Telluric Influences 

 Under this caption are to be included the supposed ef- 

 fects of : 



A. Geographical zone 



B. Climate 



C. Weather 



D. Geographical residence of species 



E. Site of nest. 



All of these have been mentioned by various writers as 

 being conditions controlling or determining the incubation 

 length; I am convinced that such influence any of these 

 may have on the length of incubation is merely that 

 of cooling the eggs and retarding the embryonic devel- 

 opment, merely adding hours or days to the true length of 

 incubation, which apparent length returns to the original if 

 the retarding influence be removed. 



It has been abundantly demonstrated with eggs in ar- 

 tificial incubation, that the duration of incubation can be 

 extended by subjecting the eggs to temperature lower than 

 the optimum, and that the degree and duration of the lower- 

 ing of the temperature have limits, beyond which the em- 

 bryo dies. Yet it is astonishing how much cooling or chill- 

 ing a set of eggs will successfully withstand in incubation. 



The writer has known of a set of house finches' eggs 

 being left uncovered all night during a spell of cold weather, 

 and yet producing a full number of normal and vigorous 

 nestlings. The structure of the egg lends itself to resistance 

 to too rapid cooling, for the shell, with its small "pores" 

 full of air, and the shell membrane, together, make a good 

 insulating medium. Brehm (quoted by Ingersoll, 110), says 

 it requires one hour and forty-five minutes at fifteen degrees, 

 Fahrenheit, to freeze a living egg; this means that it takes 

 seventeen degrees, Fahrenheit, of frost for nearly two hours' 

 exposure, to kill the developing embryo, and it is apparent 

 that this time may vary proportionately with the size of the 

 egg. It is well known that eggs in the later stages of in- 

 cubation cannot resist successfully such low, or prolonged 

 low, temperature as just mentioned, but that they do suc- 

 cessfully withstand milder degrees of frost for much longer 

 periods, especially during the early days of incubatioij, is 

 equally well known. 



It is also known that the incubation period of the do- 

 mestic hen can be prolonged to the twenty-third or twenty- 

 fourth day by judicious cooling during incubation. That 

 this effect of cooling obtains during the incubation of 

 various other birds at large is undisputed. 



Zone 



The writer has been unable to find any published infor- 

 mation bearing on the possible effect of geographical zone on 



39 



