118 povLi'iiY PRODrcrrox 



]>iiniirr (if (lniiUnii.--r]\r weakness "f ^radin.u' as a practice 

 lies ill its success as a iiictliod. l^ecaiist' in tlic second or 

 third n'ciicratioii males are ]>roduced tliat Acry closely 

 ap])roach standard, linc-hred hirds in a])])t'arancc, the temp- 

 tation is to use them for hreeding ])urj)oscs rather than to 

 purchase, first cost considered, a relatively' expensi\'e stand- 

 ard-bred bird. When tliis is done ini]iro\-einent usually 

 stojis and the ground gaiiie(l will be at least ])artially lost. 



Thus, if in Figure 4.1 the otl'sjiring of ^ and /' appear to be 

 fairly miiform, the breeder may yield to the temptation to 

 use a fine-looking cockerel from group E. '^khe offsjjring 

 on the average carry 2;") ])er cent mongrel blood instead of 

 ]'2.'t ])er cent, as indicated at G and is therefore no improve- 

 ment o\er E. In fact, in appearance many of them will not 

 be as good, because the offspring E has the benefit of the 

 cliaracteristics transmitted by a standard-bred sire. A 

 grade sire is no more jH'cjxttent than the grade females he 

 is mated with. The result is that instead of tending to become 

 more uniform the offs]iring will be less so. 



Inbreeding and Line Breeding.— Antliorities ditfer on the 

 distiui-tidii between these terms. In a. I'ecent cxliausti\'e 

 study of inbreeding Peai'l' suggests that most definitions 

 of inbreeding have been based on practical ex]iediency rather 

 than on careful analysis of the problem. He defines inbreed- 

 ing as the reduction of the lumiber of possil)le elilTerent ances- 

 tors in any generation or generations. Idiis is, of course 

 accomi)lislied through the mating of indixiduals more or 

 less closely related. 



Line breeding is a special form of inbreeding in \\hieh the 

 number of possible dill'cri'nt ancestors of a gi\-en bird is 

 reilnced in such a way as to increase the nuinbci- of appear- 

 ances of a certain ancestor in its pedigri'c. 



The diifercnce between line Ijreeiling and inbreeding is 

 shown in k'igures 51 and f)'!. 



In the third ancestral gcnei'a.tion, as shown in I'iguri, .")], 

 birds A, B, C, D, each ajjpear twice and the munbei- of 

 ancestors of K is thereby reduced one-half, 'kliat is to say, 

 K has only four different individuals as grand])arents 



^ American Natur.alist. vol. xKii, Xo. .502. 



