192 LIFE HISTORY 



The marked differences between the two forms appear later, 

 during the period of rapid growth. Grouping together the gen- 

 eral differences subsequently found, we may say that the Norway 

 rat is absolutely much heavier, relatively slightly longer, has a 

 relatively heavier brain and a heavier spinal cord, and since for 

 the same body weight as a given Albino it is younger, it has 

 when so compared a higher percentage of water in the central 

 nervous system. 



For the same age however, the percentages of water are nearly 

 alike; the percentage in the Norway rat being a trifle higher 

 (Donaldson and Hatai, '11). The relative weights of the ovaries, 

 testes and suprarenals are also greater (C. Watson, 'Q7; Hatai, 

 '14). These plus characters of the Norway tend to disappear 

 when the Norway is subjected to domestication. 



The deviations of the Norway may be expressed in another way. 

 When the body weights of Norway and Albino are the same: 



The Norway rat has a greater body length; a greater brain 

 weight; a greater spinal cord weight; a higher percentage of water 

 in the central nervous system; heavier ovaries, testes and 

 suprarenals. 



When body lengths are the same: 



The Norway rat has a smaller body weight; a greater brain 

 weight; a greater spinal cord weight; a higher percentage of 

 water in the central nervous system; heavier ovaries, testes and 

 suprarenals. 



When brain weights are the same : 



The Norway rat has a smaller body weight; a smaller body 

 length; a smaller spinal cord weight; a higher percentage of 

 water in the central nervous system. 



When the spinal cord weights are the same: 



The Norway rat has a smaller body weight; a smaller body 

 length; a greater brain weight; a higher percentage of water in 

 the central nervous system. 



Speaking generally therefore we may say that when compared 

 with the domesticated Albino, the wild Norway rat weighs more, 

 is longer and possesses a nervous system in which both the brain 

 and spinal cord are relatively larger. 



