948 
Journal of Agricultural Research voi. xxiii, No. i* 
In a previous publication ( 10 ), Knight had put forward the theory of 
varietal senility, although he did not lay special emphasis on the potato. 
His hypothesis was that all plants propagated vegetatively have their 
period of maximum vigor when at middle age and then become— 
subject within no very distant period to the debilities and diseases of old age. 
Aitken (j) applied the hypothesis to potatoes in 1837, and it was 
accepted by many later writers. Ehrenburg (6), in 1904, after reviewing 
the work of previous investigators, concluded that the idea of varietal 
senility was untenable. East (4), in 1908, came to a similar conclusion 
and notes— 
that the people who have considered this single question are unanimous in opposition 
to the hypothesis of varietal senility. 
Ehrenberg (6) considered that the hypothesis of the deterioration of 
seed stocks being due to unfavorable environment has some basis for its 
support. East (4) regarded disease as the most important factor to be 
considered in connection .with the degeneracy or running-out of seed 
stocks. Orton (15), in 1914, called attention to the so-called degener¬ 
ative diseases of “mosaic/' “curly-dwarf," and “leafroll" and to the 
probable part these diseases play in the running-out of seed stocks. The 
general distribution of these diseases and their harmful effect on the 
vigor of the plant, which has been brought out by later investigators— 
Stewart (20), Appel (2), Melhus (ij), Murphy (14, p. 33-82), Gussow (7), 
Wortley (jo), Shultz, Folsom, Hildebrand! and Hawkins (17), Quanjer 
(16 ), Shultz and Folsom (17), Krantz and Bisby (n)—has made it 
increasingly clear that the so-called running-out of seed is probably 
nothing more than a manifestation of these diseases. 
Very little attention was paid to the possibility of improving varieties 
by means of asexual selection previous to the latter half of the nineteenth 
century. It has apparently never been considered worthy of serious 
attention by investigators in the British Isles. Sutton (22), in 1899, 
and Wilson (29), in 1907, in their discussion on the improvement of 
potatoes have expressed their belief that the seedling was a finished pro¬ 
duct and that further selection was of no value in its improvement. On 
the Continent and in the United States and Canada numerous inves¬ 
tigators have attempted to ascertain its value. The meagemess of de¬ 
tails in most of the earlier published reports on selection experiments 
makes them of little value in determining the place that selection occu¬ 
pies in the improvement of the potato. The most important papers on 
the value of selection have been reviewed by East (4), in 1908, and 
more recently by Stuart (21), in 1915. East (4), after reviewing the work 
of previous investigators, pointed out that the evidence was inconclusive 
and called attention to the influences which tended to obscure the results 
obtained by these investigators. Waid (24), in 1907, at the annual 
meeting of the American Breeders Association reported the results of 
studies on hill selection of seed potatoes. The results showed quite 
clearly that the progeny of low-yielding hills remained unproductive in 
comparison to the progeny of high-yielding hills. The favorable results 
reported in this work caused considerable emphasis to be placed on this 
method of improvement. The following year Webber (25) put forward 
a method of selection, known as the tuber-unit method, which has re¬ 
ceived considerable attention. East (5), in 1910, suggested that Waid's 
results might have been influenced by the size of the seed piece used and 
the fact that Waid might not have been dealing with a pure strain, as he 
