acter as the nervous system, Avhich last has been used to place 

 the Corvidse at the apex of the avian tree. If this "tempera- 

 ture and ascent theory" of the control of the length of in- 

 cubation, with birds, be correct, it will, at least, bring about a 

 semblance of order where before more or less confusion pre- 

 vailed ; if it be fallacious, and untenable, then I can at least 

 hope that this discussion may point out the lines of inves- 

 tigation along which, in the future, ornithologists will hav^o 

 to work in order to solve what in the past has been an 

 unanswered riddle. 



Correct or incorrect this discussion may, I hope, draw 

 attention to the importance of gathering accurate data on 

 the true length of incubation with birds, and its relation 

 to their physiology and taxonomy. 



It api^ears to me that some of the previous explanations 

 as to what fixes, determines, or controls the length of avian 

 incubation have held more or less of the truth, because they 

 concern conditions which are effects of a single underlying 

 cause, but I believe that there is no experimental evidence 

 whatsoever, that differences in the size of bird, age of fe- 

 male, longevity of the parents, condition of the young at 

 hatching, the size of the egg, or its shell, or the yolk size, or 

 various telluric conditions ever permanently alter the trua 

 length of incubation; there is abundant experimental evi- 

 dence (both accidental and intentional) that variations in 

 temperature can prolong the time of incubation, and can 

 shorten it to a slight extent, and presumptive evidence that 

 this effect of variation in temperature in shortening the 

 length of incubation, does in the end bring about the large 

 variety of incubation lengths found with birds today. 



Data Needed for' Further Study of This Prohlem 



The prosecution of this study has uncovered a lament- 

 able dearth of information in many phases of the questions 

 at ifsue, and in now calling attention to a few of these de- 

 plorably incomplete chapters of ornitholog;\\ I would remind 

 ornithologists that in them they will find openings which 

 will lead to splendid opportunities for original research in 

 almost unexplored and unexploited fields, fields which may 

 soon be swept out of existence before the devastating march 

 of civilization. Data needed : 



Exact length of incubation period of birds and reptiles, 



Exact length of incubation of birds in polar and tropical 

 regions, 



The period of viability of birds' eggs. 



The weights of birds, preferably of the breeding female, 



The weights of birds' eggs, 



The effects of superheating on birds' and reptiles' eggs, 



75 



