THE PRACTICE OF BREEDING 281 



same prehistoric form. Each type breeds true. The 

 chief distinguishing character of size is firmly fixed in the 

 germ-plasm and we must come to the conclusion that 

 these radical differences have resulted from selection and 

 have become firmly established hereditary characters. 

 Similar differences in size among cattle, sheep and swine 

 supply additional evidence that size is a character which 

 may be radically changed through selection and that this 

 variation may become so firmly fixed that it may be re- 

 garded as an established characteristic of the breed. 



264. Improvement in function. The most remark- 

 able achievements in the improvement of the domestic 

 animals are undoubtedly improvements in the various 

 physiological functions of the animals useful to man. 

 Some of the most noteworthy of these are the milking 

 function in cattle, wool production in sheep, tendency 

 to fatten in meat animals, speed in horses and egg-laying 

 in the domestic fowls. A comparison of the productivity 

 of each of these types of animals with the unimproved 

 types gives ample evidence of the remarkable develop- 

 ment which has taken place through the agency of man's 

 selection. 



265. The milking function. The ability of mam- 

 malian animals to produce milk is closely correlated with 

 the reproductive functions. The mammary glands func- 

 tion primarily for the purpose of supplying a nutritious 

 and easily digested food for very young animals. Among 

 wild, forms this function persists only for a comparatively 

 brief period and its continuance is determined by the 

 needs of the young mammal. Under domestication the 

 milking function in the domestic cow represents a re- 

 markable improvement. The wild cow probably sup- 

 plied milk to her offspring only four or five months. The 



