48 C. M. JACKSON 



causing them to become relatively larger. This principle of 'use 

 and disuse' is widespread in its application as an explanation of 

 changes in the relative size of organs. Joseph ('08) suggests that 

 the relatively large heart in the smaller mammals may be due 

 to a 'physiological hypertrophy,' correlated with the more rapid 

 rate of pulsation. 



5. Within certain limits, however, an organ is normally some- 

 what larger than is necessary to perform the functional activity 

 usually demanded. Fluctuations in the amount of this excess of 

 structure (which Melzer has termed the 'factor of safety') may 

 account for a part of the individual variations in relative size 

 found to a greater or less extent in every organ. 



6. Various pathological conditions may be associated, either 

 directly or indirectly, with changes in the relative size of indi- 

 vidual organs or parts of the body. 



The foregoing illustrate conditions associated with changes in 

 the relative size of organs, and representing for the most part 

 teleological explanations, rather than actual causes of varying 

 growth rates in the different organs and parts. For any given 

 organ the immediate cause of its characteristic rate of growth is, 

 like that for the whole body, a complex of factors. These may 

 be divided into (a) intrinsic structure and chemical composition 

 of the organ and (b) its environment, especially the conditions 

 affecting the vascular supply, which transmits the respiratory, 

 nutritive and excretory materials, as well as specific hormones 

 or substances which may stimulate the organ to growth and 

 activity. 



SUMMARY 



Although the extensive data presented cannot satisfactorily be 

 summarized, some of the more important general conclusions con- 

 cerning growth and variability of the albino rat are included in 

 the following : 



1. At birth the males invariably exceed the females of the 

 same litter in average body weight. Although growth is in gen- 

 eral more vigorous in the females during the first six weeks, they 

 fail to overtake the males in the majority of litters (at observed 

 ages). At six weeks, however, the total average body weight of 



