the extremes of different eggs; there is no evidence of any 

 similar variation in the incubation period of this species. 



Further study on this point is to be made before one can 

 render a verdict for or against it. 



Size of Yolk 



It has been suggested by Pycraft (138) that an abbre- 

 viation of the incubation period with many birds has been 

 brought about by a gradual lessening of the "food yolk," 

 presumably meaning that part of the egg commonly termed 

 the yolk. 



Enunciated by so keen a student and investigator, and 

 such an original thinker, this suggestion must be considered 

 with care and attention. 



It is extremely difficult to discuss this idea in any way, 

 both because Pycraft produces no statistical or experimental 

 evidence in support of his theory, and because there seems 

 to be no information available concerning the size of the 

 yolk in various birds' eggs. 



The only publication coming to the writer's hands, 

 which throws any light on this theory is a paper by Curtis 

 (140), dealing with the factors which influence the size, 

 shape, and physical constituents of the egg of the common 

 hen. In this study (a model of its kind) the yolk is shown 

 to be the second most variable component of the egg, and 

 that of two eggs, the yolk is relatively larger in the smaller 

 egg ; also that the weight of the yolk is not determined, for 

 example, by the hen's weight alone but that it is markedly 

 modified by her hereditary constitution, physical condition 

 (state of health), stage of development (age), the season 

 of the year, and the position of the egg in the series of eggs 

 laid at the time. If these deductions by Curtis apply equally 

 well to other birds' eggs, and, until shown otherwise, it is 

 fair to hold that they do, it is difficult to see how Pycraft's 

 suggestion can be tenable, since, in general, the shorter in- 

 cubation periods are characteristic of birds laying small 

 eggs, which, as has just been shown, have relatively larger 

 yolks, in place of smaller yolks, as his hypothesis requires. 



Pycraft mentions specifically, as having reduced yolk- 

 size (or weight?) "razorbills, guillemots, (and) many gulls," 

 but it is not possible to resolve this problem as it may apply 

 to these and other species, until the relative sizes of their 

 egg yolks be determined and the periods of incubation of 

 these species be compared and viewed as to their relation to 

 such yolk sizes. While judgment must be suspended also 

 on this point, at least ijntil a considerable mass of informa- 

 tion relating to the size of the yolk in the eggs of different 

 species of birds be secured, still it seems to me that this 

 hypothesis is rather dubious. 



38 



