48 EVOLUTION OF LIVING ORGANISMS. 



tain the factor producing the crimson character, all the 

 gametes of the second parent the factor producing the 

 white character. On crossing the two factors will meet 

 in the zygotes giving rise to the hybrid offspring. If both 

 factors were now transmitted to each of the gametes of 

 the hybrid, it would of course breed true. The splitting 

 of the offspring of the hybrids is accounted for on the 

 simple supposition that the factors segregate ; that the 

 crimson-producing factor passes into half the total num- 

 ber of gametes, and the white-producing factor into the 

 other half in each hybrid parent. The gametes are then 

 " pure " in respect to the particular factor. It follows 

 that when the gametes of the hybrids fertilise each other 

 three kinds of zygotes will necessarily result ; one half 

 of the total number containing both factors will give rise 

 to new hybrids ; while one quarter with white-producing, 

 and one quarter with crimson-producing factors only will 

 give rise to pure individuals like the parents. Another 

 case in point is the "Blue" Andalusian fowl, which 

 never breeds true, being a hybrid between the Black 

 Andalusian and the splashed White, both constant races. 

 A cross between these two will always yield "Blues" 

 only ; while the Blues if interbred always split into In 

 Black, 2n Blue, and In splashed White. 



True-breeding individuals, containing only one kind of 

 gamete, are called homozygotes ; they have grown from 

 zygotes formed by the union of gametes containing the 

 same factors, and give rise to gametes of uniform value. 

 Hybrids derived from mixed zygotes are called hetero- 

 zygotes, and give rise to gametes of different factorial 

 value, each factor or set of factors passing into half the 

 total output of gametes. This conclusion is confirmed on 

 crossing the offspring of the hybrid with the homozygote, 

 when half the resulting generation will resemble the 

 homozygote, the original pure parent, and the others will 

 resemble the heterozygote. 



Now the hybrids or heterozygotes may be more or 



