452 SUMMARY OF CUllRENT llESEAECHES RELATING TO 



The evidence from palfeontology, geographical distribution, and 

 classification points to a gradual progressive process rather than 

 to abrijpt mutation. The evidence from experimental breeding points 

 both ways. 



The principal tools of the breeder are hybridization and selection. 

 Hybridization favours the occurrence of variations, but Lotsy is extreme 

 in regarding all genetic variability as the result of hybridization. This 

 is f "tly disproved by observations of Johannsen, who reports the 

 0CC7 rence of mutations in genotypically pure lines of beans, as also by 

 the remarkable series of variations observed by Morgan in an inbred race 

 of the fruit-fly. 



As regards selection, the mutationists hold that it can do nothing but 

 isolate variations ; the selectionists hold that selection can accomplish 

 more than the mere isolation of variations, inasmuch as it can, by a 

 series of selections, influence further variability. According to Castle, 

 the facts warrant adherence to the selectionist position. 



De Vries was the first to show the limitations of persistent selection. 

 But more conclusive were Johannsen's experiments with pure lines of 

 beans, which proved that selection continued generation after generation 

 in a particular line may be without result, so far, for instance, as average 

 seed size is concerned. Castle considers carefully the attempts that have 

 been made to generalize Johannsen's brilliant demonstration of the 

 principle of pure lines. In many cases mutations do occur in pure lines, 

 in defiance of the definition, and thus material for selection is afforded ; 

 but it seems clear that a race may become a ne plus ultra " pure line " as 

 regards certain characters. Thus, in the case of certain characters in 

 guinea-pigs. Castle has repeatedly attempted without success to bring 

 about a change by selecting within an inbred race. " Thus a very dark 

 form of Himalayan albino, after a certain amount of improvement by 

 selection, could not be further darkened to any appreciable extent." 



On the other hand, certain characters of guinea-pigs, rabbits, and 

 rats have been found to respond readily to selection in a particular 

 direction. Castle's experiment with hooded rats "selected simul- 

 taneously in plus and minus directions has produced one race which is 

 black all over except a white patch of variable size underneath, and 

 another race which is white all over except for the top of the head and 

 the back of the neck, which are black. The races. do not overlap at all, 

 and have not done so for many generations, though they still continue 

 to diverge from each other as the result of continued selection." In 

 similar experiments with Dutch marked rabbits it has been found possible 

 by selection to increase or decrease the amount of white at will. The 

 principle of the pure line manifestly does not apply to these cases. 



Castle's criticism of the attempts to generalize Johannsen's principle 

 of pure lines may be illustrated 1. It remains to be proved that a line 

 of beans is as devoid of genetic variation in other particulars as it is in 

 seed size. 2. It remains to be proved that all self -fertilizing organisms 

 fall automatically into pure lines. 3. The pure-line principle does 

 not apply without exception to asexually reproducing organisms. 4. 

 Attempts to extend the pure-line principle to organisms which are not 

 self-fertilizing have met with small success. 



