24 



Rats and Mice 



What is the problem to be faced ? In order to express it in the 

 most moderate terms possible, let us assume that the rat population 

 of this country on Jan. 1, 1918, was still no more than 40,000,000 — 

 that is to say that Boelter's estimate of 1909 still held good at the 

 date in question in spite of the lapse of nine years and in spite of 

 the unfortunate events of the last three years. Let us assume 

 the sexes to be equally distributed. A certain proportion of the 

 40,000,000 rats will, from one cause or another, have no chance of 

 breeding, and may be thrown out of our calculations at once. Let 

 us assume that 50 per cent, have to be so discarded. That leaves 

 us with 20,000,000 rats, or 10,000,000 pairs, on Jan. 1, 1918, as 

 our capital stock with a chance of breeding. But even breeding 

 rats must die ; we will therefore assume that 95 per cent, of our 

 10,000,000 pairs will die, by equal monthly instalments, in the 

 course of the year. We have thus assumed a natural mortality and 

 waste among the rats amounting to a total of 97*5 per cent, per 

 annum ; and by this assumption we have reduced our initial stock of 

 40,000,000 rats to 1,000,000 in the space of twelve months. This 

 waste is repaired by breeding. Let us assume that each pair of 

 rats, surviving long enough, has six litters per annum, and that 

 each litter contains on an average eight young. As will be seen 

 from the account of the breeding habits given above, both these 

 assumptions are moderate. A certain number of young will die 

 at birth ; we will assume that this accounts for 50 per cent. — a high 

 proportion. Of the survivors, many will never have a chance of 

 breeding ; we will assume this to apply to 50 per cent, of them, or 

 to 25 per cent, of all born. Those that have a chance of breeding 

 have also to die ; and as in the case of their parents, we will 

 assume that natural mortality accounts for 95 per cent, of these 

 effective rats, and we will reckon it from the moment of their 

 birth. Here again we assume a huge waste and mortality 

 probably much in excess of the natural waste. The young rats 

 are assumed to breed first when four months old and at the same 

 rate as their parents ; and the assumptions made above are also 

 made in the case of succeeding generations. 



In the accompanying table (p. 60) the results of the calculations 

 based upon the above assumptions are given, so far as they 

 concern the effective or breeding stock of rats. From this table it 

 will be seen that our effective capital stock of 10,000,000 pairs on 

 Jan. 1 might be replaced on Dec. 31, 1918, by no less than 

 41,000,000 pairs, and that in the early days of January, 1919, we 



