity 

 of the atmosphere due to it, of increasing the effect of the sun's 

 rays. Hence, as we have already seen under the preceding sub- 

 head in the instance of Pimts longtfolia and deodar, increasing or 

 diminishing altitude may often cause a species to change the aspect 

 on which it can grow. Another example of this fact is to be found 

 also in the outer Himalayas in Kumaon, where oak forest, al- 

 though chiefly confined to cool northerly and westerly aspects, 

 takes complete possession of southern slopes from the line where 

 Pinus longtfolia stops. In the Siwaliks and on the outermost ridge 

 of the Himalayas sal generally prefers northerly aspects owing to 

 the lower elevation ; but when that species approaches its highest 

 limit, it is confined to southerly slopes. The influence of altitude 

 increases rapidly as we approach the limit of tree-vegetation. 



(/-) Winds In the case of winds we have to mark their direc- 

 tion, velocity and prevalence, and their own temperature and hu- 

 midity as compared with the temperature and humidity of the air 

 of the place which they enter. Hence the influence of winds on 

 the struggle for existence between the plants of a forest crop is 

 tenfold. Firstly, by their violence they may uproot, injure the 

 roots of, break off, mutilate, distort, or stunt the trees ; secondly, 

 they may alter the relative humidity of the locality according as 

 they are drier or moister than the latter ; thirdly, they reduce or 

 raise the temperature according as they previously pass over a 

 colder or hotter region, this effect being heightened in proportion 

 to their velocity ; fourthly, provided their temperature is above the 

 dew-point or that of freezing, they delay and diminish the deposi- 

 tion of dews and mitigate the severity of frost or may even prevent 

 frost altogether ; fifthly, they may blow away the covering of dead 

 leaves and other vegetable detritus; sixthly, they always accelerate 

 evaporation ; seventhly, provided they are not too cold, they also 

 accelerate transpiration; eighthly, by constantly renewing the air, 

 they piomote assimilation ; ninthly, by causing the leaves and 

 branches to sway to and fro, they accelerate the movement of sap ; 

 and tenthly and lastly, they disseminate seed to distances propor- 

 tionate to the lightness and transportability of the seed. And all 

 these various effects may be slight or marked according as the 

 wind is intermittent and local, or constant and of long duration. 

 Of two individuals of one and the same species, that one will 

 resist better the force of the wind which possesses the broader 

 crown, since the extent of the roots will be proportional to the size 

 and vigour of the crown ; whereas if the two belong to different 



