20 Plant-Breeding 



seeing great differences arise in batches of plants, all of 

 which start apparently equal and with an equal chance, 

 that he never thinks to comment upon the occurrence. 



Having noticed that physical environments may modify 

 plants, we are now ready to consider just what changes 

 in these circumstances of plant life are most fruitful in 

 the production of new forms. 



Variation in food supply. The greater part of the 

 changes in the physical conditions of life hinge upon the 

 relative supply of food. Climbing plants assume their 

 form because, by virtue of the divergence of character, 

 they are enabled to fit themselves into places that other 

 plants cannot occupy. They rear their foliage into the 

 air, where food and sunlight are unappropriated. The 

 lower branches of tree-tops die, and the others thereby 

 appropriate the more food and grow the faster. The 

 entire practice of agriculture is built upon the augmenta- 

 tion of the food supply. For this purpose, we set the 

 plants in isolated positions, we till the ground, keep down 

 other plants or weeds, add plant-food to the soil, and prune 

 the tree and thin the fruit. 



Thomas Andrew Knight, the chief of horticultural 

 philosophers, appears to have been the first clearly to 

 enunciate the law that excess of food supply is the most 

 prolific cause of the variations of plants. Darwin sub- 

 scribes to it without reserve: "Of all the causes which 

 induce variability, excess of food, whether or not changed 

 in nature, is probably the most powerful." Alexander 

 Braun, an earlier philosophical writer on natural history, 

 said that "it appears rather, on the whole, as if the unusual 

 conditions favorable to a luxuriant state of development, 



