2 MICEOTIN^ 



widely ranging species are extraordinarily perplexing animals. 

 Althougli they attain sexual maturity and an adult appearance 

 at a very early age, their growth continues for an unusually 

 long period, during which colour, form of the skull and structure 

 of the teeth may change to such an extent that in old age they 

 may present characters widely different from those shown in the 

 earlier " adult " stages of growth. To work out the life-histories 

 of such mammals thoroughly requires a large amount of material 

 and more time than is at the disposal of a systematist busied with 

 the task of determining collections as they arrive in the Museum. 

 One general result of my work has been to reduce the number of 

 genera and species considerably. 



The Microtinae are of great general interest and merit careful 

 attention. Their structure is highly modified in relation to 

 subsistence upon a diet despised by most other Muridse, and to 

 their burrowing habits, but it retains certain primitive features 

 which seem to throw light upon the problems connected with 

 the ancestry of the Simplicidentate Rodents and with the evolu- 

 tion of their molars. The close adaptation of form and structure 

 to the needs of special functions shown by the skulls, teeth, and 

 other organs of the Microtinse deserve close study; in one 

 genus {Mimomys) at least, where we can trace a single phylum 

 forward in time, we find a good example of recapitulation. Owing 

 to the short range in time of the individual species the group is 

 of great importance to the geologist who has to correlate scattered 

 deposits like those of the Pleistocene period in Britain. 



The Microtinse become of considerable economic importance 

 in certain circumstances, as in " mouse years," when, favoured 

 by lenient weather and often by unwitting human intervention, 

 voles and lemmings swarm and inflict great losses upon agrarian 

 industries. In such years, too, riparian species occasionally do 

 serious damage to the banks of streams and canals by their bur- 

 rowing operations. When present in reasonable numbers these 

 rodents, like most other creatures, play their part in maintaining 

 the general balance of Nature, and in uncultivated lands their 

 fossorial activities endow many tracts, which would otherwise 

 be completely barren, with comparative fertility. 



Some species are apparently concerned in the dissemination 

 of disease, as, for example, the Japanese Microtus tnontebelloi, 

 which is believed to be one of the natural reservoirs of Infectious 

 Jaundice and Japanese River Fever. 



The North American genus Ondatra is of much value as a 

 fur-bearer. During the open season the flesh of this animal is 

 extensively used as food in the large towns of eastern North 

 America. 



Much remains to be done before a really satisfactory account 

 of the subfamily can be written. The present work is intended to 

 summarize existing knowledge and to direct attention to some 

 points of more general interest that require further investigation. 



