INCUBATION 
stated, does not depend upon external circumstances, nor 
can it be influenced by them without damage or loss of 
life ; for instance, no increase of heat beyond that employed 
or generated by the parent birds will hasten or quicken 
the production of the young bird, while, on the other hand, 
a lower temperature is likely to prove fatal instead of 
retarding its development. 
The consideration of these immutable laws, and the con- 
stancy of their appearance before us in such various 
shapes and forms, prove beyond all doubt the existence of 
phenomena of which we have at present but a faint 
glimpse. Do we not see the same regularity of a flxed 
time, and that the same number of days between the 
changes that occur in the maladies to which we and other 
animals are subject — are not the marked and noticeable 
changes every seven days ? That this number, recurring 
as it has and must have done in all time, doubtless led 
our species to recognize the seventh day, and our laws, 
terms of apprenticeship and imprisonment, leases of 
houses and land, and endless other matters, probably came 
into use from the observable regularity of this fixed and 
unalterable law set before us. 
It is not only in the incubation of the eggs of birds that 
this recurrence of the seventh as a period of change is con- 
stant, but in the instance of birds’ eggs it is so easily 
watched and recorded, for the development of most 
diseases is marked by the same time ; whether it be small- 
pox, ague, or any other fever, the same law appears to 
influence the state of the sufferer, and mark the changes 
that take place regularly, for better or worse. 
Even in the ordinary course of the natural production 
from the egg, we may be pardoned in remarking that much 
misapprehension exists. It is by many believed that the 
parent birds assist the young in breaking out of the egg. 
237 
