178 C. M. McCay 



the point of view of the biochemist and nutritionist is that 

 they are very subject to epidemic diseases and difficult to 

 use in any type of chemical balance work since they have the 

 habit of scattering feed badly. 



Mice have the same advantage as rats and hamsters in 

 passing from birth to old age within a period of about two 

 years. In recent years the studies of M. Visscher and associ- 

 ates (unpublished) have indicated that mice are very sensitive 

 inasmuch as small modifications of diet seem to change 

 markedly the terminal diseases of old age. 



White rats have served the nutrition laboratories for about 

 seventy-five years. Hence, their nutritional requirements 

 are better defined than most of those of the common species. 

 In spite of the long use of rats, their genetics has been little 

 studied. Some breeding studies have indicated that strains 

 can be selected which differ very much in efficiency of feed 

 conversion during growth. Some strains are also much more 

 subject to tooth decay than others. But, compared to mice, 

 the rat is an unknown in genetics. 



The assets of a rat in terms of a two year span of life and 

 ease of feeding for chemical balance studies have already 

 been indicated. Furthermore, modest knowledge has accumu- 

 lated in regard to the pathology of this species in old age. 



Since the white rat has been used more than any other 

 species for life-span studies, it may be well to scrutinize the 

 weaknesses in such research. This has become more important 

 in modern times because public interest has stimulated the 

 writing of an ever growing volume of literature based upon 

 a meagre background of experimental science. Opportunists 

 among writers have sensed easy profits from such writing 

 due to the large number of older people and the increased 

 interest by most of them in their own well being. Such 

 popular writers neither desire nor are able to evaluate critically 

 much of the meagre evidence from research laboratories. 



The greatest weakness of the white rat as it ages is the 

 development of the chronic respiratory disease commonly 

 called "bronchiectasis". This can usually be detected easily 



