49 MISCELLANEOUS PAPERS. 
uniformity, for with any change of characters the debility of the 
stock becomes more manifest and the elimination of the variant indi- 
viduals is certain. Increased yields are obtained by making the 
yield of the individual plants more uniform, even when the full pos- 
sibilities of production are not approached. The best plants of a 
highly bred variety are not conspicuously more prolific than the best 
individuals in fields from unselected seed.* 
This failure of varieties to continue to improve in vigor and 
fertility under continuous selection gives apparent support to the 
idea of De Vries that the initial selection of “ pure strains” is the 
only progress that can be expected. It has already been seen that 
attempts to secure increased vigor by close selection are impeded by 
the weakening effects of inbreeding. When selection is not ham- 
pered in this way, but is directed to characters not dependent on 
vigor and fertility, and consequently not affected by inbreeding, 
continuous progress may be made. Thus in the selections made by 
Hopkins for high and low protein content, rapid and continuous 
progress was made and an extreme was reached far in excess of 
anything observed in the original sample. The last reports show 
that the plants were no more uniform with respect to these characters 
than at the beginning of the experiment.” 
Though necessarily impeded by inbreeding, important advances 
in yield have been made by means of close selection, but the value 
of these improvements should not be allowed to obscure the fact that 
the full possibilities of production are not reached until the incre- 
ment of vigor obtained by crossing has been added. Even if the 
yields obtainable by crossing were not larger than those to be secured 
by persistent close selection, it is easier to permit crossing than it is 
to provide the very careful and skillful selection required to maintain 
high yields without crossing. To use crossing as a means of sustain- 
ing fertility, instead of relying upon selection alone, would also keep 
our stocks in more normal physiological condition, more resistant to 
disease, and less Hable to injury by adverse conditions. 
SUMMARY. 
The development of the present methods of corn breeding has re- 
sulted in greatly limiting the number of individuals that serve as a 
foundation for improved strains. The danger of this course, as 
“Tt has recently been pointed out by Mr. O. F. Cook that the apparent ad- 
vantage of selection is greatest in degenerating stocks. See Bulletin 146, Bureau 
of Plant Industry, U. S. Department of Agriculture, “The Superiority of Line 
Breeding Over Narrow Breeding.” 
5Smith, L. H. Ten Generations of Corn Breeding. Bulletin 128, Illinois 
Agricultural Experiment Station. 1908. 
141—Iv 
