SECT. 2] 



AND WEIGHT 



475 



® opening of eyes 

 ^ appearance of eye-fissure 

 " " rodent teeth 



And there is also the consideration that gestation time in some 

 animals must be arranged to suit the grazing season. This factor 

 would probably account for a good many of the divergences of 

 species from the line shown in Fig. 68. 



Again, within the individual species, birth can apparently be 

 shifted to some extent backwards and forwards. Bluhm's work shows 

 that the opening of the eye, the 

 appearance of the ears, and 

 other marks of increasing differ- 

 entiation in the mouse, occur at 

 a fixed time after conception, so 

 that the smaller the birth-weight 

 the longer the time between birth 

 and the appearance of the mark 

 in question. This relation is illus- 

 trated by Fig. 69. 



What governs the incubation 

 times of birds? The problem 

 has been much discussed, but 

 by far the best treatment of it in the literature at present is the book 

 of Bergtold. Of the 19,000 species of birds known, we have information 

 concerning the incubation periods of 625, and although most of the 

 facts are given in Table 62 Bergtold's book must be consulted for 

 the full material. The length of the incubation period varies more or 

 less with the size of the bird. 



Fere long ago pointed out that the smaller the egg the smaller the 

 incubation time: thus: 



Birth weight 

 Fig. 69. 



Duck 

 Hen 



Weight of 



Days egg in gm. Ratio 



25 739 I : 084 



21 6018 I : 0-815 



