An Introduction to a Biology 



to ensue from the cross, the more worth while will it be to 

 mate these first crosses together. But I do not press this 

 point. All I am concerned to insist on now is that if the 

 immediate result of a cross is far from what was expected, 

 this should not be cause for disappointment. If any valu- 

 able new combination of characters is to come out of a 

 particular cross, it is not to be expected till the second hybrid 

 generation. 



For instance, I have made some crosses between Jersey 

 and Ayrshire cattle with the view of finding out whether a 

 cow can be raised which has as high a percentage of butter- 

 fat in its milk as the Jersey, and gives as great a quantity 

 of milk as the Ayrshire, or at any rate approaches to this 

 ideal. I am prepared to find that the cows of the first hybrid 

 generation are even inferior to both the parent breeds in the 

 quality and quantity of their milk. But this would not make 

 me less sanguine in the hope that some cow " of noble note 

 may yet be " bred in the second hybrid generation. If the 

 two characters which I wished to combine high yield and 

 high butter-fat percentage were recessive ones, the desired 

 combination would only occur once in every sixteen indivi- 

 duals. But I do not anticipate that either yield or butter- 

 fat percentage will exhibit simple dominance. Indeed, in 

 other crosses of this kind, as between the Jersey and the 

 Danish for example, it is known that, so far as butter-fat 

 percentage is concerned, the first cross between these two 

 breeds is a blend between the two parental percentages. 

 In the second hybrid generation, therefore, we must not 

 expect four sharply circumscribed classes as in the case of 

 Fig. 3, but a very wide range of classes, all of which 

 merge into one another a very wide range of classes 

 (merging into one another) with regard to butter-fat 

 percentage, ranging from the high to the low, and a similar 

 range of classes with regard to quality. It simply remains 

 to be seen whether there can coexist in one cow the 

 high yield of the Ayrshire and the high butter-fat per- 



