166 



SCIENCE 



[N. S. Vol. LII. No. 1338 



question that the specific features are the ex- 

 pression of distinct factorial differences in 

 parts of the germ plasm of the northern and 

 southern ostrich. 



Like most of the Eatit» the two-toed 

 ostrich is degenerate in some respects and 

 highly specialized in others, compared with 

 ordinary flying birds. Degeneration is espe- 

 cially indicated by the small size of the wings 

 and the practical absence of feathers from 

 their under surface and si)ecialization (de- 

 generation) by the reduction in the number of 

 toes. The development and progress of ostrich 

 farming in South Africa had produced prior 

 to the war about a million domesticated birds 

 which afford an abundance of living specimens 

 for examination, now supplemented by the 

 importation already mentioned as well as by 

 much embryological material. The result has 

 been the demonstration that the ostrich is 

 undergoing slow degeneration in numerous di- 

 rections in connection with its plumage, wings 

 and legs, survivals of practically all stages in 

 the process being procurable. 



The principal directions along which plum- 

 age degeneration is taking place include the 

 general under-covering of the wings, the 

 single row of under-coverts, the remiges or 

 wing quills, the second and higher rows of 

 upper-coverts, the feathers covering part of 

 the leg and the under-covering of down. The 

 third digit of the wing exhibits important 

 evolutionary stages, while in addition to the 

 loss of the first, second and fifth toes of the 

 foot it can be shown that the small fourth is 

 also in process of disappearance, particularly 

 as regards its claw. Losses of the scales over 

 the big middle toe reveal that retrogression 

 has already begim on what will in time be 

 the only remaining toe. In any individual 

 bird the changes in any one direction take 

 place quite independently of those in the other 

 directions, and all proceed in a definitely de- 

 terminate manner which is the same for all 

 the representatives of the two species. The 

 experiments already carried out serve to estab- 

 lish that the losses are factorial in their na- 

 ture and that in crosses they follow strictly 



Mendelian lines. They are to be regarded 

 as retrogressive mutations resulting from the 

 dropping out of factors, the changes proceed- 

 ing in regular succession along various di- 

 rections, the succession being particularly 

 impressive in the case of the gradual loss of 

 plumes from certain of the rows of feathers and 

 in the digits of the foot (rectigradations Os- 

 born^). It is highly questionable if up to the 

 present any selection value can be attributed 

 to any of the changes. 



The present interest lies in the fact that the 

 mutative changes are common to the germ 

 plasm of both the northern and the southern 

 ostrich and are taking place in one independ- 

 ently of the other. They may indeed be pre- 

 sumed to be the same throughout the conti- 

 nent, suggesting that they are intrinsic in 

 their nature and independent of environ- 

 mental influences. If we regard the two spe- 

 cies of ostriches as distinct then we can under- 

 stand how the term parallel mutations may be 

 applied to the changes going on in both, but 

 at the same time it may appear to carry with 

 it the notion that similar mutations are pro- 

 ceeding in two disconnected and independent 

 germ plasms. If, on the other hand, we retain 

 the idea of the origin of species from a com- 

 mon stock which Darwin had in his mind in 

 the quotation given, then it becomes more in 

 harmony with fact to think of the changes as 

 taking place in germ plasm of the same nature 

 and of a common genetic origin. 



We must conceive the germ plasm to be 

 fundamentally the same for the African 

 ostrich as a whole, though certain changes 

 have taken place in parts of it which give 

 us the differences delimiting the species. On 

 this view it is easy to comprehend how the 

 same mutative changes will occur in what for 

 the sake of convenience we now distinguish as 

 two species. The mere fact that one assem- 

 blage of ostriches is bald-headed, larger and 

 differently colored as compared with another 

 and that the hens lay a different egg indicates 

 only slight differences in the germinal make-up 

 of the birds. In both the constituents of the 



2 Amer. Nat., August, 1917. 



