VARIATION 197 



Some individuals are dwarfs while others are giants in 

 size. This variation in size is due to a difference in the 

 number of cells rather than in the size of the cells. The 

 increased number of cells found in the larger individuals 

 represents excessive cell division, while the abnormally 

 small are the result of imperfect and arrested cell division. 1 



The cause of undersized individuals among animals 

 cannot always be determined. It is certain, however, 

 that insufficient food or food which is deficient in the 

 necessary elements of nutrition is a common cause of 

 undersized animals. 



It is also true that when females become pregnant 

 while still growing and immature, their growth receives 

 a sudden check. The arrested development is not so 

 much due to the effects of pregnancy as to the strain of 

 lactation. The growth of well-fed females is probably 

 not to any appreciable extent checked by pregnancy, 

 but the physiological requirements for the production 

 of milk are severe and the development of immature 

 mammalian females is abruptly arrested during the 

 period of lactation. 



Differences in size are of relatively less importance than 

 morphological differences arising from variation in the 

 relative development of the parts of the body. This 

 may be illustrated by the variations in meat animals. 

 Beef cattle may possess a broad back, well-sprung rib, 

 and a thick covering of flesh over all parts, or they may 

 lack these highly desirable qualities. 



188. Physiological variations. Changes in the func- 

 tional activities of animals are frequent and important. 

 The average domestic cow produces not more than 150 

 pounds of butter in a year, but selected herds may produce 



1 Davenport, "Principles of Breeding," p. 27. 



