104 Mutations and Evolution. 



the Japanese turtle dove. Then he shows how in different groups 

 of wild species of pigeons from all over the world, various stages of 

 the reduction of this pattern are found, beginning always at the 

 anterior end of the wing and wiping off the rows of spots until, in 

 unrelated species belonging to different groups, patterns with 

 4, 3, 2, 1 or no bars of spots may be found. The rock pigeon 

 (Colutnba lima) with usually two wing-bars thus represents a 

 relatively reduced condition, while the white-winged pigeon 

 (Melopdia leucoptera), in which the chequers have entirely 

 disappeared, is a still more advanced stage. In these cases, 

 moreover, the juvenile plumage generally represents the more 

 extensively chequered and primitive type, recapitulation thus 

 strongly supporting the author's interpretation. Whitman further 

 claims that this orthogenetic trend in the colour patterns of pigeons 

 is not a mere result of selection, but may even run counter to it 

 and gradually eliminate a pattern of spots which sexual selection 

 tended to strengthen and beautify. His monumental work places 

 in a new light the claims of orthogenesis as an evolutionary factor, 

 but space forbids a more extensive discussion of his results. 



The claims of orthogenesis have also been taken up recently 

 by Duerden (1919) for the ostriches. He believes that the 

 reduction in the plumage, in the toes and their scutellation, and in 

 other features, is proceeding progressively and independently in 

 the North and South African ostrich. He believes the evidence 

 is clear that the progress is entirely independent of adaptational 

 considerations. On the other hand, there seems no doubt of the 

 germinal origin of many of the variations. The case of reduction 

 in toes and specialization of the middle toe for running is in many 

 respects similar to that of the horse. If that is so, adaptation 

 would appear to have taken place by the progressive selection of 

 germinal variations. It does not necessarily follow, as Duerden 

 believes, that the reduction of the ostrich's toes will continue until 

 the extinction of the species results. 



The last five chapters have been devoted to an attempt to 

 bring into relation with each other such diverse things as the 

 mutation theory, the cell theory, the facts of recapitulation and 

 the evidence for the inheritance of acquired characters. We found 

 that new features of the organism may originate internally through 

 chromatin changes and express themselves centrifugally in the 

 ontogeny of the organism ; or they may originate externally as 

 organismal characters and ultimately become inherited, through a 



