14 THE COMMON RATS 



In the present connection our interest in the Norway rat is 

 due mainly to the fact that the common albino rat (M. n. a bi- 

 nus) kept as a pet or laboratory animal, and concerning which 

 we desire all possible information, is a variety of the Norway 

 rat. This relationship is shown not only by the usual methods 

 of comparison, but also by the haemoglobin crystals (Reichert 

 and Brown, '09) the shape of skull (Hatai, '07 c) and the fact 

 that the two forms interbreed freely. 



Concerning the place and time of origin of the albino strain 

 there is little information at hand. Allusions to albino rats be- 

 fore the time when the Norway rat appeared in Europe clearly 

 show that there must have been an albino strain of Mus rattus. 

 What we know of the present distribution of Albinos of Mus rat- 

 tus has been given on pages 8 and 9 in the preceding chapter. 



By some curious slip however, many of the natural histories 

 and books of reference speak of the common Albino as an Albino 

 of Mus rattus. This of course is not correct, but owing to the 

 confusion thus early introduced, it is difficult to trace the history 

 of the present albino variety 5 of the Norway. 



We do not know whether the common albino variety had a 

 single or multiple origin, or whether the colonies found in Europe 

 (Rodwell, 1858) are directly related to those now existing here. 

 Moss, 1836, mentions Albinos in or near Bristol, England about 

 1822. In their general physical characters the European and 

 American Albinos are similar (Donaldson, '12 and '12 a). Judg- 

 ing from the way in which the Albinos of other species arise, we 

 may safely assume that the present strain is derived from one or 

 more albino mutants or sports (Hatai, '12). These must have 

 been captured and the albino descendents segregated and kept 



5 Unfortunately there is one more complicating circumstance namely, the 

 existence of a melanic variety of Mus norvegicus. This melanic variety is often 

 mistaken for Mus rattus rattus because of its color, and this leads to errors of 

 statement concerning the distribution of Mus rattus and also concerning the 

 ability of the two species rattus and norvegicus to interbreed. They are in 

 fact mutually infertile (Morgan, '09) . The number of incidental allusions to this 

 melanic variety of norvegicus shows its occurrence to be widespread. See : Ed- 

 wards, 1871, 1872. Hamy, '06. PIsle, 1865. Lapicque and Legendre, '11. Schaff, 

 1891. Webster, 1892. 



