SECT. 



2] 



AND WEIGHT 



493 



pigeon (i8 days), the guinea-fowl (25 days), the duck (26 days), the 

 partridge (27 days), and the turkey (28 days). The results were 

 related to weights of embryos equal to those of the chick embryo 

 on different days, and as can be seen, the values for the other embryos 

 fell uniformly on the chick curve. The length of incubation would 

 thus appear to be a tune played as it were, " adagio " in the turkey and 

 "allegro" in the pigeon.* 



o 

 EI 

 D 



m 



H6N : MuK.ttU^Ko^ 



Partridse: 



Hen : \NKUre Wyo>«Ulte 



Pigeon 



DUCK 



Turkey 

 guinea-fowu 



Hen • Ba/»*»««ei<Ur 



% of total incubation time 

 Fig. 74- 



The possible adaptive significance of incubation time has been 

 shown remarkably in a paper by Friedmann, who found that the 

 incubation times of the cowbirds, such as Agelaioides, Molothrus and 

 Tangavius, varied according to their degree of parasitism. M. afer, 

 which is very parasitic, has an incubation time of 10 days (the 



* For this it was necessary to assume that the pre-natal growth-curves for wet weight 

 were alike, an assumption which was subsequently shown to be legitimate by Kaufmann. 

 The instantaneous percentage growth-rates of the pigeon and the hen do not begin to differ 

 until after hatching ; and this holds, according to her, for heart, liver and eye, as well as 

 for the whole body. The cells of the pigeon embryo, however, are smaller than those of 

 the chick by about 30%. 



