2 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



but the simplest mechanical arts. Yet, while our knowledge of the 

 laws of nature as they apply to machines has reached very great 

 magnitude and complexity, it is comparatively only a few years since 

 the principles of breeding have been more than a collection of unre- 

 lated traditional beliefs. The same superstitions on which the 

 shepherds of Asia based their practices at least 30 centuries ago are 

 still widely current, while the one sound principle known to the 

 ancients — selection of the best for breeding stock — is still widely 

 neglected. 



The earliest records show that the domestic animals had already 

 become much modified from their wild ancestry. The process of 

 change, however, had probably been exceedingly gradual and has 

 continued so until very recently. A thoroughly self-conscious move- 

 ment toward improvement of livestock dates back hardly more than 

 a century and a half. Robert Bakewell, of Leicestershire, England, 

 is credited with being the pioneer in this movement. 



The breeders of the time of Bakewell suspected him of possessing 

 and concealing special principles of breeding. It is often believed 

 to-day that successful breeders have some mysterious method of 

 which others are ignorant. Instead, the principles of the successful 

 breeder have been exceedingly simple. He isolates and fixes a good 

 type by careful selection and close breeding. If ambitious to take a 

 greater step in advance, he crosses types with characteristics which 

 seem to offer possibilities for a desirable combination and fixes the 

 new ideal by continued selection and close breeding. He brings 

 inferior stock up to a higher level by consistent use of prepotent 

 sires of the same improved type. The difficulty lies not so much 

 in knowing the principles as in applying them. Without skill in 

 feeding and management, the possibilities of the animals can not be 

 brought out in such way as to give a satisfactory basis for selection. 

 Selection of breeding stock, moreover, requires the best judgment in 

 estimating the merits of the animal's own performance, its conforma- 

 tion, ancestry, and previous success as a breeder, and also in giving 

 each of these its due weight. Good judgment, industry, and per- 

 sistence in following a given aim, as well as knowledge of sound 

 principles, have been the qualities which have made successful 

 breeders. 



REPRODUCTION. 



THE CELL THEORY. 



There could be no clear ideas of breeding until something was 

 known in detail of the processes through which a new individual 

 starts on his career and develops. The most important step in this 

 direction was the discovery that all living organisms are built up of 

 microscopical living units, the cells, with characteristics which do 



