198 POULTRY BREEDING 
the maturing of pullets to the laying age at the same 
time. All the theories that hatching chicks in incubators 
year after year would eventually destroy the vitality of 
a flock have been thoroughly disproved and modern poul- 
trymen give the incubator their unqualified approval. 
INCUBATION.—\W hile the art of incubation is a very 
old one, the science of it is just beginning to be under- 
stood. The Egyptian or Chinese operator uses his mud 
oven and successfully hatches chicks without knowing 
why he uses the methods he does, only knowing that his 
father and his father’s father for generations have done 
the same thing in the same way with the same results. 
Strangely enough civilized incubator operators have 
never undertaken to hatch chickens in an incubator large 
enough for a-man to enter, and today there is probably 
not a single hatching machine in this country or Europe 
large enough to allow the operator to enter it and per- 
form his work from the inside. There is no good reason 
why this is not done, for the temperature is not higher 
inside an incubator than it often is during the warmest 
days of summer in almost any state in the Union. We 
have what are called mammoth incubators in these days, 
capable of hatching thousands of eggs at a time, but they 
are controlled and operated entirely from the outside. 
Our incubator knowledge has accumulated slowly and 
through many mistakes. For years it was comparatively 
easy to hatch chicks in an incubator but hard to raise 
them after they were hatched. This trouble was attri- 
buted to the brooder used in raising them and many 
experiments were made along this line. \When it was 
decided that brooders were not at fault the trouble was 
attributed to carbonic acid gas (carbon dioxide) liberated 
in the process of hatching, and we had incubators made 
