BREEDING EXPERIMENTS WITH RATS 



PROFESSOR T. H. MORGAN 

 Columbia University 



The following experiments chiefly between the black 

 and the roof rat were undertaken as one of a series to 

 determine how far the Mendelian law of discontinuous 

 inheritance applies to wild varieties and species. Most 

 of the cases of Mendelian inheritance have been deter- 

 mined from domesticated forms in which the varieties 

 seem to differ from each other in the loss of one or more 

 characters. Until we know more about the results when 

 wild varieties are crossed with their wild species or 

 with other varieties, we can not safely apply Mendel's 

 law to the process of evolution. 



There are two species of house rats found in this coun- 

 try in addition to the common gray or Norway rat. One 

 of these is the black rat, Mm rattus, that still exists in 

 isolated communities in the north. It also infests, I 

 believe, certain ports in the southern states and Central 

 America. 



The other, the roof rat or Alexandrian rat, is said to 

 have been introduced from Mediterranean ports by ships. 

 It is generally described as a variety of the black rat. 1 

 The roof rat is gray, it has the ticked or barred gray hair 

 common to most wild rodents. It may, therefore, be 

 looked upon as the original form from which the black rat 

 has been derived. If this is the case we might expect to 

 find that the gray color is the dominant one and the black 

 the recessive, especially since in the domesticated species 

 of the Norway rat, black is recessive to gray. 



To my surprise I found that in the first generation the 



