234 PRACTICAL COURSE IN BOTANY 



it first appeared, it is called a " sport," and leads to no 

 important results; but when it is inherited by the offspring, 

 so that it is capable of giving rise to a new species, it con- 

 stitutes a " mutation." The value of a mutation to breeders 

 in saving time and trouble is obvious. Professor Hugo de 

 Vries, a Dutch botanist, was the first to call attention to the 

 importance of mutation and its bearing upon the production 

 of new species. 



265. Factors in the evolution of species. — Variation, 

 heredity, and selection are the three principal agents under- 

 lying all changes, whether for the improvement or deteriora- 

 tion of living organisms. The influence of external surround- 

 ings in keeping up a variation once begun, or in starting new 

 ones, is also a factor that cannot be disregarded. It is for 

 this reason that natural species are so much more stable than 

 those brought about by man. The former, being evolved in 

 response to natural conditions, are liable to change only as 

 alterations in their surroundings are brought about by the 

 slow operation of natural causes. But the types resulting 

 from the breeder's art, produced as they often are in response 

 to human demands and in direct opposition to the require- 

 ments of natural conditions, are in a sense purely artificial, and 

 can be preserved only by keeping up the artificial surround- 

 ings by which they were developed. Hence, the importance 

 of diligent cultivation and constant care and tillage, without 

 which the most carefully selected stocks may quickly " run 

 out " and degenerate into worthless forms. 



Practical Questions 



1. Which are the more pliable to the breeder's art, annuals or peren- 

 nials? Why? (91,93,262,263.) 



2. What advantage is gained by using buds and grafts instead of 

 seedlings in making new varieties of fruit trees? (257, 259, 260.) 



3. Would it be practicable to breed new varieties of slow-growing forest 

 trees, like oak, cypress, redwood, from seeds ? Why or why not ? (93, 

 262, 263.) 



4. Can you account for the existence of the numerous intermediate 

 forms between the different species of oaks found in nature? (255, 257.) 



