RELATIVE VIABILITY IN MAMMALS AND UIRDS. 535 



of average duration vary, but tliese ami the maximum durations 

 both show that at least under the conditions of captivity, Casso- 

 waries and Emus have a low viability in proportion to their 

 size. It is interesting to notice that the Emus, although larger 

 than most of the Cassowaries, appear to be less viable. In the 

 section on Struthiones which follows I refer to the conditions of 

 the alimentary tract of these birds. 



STRUTHIONES. 



Struthio camelits (Ostrich) 25 



S. mohjhdophanes (Somali Ostrich) . 

 Rhea americana (Common Ilhea) . 

 R. darwini (Darwin's „ ) . 



R. macrorhyncha (Great-billed Rhea) 1 1 



MetchnikofF (op. cit. p. 78) has collected some information as 

 to the longevity of Ostriches and their allies, and has come to 

 the conclusion that 35 years would be an extreme age to be 

 attained by these gigantic birds. The figures on my list show 

 much shorter maximum durations, whilst the figures of average 

 duration are still lower. It is interesting to note, that taking 

 Kiwis, Cassowaries, Emus, Rheas, and Ostriches together, not 

 only is there no direct correlation between size and avei-age 

 and maximum durations, but the familiar relation is reversed. 

 The smallest birds, the Kiwis, according to my tables, show 

 the highest viability ; next come the Cassowaries, whilst the 

 Ostriches, much the largest birds, are lowest. In the Kiwi 

 and Cassowary, the caeca and hind-gut, although moderately 

 capacious, are not specially so in proportion to the remainder of 

 the alimentary tract ; whilst in the Rheas, Emus, and above all 

 in the Ostrich, the hind-gut and cagca are relatively enormously 

 long and capacious, out of all proportion to the condition in 

 any other group of birds and recalling the structure of herbi- 

 vorous mammals. In their case there is the maximum opportunity 

 for intestinal putrefaction, and these large Struthious birds, in 

 proportion to their size, appear to have the lowest viability of 

 all birds. Capt. Flower's only high record is an Ostrich alive 

 after 1 2 years' duration. 



Summary of Aves. 



Coiistitutioncd differences iit Longevity. 



Putting together the information so carefully collected by 

 Mr. Gurney, which of course related to maximum durations and 

 potential longevity, and the results which I record here, we can 

 draw the conclusion that very reid constitutional differences 

 in viability and longevity exist amongst the difi'erent families 

 of biixls. The chief difficulty is to distinguish between low 

 viability due to special conditions of captivity and constitutional 



35* 



