Present Day Methods.—The method of the present day breeder is that of all his predeces sons who more or less consciously and intelli gently have worked for the improvement of domestic animals. The method is to select in each generation and to rear offspring from those animals which are best suited to the special purpose for which the breed is kept, be it milk, meat, wool, work or what not. Thus breeds for special purposes are created and further improved. But the work of the animal breeder is less simple than this statement might seem to imply, since in the production of a new generation of animals two parents are involved, each of which influences the character of the offspring. Each selection of animals for propa gation thus involves a double selection, selection of a sire and selection of a dam, and it is not sufficient that each be excellent for its kind. The two must be compatible in the inherited traits which they transmit. Otherwise undesir able combinations of characters may be ob tained in the offspring, or a lack of the de sired uniformity in different individuals.
The breeder's first principle of success is ac cordingly (1) Select the best individuals to breed from. His second principle is (2) Male each selected animal with one having compatible characters. This in general will mean the mat ing of like with like, selecting spates only within the same established breed or sub-breed. But here comes in a danger which must be avoided. If animals most like each other in inherited traits are mated together, brother will be mated with sister, or'cousin with cousin. But age-long experience shows that such matings very often result in diminished vigor, size or fecundity and so the careful breeder is reluctant to resort to Flose.inbreeding, and if he does so, occasionally in order to secure or preserve some particularly desirable combination of characters, he mica special precautions to avoid enfeebling effects. The breeder's third principle is accordingly (3) Avoid close inbreeding. No one can tell in ad;. vacate to what extent inbreeding may safely be followed in a particular case. Miss King, ex perimenting with rats, has found it possib'e, by selecting in each generation only the most vigorous individuals to breed from, to cost/hive mating brother with sister for more than 29 generations, without loss of vigor or fecundity in the race. On the other hand, rats inbred without such selection for vigor as was made by Miss King show marked loss of vigor and fecundity after 10 or 15 generations. He who
resorts to inbreeding should do sb with caution, keeping careful watch of the size and vigor of his race. Crossbreeding, i.e., the mating;' gether of unrelated individuals, or those of very different inherited characters, has an effect on vigor contrary to that of inbreeding. It in creases size and vigor and induces rapid growth, This is an important principle utilized by animal breeders in meat production among cattle, sheep and pigs. Distinct breeds are crossed, such as short-horn and Angus cattle, or Berkshire and Duroc-Jersey pigs. The result is to produce an animal superior to either pure breed as regarclS rapid growth and early maturity. But this superiority persists for a single generation only, so that the production of a second generation from the vigorous cross-breeds is not to be recommended. Lack of uniformity both in vigor and in other characters is quite sure to occur in the second generation, though not in the first. A fourth maxim for the breeder would be (4) Employ crossbreeding whenever practicable, to increase vigor, but do not go be yond the first cross.
While the variable second crossbred genera tion is of little use to the ordinary breeder, it may be of great value to the experimental breeder, who is engaged not in mere quantity production of a standard type of animal, as the ordinary breeder is, but in the attempt to pro duce a new type of animal better suited for some special purpose than are existing types. The experimental breeder works by the method of crossing two breeds, each of which contains a different inherited character or set of charac ters, which he desires to combine in one new race. Crossbreeding is the only method by which characters found in different races may be combined in a single new race. Seldom is the desired combination secured in the first crossbred generation. More often it will only be found in a small percentage of the second generation crossbreds obtained by breeding first generation crossbreds with each other. Such animals of the second generation crossbreds as do present the desired combination of charac ters may now be mated with each other and a new race of the desired type may thus be secured. But at first it is apt to be rather variable and several additional generations of selection are usually required fully to establish the new type. The production of a new breed is thus a difficult and expensive undertaking and should be tried only by experts familiar with the method of inheritance of different characteristics.