THE FACTORS OF EVOLUTION 209 



Since naturalists have begun during recent years to observe 

 more accurately the occurrence of jumps in variations many 

 instances have been made known both from the plant and 

 animal kingdom. The occurrence of ' single variations ' had 

 even been noticed by Darwin himself, though in view of the 

 individual or fluctuating variations he did not ascribe to them any 

 particular importance in the formation of species. Nevertheless 

 we are indebted to him for a record of several significant instances. 



In 1791, in Massachusetts, a sheep was born in an ordinary 

 flock having a long back and short bow-legs like a dachshund. 

 Because of its strange, unnatural appearance the animal was bred 

 from in the hope of obtaining a race unable to jump over the 

 hedges and fences which separate the homesteads. As only one 

 individual of this variety existed it was necessary to cross it with 

 an ordinary sheep. The first generation consisted of both bow- 

 legged and straight-legged individuals. When two bow-legged 

 sheep were crossed the character proved constant in the de- 

 scendants. Thus originated the Ancon race. Afterwards the 

 Ancon sheep were replaced by the more profitable Merinos, and 

 are probably extinct to-day. It is interesting to note that the 

 crossing of an Ancon with another sheep did not produce a 

 mixed race, but that the lambs were either bow-legged or 

 straight-legged. There are even cases on record of twins, of 

 which one exhibited the Ancon type while the other was normal. 



It is probable that our dachshunds originated by mutation 

 from greyhounds, but as the race of dachshunds is very old 

 they were kept as long as two thousand years ago in the Nile 

 delta it is impossible to obtain accurate information concerning 

 their origin. 



According to the investigation of Keller and others, it may 

 be taken as proved that the absence of horns in different races 

 of cattle, sheep, and goats, the races of goats with four horns, 

 the three-toed pigs of the Crimea, the famous tailless Manx cats, 

 black panthers, black-shouldered peacocks, the ' telescope ' and 

 multiple-tailed gold-fishes and many other animal forms owe 

 their existence to mutation. Most of the species mentioned are 

 distinguished by the fact that in in-breeding all their characters 

 remain constant. 



It seems now proved that non-continuous variability can lead 



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