THE FLOWER 



233 



conditions that tend to produce it. If he wishes to develop 

 a dwarf variety, for instance, he will take notice that over- 

 crowding, lack of nourishment, and cold tend to produce that 

 result in nature, and by acting on this hint he can direct his 

 efforts more intelligently. He will learn, too, not to waste 

 time in trying to breed a plant contrary to its nature. He 

 must not expect to gather figs from thistles by any art of 

 selection or skill in culture. By attention to Mendel's law, 

 a still further saving of time and labor may be effected. 



It is obvious, from what has been said, that a breeder's 

 chance of finding what he wants will be greater in proportion 

 to the number of individual plants he has to choose from. 

 For this reason, a horticulturist sometimes uses thousands 

 and hundreds of thousands of specimens of a single kind in 

 conducting his experiments. In this way he compresses into 

 a short space of time the advantage that nature can gain only 

 by spreading her random experiments over a long series of 

 years, or even centuries. 



264. Mutation and variation. — There are at least two 

 ways in which changes in vegetable and animal forms are 

 thought to occur: (1) 

 by the preservation and 

 fixation through selec- 

 tion and heredity, of 

 slight differences that 

 may appear from time to 

 time, such divergences 

 being called "fluctuat- 

 ing variations" ; (2) by 

 the appearance now and 

 then, due to causes as 

 yet unknown, of definite 

 and sudden changes 

 creating a new form at 



a single, though perhaps small, leap. ^Yhen such a change 

 is temporary and passes away with the individual in which 



Fig. 340. — Mutation in twin cars of corn, 

 showing the sudden variations that sometimes 

 occur, by wnich a new type may be provided 

 without the hibor of selection. 



