Mutations and Evolution , 
245 
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 
(Columba livia) with usually two wing-bars thus represents a 
relatively reduced condition, while the white-winged pigeon 
(Melopelia 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 
