476 



ON INCREASE IN SIZE 



[PT, III 



but this strict relation does not hold for many birds and is even 

 more elastic than with mammals. Thus the swift and the raven 

 have the same incubation period in spite of their different sizes, 

 while the kiwi and the hen are very similar in size but have quite 



100 



10 100 



Weight of Adult Bird in Ounces 



Fig. 70. 



1000 



different incubation periods. The lapwing, again, though smaller 

 than the woodcock, undoubtedly has a longer incubation period. 

 Nevertheless, when a broad view of the whole subject is taken, and 

 the incubation time is plotted against the adult weight on double- 

 log, paper, a definite trend does appear (see Fig. 70) and the same 



100 



1-0 lO'O 100 



Weight of Egg in Ounces 



Fig. 71. 



kind of picture is obtained when the incubation time is plotted 

 against the egg-weight (see Fig. 71). The most interesting thing to 

 notice is the slope of these two lines, which is in both cases much 

 less considerable than in the mammalian graph of Fig. 68. In other 

 words, if the weight of any mammal is multiplied one thousand times, 

 the gestation period will be prolonged by about ten times, but if 



