" PRESIDENTIAL ADDRESS. 45 
Zealand vessel, so different from the rats I had previously 
known. Adelaide specimens seen by me about 1907 were 
certainly EH. norvegicus. Dr. Borthwick informs me that 
when plague was present in Port Adelaide in 1909, both £. 
norvegicus and EH. rattus were present, the former pre- 
dominating. Recently the Black Rat has appeared near 
Adelaide in considerable numbers. Mr. E. R. Waite,4 of 
the South Australian Museum, first called attention to its 
presence and attributed its arrival to escapees from troop- 
ships. As this species, as I shall indicate elsewhere, is the 
principal rat that travels by sea and is present on manv 
vessels, any inter-State or oversea ship, however, might have 
introduced it. 
In Tasmania, Ham states that both species occur. 
In Western Australia, when I was there between 1906 
and 1909, E. rattus was the only rat met with in Perth, ex- 
cept along or adjacent to the water frontage of the Swan 
River, where £. norvegicus was also found. Similarly at Fre- 
mantle, this latter species occurred only near the wharves. 
It seems fairly certain that E. rattus had been first intro- 
duced, as might have been expected, from its being the 
species usually found on ships, and that EH. norvegicus had 
only recently arrived, but had already established itself in 
the immediate neighbourhood of shipping and water car- 
riage. 
The Number of Rats and Mice per Litter. 
The number of rats and mice born per litter is of con- 
siderable interest, as showing the potentialities that exist 
for the rapid increase of these pests in the absence of con- 
trolling factors such as dearth of food supplies. It may be 
taken as a general axiom that the various living beings that 
inhabit the earth, both plants and animals, are as regards 
+ Waite, “The Register,’ Feb. 12th, 1916. 
