THE BLACK OR SHIP RAT 595 
Bonhote (Proc. Zool. Soc., London, t912, 6) found on crossing 
Srugivorus with alexandrinus that the former was apparently a simple 
Mendelian dominant to the latter ; and while the heterozygous frugzvorus 
gave a proportion of pure alevandrinus, the latter always bred true. 
By the mating of /ruagzvorus with alexandrinus thirty “ hybrids,” all being 
in appearance typical white-bellied /ragzvorus, were produced. Five 
pairs of these hybrids were mated, and their progeny consisted of 
17 (apparent) frugevorus, 5 alexandrinus, 7 fawn frugivorus, and 1 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 7. vatfus behaves in turn similarly as a dominant to 
Jrugivorus*; his results are, however, complex, and not easy to disen- 
tangle, probably because both vattws and frugzvorus include a large per- 
centage of heterozygous individuals. The hybrids of rattus x frugivorus 
on being paired together gave a progeny of numerous black rats, fewer 
JSrugivorus, and, as in the case of the cross frugzvorus 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 (Amerzcan Nat. xliii., 182, 1909) 
confirm in part these results deduced from de I’'Isle’s records ; he found 
that whichever way the cross was made the progeny of frugzvorus 
x vattus 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, 7 vattus, 
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, ractus 
1 Although de l’Isle throughout his paper calls his light-coloured rat a/exandrinus, 
his description of it as having the ventral hairs white to their bases leaves no room 
for doubting that his specimens were of the /rwgzvorus race as defined by Bonhote. 
2 De l’Isle’s “semi-alexandrine ” corresponds apparently to Z. +. alexandrinus as 
defined here. 
