THE XEW MEWS ABOUT REVERSION. 



By C. B. davenport. 



(Read April 23, igio.) 



To the general principle that " like begets like " striking excep- 

 tions are not infrequently found. These exceptional cases fall into 

 two classes. In the one class the child possesses some character 

 not visible in either parent but found in a grandparent e. g., blue 

 iris in the chlidren of brown-eyed parents. This is known as 

 atavism. In the other class, the child possesses some character not 

 visible in immediate relatives but found in some remote ancestor or 

 even ancestral species; e. g., the reappearance of the Jungle fowl 

 plumage among domesticated poultry. This is known as reversion. 



Of these two phenomena reversion is the more striking and the 

 explanation that has been current until recently, even among biolo- 

 gists, and is still common among breeders is essentially that of 

 Darwin^ — "An inherent tendency to reversion is evolved through 

 some disturbance in the organization caused by the act of crossing." 



The new explanation is based on the principle that the unit of 

 inheritance is not the individual but some characteristic of an organ- 

 ism. Paternal or maternal characteristics are not inherited en 

 masse for the most part but each character independently of every 

 other. A second principle, no less important, is that inheritance is 

 not truly from the parents but from the germ plasm ; it is not the 

 adult characters that are inherited but something in the germ cells 

 that will eventually determine those characters. One may dispute 

 the hypothesis of pre-formation in the germ but one can not deny 

 that the tgg of an ox is predetermined, certain conditions being 

 fulfilled, to develop into an ox while the egg of man is similarly 

 predetermined to develop into a man. There are probably numer- 

 ous points of difference in the minute chemical structures of these 



^Variation of Animals and Plants under Domestication," Chapter XIII. 



291 



