•194 Crossmg the North and South African Ostrich 



stant, that all were equally fertile and that breeding at random took 

 place then a single pair of mature birds would on the average give rise 

 to only a single mature pair. At the beginning we may allow that the 

 mutation occurred in the germ plasm of, say, a single bird, and repre- 

 sented a double or duplex dose and was dominant. Such a bird mated 

 with one in which the change had not taken place would give offspring 

 all of which would be simplex dominants for baldness. Only two of the 

 progeny would reach maturity and mating with two nulliplex individuals 

 would give four mature birds of which two would have the factor simplex 

 and two would be nulliplex. Thus on the conditions postulated the 

 number of birds showing baldness would never increase beyond two and 

 both would be simplex, that is, the new character would retain the same 

 proportion throughout the history of the race. There would be no 

 swamping of the character and no increase of individuals showing it. 

 In the same manner if the factorial change took place in the germ 

 plasm of a number, x, of birds simultaneously, their influence on its in- 

 troduction would be to the extent of 2x, if all the progeny were simplex. 

 If matings took place between simplex pairs instead of between simplex 

 and nulliplex then the result at maturity would be one duplex dominant, 

 two simplex dominants and one nulliplex on the average for each two 

 pairs ; in other words, the mating would result for the time being in a 

 loss of one-fourth of the number bearing the character, but the original 

 number would be restored if the duplex dominant paired with a nulliplex, 

 for two simplex individuals would result. 



Thus under the conditions stipulated — a new character of no selection 

 value, a stable population, free intermingling and equal fertility — con- 

 ditions which it must be admitted are closely approximated in the 

 natural life of the ostrich, the complete introduction of a new unit 

 character in duplex form would occur only by the germinal change 

 taking place as many times as there are individuals making up the 

 race. On the other hand the character could be introduced in a simplex 

 form by the change being effected in half the number of individuals. 

 It follows that unless a new character has some selection value it can- 

 not be bred into a race ; it must be introduced de novo for each homo- 

 zygous increase and half the number of times for the heterozygous 

 increases. 



As regards the bald patch therefore the germinal change must have 

 been effected as many times as there are individuals making up the 

 northern race, for the experiments have proved they are all homozygotes- 

 We can scarcely conceive that the alteration would be carried out simul- 



