A WARNING FROM NATURE 
Yawning is commonly said to be a means of ventilating the lungs, but ventilation appears 
really to be a very small part of the act. It seems more likely that it is a warning 
of drowsiness and need of sleep. On this hypothesis it could easily be shown to be 
useful in natural selection. Suppose an animal traveling in the open is suddenly 
overcome with fatigue and the need of sleep: if he went to sleep on the spot he would 
be exposed to enemies and perhaps perish. The yawn warns him to look for shelter and 
rest, and there seems to be a slight quickening of the faculties after yawning which 
aids the animal to get to a place of safety. The act may still be of use to man in a 
similar connection, warning him when he has worked long enough and needs to seek 
repose. This explanation is hypothetical, as the subject has been little studied; but 
yawning appears at any rate to be a very primitive function, going back far beyond 
the mammals in the scale of evolution. Photograph from the Nursery Studio, 
Washington, D.C. (Fig. 5.) 
