30 APPLICATION OF PRINCIPLES OF HEKEDITY TO BREEDING. 



The following statement is made by Dr. Raymond Pearl and Mr. 

 Frank M. Surface in Bulletin No. 166 of the Maine Agricultural 

 Experiment Station: 



There is a rapidly accumulating mass of evidence that the chief, if not the entire, 

 function of selection in breeding is to isolate pure strains from a mixed population. 

 It is found in actual experience impossible to bring about by selection improvement 

 beyond the point already existing in the pure (isolated) strain at the beginning. 



These writers do not here distinguish between the effect of selec- 

 tion in self-fertilized and cross-fertilized species, but what is said does 

 apply to close-fertilized species strictly, where hybridizing is not 

 practiced, and with certain limitations it also applies to cross- 

 fertilized species, as will be seen later. 



If the conclusion that selection of fluctuating variations is without 

 effect is correct, then it follows that after we have by trial found the 

 best individuals in a crop propagated vegetatively we have gone as 

 far as selection enables us to go, except as immediately stated below. 



But there is a second type of variation in vegetatively propagated ^ 

 crops which can be affected by selection. Each individual plant is 

 endowed with a certain number of hereditary characters. These 

 characters may or may not come to complete development under 

 given environmental conditions, or some of th-em may reach com- 

 plete development while others may fail to do so. In so far as this 

 failure to develop is due solely to environmental conditions selection 

 is without power to modify the crop. But it would appear that 

 from time to time, or perhaps more or less continuously, changes are 

 going on with reference to these hereditary characters by which 

 their tendency to develop under given conditions changes; so that 

 in a crop like potatoes we may in time get a good many varieties 

 from the descendants of a single individual. But these varieties, in 

 the main, arise by certain hereditary characters becoming latent or 

 possibly in some cases disappearing altogether. Again, it may be 

 that the tuber with which we start a race may have a good many 

 latent characters in it whose tendency to develop may subsequently 

 increase, so that occasionally we get a variety which differs from that 

 with which we started by the development of certain characters 

 which were not patent in our original stock. For instance, a white 

 variety may produce tubers with colored skin. Color is especially 

 likely in white varieties to occur in the vicinity of the eyes of the 

 tuber. 



The more usual variation which occurs in such cases is for charac- 

 ters that are present to become latent, so that we are more likely to 

 get light color or white from colored stock. 



What has been said about variation in vegetatively propagated 

 plants appKes also to bud variations, or the so-called ^'bud sports." 



165 



