frequently, or kept from its nest too long, or through the 

 parent birds being inattentive. 



This effect on incubation has long been known to poul- 

 try raisers; it is, however, an effect not altering the true 

 length of incubation, but merely one of cooling and retarda- 

 tion of embryonic development. Such conduct of parents 

 does not affect the true or specific incubation length. 



Longevity 



In a valuable paper on longevity in birds, Gurney (132) 

 suggested that there might be some relation between bird 

 longevity and the length of incubation. 



It is nearly impossible to reach any definite conclusion 

 on this suggestion since very little is known on that variety 

 of longevity which is most likely the only one which affects 

 the fluctuations of bird population and their correlated 

 biologic results. There are nearly a hundred records relat- 

 ing to the ages to which various species of birds live in 

 captivity or when domesticated, but this is potential 

 longevity. What is lacking, however, is information on 

 the mean or average longevity, the length of life which birds 

 attain in nature, under normal conditions of life's pressure 

 for and against them. Brehm (132) thought that longevity 

 was more or less correlated with size, and there are some 

 indications that within the Class this is true, but it fails to 

 hold good when comparing species of differing Classes. 



A curve was plotted from the longevity data given by 

 Gurney and gathered by the writer from other sources and 

 placed in juxtaposition with the curve of incubation lengths 

 of the same species; it showed no correlation between the 

 two. It is safe, from the present data, to hold that length 

 of incubation and longevity have no relation in fact, a con- 

 clusion which H. Milne-Edwards (133) reached many year* 

 ago. 



State of Young at Hatching 



Precocious, 



Altricial, 



Completeness of development. 



In these three conditions, given as determining factors 

 affecting the length of incubation, there are more or less 

 confused, it seems to me, two distinct ideas; the first is that 

 of precocity in its usual sense, i. e., the state of self -helpful 

 activity and semi-independence of young birds at hatching; 

 and the second is that of the condition of the young being 

 well on towards comj^leteness of development at hatching. 

 The first conception may or may not include the second, 

 while the second always includes the first — a newly hatched 

 duckling is typical of the first, while a newly hatched mega- 

 pod bird is typical of the second. In considering precocity 



27 



