THE BLACK OR SHIP RAT 



S95 



Bonhote {Proc. Zool. Soc, London, 191 2, 6) found on crossing 

 frugivorus with alexandrinus that the former was apparently a simple 

 Mendelian dominant to the latter ; and while the heterozygous frugworus 

 gave a proportion of pure alexandrinus, the latter always bred true. 

 By the mating oi frugivorus with alexandrinus thirty " hybrids," all being 

 in appearance typical white-bellied frugivorus, were produced. Five 

 pairs of these hybrids were mated, and their progeny consisted of 

 17 (apparent) frugivorus, 5 alexandrinus, 7 ia.-wn frugivorus, and i fawn 

 alexandrinus — the Mendelian expectation being 18, 6, 6, 2 for these 

 respective kinds. The fawn types are novelties, arising probably from 

 the absence from them of black pigment. 



The classic experiments of de I'Isle, made shortly after, and unfortun- 

 ately in ignorance of, Mendel's great discovery, seem to indicate that 

 the dusky race r. rattus behaves in turn similarly as a dominant to 

 frugivorus ^ ; his results are, however, complex, and not easy to disen- 

 tangle, probably because both rattus a.nd frugivorus include a large per- 

 centage of heterozygous individuals. The hybrids of rattus y. frugivorus 

 on being paired together gave a progeny of numerous black rats, fewer 

 frugivorus, and, as in the case of the cross frugivorus by alexandrinus, 

 two novel types, viz., one relatively abundant, with light-coloured back 

 and dark belly, called by de I'Isle the "semi-alexandrine"^; the other, 

 much rarer, with dark back and white belly. The " semi-alexandrines " 

 bred true. Morgan's experiments (^American Nat., xliii., 182, 1909) 

 confirm in part these results deduced from de 1' Isle's records ; he found 

 that whichever way the cross was made the progeny oi frugivorus 

 X rattus were black ; in the first generation he raised thirty-two black 

 individuals, but they showed some variation in the degree of their 

 blackness. On pairing these hybrids, Morgan obtained a litter of four 

 black and one grey, the blacks varying in shade as in the first genera- 

 tion ; although the number bred is too small to yield any precise result, 

 it indicates so far as it goes that the two colours follow Mendel's law. 



It may be suggested that both alexandrinus and frugivorus 

 have contributed to the formation of the black race, r. rattus, 

 which has arisen, perhaps on many distinct occasions, in response 

 to the exigencies of a new environment, namely, that afforded by 

 an exclusively parasitic life in temperate Europe. The fact that the 

 curve of the hind foot measurements shows a greater number of apices 

 than does the hind foot curve of either of the two " wild " races is 

 explicable on the assumption of such a multiple origin ; further, rattus 



' Although de I'Isle throughout his paper calls his light-coloured xzX alexandrinus, 

 his description of it as having the ventral hairs white to their bases leaves no room 

 for doubting that his specimens were of the. frugivorus race as defined by Bonhote. 



^ De I'Isle's " semi-alexandrine " corresponds apparently to E. r. alexandrinus as 

 defined here. 



