CHAPTEE XXXIX 

 VARIATION, MUTATION, AND ORIGIN OF SPECIES 



472. Variations of plants. One of the foundation principles 

 of scientific farming and gardening is that seeds will grow into 

 plants like those which produced them. Not only is it assumed 

 that grains of corn will grow into corn plants and beans into 

 bean plants, but also that any special variety of sweet corn will 

 produce its like, yellow-eyed beans their like, and so with mul- 

 titudes of familiar cases. Closer observation, however, shows 

 that no two of the hundreds or thousands of plants raised from 

 the seeds of a single parent plant will be exactly like each other 

 or the parent. Generally the variations are very slight, and most 

 of them fail to continue themselves in succeeding generations 

 so as to establish new varieties of plants. 



473. Variations in one direction. While variation generally 

 goes on in all directions, so that one of a brood sprung from a 

 given parent will be smaller and another larger, one more and 

 another less hairy than the parent plant, and so on, it is not 

 uncommon to find what may be called definite variation, in 

 which the changes all lead toward a definite new type. The 

 behavior of lowland forms planted in alpine regions (Sec. 464) 

 is a good instance of the kind. It is well known, too, that seed 

 from northern localities when planted farther south will produce 

 earlier crops than can be obtained from southern seed. American 

 varieties of onion, after being grown for a series of years in Eng- 

 land, become habituated to the longer mild season there, and 

 when the seed is brought back to America the plants grown from 

 it fail to mature their bulbs before the coming of the frost. 



Such facts as these seem to indicate that characteristics which 

 have been impressed upon the plant by external influences, such 



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