368 



Inbreeding, Selection, and Heterosis 



inbreeding and that a harmful result, when it occurs, is not 

 caused by some mysterious force that produces weaknesses in 

 some undefined way. Second, there are organisms in which in- 

 breeding is not harmful and also organisms in which it appears to 



tilUtrf f 



',2 iH 16 



tw*«rt self-f 

 ferti)ized > 



// \/ \A/ X f X y X T^ 



Fig. 100. Shull's first demonstration, of heterosis in maize. The top line 

 of ears consists of cross-bred types differing in the number of rows. Con- 

 tinual self-fertiHzation of these strains produced the vastly inferior types 

 shown in the middle row. The fine, large ears of the bottom row resulted 

 from certain crosses between self-fertilized lines and show heterosis. From 

 an exhibit at Omaha, Nebraska, before the annual meeting of the American 

 Breeders Association. (Photogi-aph made Dec. 7, 1909. Courtesy of Dr. 

 G. H. Shull.) 



be beneficial. Some plants, such as peas, beans, wheat, and oats, 

 regularly produce seed by self-fertilization and do not seem to 

 benefit if they are cross-fertilized. An experiment in brother 

 and sister matings in Poland China swine, which is as close in- 

 breeding as can be carried out in such animals, was made at 

 the Minnesota Agricultural Experiment Station. After eight 

 successive generations on the whole no loss of vigor resulted. 

 As for human beings, the Ptolemies of Egypt formed a succession 



