304 MUTATIONS AND EVOLUTION. 



germinal differences which have appeared in the past, wholly 

 apart from any considerations of welfare, and now serve to 

 demark the ostriches of the more extreme parts of the continent. 

 An interming-ling, however, occurs in intermediate geographical 

 areas, as indicated by the intermediate characters of the other 

 two species which have been erected, namely, the East African 

 ostrich, S. massmcus, and the Somali ostrich, S. molyhdophanes. 



If we concede that the ancestral ostrich had uniform charac- 

 ters, and that the differences which have since arisen are of no 

 welfare value, the latter cannot have become dominant as a result 

 of natural selection. We shall have to allow that to complete 

 the full establishment of a mutation within a fixed portion of the 

 race the necessary germinal changes must have been effected as 

 rtiany times as there are individuals representing the mutating 

 portion. For where we are concerned with a numerically fixed 

 population, a mutation of no selection value, equal fertility and 

 opportunity for mating, conditions which seem nearly approached 

 in the case of the wild ostrich, then, as Mr. G. H. Hardy* has 

 shown, a mixed population falls into a stable condition of 

 equilibrium with regard to the proportions of the two homozy- 

 gous and the heterozygous forms after a single generation, and 

 this position is thereafter maintained. A new mutation of no 

 selection value can ordinarily increase in number in a homozy- 

 gous form only for as many times as it makes its appearance de 

 novo. It cannot be bred into the race, but must appear indepen- 

 dently for each increase. 



The conditions surrounding the bald patch, for example, are 

 particularly convincing on this point. It is a mutation which 

 has arisen in the northern race, the feathers falling out when 

 the chicks are about three months old. It is probably altogether 

 intrinsic in its origin, as it is in the highest degree improbable 

 that any environmental conditions could have brought it about 

 which would not be also effective in the case of the southern 

 bird. Though representing an absence of feathers, experiments 

 have proved that it is homozygous and dominant over the pre- 

 sence of a covering in the southern bird. It is hardly conceivable 

 that it has any bearing on the welfare of the bird, and therefore 

 has in no measure been influenced by natural selection. Probably 

 it is a part of the general plumage degeneration going on in 

 the ostrich, to be considered later. Under the conditions already 

 postulated one or more mutated individuals could not in the end 

 influence the race in the matter of increase; a pair in which the 

 mutation had appeared would on an average produce only another 

 pair of mature progeny. To establish the character throughout 

 the members of the race, the factorial change must occur de novo 

 as many times as there are individuals making up the race. 



In the same manner the opening of the shell-pores in pits 



* R. C. Punnett, " Mendelism," 3rd ed., London, 191 1. p. 136. 



