CHAPTER V 



THEORIES OF HEREDITY (Continued] 



I. REVERSION. 



BY Reversion or Atavism we understand the reappear- 

 ance of ancestral traits which have been absent or latent 

 in the race for one or more generations. These atavistic 

 characters must not be confused with phenomena which, 

 though seemingly due to a throwback or reversion, are in 

 reality caused by arrested development. As each indi- 

 vidual repeats during its ontogenesis the general stages of 

 the line of its animal ancestry, it will sometimes happen 

 that an organ, when arrested at a certain stage of its em- 

 bryonic development, will present the type of some ances- 

 tral species : thus, a cleft-palate or a hare-lip is due to 

 nothing but to defective union of the two primitive upper 

 jaws, which is a persistent trait in certain lower animals. 

 Such cases are not instances of reversion, for the organ in 

 question is not absent in any one generation, but is repre- 

 sented in each generation in its normally developed form. 

 In true reversion we may distinguish between those cases 

 where only one generation is skipped, the grandchild taking 

 after the grandparent, and those cases where an individual 

 shows traits of a distant ancestor, often of a different, 

 though related, species. 



(a) REVERSION TO GRANDPARENTS. 



In the crossing of distinct varieties there is a great ten- 

 dency for the offspring to revert to one of the parent types 

 after a few generations. It is possible that even as early 



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