440 Tricotylous Races. 



cotylous seedlings from seed which had l)cen bought or 

 obtained from some other source, or with plants which 

 had been found by chance (Oenothera, Silene). Where 

 necessary, this year is denoted in the table by I (first 

 generation). The numbers which succeed each other in 

 the row to the right of it refer to the first and the follow- 

 ing generations. Thus for instance in Clarkia there were 

 as many as 16% tricotyls in the harvest of the first gen- 

 eration, 64 in those of the second and 79 in the third. 



The point which this table is intended to illustrate 

 is best brought out by a comparison with the series of 

 figures given on page 392 for the half races. In that 

 case, a selection continued from four to six years, did 

 not bring this value, as a rule, further than 2 to 4%, and 

 only in exceptional cases attained 15 to 20%. In this 

 case, on the other hand, 55% is attained in two or three 

 generations. In the half race a continuation of the 

 selection would presumably not have led to any con- 

 siderable increase, a fact which is demonstrated by the 

 experiments with Amarantiis and Scrophularia which 

 were continued over a longer period of time.. In this 

 case, however, selection is as a rule very effective, inas- 

 much as it can increase the best representatives of the 

 race, in a veiy short time, to a hereditary capacity of 

 80 to 90%. 



Therefore there can be no doubt that entirely differ- 

 ent factors are at work in these two cases. In the former 

 there occurred only races with half curves on which se- 

 lection has little effect. In this case, however, tliere oc- 

 ciu-red, besides these, the highly variable intermediate 

 races which are extraordinarily susceptible to selection 

 and to external conditions of life. They were easily 

 isolated, either because one or more examples of them 



