which grew to maturity. This experience discloses but one 

 more of the many interesting points embraced in this prob- 

 lem, which are j^et to be thoroughly studied and settled. 



New Eacpla n ation s 

 Body-weight : egg-white index, and incubation length. 



Newton (25) says that "the size of the Q^g is generally, 

 but not at all constantly, in proportion to that of the 

 parent," attributing this relation to the necessity of the 

 parent being able to completely cover and incubate all the 

 eggs it lays in a set. This relation between the size of the 

 body and of the o^gg is only general. It is also true, in a 

 loose way, that the smaller is the bird, the smaller is its ^gg. 

 There are many notable exceptions to be found on either 

 side of the equation; the kiwi (168), megapods (16), some 

 gallinaceous birds, and the barbary duck (161) lay eggs 

 very large in proportion to their bodies, while with storks, 

 cuckoos and several other species the reverse holds true. 



These facts, and my own study of the weights of birds 

 and their eggs, suggested to me that there might be some 

 relation between the ratio of these two weights and the 

 length of incubation of the ^gg. This possibility was in- 

 vestigated far enough to demonstrate that there is not the 

 least relation discernible between the two, hence it is men- 

 tioned here, only to be dismissed. 



A Ne-w Explanation Based on Physiologic Grounds 



Most of the explanations previously propounded seem 

 inadequate, and more or less illogical, because they have 

 been based on passive conditions, such as an anatomical 

 character (size of body), or on a histologic character (size 

 of egg), or have been built on effects which merely retard 

 or suspend embryonic development, eifects which do not 

 alter the true or specif c length of incubation. None has 

 been directly correlated with an active biologic or a physi- 

 ologic process or condition, which alone, it seems to me, can 

 directly modify so vital a span as is the true duration of 

 incubation. 



Only two of these past theories have been founded on 

 physiologic grounds — longevity and the health of the par- 

 ents. The first cannot at present be said to have any bear- 

 ing at all on the duration of incubation, and the second is 

 inseparably related to another which seems to have an all- 

 important influence on the true length of incubation, a con- 

 dition now to be taken up and discussed. 



It is hard to realize that a fertilized egg is, during 

 incubation, not an organ or a detached part of a preceding 

 bird, but that it is in reality a growing neto individual^ 

 and as such must respond, or be subject, to control by special 



44 



