Ornithology,. , 175 



hient, &ndl other circurnstances which have hitherto eluded the 

 observation of naturahsts, and are uniform and constant, as long 

 as the causes which operate in their production, continue to act. 

 At first sight it might be supposed that the difference of temperature 

 is the difference in the intensity or color, in shells produced in dif- 

 ferent climates. It might be supposed too, that the different 

 depths at which shells are found in the ocean, the medium in 

 which they live being thus very different, would occasion great 

 diversity in color. Near the surface, where the heat is greatest) 

 if the operation of this cause were considerable, the colors of shells 

 should be expected to be most vivid, and as the depth increased, 

 at least to a certain extent, the intensity of color should be dimin- 

 ished. But it has been observed in bivalve shells which are found 

 at great depths, such as some species of oyster and spondylus, that 

 the lower valve, which is attached to the rock, is almost always 

 white or colorless, while the upper valve often exhibits bright and 

 vivid colors; but this difference cannot be ascribed to the differ- 

 ence of temperature, for in both valves it must be the same as the 

 matter secreted, for their formation is prepared by the same organs, 

 and is deposited in a similar manner; and indeed they are alto- 

 gether placed in the same circumstances, and the same causes, 

 excepting that the upper valve is exposed to the rays of light, and 

 is therefore colored, while the lower valve is removed from the 

 action of this cause, and is colorless. 



The same difference is observed in the valves of other shells, 

 which are produced in similar circumstances. The different spe- 

 cies of pholas which make their abode in calcareous or coral rocks, 

 and the Teredo navalis or ship worm, which pierces wood, and 

 makes it its habitation, are usually colorless. Those testaceous 

 animals too, which live at great depths in the ocean, and are thus 

 far removed from the influence of light, are also distinguished by 

 very white colors, or are entirely white. 



ORNITHOLOGY. 



NO. vr. 



Incubation. It is probable birds are endowed with an instinct- 

 ive power of regulating the necessary heat for this purpose; of 

 course, should the heat of the air, together with the natural warmth 

 of the body, on the close contact of the bird to the eggs, be too 



