140 Degeneration in the Ostrich 



The degenerative influence, having resulted in the complete or partial 

 loss of a plume, then begins to attack the next one or two in the 

 ordinal sequence of the row, and thus a regular, successional reduction 

 is maintained for the row as a whole, though not more than one or two 

 members are in a retrogressive phase at the same period. This appears 

 to be the limit of the degree of retrogressive activity for the time being 

 as regards any row, an independence for each end being maintained, and 

 also for the different rows. The succession of losses is never irregular or 

 haphazard, as would be the case if a plume disappeared in the course of 

 the row instead of from one or other of the extremities. 



That in the end the factors become completely lost to the germ plasm 

 may be inferred from the fact that missing members are wanting from 

 the earliest appearance of the plumage, and only re-appear in the naked 

 part of the row in certain strains in which their degradation is not yet 

 completed. Examination of embryos when first showing feathers reveals 

 similar absences, as do chicks when first hatched and the adults later. 

 The degenerative losses are therefore not effected during ontogeny, but 

 during the complex mitotic germinal changes which intervene between 

 one generation and the next. Dr C. B. Davenport writes' : " We are 

 ignorant of the specific nature of the machinery that determines phylo- 

 genetic variations, but we have reason to think that it is located in the 

 germ plasm and that the karyokinetic phenomena, especially the move- 

 ments of chromosomes at and around the time of fertilization, have a 

 great deal to do with such phylogenetic change." Prof Bateson in his 

 Presidential Address in Australia also remarks : " It is to be inferred 

 that these fractional degradations are the consequence of irregularities 

 in segregation." The phylogenetic changes which Bateson, Davenport 

 and others usually have in mind in this connection are the larger, sudden, 

 detached changes regarded as discontinuous variations (Bateson) or 

 mutations (de Vries). Their fortuitous isolated character can be readily 

 understood if we conceive of them as resulting from irregularities during 

 mitotic division and reconstruction ; but it is manifest that the con- 

 tinued, determinate phylogenetic changes exhibited by the ostrich plume 

 could not be a product of mitotic irregularities. The facts call for a 

 regular succession of germinal changes of a uniform character in the 

 same direction, continued generation after generation; they must be 

 sequential and cumulative, and the results would admit of accentuation 

 by selection. 



Several writers have drawn attention to the fact that the occurrence 

 1 Amer. Nat. Vol. l. August, 1916. 



