42 A. Comfort 



been regularly at stud may enter the record at any age if they 

 produce a thoroughbred foal; some of these have probably 

 been missed. But on the basis of mortality rates calculated 

 for all the cohorts at 4 years of age, omitting subsequent 

 entrants, it appears that this bias is only enough to account 

 for a small part of the difference observed. The 1900 and 1910 

 cohorts gave much lower figures for mean and median ex- 

 pectation of life than either of the earlier samples. This 

 apparent fall in performance might be influenced (1) by the 

 1914-18 war: the increased losses are concentrated early in the 

 1910 table and about 10 years later in the 1900 table, (2) by a 

 change in breeding policy — mares over 23 are rare in recent 

 volumes, being now apparently put out of stud at earlier ages. 

 A secular trend in lifespan cannot be assumed without scoring 

 further cohorts, but there is some prima facie evidence of it 

 here. 



The mean expectation of life of Arabians was significantly 

 higher than that for any cohort of thoroughbreds {e^^^ = 

 18-81 ±0-66 years) and the rate of decline slower. These 

 lives are spread too thinly over too long a period for any 

 secular trend to be made out. 



Maximum age records 



The highest ages in the series were reached by Arabians, 

 three mares reaching 31 years, and one dying in its 33rd year 

 (born 1911, died 1943; last covered, but barren, 1942); these 

 ages may only indicate more conscientious returns for Arabians 

 past breeding age, compared with thoroughbreds. The two 

 oldest thoroughbred mares in the sample were alive at 30 

 years. The Stud Book has not been searched in detail for 

 higher records — the oldest mare so far encountered (Blue Bell, 

 by Heron out of Jessie) was foaled in 1851 and died in 1885 at 

 the age of 34. Pocahontas (Stockwell's, Rataplan's and King 

 Tom's dam) was foaled in 1837, died in 1870, and bore her 

 last foal in 1862. The frequency with which such ages are 



