98 THE STORY OF THE BIRDS. 
in these parts are much increased. It is said that the 
cruel experiment has been tried of baring and burn- 
ing the breast of a common rooster, whereupon he at 
once began sitting on eggs to cool himself. 
All this, if true, leaves the increased circulation at 
this particular season yet to be accounted for, but in- 
cubation itself may have come about in such a man- 
ner as to develop both the habit and congestion to- 
gether, for the mere act of sitting long in one place 
tends to make the breast warm, while the increasing 
heat of the hatching eggs tends also to heighten the 
temperature. Now the rhythm, or regular recur- 
rence, of a physiological condition is well known to 
be easily established, just as we train ourselves to 
hunger or to desire sleep at certain intervals. Birds 
exhibit these physiological rhythms in other ways 
than this. 
Since so many birds even after laying one egg be- 
gin at once to sit before others are deposited, it may 
be that incubation began in the act of laying. It is 
well known that an egg begins to hatch even before 
it is laid, and in many reptiles its embryo is thus per- 
fected within the body. It is not impossible that such 
birds as lingered long over the prior eggs before or in 
the act of laying the succeeding ones, or rested on 
them a while from the exhaustion of the act, or possi- 
bly took to roosting on them to protect them, had their 
young more surely and quickly hatched out, and 
hence, by inheritance of the tendency, natural selec- 
tion could easily intensify the habit. 
Besides the sun-hatching tendencies exhibited by 
