IIEIATIVE VIABILITY IN MAMMALS AND BIRDS. 515 



Mr. J. H. Gurney gives [loc. cit. p. 39) one dovibtfvil case of a 

 Wild Duck of 29 years, and several well authenticated examples 

 of various ducks ranging from 23 years downwards. The figures 

 of maximum duration on my list are almost uniformly high, the 

 best being a Yellow-billed Duck of 26 years', a Chilian Pintail 

 of nearly 23 years', and a Red-billed Tree-duck of over 20 years' 

 duiution. The figures of average duration are reduced in many 

 cases by the inclusion of a number of veiy short-lived birds, either 

 ducklings or new arrivals, but none the less show a fairly consistent 

 high viability. 



Summa?'i/ of Anseres. 



The first question that comes into special prominence in the 

 case of Swans, Geese and Ducks is the validity of the units on 

 which these averages are made. The individuality of these birds 

 is difiicult to distinguish, and as in the period to which the 

 figures relate, there was no system of marking the individuals, 

 mistakes must have been easy to make, where opportunity 

 occurred. The swans, geese, and a good many of the larger 

 ducks, such as Sheldrakes, were kept in separate pens, and the 

 chances of confusing individuals were very small. In the case of 

 birds like the Mandarins, Summer Ducks, Wild Duck, Pin- 

 tail, Teal, and so forth, a good many were kept in the same 

 ponds, and mistakes as to individuals must have taken place. 

 However, the numbers at any particular time during the whole 

 period of 33 years were never very large ; and if the figures of 

 cases where individuals were very few and were kept separate be 

 compared with those where confusion was more likely, the 

 inference can be drawn fairly, that on the whole such mistakes 

 have corrected each other, and that the results are fairly valid. 



It is to be noticed that the familiar coi-relation between size 

 and longevity does not appear in these figures of Anserine birds, 

 although it may apply to potential longevity. There is very 

 little to choose between the ducks and geese, and both ducks and 

 geese on the whole show higher maximum and average durations 

 than occur for swans. In all the Anseres the alimentary canal is 

 relatively long, arranged in a somewhat complex fashion, and 

 narrow in calibre; the hind-gut and csecaare, in comparison with 

 most other birds, rather long, and caj^acious (except in the 

 Mergansers, where the caeca are vestigial). They are highly 

 intelligent birds, readily adapting themselves to new conditions 

 and realizing quickly that they have nothing to fear from man. 

 Except that the space given to Swans has been rather limited, 

 the conditions have been good, as these birds, whether from the 

 tropics or not, have been invariably given access to open air, and 

 in most cases have spent their whole time in it. The figures of 

 average and maximum duration show that these birds have a good 

 viability in captivity. None the less, as the conditions are rather 

 unusually favourable, I think the inference can be drawn that, in 



