Ornithology. ' 73 



To those who wish to make conchology a study, we can only 

 recommend them some of the late works of the authors 

 heretofore enumerated. 



Having given the general terms applied to the parts of 

 shells, we next purpose to inquire what is the nature of their 

 substance, and in what way it is produced by the animal, and 

 how it is enlarged as the animal increases in size. These 

 topics shall be the subject of these essays, which may be 

 conveniently divided into the following sections. 1. Of the 

 constituent parts of shells. 2. Of their formation. 3. Of 

 the colors of shells. 4. Of the formation of the umbilicus 

 and protuberances, &c. 5. Of the pearl and pearl fishery. 



ORNITHOLOGY. 



NO. III. 



Eggs of birds. By experiment it appears that birds do 

 not instinctively know the necessary time of incubation; for 

 unincubated eggs of a bird have repeatedly been taken and 

 placed under another of the same species, which was on the 

 point of hatching, and vice versa, those on the point of hatch- 

 ing into the nest of such as had only began to sit ; and in both 

 cases the young were brought to maturity. Birds will some- 

 times discriminate the egg of another species put into their 

 nest, and will turn it out ; but they will frequently breed up 

 the young of another when exchanged, provided they are of 

 the same age, and not very large when the experiment is 

 made. 



Those who suppose a bird capable of producing eggs at 

 will, or that any bird is excited to lay more eggs than usual 

 by daily robbing their nest, are certainly mistaken. In a 

 domesticated fowl it is probable the desire of incubation may 

 be prolonged by leaving a little or nothing in the nest to sit on. 

 It will therefore lay the number allotted by nature, which is 

 determined before the first egg is produced. If it is prevented 

 from incubation by any means whatever, it may begin again 

 to lay in five or six days ; but there is always an interval of a 

 few days, and sometimes as many weeks, which must wholly 

 depend on the age and vigor of the bird. When it happens 

 that a fresh lot of eggs is laid with only a few days' interval, and 

 that perhaps in the same nest, it is deemed a continuation for 

 want of nice observation ; but we are not to look to domes- 

 ticated animals for natural causes, for those are taken from 

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