482 ON INCREASE IN SIZE [pt. iii 



time to make an equivalent birth-weight of bird than of mammal. 

 (The hatching-weights are here obtained by taking 75 per cent, 

 of the egg-weight in grams, the remaining 25 per cent, being 

 of course divided between shell-weight, weight of membranes 

 left behind, weight of water-vapour evaporated during incubation, 

 and weight of material combusted in the same period.) It is also 

 evident that the largest bird is, as regards birth-weight, 250 times 

 as small as the largest mammal. We are thus left with the following 

 three considerations (which apply wholly to birth-weight) : 



(i) Although there are mammals as small as the smallest birds, 

 there are no birds as large as the largest mammals. In fact the largest 

 bird is only a little larger than the half-way point on the mammalian 

 line. 



(2) The time required to make a given weight of bird is always less 

 than that required to make a given weight of mammal, as may be 

 roughly expressed by the following table: 



(3) The prolongation of the incubation time caused by raising 

 the hatching-weight a given amount is not so considerable as the 

 prolongation of the gestation time caused by raising the birth- weight 

 by the same amount. 



The bird is therefore much more rapid than the mammal in its 

 development, and one may well ask whether this is not an adaptation 

 to life within the egg. In Section 9 and in the Epilegomena the 

 conception of the "cleidoic" egg will be developed, but without 

 forestalling those discussions, it may be said here that eggs such as 

 those of reptiles, birds and insects, with their isolation from their 

 terrestrial environment, quite unlike the close dependence of many 

 aquatic eggs upon the sea, are closed systems, characterised, as it 

 seems, by a definite type of metabolism in which protein breakdown 

 is suppressed and uric acid takes the place of urea and ammonia 

 as nitrogenous waste products. If, then, there are serious problems 

 confronting animals which make their embryos develop in closed 



