222 THE NEW AGRICULTURE. 



three : hybridization, or the cross-breeding of species ; cross- 

 fertilization, or the inter-breeding of varieties; and selection, 

 this latter being by far the most potent factor of her process. 



Owing to the fact that almost all plants produce seeds and 

 almost all animals annually pair, there is ever a tendency to 

 increase in a geometrical ratio. From this, exceedingly rapid 

 multiplication of numbers comes inevitably a struggle for 

 existence for food, water, light, air; for protection, and for 

 the privileges of propagation. If we bear in mind that in- 

 dividual specimens of the same species are continually show- 

 ing slight differences of structure, character, etc., and that 

 by means not always readily apparent she occasionally pro- 

 duces sports and monstrosities, the manner of her procedure 

 becomes immediately apparent. A series of frosts will sweep 

 over a region killing almost all the plants of a given kind ; 

 only those which are sufficiently qualified to survive the test 

 are spared. Nature has made a selection. Plants thus spared 

 mature seed and another generation comes upon the scene. 

 The same climatic conditions again prevail ; not so many plants 

 die, 'but all the weaker ones perish. Or the second visitation 

 may be more severe than the first, in which case again only 

 a few plants will survive, but these will be still hardier than 

 any hitherto grown. It is thus that forms which could at 

 first bear only a temperate climate ultimately become able to 

 endure a very severe one. 



Again, natural selection may be made by the desert char- 

 acter of climate and soil. Long continued drouth may de- 

 stroy millions of plants where one is permitted to survive be- 

 cause of some slight variation of form or constitution which 

 enables it to obtain sustenance where the others have failed. 

 The living plant will mature seed, and to some of its offspring 



