136 INBREEDING AND OUTBREEDING 



amount of seed produced by hand pollination under the 

 most favorable circumstances, necessitates the using of 

 the best ears obtained for planting in order to have 

 enough plants upon which to make any fair observations. 



These factors tend to prevent the attainment of com- 

 plete homozygosity. Nevertheless, all the evidence at hand 

 indicates that the four strains of Learning corn which have 

 been continuously self-fertilized for twelve generations 

 are now very nearly, if not completely, homozygous in all 

 inherited characters. As stated before, this evidence com- 

 prises cessation of reduction in size and productiveness, 

 of reduction in variability, and of change of average row 

 number and other characters. But there are still other 

 ways of testing the proposition. On the theory that in- 

 crease in growth results from crossing when the individu- 

 als united differ in respect to some inherited qualities, if 

 no increase results, then the parents have no differences. 

 These strains have been tested in this way by crossing 

 different plants within a strain and comparing the crossed 

 plants with self ed plants. While some increases in growth 

 resulted from such crossing they were balanced by de- 

 creases in other cases, so that the inconsistencies are most 

 likely due to difficulty in securing an accurate test. At 

 the same time one should not shut his eyes to the possi- 

 bility that some of the strains have reached complete 

 homozygosity, while others, as yet, have not ; although no 

 sure evidence of such a state of affairs has been obtained. 



Most of the direct experimentation to determine the 

 effects of inbreeding has been with cultivated plants and 

 domestic animals. The question will undoubtedly be asked, 

 therefore, as to whether the results would have been the 

 same had wild species been investigated. It would be 



