There is support to the further idea that the optimum 

 may A^ary according to the stage of the incubation. Job (95) 

 says that for ducks (sp?) the best temperature during most 

 of the incubation is 103° F., and slightly before or at hatch- 

 ing it is 104° F. One might predict this. It is a fact that 

 with many birds, as the incubation period nears its close, 

 the bird's belly becomes more and more bare, permitting a 

 closer and closer contact of parent to eggs, a condition 

 facilitating heating of the eggs; and, too, that towards the 

 close of incubation the eggs themselves, especially in a large 

 set of eggs, produce and disseminate heat (160) ; these two 

 facts forming a combination calculated to subject the eggs 

 to a gradually rising temperature as the incubation nears 

 completion. 



Furthermore, on the basis of what is known about avian 

 (embryonic and post-embryonic) temperatures, this gradual 

 rise of the incubation temperature would square with what 

 goes on within the egg as it is incubating. I am not doing 

 violence to the facts, nor yet making an overdraft on the 

 imagination, in believing that a new bird, in its embryonic 

 development, climbs up a series of evolutionary levels, from 

 low to high, and that under these circumstances one would 

 expect to find lower temperatures more fitting to the early 

 parts of the incubation period, and higher temperatures in 

 the later portions of the period, dirt'erences which may be 

 slight, yet none the less significant. The optimum incubat- 

 ing temperature (in artificial incubation) for the domestic 

 hen is given as 102° F. (early) and 103° F. (late) (33); 

 for ducks (sp?) for the first three weeks as 102° F., and the 

 last week 103° F. (34) ; for the ostrich 101° F (160), and 

 for the rhea (Rhea Americana) as 103° F. (13). 



There is a large field for research in this question of 

 temperatures of artificial incubation ; there is also an en- 

 gaging and unexploited field for investigation in the tem- 

 perature conditions of the nest in natural incubation; special 

 thermometers have been constructed to register this tem- 

 perature, but the data are too few to require special notice 

 here. This method of study might be undertaken with the 

 aid of the ordinary clinical thermometer, and it remains 

 to, and should, be vigorously prosecuted. 



While it is known that there is a slight upward swing 

 of the optimmn temperature towards the end of artificial 

 incubation, it has not yet been demonstrated in natural con- 

 ditions. Still so much points to the high probability of its 

 existence also in natural incubation that one can safely 

 accept it as tentatively demonstrated. 



If there be an optimum incubation temperature, which 

 A'aries with the species and, also, according to the degree 

 of embryonic development in the egg, it would seem safe 



48 



