The main points that have been brought out in this paper are as follows:—
- (i) Two varieties (and intermediates) of M. rattus are found in Egypt.
- (ii) The hind foot is taken as a standard measurement and curves of this measurement drawn.
- (iii) These curves show three apices, the first being very small.
- (iv) These three apices correspond with the length of the hind foot typical of the three subgroups of M. rattus in the Oriental Region described by myself some years ago.
- (v) Hence it is argued that these three apices probably represent mutations, and that the first and smallest apex represents the Jalorensis subgroup found in the Oriental Region, but which from some cause or other is practically non-existent in Egypt. Since, however, a mutation. cannot be destroyed, we still find it in a small percentage of individuals ready under favourable circumstances to increase. It is pointed out that this may account for the very quick way in which some species accommodate themselves to altered surroundings, since if a favourable mutation is present in the species very few generations would suffice to make it the dominant form.
- (vi) In M. rattus we found that the length of the hind foot was also correlated with external differences, and that the white-bellied M. tectorum had a longer hind foot than the darker M. alexandrinus. Although in the Oriental Region the pure white underparts is characteristic of one of the subgroups, yet in that region it is correlated with a different sized hind foot to that with which it is associated in Egypt. Therefore the length of the hind foot and the colour of tlie underparts, although both pure mutations, can probably be inherited independently.
- (vii) An examination of the hind-foot curve in M. norvegicus shows also three apices, but in this 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 species.
- (viii) It is suggested that this enables us to understand how several very nearly allied forms of the same species (e. g. in Mus jerdoni and some of the Sciuridæ) 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 M. alexandrinus were found to breed perfectly true.
- (x) Lastly, this paper claims to show that Mus 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 Mus 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 found 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 characters of these varieties is shown to be present in another species, M. norvegicus, although it cannot be distinguished in any particular individual. It is further 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 another species, namely, the house-mouse, Mus musculus. M. m. gentilis has the hairs white to their bases, while M. m. orientalis has slate-coloured bases. Both these forms occur in Egypt.