An Introduction to a Biology 



birds which you yourself have bred. With regard to these 

 you will say, " I, at any rate, know that the parents of these 

 birds were blue, as I bred them myself." But you will have 

 no better luck ; the blacks and whites will appear again in 

 about the same numbers. Still you will say to yourself, 

 " This black and white impurity looks as if it must have 

 been introduced very shortly before I bought my birds, so 

 that perhaps I have no right to expect that all traces of it 

 will disappear in a paltry two generations. I will con- 

 tinue to breed from the blues only, always throwing away 

 the blacks and whites, and ultimately, and I hope before 

 very long, I am bound to obtain a race of pure-breeding 

 blues : it stands to reason that if I go on breeding in this 

 way long enough, I shall at last obtain a pure strain." Well, 

 it may stand to reason that it will happen. It won't happen. 

 For however many generations the blue Andalusians are 

 bred together they will produce blacks, blues, and whites in 

 the following proportions : 



25% Blacks. 50% Blue Andalusians. 25% Whites. 



Are not those percentages familiar to us ? Do they not 

 suggest that the blue Andalusian is a hybrid like the hybrid 

 tall pea, and that the blacks and the whites are the two 

 pure forms corresponding to the pure tall and the pure 

 dwarf ? There is a very easy way of finding out if this 

 supposition is true. If it is true, one of the 25 per cent, blacks 

 crossed with one of the 25 per cent, whites should give blue 

 Andalusians only. This is indeed what actually occurs, as 

 shown in Fig. 2. If this diagram is compared with the 

 corresponding one for the peas (Fig. 1), the following dif- 

 ferences will be seen. There is no question of dominance 

 here ; there is no exclusion of one character from the first 

 generation by the other character. Black and white meet 

 and compromise in a blue ; there is no victory for one of 

 them, as in the case where tall and dwarf met ; so that we 



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