162 ANIMAL LIFE AND HUMAN PROGRESS 



This " breaking of the type," as the breeder terms it, is, for 

 him, one of the most familiar of phenomena. How are we to 

 reconcile it with the conception of gametic purity ? Perhaps a 

 simple case may serve as an illustration. 



Let us suppose that our original cross is between two strains 

 which differ in the form of the comb as well as in the colour of 

 the plumage, and that our cross is between a black rosecombed 

 bird and a white bird with a single comb. If certain strains of 

 these are used the offspring are black rosecombed birds. When 

 bred together they produce four kinds of offspring, viz. black 

 rosecombs, white rosecombs, black singles and white singles, 

 and the proportion in which these four forms appear is 

 approximately 9:3:3:1. Two new forms have appeared 

 white roses and black singles, or, as the breeder might prefer 

 to put it, the types with which we started have been broken. 

 The explanation becomes clear if we regard it, not from the 

 standpoint of the original parents as a whole, but from that 

 of the two contrasting pairs of characters, black and white 

 plumage, rose and single comb, which went into the cross. In 

 the first cross black is dominant as before, while in the next 

 generation blacks and whites are as 3 : 1. So also rose is 

 dominant in the first cross, while the next generation consists 

 of roses and singles in the ratio 3:1. Any given gamete arising 

 from the first-cross birds is pure for the property of transmit- 

 ting either black or white. At the same time it is pure as 

 regards the property of transmitting either rose or single comb. 

 But the plumage factor and the comb factor are independent 

 of one another. Though the first cross has been formed by the 

 union of a black rose with a white single gamete, the gametes 

 that arise from it consist of black singles and white roses, as 

 well as of the two parental types. And the breeding results 

 force us to conclude that these four kinds of gamete are pro- 

 duced by such birds in equal numbers. An orderly redistribu- 

 tion of the factors takes place somewhere in the cell divisions 

 which lead up to the formation of gametes in the cross-bred. 

 Though the original parental types are broken and fresh types 

 appear in the second generation, this in no wise militates 

 against the conception of the purity of the gametes. And 



