48o ON INCREASE IN SIZE AND WEIGHT [pt. iii 



as we have seen is only true within wide Hmits, for the ostrich and 

 the kiwi have equal incubation lengths, yet the ostrich's egg weighs 

 3I lb. while that of the kiwi weighs less than i lb. It is natural that 

 if incubation time depends to some extent directly upon egg- weight, 

 it should depend upon bird body-weight ; for as Huxley has shown, 

 the egg-weight varies closely with the body-weight, though the eggs 

 of large birds are not as large as they should be in proportion. Lastly, 

 Pycraft had a theory that incubation time depended on yolk-weight, 

 but as neither he nor anyone else accumulated any data with which 

 to test the hypothesis, and as it is not in any case a very attractive 

 one, it may be dismissed at once. 



In Bergtold's view the body-temperature of the parent bird is the 

 important factor. It is likely a priori that the larger the bird the lower 

 its body-temperature, and a degree or two may make a big difference. 

 Bergtold gives in his book a long list of bird temperatures and it 

 certainly seems that the smaller the bird the higher the reading, 

 but unfortunately the data are as yet too few for it to appear whether 

 the exceptions noted above as destroying other theories are abolished 

 on this one. Bergtold's theory is complicated by various taxonomic 

 considerations, in which he supposes, following Sutherland, that the 

 higher a bird is taxonomically, the higher its temperature. As the 

 smaller birds (and mammals) are believed to be the most recent 

 palaeontologically, this may well be the case. 



In favour of Bergtold's view are the experiments of Heinroth who 

 reported that the eggs of the Egyptian goose hatch in 28 days under 

 a common hen and in 30 days under a Muscovy duck. It is known 

 (see Fig. 83 a) that within narrow limits, the speed of embryonic 

 development in ordinary hen's eggs can be controlled by temperature 

 regulation. "The diminishing size of birds," says Bergtold, "ac- 

 celerated the metabolic rate, elevated the body-temperature, and 

 so shortened the incubation period." According to Bergtold the 

 scanty data of reptilian incubation times support his temperature 

 theory. 



Returning now to the comparison between mammals and birds 

 which was raised by Figs. 68 and 71 in which the slope of their 

 weight/incubation-time lines was seen to be different, it is interesting 

 to plot the two on the same graph, as is done in Fig. 72. It can 

 now be seen that not only are the slopes different, but the 

 absolute values are also different, so that on the whole it takes less 



