54 A. Comfort 



of the contribution, under these breeding conditions, made by 

 short-hved parents to the next generation. There is, effect- 

 ively, intensive spontaneous selection for longevity, which is 

 not a deliberately-sought character in racehorses. Murie's 

 work on sheep (1944) suggests that for some large mammals 

 the wild adult survival curve is not very different from that 

 in domestication. If this were true of wild horse populations, 

 longevity of the dam, and probably, in the presence of com- 

 petition between males, longevity of the stallion, would have 

 a large selective advantage; the upper limit of the lifespan 

 would have in this case to be fixed by something other than 

 decline of selection pressure. 



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