Xiv PROLOGUE 



In enumerating the qualifications of the rat as a laboratory 

 animal, and in pointing out some of its similarities to man, it 

 is not intended to convey the notion that the rat is a bewitched 

 prince or that man is an overgrown rat, but merely to em- 

 phasize the accepted view that the similarities between mam- 

 mals having the same food habits tend to be close, and that in 

 some instances at least, by the use of equivalent ages, the re- 

 sults obtained with one form can be very precisely transferred to 

 the other. 



It is proper to recognize, however, that, considered from the 

 laboratory standpoint, at least three major deficiencies are pre- 

 sented by the rat. Its small size makes some types of experi- 

 mental work either difficult or impossible. Its resistance to 

 various forms of infection, diphtheria and tuberculosis, for ex- 

 ample, makes it unsuitable for certain classes of studies, while 

 its susceptibility to a form of lung infection, sometimes called 

 rat pneumonia, has proved thus far a serious obstacle to studies 

 on senescence. It is possible that this last defect may be reme- 

 diable, but the others are inherent. 



In assembling this information the aim has been to give, in 

 tabular form especially, such data as can be presented in quan- 

 titative terms and also such as are closely related to age and to 

 the critical periods of development, since the experimental re- 

 sults obtained often depend in such an intimate way on the 

 exact condition of the animal at the time of the test. 



