The Influence of Genetic Constitution 



vigour, in the form of better homoeostasis (Robertson and 

 Reeve, 1952). Hybrids can, like inbreds, be employed for trans- 

 plantation experiments. Like inbreds, too, they may all die of 

 a single cause, and will do so as a rule with greater unanimity 

 in regard to age. 



Random-mated material, when mating is genuinely random, 

 and not occurring within an already highly-inbred colony or 

 between such colonies, produces animals, the strongest of which 

 exhibit a vigour and life-span approaching that of hybrids, 

 more variable than hybrids, unsuitable for transplantation 

 experiments, showing a variety of causes of death more closely 

 resembling that in human populations, and, in general, resem- 

 bling such populations more closely than hybrid or inbred lines. 



The choice of inbred, hybrid, or random-bred material, 

 when it is not dictated by the fact that no pure lines are avail- 

 able, will depend upon which of these attributes are most useful. 

 The type of material must, however, be correctly stated, since 

 it greatly affects the interpretation of results. Probably the most 

 valuable approach to the comparative study of ageing, though 

 not always a practicable one, would be a scheme of research 

 carried out in parallel upon all three types of strain, in an 

 animal which is already genetically familiar. 



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