lino.] VARIKTIKS OF MVfi liAms IN F.CVI'T. (5()5 



It is pointcMl out tlmt this iii;i_y account ior the \crv (juick wjiy 

 in which some species accomiiiodiite themselves to jiltered suiround- 

 ings, since if a favourable mutation is present in the species very 

 few generations would suffice to make it the dominant form. 



(vi.) In M. rattns we found that the length of the hind foot 

 was also correlated with external differences, and thnt the white- 

 bellied M. tectoruvi had a longer hind foot than the darker 

 J/, cdexandri'iius. 



Although in the Oriental Region the pure white nnderpnrfs 

 is characteristic of one of the subgroups, yet in that region it is 

 correhited with <i, different sized hind foot to that with which it is 

 associated in Egypt. Tlierefore the length of the hind foot and 

 the colour of tlie underpart.s, although both pure mutations can 

 probably be inherited independently. 



(vii.) An examination of the hind-foot curve in J/, iioriy'i/ic/rs 

 shows also three apices, but in thi.s case there is no colour- 

 character by which the mutations may be distinguished externally. 

 The length of the hind foot as a mutation is therefore a character 

 common to two or more sj^ecies. ■ 



(viii.) It is suggested that this enables us to understand how 

 several very nearly allied forms of the same species (e. g. in J/ks 

 jerdoni and some of the Sciurida?) may exist in the same locality 

 without losing their characteristic differences, however small 

 those differences may be. 



(ix.) Further investigation on these lines is required by means 

 of experiments in heredity, and so far as the author has. carried 

 these out the pure varieties of M. tectorum and J/, cdexandrinns 

 were found to breed })erfectly true. 



(x.) Lastly, this paj)er claims to show that JIns rattus is a species 

 containing many slight but definite mutations which, as far as 

 the evidence goes, breed true when paired together, and that the 

 apparent innumerable and indefinite variations are merely due to 

 these animals being carried all over the world and mixing together 

 in the large seaport towns. 



In short : — The varieties in Mas rattus appear to run on definite 

 lines and to have arisen as mutations, they are therefore inherited 

 on Mendelian lines. Of the three main varieties fovand in the 

 Oriental Region only two occur in Egypt, but this paper shows 

 the presence of the third, though in very small numbers. One of 

 the charactei's of these varieties is shown to be present in another 

 species, J/, iionvi/icas, although it cannot l)e distinguished in any 

 particular individual. It is fui-ther hinted that many of these so- 

 called species which are very closely allied have probably arisen as 

 mutations, and that it is due to this fact that they are able to exist 

 side by side under precisely the same conditions and yet preserve 

 their characters intact. It may also be noticed that the colour- 

 character of the hairs on the underparts is also found in anotlu^i; 

 species, namely, the house-mouse, HIus musciohos. M. m. yent'dis 

 has the hairs white to their bases, while M. in. orientcdis has slate- 

 coloured bases. Both these forms occur in Egvpt. 



