vii HEREDITY, VARIATION 113 



This ever-present and all-pervading variability is probably 

 the most important of the contributory factors of evolution, 

 and must never for a single moment be lost sight of. 



Powers of Increase of Plants and A nimals 



Of almost equal importance with ever-present variation 

 is the power which all organisms possess of reproducing their 

 kind so rapidly as to be able to take possession of any 

 unoccupied spaces around them, and in many cases to expel 

 other kinds by the vigour of their growth. 



The rapidity of increase is most prominently seen among 

 vegetables. These are capable, not only of a fivefold or 

 tenfold annual increase, as among many of the higher 

 animals, but one of many hundred or even thousandfold 

 annually. A full-grown oak or beech tree is often laden 

 with fruit on every branch, which must often reach 100,000, 

 and sometimes perhaps a million in number, each acorn or 

 nut being capable, under favourable conditions, of growing 

 into a tree like its parent. Our wild cherries, hawthorns, 

 and many other trees, are almost equally abundant fruit- 

 bearers, but in all these cases it is only rarely (in a state of 

 nature) that any one seed grows to a fruit-bearing size, because, 

 all having a superabundance of reproductive power, an equili- 

 brium has been reached everywhere, and it is only when 

 some vacancy occurs, as when a tempest uproots or destroys 

 a number of trees, or some diminution of grazing animals 

 allows more seedlings than usual to grow up, that any of 

 the seeds of the various trees around have a chance of 

 surviving ; and the most vigorous of these will fill up the 

 various gaps that have been produced. 



But it is among the herbaceous plants that perhaps even 

 greater powers of increase exist. Where our common fox- 

 glove luxuriates we often see its tall spikes densely packed 

 with capsules, each crowded with hundreds of minute seeds, 

 which are scattered by the wind over the surrounding fields, 

 but only a few which are carried to especially favourable 

 spots serve to keep up the supply of plants. Kerner, in his 

 Natural History of Plants, tells us that a crucifer, Sisym- 

 brium Sophia, has been found to produce on an average 

 730.000 seeds, so that if vacant spaces of suitable land 



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