TYPES OF VARIATION 239 
tions are commonly called “ sports ’’; they are of little economic 
value, owing to the impossibility of predicting their appearance 
and to the readiness with which they disappear. If inbred with 
the parent stock they frequently cease to reproduce. The better 
method for improvement is the slow one of gradual selection 
from time to time of all variations which tend toward the ideal 
i 
Fig. 123.—Eggs of different size, showing functional variation. Upper and lower 
rows laid by different hens. The difference in size is a result of functional variation due to 
varying rapidity in the development of the ovum and variations in glandular secretions in 
the oviduct. 
type. A typical example of mutation is the rumpless or tailless 
bird (Fig. 124), which shows an inability to reproduce when 
closely bred, and when bred to tailed birds produces very few 
rumpless progeny. . : 
Causes of Variation.—All variations are influenced to a greater 
or less degree by two groups of causes,—namely, internal and 
external. Accurate knowledge pertaining to this first group of 
influences is so limited, and the subject so complex, that it is of 
