THE PRACTICE OF BREEDING 287 



in the practices and results of animal-breeders can be 

 explained upon the basis of selection alone, but it is quite 

 certain that without selection little practical use could 

 be made of the known laws which govern the transmission 

 of characters. Man's chief agency in the improvement 

 of animals has not been a conscious effort to bring about 

 a series of variations or mutations of a certain kind, but 

 it has rather been in the direction of preserving such 

 valuable characteristics as have been already in existence 

 or have appeared through variation. These character- 

 istics have been intensified by judicious matings and 

 perpetuated as a result of the keen insight of the skillful 

 breeder. The successful breeder has ever in mind an 

 ideal. He is at all times alert to detect variations which 

 approach this ideal. 



Darwin believed that most of the improvement wrought 

 in domestic animals was due to minute or continuous 

 variations from the less desirable to the more desirable. 

 This belief has been general among animal-breeders 

 themselves. Working upon this assumption, the work of 

 the breeder consisted merely in accurately observing the 

 variations which tended in the desired direction. 



But continuous variation assumes a perfect series of 

 infinitesimal steps, each grading into the one higher or 

 lower. Such continuity exists in the growth of animals 

 from birth to maximum development. Continuous varia- 

 tion is also illustrated in the physical world by changes 

 in temperature. In the history of the improvement of 

 domestic animals there are many examples of marked 

 and sudden variations in the offspring which cannot 

 be continuous. Such variations have been called dis- 

 continuous variations or mutations. It is certain that 

 much of the improvement in the domestic animals has 



