HYBRID VIGOR OR HETEROSIS 163 



A somewhat similar series can be arranged with animals, 

 although usually in wide crosses if the hybrids can be 

 obtained at all they are as large or larger than the average 

 between parents. A satisfactory interpretation of the 

 vigor of hybridization must take all these facts into con- 

 sideration, even though they may not be the result of the 

 operation of one single law. 



