352 GENETICS IN RELATION TO AGRICULTURE 



have been reported. While many first generation crosses between 

 different species are more vigorous than either parent, others are known 

 to be exceedingly weak. Unless repeating crosses which have already 

 been made, the hybridizer of species is exploring the unknown and there 

 is always the possibility that his results may be of interest to science as 

 well as of practical value. 



The Svalb'f Method of Creating Populations. Progress in plant im- 

 provement by means of hybridization experiments will always be limited 

 by the available supply of experts as well as by facilities and time. Any 

 method, therefore, that enables the breeder to secure desirable new 

 combinations of parental characters without the enormous amount 

 of detail involved in a system of pedigree culture, is worthy of serious 

 consideration. Such a method was devised by Nilsson-Ehle and has been 

 used at Svalof with success. According to Newman, "two known sorts 

 are crossed and the whole progeny from all second and succeeding gen- 

 erations is sown together en masse. The object of this plan is to allow 

 the severe conditions of winter and early spring to either destroy or 

 expose the weaknesses of as many of the more delicate combinations as 

 possible. In the latter case the breeder is given an opportunity to assist 

 nature in her work of elimination by practising a form of mass-selection. 

 While there is thus effected in a very simple manner a gradual weeding 

 out of a great mass of unfit combinations, the progeny of a crossing at 

 the same time gradually assumes the character of an ordinary mixed 

 population, the different combinations becoming automatically constant 

 as time passes. The advantages of working with constant forms will be 

 appreciated by all breeders as will also the fact that through the above 

 arrangement the number of combinations which may arise through the 

 repeated segregation of inconstant forms in each succeeding generation 

 will have increased immensely. . . . While the above system requires 

 a considerable length of time before any definite results can be reached, 

 yet it requires very little work until the time comes to make selections. 

 Numerous crossings of this kind may therefore be carried forward with 

 the regular work and thus provide a constant source of new material." 



