DOMESTIC ANIMALS 



39 



cross-breeding, line-breeding, natural- 

 breeding and grade-breeding. Quite 

 extensive data on inbreeding have been 

 collected by Miles and bis discussion is 

 followed in part in tbe present account. 

 Inbreeding — I n tbe first place mucb 

 difference of opinion prevails as to the 

 closeness of relationship necessary to es- 

 tablish a case of inbreeding. Inbreeding 

 has been variously defined as "breeding 

 between relatives without reference to 

 the degree of consanguinity," "breed- 

 ing from close relations," "breeding 

 from the same family" and "pairing of 

 relations within the second degree of 

 cousins twice or more in succession." 

 In general inbreeding is used to mean 

 breeding between close relatives with- 

 out defining the exact relation. Inbreed- 

 ing may mean a single pairing of 

 relations, or several, or even the contin- 

 uation of the process. 



Since the days of Bakewell, who be- 

 gan his work in 1760, all noted breeders 

 have practiced inbreeding to a greater 

 or less extent. It seems to be necessary, 

 in fact, in establishing a breed. Cer- 

 tain desirable characters appear in one 

 or two members of a family and form the 

 basis of a breed. Some of the families 

 of Herefords were inbred for 80 years 

 or more without the infusion of outside 

 blood. In the practice of inbreeding 

 many of the closest possible pairings 

 have been allowed, for example brother 

 and sister, father and daughter, etc. 

 Atwood developed a fine flock of Meri- 

 nos from one ewe without any outside 

 blood. Hundreds of other examples of 

 close and continued inbreeding are on 

 record. In all cases, however, inbreed- 

 ing has been practiced in order to pre- 

 vent the admixture of other tendencies 

 in the preservation of desirable ones. 

 In other words, breeders have selected 

 their breeding animals for their posses- 

 sion of the required characters and with- 

 out reference to the relationship be- 

 tween the animals. Inbreeding is a 

 merely accidental process in the forma- 

 tion of a breed; it is a means to an end. 

 Inbreeding insures the fixing of tenden- 

 cies more quickly than by any other 

 method of breeding. If great care is not 

 exercised, however, defects are likely to be 

 established and perpetuated. Moreover, 

 inbreeding may result in a loss of vigor 

 and fecundity. It should, therefore, 

 never be practiced without a definite ob- 

 ject in view. In fact it is almost never 

 to be recommended in ordinary farm 

 practice. 



So called "line breeding" i s a modified 

 form of inbreeding, the main difference 

 being that in line breeding a little more 

 latitude is given to the closeness of re- 

 lationship. Interbreeding is allowed be- 

 tween members of a few families which 

 have descended from a common ancestry. 

 In other words the animals selected for 

 breeding purposes must be in the same 

 line of descent. It is obvious that if a 

 breed of animals be originated by in- 

 breeding, the large number of animals 

 ultimately produced will allow breeding 

 between animals of quite wide relation- 

 ship. They will, however, all be in the 

 same "line" of ancestry. Consequently 

 line breeding is essentially nothing 

 more nor less than the logical extension 

 of inbreeding after a large number of 

 breeding animals in one line have be- 

 come available. Both inbreeding and 

 line breeding suffer from the defects of 

 exclusiveness. All animals from out- 

 crosses are excluded. As suggested by 

 Warfield, if the family should become 

 too large it would get out of control, and 

 the feature of exclusiveness would be 

 lost. The purpose of inbreeding and 

 line breeding is to secure as nearly as 

 possible an absolute indentity of blood. 

 It may be desirable to have only one 

 type in a herd, but this is true only 

 when the type is perfect and without 

 defect. Since such an attainment has 

 thus far been impossible, it is desir- 

 able to have a slight variation in the 

 herd in order that selection may be made 

 of those animals which most closely ap- 

 proach the ideal in the mind of the 

 breeder. Exclusive inbreeding or line 

 breeding leads ultimately to loss of 

 vigor and size as shown by the white 

 park cattle of England which weigh 

 only 550 to 700 pounds. 



Natural breeding aS defined by War- 

 field, or, as it is sometimes called, out- 

 crossing or mixed breeding, is breeding 

 with the object of securing the best 

 progeny by all means at the disposal of 

 the breeder. This is nothing more nor 

 less than artificial selection. Natural 

 selection tends to the perpetuation of 

 the average individual of the species. 

 In the case of cattle, for example, cows 

 which give too little milk for the proper 

 sustenance of the calf would be gradu- 

 ally eliminated by natural selection. 

 Cows which give much more milk than 

 required by the calf would be weeded 

 out for the reason that the udder would 

 be too large and clumsy and might im- 

 pede the progress of the cow in escaping 



