The Biology of Senescence 



'vegetative hybridization' of this type, it seems clear that the 

 last word has yet to be said upon the nature of induced vigour. 

 The whole problem is one which might be of considerable 

 interest to gerontology, since in some cases Vigour' appears 

 capable of induction post-conceptually, or even post-natally. It 

 is important to notice, however, that there is no clear evidence 

 at present to show that the vigour and longevity obtainable by 

 true heterosis are greater than those existing in wild strains. 

 Heterosis should be regarded, in all probability, as the restora- 

 tion of 'wild' vigour, whether by restoring heterozygy or by 

 other processes, in lines which have lost that vigour through 

 inbreeding. Whether the results of heterosis can be superior 

 to those of wildness , in longevity or otherwise, remains to be 

 demonstrated. 



4-3 Sex Differences 



In most animals which have been studied, the male sex is 

 the shorter lived. This is true in organisms as dissimilar as fish 

 (Bidder, 1932; Wimpenny, 1953), spiders (Deevey and Deevey, 

 1945, Figs. 34, 35), Drosophila (Alpatov and Pearl, 1929; 

 Bilewicz, 1953), Habrobracon (Georgiana, 1949), Tribolium 

 (Park, 1945, Fig. 36); water-beetles (Blunck, 1924) and man. 

 In exceptional cases the preponderance of male mortality can 

 be reversed. Thus Woolley (1946) found that in crosses between 

 dba female and c57 male mice, the virgin females of the F x had 

 a mean life of 27 and the males 29 months: in the reciprocal 

 cross, the females lived 30 and the males 33 months. Males of 

 Rattus natalensis outlive the females (Oliff, 1953). Darwin (1874) 

 regarded the shorter life of the male as 'a natural and con- 

 stitutional peculiarity due to sex alone'. Attempts have also 

 been made to explain it in genetic terms (Geiser, 1924; Gowen, 

 1931, 1934). Gowen constructed life- tables for Drosophila inter- 

 sexes and triploids, and concluded from his results that chromo- 

 some imbalance in itself exerted an adverse effect on life-span. 

 In most of the forms where full life-tables have been made, the 

 bias of mortality against the male follows the rule of greater 

 vigour in the homogametic sex. McArthur and Baillie (1932) 

 pointed out that if the lowered vitality of the male was due to 



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