186 Crossing the North arid South African Ostrich 



Thus, as in the case of the loss of scales over the big toe, the evidence 

 is conclusive that the presence of the claw follows the Mendelian pro- 

 portions for heterozygotes when breeding takes place. Where the 

 parents are nulliplex as regards the factor no claw usually appears in 

 the progeny, but where a clawed individual mates with a clawless the 

 structure appears in practically half the progeny. 



With the small proportion of clawed to clawless individuals among 

 both northern and southern birds it is to be expected that most of the 

 clawed ostriches will be heterozygous as regards the factor for the claw 

 for in a mixed assemblage the chances of a clawed bird mating with 

 another clawed one are very remote. A clawed bird will almost certainly 

 be the progeny of one clawed and one clawless parent, and hence will be 

 a simplex dominant, and when in turn mated with a non-clawed bird 

 will give progeny half of which are again simplex clawed and half 

 clawless. 



Discussion. 



The northern and the southern ostrich illustrate in a clear manner 

 how distinct species of animals may arise on the basis of germinal or 

 factorial changes. It may be assumed that both are the descendants of 

 an original stock in which the characters were all alike, and that in the 

 course of time alterations have taken place in the germ plasm which 

 give the marked differences now separating the two. Whether the 

 changes have any adaptive value or not will be discussed later. Con- 

 sidered as a whole the broad genetical conditions are fairly simple, as 

 no other representative of the Ratitae exists in Africa with which in- 

 dividual ostriches could have hybridised. As we know the two-toed 

 ostrich to-day, it may be assumed to have evolved entirely within the 

 continent, though in Tertiary times extending into Eurasia as far as 

 Southern India, fossil remains having been found in the Siwalik deposits. 

 That the two species had a common origin, and are not yet far apart, 

 may be inferred from the fact that they interbreed and the offspring are 

 fertile, also that similar degenerative changes are going on in the germ 

 plasm of each (parallel mutations). 



Factorial Constitution. The results from the crossing of the two 

 species, though admittedly very incomplete, afford certain evidence as 

 to the present germinal constitution of the ostrich, and indicate directions 

 along which changes are taking place. Everything points to the dis- 

 tinctive characteristics of the two species as having separate factorial 



