NUMBER OF YOUNG! NORWAY 337 



and he himself in 1902 examined one female bearing 16. Donald- 

 son (MS '09) also noted in a rat taken in Paris, 16 fetuses. 



The India Plague Commission reports ('08) that the average 

 number of fetuses found in females was 8.1 from a total of 12,000 

 Norway rats. 



According to Lantz ('10) the maximum size of- litters recorded 

 in England (Field) are 17,19, 22 and 23; in India however, 14. 



The maximum numbers just given as recorded in England are 

 not trustworthy as they represent merely the number of young 

 found in a single nest. Since two different litters are sometimes 

 reared in the same nest the inference from the number in the nest 

 to the number in the litter is not convincing. Lantz ('09) as- 

 sumes the average litter (in north temperate latitudes) to be 

 about 10. This is what Miller ('11) (vide infra) and Crampe 

 ('84) (vide supra) found. 



Miller ('11) observed in a group of eight litters 7-12 young in a 

 litter, with an average of 10.5. 



Recently King ('23) has studied the litter size and sex ratio in 

 a series of caged Norways undergoing domestication. 



In this instance 309 litters were examined. 



In the Norway, as in the albino strain, the first litter cast 

 is relatively small and the next two litters tend to be the largest 

 of the series cast. In later Utters the number of young gradually 

 decreases, dropping to an average of 4.8 in the final litter of the 

 series (table 184) . The 309 litters cast by Norway females contain 

 an average of 6.0 young: the data for the albino strain gives an 

 average of 6.1 young per litter (table 6). The average size of 

 the litter is therefore practically the same in the two forms; 

 but as litter production in the albino strain is greater than that 

 in the Norway strain, it appears that domestication has in- 

 creased the fertility of the rat, as it has of many other animals 

 (Darwin, 75). 



Weight at birth. King ('23) found before suckling the weight at 

 birth to be 5.34 grams for the males and 5.09 grams for the females. 



Sex ratio. Lantz ('09) and others state that among trapped 

 animals the males are in excess. Donaldson ('12) found the same 

 in trapped series taken in Paris and London. In a small series 

 trapped in Vienna however, the females were in excess. 



