SELECTION IN PLANT BREEDING 201 
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 sel f-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. ErrectS OF INBREEDING IN Matizp. 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loss of vigor from the fact that they are withdrawn from hybrid com- 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, Lxx1.—14. 
