240 PRINCIPLES OF POULTRY BREEDING 
little value to the poultry breeder. The small breeder, and even 
the casual observer, 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 
Fia. 124.—Rumpless birds—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 
