154 Degeneration in the Ostrich 



it has been established that " the baldness behaves as a homozygous 

 dominant, all the cross-bred chicks showing it at about the same 

 age as do the northern chicks, and segregation takes place in the 

 F^ generation. 



It may be presumed that in the past some germinal change took 

 place in the northern bird as compared with the southern, as a result 

 of which the feathers began to fall out from the crown of the head 

 at a certain age. Whatever the change was, it is transmissible and 

 behaves as a definite Mendelian character, dominant in crosses ; the 

 factor for baldness is dominant over one for a covering of feathers over 

 a definite area. The factor however only becomes effective when the 

 chick is about three months old (delayed dominance). It is a direct 

 instance of factorial change occurring in a limited portion of the 

 ostrich race, since no corresponding effect is shown by the southern 

 bird. Naturally it is through such changes, restricted to certain parts 

 of a race, that specific distinctions arise. All the other degenerative 

 changes here described occur throughout the continent. If,.following 

 most systematists, we regard the southern and northern ostriches as 

 distinct species then the degenerative changes occurring in both may 

 be classed as "parallel mutations," presumably taking place in the parts 

 of the germ plasm which are common to the two species ^ 



The production of baldness in the ostrich, like that of the loss of 

 the leg contours, has a different germinal and evolutionary significance 

 from the other plumage losses hitherto noticed. Among the coverts, 

 for example, the reduction takes place in a definite ordinal sequence, 

 from one end or the other of a row, not a number at a time involving 

 a special area ; moreover, it takes place in piecemeal fashion and the 

 factors are ultimately lost, the individual plumes no longer making 

 their appearance at any stage of development. Presumably the changes 



1 Prof. T. H. Morgan [I.e. p. 47) has strongly emphasized that it is unsafe to judge as 

 to the nature of the mutation from the appearance of the character alone ; that the 

 appearance of a similar character in related species does not necessarily represent a 

 similar mutation, due to a change in the same gene or determinant, even though they may 

 have many of these in common. He instances two different white-flowered races of sweet- 

 peas which represent different mutations ; three independent mutations have produced 

 white birds ; and by the refined method of localizing genes in the chromosomes establishes 

 the different mutative origin of several of the closely related eye colours of Drosophila. 

 All these however are concerned with such complex changeable characters as colour in 

 which many factors are concerned, and it by no means follows that similar possibilities 

 hold in the case of more fundamental structural parts of the organism. For these it 

 would surely appear to be safe to assume that corresponding changes in related species take 

 place in parts of the germ plasm which they inherit in common from the ancestral stock. 



