2 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 
hut 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 ta"ke 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 
