4 Memoirs of the Indian Museum. [Voi.. I, 



Mr. Bonhote admits that a series of M. bukit agreed exactly with a series sent 

 home by Mr. I^yle from Assam and recorded by Mr. Bonhote himself as M. jerdoni. 

 Nevertheless, not only does he retain the name of M. bukit, but he places it in a different 

 subgroup from M. jerdoni, namely, that of M. griseiventer. M. pellax is practically a 

 prototype of M. jerdoni, but he separates them. Thomas found M. pyctoris to be 

 identical with M. nitidus, but they are here placed in separate subgroups, the latter 

 being placed in that of M. griseiventer although it has been specially noted that the 

 pyctoris subgroup represents the nitidus group of Thomas. 



THE CAUSES OF CONFUSION. 



It is difficult for one who is an absolute tyro like myself to discuss, however super- 

 ficially, the causes that have led to the present confusion. It is only the great practical 

 importance of the subject and the knowledge that many of those who are working at 

 rats in connection with plague have no pretensions to be trained zoologists, that em- 

 boldens me to touch on it at all. The leading fault seems to be one that is all too com- 

 mon in many branches of zoology, namely, an undue tendency to manufacture new 

 species out of the most trifling deviations from type, accompanied by neglect to give 

 due weight to the following factors of variation : — 



1. Local variations due to environment. 



2. Developmental variations. 



3. Normal variations and sports. 



LocAi, Variations. — In recent literature the most instructive instance of this fac- 

 tor is that of the Mus musculus (House Mouse) of North Bull, an island in Dublin Bay. 

 It is less than a century since this island was cut off from the mainland, but already a 

 large proportion of individuals have developed a pale sand-coloured coat with light buff 

 or yellowish belly to harmonize with the environment of sandhills. In St. Kilda and the 

 Faroes, in both of which it is known that the time available for variation is short, 

 specialization has gone much further and types have been produced differing im- 

 mensely from the parent type in general size and the relatively much increased hind 

 foot, in addition to exhibiting comparatively minor variations in colour. How far those 

 local variations can carry the enthusiastic species-maker is seen in Captain Barrett- 

 Hamilton's paper on the variations of the Long-tailed Field Mouse, in which most de- 

 tailed descriptions are given of 16 subspecies. Judging from these instances we must 

 expect to find slight variations in all island forms, so that the present policy of giving 

 each of these a new specific name opens up a vista full of the direst complexity. 



DevEi^opmentai, Variations. — Immature specimens, particularly where they 

 do not form part of a series, are liable to be made into new species, as the variations 

 are sometimes extreme. If this major error is avoided, a source of great confusion 

 remains in the erroneous and misleading measurements that may be recorded. I 

 shall not deal with the subject at present, as it will be taken up at length later on. 



Normal Variations. — As has already been indicated, these are too frequently 

 neglected altogether. There is already a considerable amount of literature dealing 



