3^4 MUTATIONS AND EVOLUTION. 



In a paper, " The Trend of Genetics," Dr. A. F. Shull* asks 

 the pertinent questions: "If a gene is altered in a given way 

 to-day, is it likely to be further altered to-morrow ; and especially 

 i? it more likely to be further altered again in the same 

 direction than in any other way? . . . Can the direction of 

 spontaneous alteration be predicted?" Probably all will admit 

 that the somatic expressions of degeneration in the ostrich are 

 so much evidence of genetic change, and as, with good reason, 

 predictions for the future can be made, this may certainly be 

 accepted as an affirmative answer to the questions proposed. 



Summary. 



The conditions encountered in the ostrich appear to call for 

 mutative germinal changes of three types : 



(a) Fortuitous, disconnected mutations in numerous direc- 

 tions which provide the small hereditary characters distinguishing 

 the various individuals or strains ; especially noticeable among 

 the plumes, which have been studied intensively, and show an 

 almost infinity of germinal variations within prescribed limits. 

 They are the haphazard factorial changes presumed to result from 

 mitotic irregularities during gametogenesis. Any of the varia- 

 tions incidentally conferring an advantage over the others would 

 become subject to natural selection, and the result would be a 

 preponderance of the individuals possessing them ; but they are 

 manifestly too insignificant to count in the large issues involved 

 in the struggle for existence. They have probably no influence 

 on the evolution of the ostrich. 



(b) Recurrent, discontinuous changes of the same factor or 

 factors, gradually affecting all the members of some restricted 

 portion of the race ; especially exemplified by the well-marked 

 distinctions between the northern and southern birds, as regards 

 baldness, dimensions, colour, and the nature of the egg-shell. 

 Presumably they are the result of isolated factorial changes due 

 to some intrinsic condition of the germ plasm of a part of the 

 race, or to some uniform influence acting upon it. Incidentally, 

 they may confer an advantage, but it is highly questionable; they 

 would afford specific evolution apart from any selection value. 



(c) Regular, successional changes in definite directions 

 throughout the race, giving the various intermediate stages in 

 several retrogressive orthogenetic or rectigrade series ; the stages 

 reached at any one time varying somewhat in the different 

 individuals, showing they proceed with a measure of independence 

 both as regards any particular individual and any particular trend. 

 Exemplified in the ordinal succession of losses from the under- 

 and upper-coverts and remiges, the down, and the digits of the 

 wing and foot. Presumably they are the result of some orderly, 

 continued factorial degradation or loss in the germ plasm. 



* 20th Mich. Acad. Sci. Rept., 1918. 



