OF NATURE, 
*3 
§* 1 2 ® 
The fceeundated egg requires a certain, and 
proportionate degree of heat for the expanfion 
bf the ftamina of the embryo . That this may 
be obtained, nature operates in different man- 
ners, and therefore we find in different claffes 
of animals a different way of excluding the 
foetus. 
The females of quadrupeds have an uterus , 
contrived for eafy geftation, temperate and 
cherifhing warmth, and proper nourifhment 
of the foetus^ as molt of them live upon the 
earth, and are there fed. 
Birds i in order to get fubfiftence* and for 
other reafons, are under a neceffity of fhifting 
place •, and that not upon their feet but wings. 
Geftation therefore would be burthenfome to 
them. For this reafon they lay eggs, covered 
with a hard fhell. Thefe they fit upon by a na- 
tural inftindt, and cherifh till the young one 
comes forth. 
The cftrich and caffowary are almoft the 
only birds, that do not obferve this law ; thefe 
commit their eggs to the fand, where the in- 
tenfe heat of the fun excludes the foetus. 
G 2 Fifhes 
