J. K. DlTKHDKS \m 



a rwult of mmu» inU^nml vitnlJHtio foixv. acting alto^rth«T in(l«'|M'mlently 

 of ext<?riml inHuonces. and jmK^otMlinjr alon^ drfiniU* linoH, irr«'H|K*ctiv<' 

 of mlaptivo (*<>n8i(lt>nitions, sonns t<> he gaining ground at the prcwiit 

 time among l)ioIogi8t.8'. The degeneration phenomena pn^senU'd hy the 

 ostrich apjH»ar t4» conatitnte as clear an example in support < if it ;us <•( mid 

 be addiieiH), while the gent'tical result.s se<'m to atVonI what lm,s hitherto 

 been lacking, namely, the cliroet application of mutation and Mendelian 

 principles U* continuous determinate changes, such as confront th<' com- 

 panitive anatomist and the palaeontologist. The main evolutionary 

 conception associated with mutation is fortuitous discontinuity, but in 

 the ostrich we |)erceive how discontinuous changes in the individual 

 may proceiHi along definite lines and result in determinate continuous 

 evolution for the race as a whole. The loss of sailes or single feathers 

 in individual birds may seem to be nothing more than haphaziinl chance 

 occurrences, but when considered for the race they indicate an onlerly 

 progress towards definite end results. 



Establishment of Characters. If none of the changes which have 

 tAken place between the northern and southern ostrich have any selec- 

 tion value we may well enquire how the differences have actually become 

 established. Undoubtedly geographical isolation ;us regards North and 

 South Africa has played some part. Whatever intermediate forms may 

 be found in the intervening areas, the ostriches in the more extreme 

 parts of the continent must have evolved independently on one another 

 for long ages, though not to such a degree as to bring about infertility 

 between them. Some changes, such as the bald patch, and those con- 

 nected with the size and colour of the body and the nature of the egg, 

 are now distinctive between the two races, while others, such as the loss 

 of plumage, the loss of the claw on the small toe, and of scales on the 

 large toe, are common to the ostrich race as a whole. 



Assuming the characteristics for the race to have been the same 

 originally, and that the distinguishing features of to-day have no selec- 

 tion value, we may first enquire how, for example, such a unit character 

 as the bald head patch has come to be dominant and duplex for the 

 northern species, while altogether absent from the southern. • On the 

 factorial theorj' of variation we a.ssume that some definite, hereditable 

 change took place in the germ plasm of the northern ostrich, a,s a result 

 of which the feathers fall out from the top of the head at a cerUiin age. 

 If we further admit that the number of ostriches for the area was con- 



* C. B. Davenport, •* The Form of Evolutionary Theory that modern Genetical Research 

 seems to favour." Amer. Sat. Vol. l. Aug. 191G. 



