nest produces, not variability of the incubation period, but 

 differing lengths in different species. Thus, by this explana- 

 tion, the tree-nesting ducks and the European vulture (Vul- 

 ture monachus) have long incubation periods because of 

 their nesting places being so located as to have security from 

 ordinary foes. 



This, however, does not explain the long incubation 

 periods of ground-nesting Anatidae, such as a goose or a 

 swan, nor can it be reconciled with the short incubation 

 period of a flicker, with which species the nesting place is 

 reasonably free from intrusion by foes, or the short periods 

 of cliff-nesting swallows. 



The writer believes that all of these supposed influences 

 on the true length of incubation discussed above really bring 

 about only a retardation or a suspension of embryonic de- 

 velopment, a suspended animation in ovo, as it were, and 

 that in no way do they affect the true length of the incuba- 

 tion period, which, when distorted, is resumed with the next 

 incubation or the. following generation, granting the en- 

 vironmental conditions be normal. From the point of view 

 of this study, one must carefully differentiate between a 

 change of incubation length which is temporary, and affect- 

 ing only one set of eggs, and one which is permanent, and 

 passed on to succeeding generations. 



Assuming, as one must, that chilling the eggs during 

 incubation does produce such effects, the question must be 

 interjected, "Do the eggs of all birds withstand equally well 

 similar degrees of chilling ? " It must be considered because, 

 if they do not, those birds whose eggs endure successfully 

 the longer periods and greater degrees of cooling will ex- 

 hibit a greater variability in their incubation periods, the 

 same being a source of perplexity in studying the problem 

 in hand. 



As has been intimated before, I believe that it will be 

 found by future investigations (if it has not already been 

 so determined) that eggs of the more primitive birds will 

 remain viable under adverse conditions much longer than 

 will those of the highpr birds under like conditions ; in other 

 words, under like conditions of cooling, rough usage and 

 neglect, a duck's egg will be viable long after a robin's egg 

 will be spoiled. I doubt if thrushes', warblers', finches' and 

 swallows' eggs would hatch if subjected to a course of treat- 

 ment as was given to some eggs of "shore birds" by Beebe 

 (114). This observer gathered a number of eggs of terns, 

 skimmers, gulls, green herons (all the ^ggs being well on 

 in the incubation stage) for purposes of embryologic study, 

 which were carried about for several days without any sus- 

 picion of their remaining viable; yet several hatched on 

 being placed in an incubator, and produced normal "chicks," 



43 



