ferent bird temperature conditions, for it has been shown 

 (162) that the eggs of the Eg^q^tian goose hatch under a 

 common hen in twenty-eight days, and under a muscovy 

 duck in thirty days. One more suggestive contribution to 

 the experimental evidence going to support the connection 

 between temperature, size, and incubation length can be 

 given here; Milne-Edwards (38) proA'ed "that abnormal 

 elevation of incubation temperature during the first period 

 of incubation tends to diminish* the size of the chick, and 

 to produce dwarfs, though shortening the period.'''' 



If it be assumed that birds have in general grown 

 smaller as they evolved upward, the question can be inter- 

 polated here (in fact, it can be considered the "acid test" of 

 the truth of the assumption), have they benefited by their 

 smaller size? It would seem so, since the efflorescence of 

 the avian tree is made of the Passeres, practically all of 

 which are A"ery small. Perhaps the diminishing size of birds 

 steadily accelerated their metabolic speed, which elevated 

 the body temperature (or vice versa), and initiated a physi- 

 ologic cycle, which is still revolving in ever diminishing 

 circles. 



If a rise of temperature in an artificial incubator can 

 diminish the size of the chick, and shorten the incubation 

 period, is it impossible to have the same thing happen in na- 

 ture? Because one cannot see or measure the slight decrease 

 in size, following an equally slight elevation of temperature 

 in a given species, is it not, however, possible that thousands 

 of such slight elevations of temperature, and of such minute 

 reductions in size, can be cumulative, and end in results 

 which are seen as existing conditions of today. It seems to 

 me not only possible, but highly probable. 



For the sake of convenience, I will term the explanation, 

 (just elaborated), of factors fixing or controlling the true 

 length of incubation "the temperature and ascent theory." 

 A careful search through literature has disclosed but two 

 hints that avian "lowness" or "highness" might have some 

 effect on the incubation length. Both are more or less in- 

 direct, and the first is given in a few words by Fiirbringer 

 (102), who apparently considered this explanation, only to 

 reject it by saying, "Thus the number of incubation days, 

 which arranges itself more according to the size of the egg 

 and bird, than to the relationship boundaries, can be of no 

 great taxonomic* significance," with which conclusion I can- 

 not concur; the second is a suggestion made by Gadow 

 (150), who held that the developmental period (embryonic 

 plus nest life) stands in direct relation to the degree of "low- 

 ness" or "highness" of the bird, and that the "highest" birds 



•Italics by W. H. B. 



65 



