THE PROBLEM OF THE ORIGIN OF SPECIES. 35 



of but one interpretation. This pigeon is a not-distant relative of the rock-pigeon, has a 

 similar gray ground, and is therefore probably moving in a parallel direction, only more 

 advanced. 



(3) The spots are found at the posterior end of the wing, near the upper edge, on one 

 to three tertials and on a few long coverts. In some cases they occur also on a few of the 

 second row of long coverts, 15 but here they are always very small and completely concealed. 

 They are thus in the position occupied by vanishing spots generally. 



(4) The adult plumage makes no advance in the number of spots, and some spots 

 (second row of long coverts), visible in the young, are completely concealed in the adult. 

 This indicates degeneration unmistakably. 



(5) The stock-dove, although sometimes having a concealed third bar of few spots, 

 never appears in chequered dress. It seems to have moved so far in the opposite direction 

 that no reversal of course is now open to it- 

 Taking the chequered pattern as the earlier one, the various conditions of 



chequers and bars in rock-pigeons, domestic races, and indeed in all the wild pigeons, 

 become almost self-explanatory. We could not explain satisfactorily how just two 

 bars could arise de novo in one species, three in another, twelve in another, and so 

 on. The repetition of de novo origins would become ever more incredible. Making 

 phylogeny our guide as to the starting-point, we find it comparatively easy to 

 thread our way through the maze of patterns existing among 500 or more species 

 of pigeons, and even to trace affinities farther back in the bird world. 



The orthogenetic process is the primary and fundamental one. In its course 

 we find unlimited opportunities for the play of natural selection, escape the great 

 difficulty of incipient stages, and readily understand why we find so many condi- 

 tions arising and persisting without any direct help of selection. 



Charles Darwin: 



As natural selection acts solely by accumulating slight, successive, favorable variations, 

 it can produce no great or sudden modification. (Origin of Species, Chapter XIV, p. 421.) 



Slight individual differences, however, suffice for the work, and are probably the sole 

 differences which are effective in the production of new species. (Animals and Plants, 

 Vol. II, Chapter XX, p. 233.) 



As modern geology has almost banished such views as the excavation of a great valley 

 by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief 

 of the continued creation of new organic beings, or of any great and sudden modification 

 in their structure. (Origin of Species, Chapter IV, p. 98.) 



August Weismann: 



The simultaneous modification of numerous co-functioning parts, in essentially different 

 ways, yet in harmonious functional relations, points conclusively to the fact that something 

 is still wanting to the selection of Darwin and Wallace. (Germinal Selection, p. 22.) 



We know of only one natural principle of explanation for adaptation, that of selection. 

 (Ibid., p. 61.) 



The three principal stages of selection — that of personal selection, as held by Darwin 

 and Wallace; that of historical selection as upheld by Wilhelm Roux in the form of a 

 "Struggle of the Parts"; and, finally, that of germinal selection, the existence of which I 

 have endeavored to establish — these are the factors that cooperate to maintain the forms 

 of life constantly capable of life. (Ibid., p. 60.) 



16 Rudimentary spots on the third row of coverts — a fourth bar — were later found (1909); these are shown in plate 

 13— Ed. 



