SECT. 2] AND WEIGHT 483 



boxes, especially with regard to the disposal of incombustible waste, 

 is it not possible that their incubation time would naturally tend to 

 be shorter than that of beings such as mammals which can con- 

 veniently excrete their embryonic waste products through the 

 maternal kidneys? It is perhaps justifiable, therefore, to see in Fig. 72 

 the results of the closed-box system, development inside it being 

 adaptively hastened. It would be very interesting to have parallel 

 sets of data for insects and reptiles, and one might predict that they 

 also would take relatively shorter times than the mammals, but so 

 far I have not succeeded in finding any data from which graphs 

 could be constructed. 



As for the comparatively mean size of the largest bird at hatching 

 compared with that of the largest mammal at birth, it has been 

 probably more than once suggested that eggs above a certain size 

 would begin to suffer from prohibitive mechanical difficulties. An 

 egg large enough to produce a bird as big as an elephant at birth 

 would require, either internal struts, which would be impracticable, 

 or else an extremely thick shell (see Friese's work, p. 239) which 

 would raise great difficulties with respect to gaseous exchange. It 

 is likely, therefore, that 100 days is the extreme limit to which ovi- 

 parous animals can prolong their incubation time (without hiber- 

 nating), as against the 600 or more which are possible to mammals. 

 Is this connected with the extinction of the Aepyornis? 



It might well be asked at this point how it was that the extinct 

 reptiles attained their prodigious size if they were oviparous, and the 

 answer seems to be that for the most part they were not. Some form 

 of ovoviviparity was common, judging from the numerous finds of 

 small skeletons within the abdominal areas of the larger ones. Whether 

 these were really embryos or perhaps rather remnants of undigested 

 food is not yet, and probably never will be, certain, but the question 

 has been discussed by Fraas; Liepmann; van Straalen, and others 

 and the general opinion is that they should be regarded as embryos. 



We may conclude that the relatively rapid development of birds 

 is an adaptation to cleidoic life, perhaps associated with the high 

 temperatures of birds. 



It is interesting that hibernation of embryos is not unknown. The 

 best known case of this is probably the silkworm, the embryo of 

 which spends about 8| months in a more or less quiescent state, not 

 advancing to any extent with its development. Dendy reported in 



31-2 



