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.) 
+. Can you account for the existence of the numerous iztermediate 
forms between the different species of oaks found in nature? (255, 257.) 
