342 HEREDITY, 



the light gray, and in the combinations already known that repro- 

 duce the types seen in the parents and grandparents. 



This second type of heredity is particularly interesting to horti- 

 culturists and breeders. It is probably crossings of this kind that, in 

 the second generation, when the plant hreaks^ as the horticulturists 

 say, produce new varieties comparable to these black and light-gray 

 mice, that may be superior in value to the primitive parents. 



It is possible that there may be other laws of heredity than those of 

 Mendel, but as the work of investigation proceeds the less this seems 

 likely; in any case, we do not as yet know any absolutely certain 

 example. I will not dwell, therefore, upon this aspect of the subject. 



The explication of Mendelian heredity has had numerous theo- 

 retical consequences. A number of facts that were very difficult to 

 explain have become quite clear ; this is notably the case with atavism 

 (from the Latin atavi/s, an ancestor). 



Atavism is a singular phenomenon which consists in the unex- 

 pected ajDpearance in breeds of animals or plants of a form (ata- 

 vistic) that recalls in whole or in part an ancestor that disappeared 

 a number of generations ago. As a first case I will recall the 

 classical example given by Darwin, He crossed well-marked and 

 constant breeds of pigeons, two black, a white with a red tail, and a 

 white fantail, and at the second generation obtained, among others, 

 a magnificent blue pigeon with white crouj^, double black w^ing bars, 

 tail barred and bordered with white — in fact, having every character- 

 istic mark of the rock pigeon, the wild Columba livia, which is 

 almost certainly the ancestor of our domestic pigeons. The follow- 

 ing diagram will show the details of the crossing: 



White fantail. Black barb. White with red tail. Black barb. 



I , I I I 



I I 



Dusky colored pigeon. Dusky colored pigeon. 



I 



A varied progeny, among which was an atavistic form. 



I described to you a few moments ago a crossing which certainly 

 furnished atavistic forms. I refer to the crossing between white 

 mice and yellow mice with red eyes. These two forms, that breed with 

 perfect constancy, have each had parents identical with themselves 

 for many generations, and yet the products of their union are all 

 mice identical, or nearly so, with the field mouse, which is probably 

 closely related to the wild ancestor of our house mice. 



In order to explain this reappearance of an ancestor, Darwin 

 and Weismann, to cite only them among many others, have greatly 

 complicated their theories of heredity. They have thought it neces- 



