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 



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 



