118 Plant-Breeding 



and (c) fertility of offspring. The relative values of these 

 factors varies with different plants. In the carnation, for 

 instance, factor (a) of cross-fertilized plants was 9 per cent 

 greater than in self -fertilized plants, (b) was 16 per cent 

 greater, and (c) was 54 per cent greater; in tobacco, 

 factor (a) was 33 per cent less than in self -fertilized plants, 

 but factor (b) was 28 per cent greater and factor (c) 3 per 

 cent greater. Even when the fertility of the mother 

 plant is greatly reduced by hybridizing with a distinct 

 species and the hybrids themselves are sterile or very 

 infertile, they nevertheless often show extraordinary vigor, 

 that is, (b) is often greater in hybrids than in pure-bred 

 plants, but factors (a) and (c) are usually less. In plant- 

 breeding the importance of this increased vigor is very 

 great (Figs. 32 and 33). 



The outright production of new varieties. The reader is 

 waiting for a discussion of the second of the great features 

 of crossing, the summary production of new varieties. 

 This is the subject that is almost universally associated 

 with crossing in the popular mind, and even among hor- 

 ticulturists themselves. It is the commonest notion that 

 the desirable characters of given parents can be definitely 

 combined in a pronounced cross of hybrids. There are 

 two or three philosophical reasons which somewhat oppose 

 this doctrine, and which we will do well to consider at the 

 outset. In the first place, nature is opposed to hybrids, 

 for species have been bred away from each other in the 

 ability to cross. If, therefore, there is no advantage for 

 nature to hybridize, we may suppose that there would be 

 little advantage for man to do so ; and there would be no 

 advantage for man did he not place the plant under condi- 



