on hypothetical grounds alone. Thus, this relation of metab- 

 olic intensity, size, and length of incubation may account 

 for the loose relation known to exist between a bird's size 

 and its incubation length, a relation admitted in the discus- 

 sion on that theory. It seems to me that there is direct 

 experimental evidence of the effect of differing temperatures 

 on the incubation length ; it has been shown that an optimum 

 temperature is the most important of the three factors 

 necessary to successful incubation, and that this optimum 

 emanates from the incubating parent, and that with hens' 

 eggs, their usual minimum duration of incubation can be 

 shortened a few hours by carefully raising the temperature 

 of the incubator a little above the usual optimum, which 

 is identical under the hen and in incubator practice. 



Until evidence is forthcoming to show that these two 

 conditions do not apply to all other birds' eggs, it seems 

 tenable to believe that as birds have slowly risen in evolu- 

 tionary height, their temperatures have also been corre- 

 spondingly elevated, and that these increasingly higher tem- 

 peratures have gradually shortened the periods of incu- 

 bation. What occurs with a hen's egg during one incubation 

 period through a slight elevation of the incubator tempera- 

 ture, has taken place in lesser degrees, in nature, for count- 

 less bird generations: each increment of temperature eleva- 

 tion, however slight, added by each succeeding ascending 

 generation, has correspondingly influenced the length of in- 

 cubation, always resulting in some shortening. This process 

 has been, probably, exceedingly slow, consuming ages in 

 accumulating a patent change, both in the body temperature 

 and in the incubation period yet, in the end, as we see it 

 today, it has swept on with striking results, the Passeres 

 having forged ahead, through their very high temperatures, 

 to a fourteen-day period, while the ostrich and other birds 

 with low temperatures are marooned at forty-two (or more) 

 days. From the foregoing it seems, to me, impossible to 

 escape the conviction that the true length of incubation is 

 -fixed or determined by the temperature of the incubating 

 parent, long with low temperatures, and brief with high 

 temperatures. Inasmuch as there is a goodly amount of 

 support to Sutherland's law of low temperatures with 

 "low" birds and higher temperatures with "high" birds, I 

 would tentatively enunciate the idea that the true length of 

 incubation is determined by the bird's position in the avian 

 scale of life, basing this hypothetical law on the assumed 

 relation of temperature elevation to incubation length, and 

 of temperature elevation to the species' position in avian 

 taxonomy. 



If the various data collected together in this study are 

 now to be examined and interpreted from the viewpoints 



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