RATS. 279 



black Rats, and likewise the common Mouse, all of which follow man in his peregrinations, and 

 which to a certain degree are dependent upon man, and may, therefore, be termed semi-domestic 

 animals, are subject to a greater degree of variation than those species which hold themselves aloof 

 from him."* 



The cases, however, do not seem parallel. If the change means anything in these instances, it is 

 not the variation of a new breed, but the formation of a new species. We know that in the case of 

 the very same species the individuals which have settled in Europe show no difference in any 

 part of it ; that those which have colonized North America and the greater part of South America 

 have as yet remained constant ; that the same is the case with the emigrants into Africa, India, 

 China, Australia, and New Zealand. But we are, perhaps, dealing with an animal which has consti- 

 tutionally great powers of enduring change of condition of life. Next, wherever man has been 

 present to watch the process of variation, no variation has taken place, but in desert places like 

 the Galapagos, or nearly uninhabited, like Ascension Island, there variation has shown itself ; there- 

 fore isolation would seem to have something to do with it. It is as if in ordinary circimistances the 

 inclination to change of form induced by change of condition of life was never allowed to operate, or 

 each step towards change obliterated as soon as made by the prepotent influence of fresh blood from 

 the normal type ; but access to this being prevented in isolated situations, the progress to change is 

 able to go on uninterrupted. The changes in other places may probably only be postponed, not 

 refused. As just remarked, we must remember that we are dealing with an animal which has 

 been exposed to change only for a very short period — the Brown Rat not for a hundi-ed j^ears, the 

 Black Rat only for a slightly longer period ; and sure enough it is in the species which has been 

 longest exposed to it, viz., the Black Rat, that the most decided of these alterations have been 

 observed. 



One of the most interesting points connected with the distribution of Rats and Mice, is the 

 occurrence of many species in AustraKa. It was for a long time thought that that continent possessed 

 no placental mammals. But not only is this not the case, but the number of placental species is 

 very considerable. They are, however, all rodents and bats, and the rodents all belong to this 

 family. The Frontispiece sufficiently illustrates the similarity between one of the Australian pla- 

 cental Mice and the marsupial Antechini. It is to be noticed, that the Australian species all belong 

 to the Old-world tj^^e and not to that of the American species. 



Almost the only jjart of the habitable globe where Rats and Mice are absent is the Arctic 

 regions, and the elevated steppes of Central Asia. There they are replaced by Voles (Field- 

 mice). Even in the burning Sahara the little Mouse makes its home, and the species there found 

 are, like many others of her creatures in the same or similar circumstances, pro\'ided by Nature with 

 a disguise which secures theii- safety by their modest garb, being dressed in a livery of the same 

 colour as the soil on which they pass their lives. 



In warm climates the species attain the greatest size, the Bandicoot Rat of India (not the 

 Bandicoot of Australia, which is a totally different animal), M. gigaxteus, one of the largest being 

 upwards of two feet in length. 



Fossil remains of several species have been found in tertiary deposits in France and Germany. 



Shakespeare says "there be land-rats and water-rats;" it is only in Australia, however, that 

 this can be said with truth. Our "Water-Rats are not Rats, but Voles (Arvicol.^) ; but two or three 



* Waterhodse, op. cit. 



