Biological Approach in Study of Ageing 13 



Zoological Society's collection in London. The choice of 

 species in these cases was determined entirely by the number 

 of scoreable lives available. In consequence the data are 

 practically confined to species breeding readily in captivity 

 and kept in considerable numbers. The wolfhound data were 

 obtained from a kennel book which was exceptional in that 

 the breeder had ascertained, and recorded, the subsequent 

 fate of most of the dogs she sold (Fig. 2). 



The most evident feature of the survival curves which we 

 have obtained is their similarity. They all approach a straight 

 line over the whole adult period, when survivorship and time 

 are plotted on arithmetic co-ordinates. Such a distribution is 

 unusual in comparison with the log-linear decline usually 

 found in wild populations and the plateau of low adult 

 mortality in fully domestic animals. It indicates (1) that the 

 number of animals of a cohort which die in unit time is con- 

 stant, (2) that the force of mortality rises continually with age, 

 since each batch of deaths is an increasing fraction of the total 

 surviving, (3) that the distribution of lifespans is rectangular, 

 there being no commonest age of death after the initial infant 

 mortality is past (Fig. 3). 



The survival curves of the two species of sheep (Ovis 

 musimon and Ammotragus lervia) and of the wolf X timber 

 wolf hybrids in the London Zoo are practically identical. 

 That of Grecian wild goats, also in the London Zoo, is closely 

 similar in form, but declines at rather more than half the rate. 

 The lifespan of domestic goats appears, from some initial 

 figures which are not yet complete, to be very much shorter 

 than that of wild goats. The figures we obtained for wolf- 

 hounds showed a large sex difference in mortality; they also 

 indicate that there is little difference between age-mortality 

 pattern in a cohort of highly inbred pedigree dogs and in 

 dingoes, which are wild-type, probably feral, dogs. 



In examining figures of this kind it is possible to see whether 

 any of a population of animals has, in fact, reached old age by 

 comparison with maximum age records. If we obtain a series 

 of curves in which the survivorship can be followed through 



