ALBINO RAT VISCERA, GLANDS, EYEBALLS I WEIGHTS 117 



Our experience with rats for maii}^ years, however, shows that 

 they can not be kept in normal health and reared for any length 

 of time without great care and a highly complex diet. For a 

 time rats may grow and multiply under almost any condition, 

 but if one takes the trouble to record the fertility, death rate, 

 growth rate, and so forth, the importance of the proper food and 

 surroundings will at once be recognized. The relations between 

 the body and organs in weight are clearly modified according to 

 whether the growth of the individual was retarded, accelerated, 

 or normal. Although I have not arranged the data to demon- 

 strate such modifications, the records for such a demonstration 

 are at hand. Recent work by Watson ('10) shows more or less 

 histological alteration of various organs following the different 

 kinds of food given to the rats. 



2. Different strains. Another cause of fluctuations in the rec- 

 ords may be due to the use of different strains of rats. How 

 many strains are there, and how these strains originated or fur- 

 ther how stable such strains will be under varying conditions we 

 do not yet know. 



Nevertheless, it has come to our notice that certain strains, 

 represented by rats coming from a new source, tend to show 

 constantly some degree of deviation always in the same sense. 

 For example, we had a strain called ''the Ridley Park strain." 

 The rats belonging to this strain had for a given body weight a 

 relatively shorter tail and body and possessed a markedly heav- 

 ier brain. We had again a strain called ''the summer strain" — - 

 animals which had been born and reared under unfavorable sum- 

 mer conditions. The rats belonging to this strain had a small 

 body weight for their age and an unusually small nervous system. 

 Such strains may occur under various climatic, nutritional or 

 other environmental conditions. It is important therefore to 

 consider the conditions of the particular colony of rats which 

 may be used for any series of observations. If the fluctuations 

 found are not merely statistical, but deviate constantly in one 

 direction, this fact must be given its proper weight when com- 

 paring these animals with those represented in the tables here 

 presented. 



