302 MUTATIONS AND EVOLUTION. 



species with characters havinj:^ no particular advantage over the 

 old. 



Conditions represented at the present time by the two-toed 

 African ostrich, Struthio^ appear tc» throw some light ttpon the 

 various kinds of mutations to which a race may be subject, and 

 the nature of the evolution which it may undergo in consequence. 

 They seem to afford strong support for the view that intrinsic 

 mutations may arise quite apart from any adaptive or utilitarian 

 considerations, and ultimately come to influence the race either 

 as a whole or in part without the invocation of natural selection. 

 Discontinuous mutations, deemed to have been of a recurrent 

 nature, now serve to distinguish the northern race from the 

 southern, while successional cumulative mutations of a degenera- 

 tive nature affect the race as a whole, with differences of degree 

 represented by different strains. In addition, individual ostriches 

 and strains present the infinity of small, germinal variations 

 characteristic of species generally. 



Individual or Strain Mutations. 



The small, fluctuating, germinal differences are well exempli- 

 "fied by the valuable plumes of the ostrich, which call for an 

 intensive study on the part of the ostrich breeder. An almost 

 indefinite number of variations are represented, connected with 

 the dimensions, shape, density, texture, strength and lustre of the 

 feather as a whole, as well as of its constitttent parts — shaft, 

 barbs and barbules. These constitute the " points " of the feather 

 expert, and can often be differentiated only by the trained eye. 

 From practical experience the breeder knows full well that the 

 smallest structural details of the plumes are hereditary, and that 

 the most desirable of them must either be present or " bred into" 

 his flock if he is to succeed ; he can only modify his strains by 

 crossing with other equally fixed strains. Differences dependent 

 upon feeding and the general management of the birds are 

 afforded in plenty, the feather growth being extremely sensitive 

 to the nutritive condition, but these are not part and parcel of 

 the fixed constitution of the flock. Other structural parts of the 

 body would no doubt be found to be as minutely variable as are 

 the plumes, but have not called for the same special study. In 

 its plumes alone the ostrich unquestionably supports Prof. 

 Caullery's dictum that " a species is nothing but the sum of an 

 infinity of genotypes," and the minute fluctuating variations may 

 well be the result of small mitotic irregularities during gameto- 

 genesis. 



While in farming practice the small plumage variations 

 determine which birds will be used as breeders, in a state of 

 nature it is im^xissible to conceive of any circumstances in which 

 they could have any bearing on the welfare of the individual. 

 They manifestly represent small, casual, germinal changes, ver}^ 



