1 92 CAUSES OF VARIATION 



SECTION IX REVERSION AND ATAVISM 



These two terms are used to designate characters appearing 

 in the offspring but not visible in the parents. " Reversion " is 

 used to indicate resemblance to a comparatively near-by ancestor, 

 not the parent, while " atavism " refers to exceedingly remote 

 ancestors, sometimes of other and foundation species. 



Thus, if a dash of impurity of blood enters a herd, it will 

 appear occasionally for many generations. This would be 

 spoken of as a " reversion to the strange blood." If a sire or 

 dam has some peculiar character, like white stockings in horses, 

 a peculiar horn in cattle, or a habit in man, it is likely to appear 

 from time to time in future generations, even after its real 

 origin is forgotten. This is a reversion. English breeds of cattle 

 are developed from the ancient wild white cattle of Britain, and 

 the occasional appearance in all these breeds of a white calf 

 with red or brown ears, lower legs, and tail brush is to be 

 expected. It is a reversion, not a proof of mixed blood. Of 

 course the animal so marked is useless for breeding purposes, 

 but no reproach to the herd, and none necessarily to the dam that 

 produced it, for reversions for the most part seem to come singly. 



Atavism, on the other hand, goes farther back. For example, 

 mammals during their early embryonic development still show 

 traces of the gill slits, thus betraying their undoubted one-time 

 connection with the same stock which gave rise to the aquatic 

 animals. These gill slits occasionally persist, failing to close, 

 and give rise to the abnormality known as " cervical fistula." It 

 is an undoubted atavistic abnormality, an extreme case of 

 course, because of its antiquity. 



Cases of this kind are to be carefully distinguished from mere 

 meristic variations. For example, the sudden appearance of a 

 three-toed horse would be regarded as atavistic, for all horses 

 once had three toes ; but a sixth digit in man is certainly not 

 atavistic, for we have no evidence that man ever- possessed 

 normally more than five digits. The criminal instinct in certain 

 men is undoubtedly atavistic, showing not so much a delight 

 in evil doing as an entire absence of the higher sense of right 

 doing. 



