62 OKTHOGENETIC EVOLUTION IN PIGEONS. 



reach such a definitely localized and apparently rigid character, it might seem that 

 evolution had come to a halt, that the character was caught in a mutation-trap, 

 from which there could be no escape. But even while we are looking at this picture 

 the selfsame process that produced it may be preparing another picture to appear 

 after a molt in more or less different guise. Can a single individual mutate from 

 plumage to plumage, i.e., become a new species? 



When we look around among allied species and see these same bars reduced to 

 about half dimensions in the rock-pigeon of Manchuria {Columba rupestris, text- 

 fig. 23), reduced to mere remnants of two to six spots in the stock-dove {Columba 

 oetias), carried to complete obsoletion or to a few shadowy reminiscences in the 

 secondaries in Columba rufina of Brazil (pi. 46), gone past return in some of our 

 domestic breeds and in many of the wild Columba? 17 — when we see all these stages 

 multiplied and varied through some 400 to 500 wild species and 100 to 200 domestic 

 breeds, and in general tending to the same goal, we begin to realize that they are 

 not to be regarded as permanent halts, but rather as slowly passing phases in the 

 progress of an orthogenetic process of evolution, which seems to have no fixed goal 

 this side of an immaculate monochrome — possibly none short of complete albinism. 



Even in cases where natural selection has probably played a conspicuous part 

 in modifying and beautifying these marks, e.g., in the crested pigeon of Australia 

 (Ocyphaps lophotes), we find that the reducing process has not been brought to a 

 standstill. Indeed, a careful comparison of the juvenal and adult plumages in both 

 sexes shows that differentiation has been gradual and continuous and that it is still 

 in progress in the bar of the long coverts, the homologue of the anterior bar in the 

 rock-pigeons. What we see now going on in this bar has been already achieved in 

 the white-bellied plumed dove 18 (Lophophaps leucogaster) of the same country, and 

 is now progressing down the next and last bar. 19 A most striking demonstration of 

 progressive orthogenetic differentiation, still advancing and even cutting through 

 the brilliant coloring, which, in part, we attribute to natural selection. 



SUMMARY. 20 



The wild rock-pigeons present two very distinct color-patterns: (1) the chequered 

 type and (2) the barred type. 



Two black wing-bars on a gray ground have always been held to be the more 

 primitive pattern, and birds of this pattern are supposed to represent the typical 

 Columba livia. 



The form with black chequers evenly distributed over the wing and back, 

 although once named Columba affinis, as a distinct species, was regarded by Darwin 

 as a variety derived from the two-barred rock, and his opinion has stood undisputed. 



It appears from a comparative study of many species of wild pigeons, and from 

 a study of the variations in domestic species, that the relationship is just the reverse: 

 C. affinis is the original rock-dove and C. livia is the derived type. Domestic pigeons 

 have come from both sources. 



" Illustrations of some of these are given in Chapter VII. — Ed. 



18 A good picture of this dove was given by Mr. D. Seth-Smith, in the Avicultural Magazine, December 1906. 

 (The wing-bars of this bird are here shown in text-fig. 9. — Ed.) 



19 A detailed account of this case is now in preparation (the account was not completed. — Ed.). 



20 The abstract of an address of 1903 (Biological Bulletin, 1904, Vol. VI, p. 307) is used as a summary for this 

 chapter. The four final paragraphs have been substituted by the editor, from the manuscripts for the completed 

 chapter, for the two final paragraphs of the author's abstracts. 



