1907.] 



Dr. Hossack : The Rats of Calcutta. 



15 



The law of age variation, which I have already laid down, enabled me to settle this 

 definitely. If, as seemed probable, differing sizes were due simply to age, then by 

 arranging them in series according to size one would expect to find that as the size 

 increased the relative proportions of the hind foot and ear would regu'arly decrease. 

 How completely the result justifies the anticipation will be seen from the following 

 table, where the gradation of the percentage of the length of the hind foot and the ear 

 is perfect. 



Table showing gradation of measurements in different sizes (i.e., ages) of Mus rattus. 





Total length of body in centimetres 



Percentage of body-length. 















. Tail. 



Hind foot. 



Ear. 



13—15 



. . 



135-1 



22-8"^ 



137 



15—16 



. . 



125-8 



20-1 



12-7 



16 — 17 



. . 



123-5 



19-9 



12-6 



17—18 



. . 



125-0 



19-2 



I2-I 



18—19 





I20-I 



l8-2 



11-9 



ig — 20 



. . 



124-4 



i8-i 



II-8 



An examination of the skulls of the series confirms the result of the body mea- 

 surements, and almost equally strongly indicates that the gradation of size is simply 

 one of age. There are one or two slight discrepancies in the case of some of the mea- 

 surements, particularly that of the interorbital breadth, a very small measurement in 

 which an error of j-q- of an inch is an error of over i per cent, in relation to the total 

 length of the skull, and 8 per cent, of the actual measurement. In the most im- 

 portant measurement, however, that of the breadth of the cranium, the gradation 

 is perfect, ranging from 40-4 per cent, for rats of only 13 to 15 cm. in length to 35-5 

 cm. for the largest sized rats from 19 to 20 cm. in length. It will be remembered 

 that in the young rats 9-10 cm. long the cranial breadth was as much as 50 per cent. 



