72 HENRY H. DONALDSON 
which the Norway rat has undergone in its migration across 
Europe and over seas to the Americas. 
From our experience with the wild Norways of North America, 
as found in Chicago and Philadelphia, there is no reason to think 
that such an established strain exists in the United States, al- 
though there appear to be slight differences characteristic for the 
rats from different stations, and in one instance the rats from a 
restricted locality near Philadelphia showed a brain weight 
decidedly lower than that of our standard series. We do not 
look on this latter group however as representing an established 
strain. 
It was thought possible nevertheless that such a strain might 
be found in western Europe and I decided therefore in the summer 
of 1909 to test the matter by collecting both Norway and albino 
rats from Vienna, Paris and London in order to compare these 
in respect to their central nervous systems with specimens of 
both forms as observed in Philadelphia. 
Historically, nothing is known of the albino variety of the 
Norway rat, not even whether the Albinos found in Europe and 
those in North America have had a common origin. 
Concerning the wild Norway rat, we are a trifle better informed. 
Mus norvegicus is reported by Pallas to have entered Europe by 
way of southern Russia about the beginning of the eighteenth 
century. It arrived in England, by ships, about 1728-1729 and 
in Paris about 1758, but of its first arrival in Vienna, I have found 
no mention. The date of its arrival on the eastern seaboard of 
the United States was about 1775 (Lantz, ’09). Thus the wild 
Norway rat has been in Europe for nearly two hundred years, and 
in the eastern United States for about a hundred and thirty-five 
years. It seems most probable that the albino variety has been 
established since the appearance of the wild Norway rat in Europe. 
For the opportunity to make these studies on the rat at the 
several European stations, I was indebted to the courtesy of 
colleagues in each city, and it is a pleasure to present here my 
thanks to all of them for their unfailing kindness and interest. 
At Vienna, Professor Obersteiner obtained for me a table in 
