REVERSION 65 



many supposed cases of atavism are merely cases of 

 normal variation, as ^vhen a tall man with short 

 parents is set down as a case of atavism because he 

 is known to have had a tall ancestor. Other cases 

 of so-called atavism are better called cases of rever- 

 sion, of return to the ancestral type. Karl Pearson 

 gives separate definitions of the terms atavism 

 and reversion, but as the terms are often used inter- 

 changeably, or conversely, we may content ourselves 

 with speaking of reversion alone. Thus when 

 Thomson says, " what are called reversions are prob- 

 ably in many cases misinterpretations," he means 

 just what Mitchell means when he says that "atavism 

 is, in fact, a misleading name covering a nuuiber of 

 very diti'erent phenomena." 



It is scarcely necessary to multiply in.stances of 

 reversion, but I may quote a typical one recorded by 

 Cossar Ewart. The Edinburgh professor mated a 

 pure white fantail cock-pigeon with a cross between 

 an "owl" and an "archangel." "The result was a 

 couple of fantail-owl-archangel crosses, one resembling 

 the Shetland rock-pigeon, and the other the blue 

 rock of India. Not only in colour, but in shape, 

 attitude, and movements there was an almost com- 

 plete reversion to the form which is believed to bo 

 ancestral to all the domestic pigeons." 



The generally accepted explanation of reversion 

 is that it is due to the sudden activity of ' latent 

 ancestral units " ; or, in the words of Thomson, " that 

 characters may be latent for a generation or for 

 generations, or, in other words, that certain potenti- 

 alities or initiatives which form ])art of the heritage 

 may remain unexpressed " — to tind expression at hist 



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