levels of taxonomy. The length of incubation with the 

 ostrich, and its temperature, would indicate for this species 

 a higher physiologic level, taxonomically, than does its 

 anatomy indicate for its morphologic level. It is higher 

 physiologically than morphologically. 



Whatever may be the final decision as to the relation of 

 a bird's "lowness" or "highness" to its length of incubation, 

 I am convinced that there w^ill be but one conclusion as to 

 the correlation of incubation temperature and the length of 

 incubation, and that it is highly probably that, ultimately, 

 the last word on this question will have to be said through 

 determination of temperatures taken between the eggs and 

 under the incubating parent (real or foster). 



CoUateral Evidence 



Perhaps some additional light can be thrown on the 

 .theory in hand, through a review of the incubation condi- 

 tions which prevail amongst birds' nearest relatives, the rep- 

 tiles, and by a scrutiny of the effects of temperature on eggs 

 other than those of birds and reptiles. H. Milne-Edwards 

 (38) states that the period of incubation of silk-worm eggs 

 can be prolonged to fifty days if subjected to low tempera- 

 tures, that they hatch in thirty-four or thirty-six days if 

 kept at a temperature of from 77° to 86° F., and in sixteen 

 to eighteen days if they are maintained in a temperature of 

 86* to 95° F. Though it is true that these eggs belong to a 

 creature in a class widely separated, and very divergent 

 from the Class Aves, it is still quite suggestive that with 

 them is found a high grade of elasticity in the length of in- 

 cubation, which is governed by variations in the temperature 

 conditions; this is mentioned to illustrate how potent an in- 

 fluence variations in temperature can have on embryonic 

 development. The close relationship between reptiles and 

 birds warrants a careful examination of the facts touching 

 on incubation of reptile eggs ; it is highly possible that the 

 incubation of such eggs is subject to the same variety of 

 control as those of birds, and that the same "temperature 

 and ascent" theory applies as well to reptiles', as to birds' 

 eggs. In line with this thought one may recall that the 

 evolution of "parental care" shows a series of steps from 

 low to high, broken it must be said, however, by striking 

 exceptions; the evolution of parental care is well seen in 

 reptiles and birds, taken as a whole, and has been said (155) 

 to be associated "with the need of higher temperatures for 

 development." 



Reptile eggs, in all probability, have a normal minimum 

 duration of incubation, which may be peculiar to each genus, 

 i. e., a specific incubation length; such eggs withstand suc- 

 cessfully a much longer and some a far more severe chilling 



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