40 
ZOOLOGICAL LITLUATUKE. 
the correctness of the aiithor^s conclusions would have been 
beyond any doubt if a part at least of the experiments had been 
made with examples obtained from Egypt : — 
The author commences by showing that the characters of form and ana- 
tomical structure are absolutely the same in Mus rattus and M. alexandnuus, 
and that the distinctive cliaracters given by Blasius do not hold good, if 
numerous fresh examples are examined. M. tectorum and M. leucogastcv 
(Pictet) are merely nominal species. Experience shows us that every distinct 
European species of 31ns is distinguished by certain peculiarities in habits ; 
but there is an absolute identity in this respect between 3f. rattus and 3f. 
alexandrinus \ in fact the only distinctive difference is one of colour, the 
former species being intensely black above, which colour gradually passes 
into the greyish of the lower parts, the latter being pure white below, pass- 
ing into a brownish grey above. 
These extreme types of coloration are, however, united by a series of in- 
termediate forms. As regards the question Avhich of the two forms is the 
original stock, the author refers to the fact that tlie species of numerous 
natural genera show a similar system of coloration, and states, as his con- 
clusion, that, as 31. alexandrinus represents the general coloration of 3Ius, 
we must consider this species to be the parent stock from which the black 
31. rattus is a descendant. In like manner the uniformly coloured 31. onus- 
culus is to be looked upon, not in the light of a specific t^^pe, but as the de- 
scendant of M. imertus (Savi), which continues to show the typical colora- 
tion of the genus 3£us, 
After having thus zoologically demonstrated the specific identity of the 
black- and white-bellied Rats, he was desirous of obtaining additional and 
conclusive proof. He gives a most instructive account of his experiments, 
crossing both races and producing hybrids which did not show any decrease 
in the power of reproduction, even after the offspring of the same parents, 
and of the same litter, had been paired through four generations. He ob- 
served some very curious facts in the course of these experiments : when a 
male 3£. rattus was crossed with a female 3t. alexandrinus, black Rais 
only (like the father) were produced; but when the sexes were re- 
versed, half of the offspring were like tlie mother, the other half like the 
father. The author has a very ingenious explanation for these different 
results : there is, he says, in the first case, a union, and in the second an an- 
tagonism of two agencies, simultaneously at work during reproduction, viz. the 
influence of the male parent over the female, and the influence of the acclima- 
tized race over the exotic. We cannot follow the author mto all the details 
of his experiments; but after having obtained the black form from the union 
of two pure Alexandrine Rats, he came to the irresistible conviction that all the 
individuals experimented upon must belong to one and the same species. 
Attempts to cross 31. rattus with 3£. decumanus were entirely unsuc- 
cessful. 
In the concluding chapter, the author enters into the probable history of 
the migration of 31. rattus. Neither the black- nor the white-bellied forms 
are very recent additions to the French fauna; they are found inland in 
fields, not in or near the seaports. The centre of creation of this species is 
Arabia ; hence it spread over the soiilli-eastern coasts of the Mediterranean, 
and was imported into France towards tlic end of the tAvelfth century. 
