SELECTION IN PLANT BREEDING 



20I 



In closing I should like to call attention to a fact both of evolu- 

 tionary and of commercial importance. The first generation of crosses 

 between nearly related types generally grows more vigorously than the 

 pure types themselves. If the fertility is not impaired;, they even fruit 

 more freely. This is undoubtedly the explanation of Burbank's quick- 

 growing hybrid walnuts, but if they were self-pollinated and grown for 

 another generation a large percentage of the progeny would lose this 

 character. In naturally self-pollinated types like tobacco, one sees the 

 phenomenon expressed as greater vigor in a cross ; in a continually inter- 

 crossed species like maize the same thing is shown by a loss of vigor 

 when the plants are self-pollinated. It is clear then that if pure strains 

 of maize are gradually isolated by selection, by the same token they lose 

 in vigor and productiveness. The original mixed strain may contain 



Fig 5. Effects of inbreeding in Maize. Outer ears inbred four generations. 

 Middle ear the result of their crossing, first generation. 



sub-strains some of which are much more productive than others. The 

 less productive types may be discarded, but at the same time there is a 

 loss of vigor from the fact that they are withdrawn from hybrid com- 

 binations. The logical procedure, then, is to isolate two high-yielding 

 types, combine them by hybridization, and grow only the first genera- 

 tion of the cross. This is not mere theory, for by using such methods 

 I have obtained from 100 to 200 bushels of shelled corn per acre on 

 small plots. Unfortunately, this method can not be used to advantage 

 on many crops, but in the case of maize the procedure is simple. There 

 are many breeders using the isolation method of improvement. From 



VOL. LXXII. — 14. 



