76 PROBLEMS OF EVOLUTION 



more correctly, he is a new feature in the environment, and it 

 often happens that a tree or a flower accommodates itself to him 

 and his caprice rather than to vegetable competitors. Were the 

 interaction of plant upon plant left unrestricted, man's favourites 

 would have little chance of survival. Give your weeds a fair chance, 

 and you will see only a few more generations of your wheat. 

 Forest A forest fire must mean a grand opening for a readjustment 

 es in the relations between competitive species of trees, and there 

 is reason to believe that, before man learnt the art of kindling 

 fire, conflagrations were of sufficiently frequent occurrence to 

 help on the evolution of plants. Volcanic eruptions must have 

 occasionally set fire to surrounding vegetation, and it is quite 

 possible that accumulations of dead leaves may have heated and 

 smouldered, and at last burst forth into flame. More often, 

 probably, a flash of lightning started a conflagration. A great 

 forest fire gives a grand chance not only to enterprising species 

 of trees, but also to smaller plants, whose seeds seem to lie 

 dormant waiting for such an opportunity. 



To show how important it is that an organism should be ready 

 to meet an occasional emergency, I will mention what Kerner 

 gives as the reason why nearly all the trees in northern latitudes 

 have developed a means of shedding their leaves and have become 

 deciduous. Snow, he maintains, is the cause of the phenomenon. 1 

 The few native evergreens that we have are able through the 

 slipperiness of their leaves, or the small surface they expose, or 

 through the elasticity of their twigs, either to shift the snow or 

 remain unharmed by its weight. But most of our forest trees 

 have very different foliage. Kerner describes the disastrous 

 effects of a heavy fall of snow in autumn or spring when every 

 tree is in leaf. In our mild island climate this is not likely to 

 happen so often as in Germany. But most of us can remember 

 an instance of a cedar tree losing some of its big boughs through 

 the weight of snow. And we may feel sure that many tropical 

 trees, exposing as they do a great breadth of leaf, would be 

 impossible in countries in which snow occasionally lies deep. 2 



* Natural History of Plants, \. 358. 



2 No doubt cold hastens the fall of the leaf, i.e., sets the machinery in motion. 



