WALTZTXG RATS. I 



flark-bellied ones always breeding true and the heterozygous 

 light-bellied ones giving a pro^Jortion of pure alexandrinvs. 



Tliis entirely bore out the suggestion put forward by Mr. Bonhote 

 in a former paper {loc. cit.) that the main varieties (or sub-groups 

 as he had called them) of Mus rattns were true Mendelian 

 mutations. It would thus be understood how forms (which were 

 at present considered as closely allied species) and which differed 

 only in small but constant characters, were able to exist side by 

 side under practically identical conditions without those characters 

 becoming blended or one of them being lost, as would be bound to 

 occur if natural selection were the only active foi-ce in evolution. 



With regard to the heredity of the fawn — the original specimen, 

 a male, which showed the characteristic white uuderparts of M. 

 tecloruDi (its parents), was mated to an alexandrinus 5 ■, 'I'l^d the 

 resulting F^ generation, some thirty in number, were all typical 

 vvhite-l)ellied tectorum. 



Five pairs of these were mated and gave in the F., generation : — ■ 

 17 Tectorum, 5 Alexandrinus, 7 Fawn T., 1 Fawn A. 

 the expectation 



being IS „ 6 ,. 6 „ _ 2 „ _ 



In addition, there were three individuals that died before it 

 could be determined whether they were alexandrhiKs or tectorum, 

 but they wei'e not Fawn, as these could be distinguished at birth. 

 The fawn colour, which was probably due to the absence of black, 

 was, therefore, recessive to the normal grey coloviring but might 

 occur in either of the norn)al forms. Thus a rat having an 

 absence of black and presence of the Alexandrine (slaty underparts) 

 character was whitish fawn in colour, since the number of hairs 

 which should contain black was much greater and in the absence 

 of the black factor these hairs were colourless. 



It might be noticed that in young Fawn Rats the eye was 

 ruby coloured, as in the case of the Cinnamon Canary, becoming 

 quite dark as maturity was reached^ 



The " waltzing " i-ats, of which altogether four had been 

 produced, all appeared in the F^ genei-ation, but were not all from 

 the same parents. Of the matings for the F„ genei'ation only 

 one (daughter x father) produced sound young. Ai^art from the 

 " waltzing " character, three individuals were born blind, and in 

 two of these cases the eyes were undeveloped and the optic nerve 

 absent, and in addition many of the young that were reared were 

 so weakly that they had to be killed, and others died before being 

 weaned. 



From certain causes, therefoi-e, which are not very clear, partly 

 due to inbreeding, partly probably to environment, and partly pos- 

 sibly to their breeding at too early an age (6 months), a degeneiate 

 and defective generation was produced, and one of the i-esults 

 was to bring about in certain individuals a character (" waltz- 

 ing"), which in a nearly allied species was shown* to have a 

 Mendelian inheritance. 



* Uarbishire, ' Biometrika,' i. pp. 101, 165, 282 (1902), ibid. iii. p. 1 (1903) : 

 G. von Guaita, Ber. Naturg. Gts. Freiburg, x., xi. (1898) (1900). 



