196 Crossing the North and South Africmi Ostrich 



however that in all cases where an ostrich showing the break has 

 been paired with one in which it is wanting, the bird has proved itself 

 to be heterozygous, giving progeny of which approximately half display 

 the break and half the continuous scutellation. Thus while baldness 

 is a mutation fully established for the northern race and germinally 

 duplex, the loss of scales from the third toe is a mutation only partly 

 established and germinally simplex. A character in course of intro- 

 duction within a race will for a long time be mainly in a simplex or 

 heterozygous form ; later, as the mutation appears de novo in more 

 and more individuals, the population will tend to consist of duplex, 

 simplex and nulliplex birds, until in the end all will be duplex or 

 homozygous. 



As regards the race as a whole the claw on the fourth toe reveals 

 conditions somewhat similar to those of the loss of scales over the middle 

 toe, but is a character which has almost disappeared. Experiments 

 have shown that the presence of the claw is dominant over its absence, 

 and matings with a nulliplex individual give progeny half simplex 

 dominant and half nulliplex, proving that the clawed individuals are 

 heterozj^gotes. This again is what would be expected considering the 

 small proportion of clawed birds, and the remote likelihood that a hetero- 

 zygote would mate with a heterozygote. If only heterozygous individuals 

 are to be found then any further loss of the factor will presumably take 

 place as a simplex, and to be completely lost to the race the change 

 must take place as many times as there are heterozygous birds. As 

 for the introduction of a new character so for the loss of an existing 

 character, it cannot be bred out under the conditions postulated, but 

 must drop out germinally. 



Specific distinctness of Northern and Southern Ostrich. Whether the 

 northern and the southern ostrich are to be regarded as separate species, 

 or only as sub-species or varieties of a single species, raises the ever- 

 recurring, but undefinable question as to what constitutes a species. In 

 the foregoing we have available all the data which the systematist could 

 possibly desire to enable him to reach a decision. A germinal character, 

 baldness, occurs in one, but is wanting in the other, while the dimen- 

 sions and colours of the body as well as certain features of the egg are 

 also distinctive and germinal. The characters are retained when the 

 members of one race are subjected to the same environmental conditions 

 as the other, showing they are not dependent upon external circum- 

 stances. They can all be regarded as distinct elementary characters in 



