OF N A T U R E. 83 



The fecundated egg requires a certain, and 

 proportionate degree of heat for the expanfion 

 of the Jlamina of the emhryo. 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 

 fcetus. 



The females of quadrupeds have an uterus^ 

 contrived for eafy geilation, temperate and 

 cheriffiing warmth, and proper nourifhment 

 of the fcetus^ as moft of them live upon the 

 earth, and are there fed. 



Birds^ in order to get fubfillence, and for 

 other reafons, are under a neceflity of Ihifting 

 place ; and that not upon their feet but wings. 

 Geftation therefore would be burthenfome to 

 them. For this reafon they ky eggs, covered 

 with a hard fhell. Thefe they fit upon by a na- 

 tural inftindl, and cherifli ^ill the young one 

 comes forth. 



The ojlrich and cajfowary are almofl: 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 ihtfatus, 



G 2 Fifies 



