EVOLUTION 973 



reappearances of a feature seen in some of the Lemuroids or Half- 

 Monkeys. Similarly, extreme hairiness in man has been interpreted 

 as an atavism, but the interpretation may be erroneous. Hyper- 

 trichosis, as it is called, may be a novel plus-mutation. 



(2) The term "atavism" is used to denote the hereditary re- 

 appearance of a character not seen in the parents, but known to 

 have occurred in a definite ancestor belonging to the stock. Thus a 

 child may have the peculiar hazel eyes or a peculiar lock of hair 

 characteristic of a great-grandparent, and not expressed in the 

 intervening lineage. To such cases the term "reversion" is often 

 restricted. Thus in his Grammar of Science (1900) Karl Pearson calls 

 reversion "the full reappearance in an individual of a character 

 which is recorded to have occurred in a definite ancestor of the same 

 race", while "atavism" is restricted to "a return of an individual 

 to a character not typical of the race at all, but found in allied 

 races supposed to be related to the evolutionary ancestry of the 

 given race". This would be a useful distinction between atavism 

 and reversion, but unfortunately some scientific writers have used 

 the two terms in the very opposite way, applying "reversion" to (i) 

 and "atavism" to (2). 



(3) It remains justifiable to use "atavism" and "reversion" as 

 synonyms denoting the hereditary reappearance of characters which 

 were latent in the parents at least, but which were expressed in 

 definite — ^not problematical — ancestors, near or remote. It need 

 hardly be said that an atavism is not necessarily a deterioration, it 

 may be a throw-back to a higher degree of differentiation. That 

 depends on the direction in which the species or stock is evolving in 

 relation to its ancestors. 



Examples. — ^A dovecot with carefully bred pigeons was left to 

 itself for some years, after which it was found to contain numerous 

 blue pigeons, resembling in many ways the wild rock-dove {Columba 

 livia), believed to be the ancestor of all the domestic breeds of 

 pigeon. In exact experiment this reversion to the rock-dove type 

 has been repeatedly observed. Cultivated flowers and vegetables, 

 such as pansies and cabbages, sometimes produce forms hardly 

 distinguishable from their wild progenitors. The nectarine, which 

 is derived from a peach, may produce what is practically a peach 

 again; the white flowering-currant, which is derived from the 

 common red form, may have branches with red flowers. In a horn- 

 less breed of cattle, derived originally from a homed breed, a homed 

 individual may suddenly reappear. A dark bantam hen, crossed 

 with an Indian Game Dorking cock, produced amongst others a 

 cockerel almost identical with a jungle fowl (Gallus bankiva), i.e. 

 with the original wild stock (Cossar Ewart). Similarly, in his horse- 

 zebra hybridizations, Prof. Cossar Ewart obtained forms whose 

 stripings were at least plausibly interpreted as reversions to an 



