The Influence of Genetic Constitution 



cumulative disadvantage in longevity accruing to 'youngest 

 sons of youngest daughters'. In this connection Strong has how- 

 ever stressed, on a number of occasions, the need for further 

 information on the relation between longevity and cumulative 

 parental age in human genealogies. Such information is un- 

 fortunately hard to come by, and no large-scale study has yet 

 been published. Lansing's effect might well be sought in the 

 parthenogenetic Gladocera. The age of the mother affects the 

 rate of development, and probably the longevity, of young 

 Daphnia. Green (1954) recently found that the size o£ Daphnia at 

 birth determines the instar in which maturity occurs, the largest 

 becoming mature earliest. The birth size itself depends upon 

 maternal age, being highest (in D. magna) in the third brood. 

 Since the pre-mature phase is the part of the life-cycle in which 

 most variation occurs, the mature phase being usually of fixed 

 length, early developers might be expected to be significantly 

 shorter-lived than late. But R. H. Fritsch, at the Justus Liebig 

 School in Giessen (unpublished) has compared the longevity 

 under carefully standardized conditions of successive genera- 

 tions of Daphnia raised wholly from first, third, and sixth hatch- 

 ings, and finds no significant trend in any of the orthoclones, 

 the mean life-span in all remaining at about 30 days. 



4-2 Heterosis or Hybrid Vigour 



Abnormally long-lived animals can regularly be produced by 

 crossing certain pure lines, not themselves unusually long-lived, 

 the effect being greatest in the hybrid F x and their offspring and 

 declining rapidly on subsequent inbreeding. This is, in fact, the 

 simplest method of increasing the specific age in many already 

 inbred laboratory and domestic animals. Striking examples of 

 this effect (heterosis) in increasing longevity have been recorded 

 in mice. Gates (1926) by crossing Japanese waltzing with 'dilute 

 brown' strains produced a generation which was still actively 

 breeding at 2 years of age. Comparative life-tables for crosses 

 exhibiting extreme hybrid vigour do not seem to have been 

 published. 'Super-mice' produced by heterosis develop pre- 

 cociously, reach a large size, and remain in active reproduc- 

 tion much longer than their parents, thereby exhibiting a 



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