2oo 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, 2 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 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. 



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. 



