148 A MANUAL OF MENDELISM 



multiplicity of their descendants. He thus inferentially 

 refers to his lack of success with crosses and explains his 

 success with single-ear selections : " My selections have 

 chiefly been from natural sorts, which soon show their 

 propensities, and often prove constant, having in all 

 probability undergone reproduction before being selected. 

 . . . Sports may be regarded as the gift of nature to 

 man." Shirreff was half a century too soon. 



Fifty years later, Mendel's work having become known 

 in the meantime, Nilsson-Ehle saw that the second 

 generation, that is, the hybrids' progeny, contains all 

 the types likely to result from the cross and that the 

 descendants of every second-generation plant must be 

 kept by themselves as a separate cultxire. Then the 

 process of selection may begin : the most promising 

 in yield and purity being retained, the others destroyed. 

 The cultures retained are constantly " rogued " till the 

 fifth generation, or later, if necessary, when the stock 

 is usually large enough to be first tested for yield. In 

 a year or two, field trials can be made, and a final 

 judgment arrived at. 



Nor is improvement in the animal world less neces- 

 sary. The present position in the United Kingdom 

 may be taken by way of example. She is regarded as 

 the most successful of stock-breeding countries, for the 

 reason that her horses for draught and speed, and her 

 bullocks for beef, sheep for mutton, and pigs for pork 

 are the best in the world. But, with the exception of 

 the race-horse, which, through the three centuries of 

 its history, has been selected upon race-course results, 

 these animals excel in qualities which are judged by 

 the eye and hand. With animals whose fitness for their 

 purpose can be tested only by carefully collected 



