200 THE POPULAR SCIENCE MONTHLY 
It is sometimes somewhat difficult to see why selection of this kind 
should yield results slowly. There are indeed many points concerning 
which little is known. One may picture to himself, however, that 
where crossing is always likely to occur and where the apparent char- 
acter is in reality a combination of a number of separately inherited 
characters, many thousands or even millions of individuals would have 
to be grown to run a fair chance of obtaining the most desirable com- 
bination. By growing a few individuals in which the desired character 
is intensified in successive generations, the combination wanted may 
be obtained with the use of smaller numbers. 
I have stated that nothing can be accomplished by selection after a 
pure line or genotype as Johannsen calls them is isolated, unless a new 
transmissible variation is produced by nature. The questions then 
arise: how often may such changes be expected? and, what is their 
nature? Such changes are of two kinds,’ progressive where a new 
character appears, or retrogressive where a character is lost. But little 
can be said as to their relative frequency. Undoubtedly some species 
are in a more unstable condition than others and give more of such 
variations, as de Vries has already suggested. On the other hand, cer- 
tain unknown combinations of external conditions may favor germ- 
cell changes. They are both rare, the progressive changes being rela- 
tively much less frequent than the retrogressive changes, but they are 
sufficiently common for several to have come within the knowledge of 
every experienced breeder. 
There is another type of variation much more closely related to 
changes occurring in “ pure lines > than is generally supposed. I refer 
to what is commonly known as bud variation or vegetative sports. 
Retrogressive variations of this kind are probably no rarer than the 
same kind of changes occurring in pure lines. No authentic progressive 
variations (as distinguished from digressive) are known. In my own 
experience in growing eight hundred species and varieties of tuberous 
solanums (largely potato varieties), fifteen retrogressive variations have 
been noticed, and the changes that occurred were exactly like those 
occurring in seed-propagated strains. 
The relative value of progressive and retrogressive variations 1s 
difficult to estimate. In organic evolution the former must have been 
far more valuable; commercially the latter are often of great worth. 
We may cite, for example, the great value of the bush or dwarf varieties 
of beans, peas and tomatoes that have originated as retrogressions. 
2 De Vries also gives a third kind, digressive variations, such as occur when 
a character previously possessed by but latent in the plant appears. This class 
is unnecessary. Digressive characters appear either through the loss of a com- 
plementary inhibiting factor or the gain of a complementary factor necessary 
for it to become active. 
