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 character 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 combination. 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,[1] 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, certain unknown combinations of external conditions may favor germcell changes. They are both rare, the progressive changes being relatively 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 is 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.
- ↑ 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 complementary inhibiting factor or the gain of a complementary factor necessary for it to become active.
