240 PRINCIPLES OF POULTRY BREEDING 



little value to the poultry breeder. The small breeder, and even 

 the casual observ^er, cannot, however, fail to note the great 

 variation in breeds of poultry owing to diverse conditions in their 

 environment. By this is meant all the external conditions of 

 their life; as feed, climate, housing, enemies, and especially with 

 young growing birds, their range. All internal processes of devel- 

 opment are dependent upon external influences for their natural 

 expression ; hence the breeder has every incentive to create exter- 

 nal conditions which will conduce to the growth and highest 



FlQ. 124. — Rumpless birda — a common example of spontaneous variation or mutation. 



development of the individual, and these conditions will in them- 

 selves contribute to the development of the particular type or 

 variation which is desired. 



Atavism. — By atavism is meant the invariable tendency of 

 individuals to revert to the original type. It is sometimes called 

 reversion, retrogression, or breeding back. It differs from the law 

 of heredity in the fact that the characters cropping out represent 

 ancestry more or less remote rather than that near at hand. An 

 excellent example of atavism is the frequent hatching of black 

 offspring from apparently pure-bred Barred Plymouth Rock 

 matings. This is a reversion in type to the original Black Java 



