PROBLEM OF ORIGIN OF SPECIES 49 



Natural selection could not be supposed to originate or to guide 

 the first indifferent stages of new characters. Mutation would be 

 equally helpless, and each step would leave a gulf of discontinuity, a 

 miracle that nature seems to abhor. 



Turning from theoretical impasses to the facts, let us compare the 

 two patterns. 



In the checkered pattern all the feathers are marked alike no 

 regional differentiation. In the other type we have a conspicuous 

 local differentiation, suggesting at once a higher stage of evolution. 

 Checkered wings are to be found which vary all the way between a 

 uniform marking and the barred type. If we arrange a number of 

 unequally checkered wings in a series, running from the most to the 

 least checkered, we shall see that the pattern approaches more and 

 more nearly to that of two bars, as the checkers diminish in size and 

 number. We shall notice that the pigment is reduced more rapidly 

 in the anterior than in the posterior part of the wing. 



As checkers are reduced, they gradually lose their sharp ends, and 

 approximate the square or rounded form seen in the elements of the 

 typical bars. The series shows a flowing gradation, that may be read 

 forward or backward with equal facility. Darwin's view takes the 

 bars as the starting-point and reads forward. Taking the checkered 

 condition as the point of departure, the variation runs just as 

 smoothly in the opposite direction. We here meet an ambiguity 

 that is everywhere present in color-pattern problems an ambigu- 

 ity that is frequently overlooked with disastrous consequences. 

 The only way to eliminate the difficulty is to take our evidence from 

 several different sources, and when agreement is found for one direc- 

 tion, and disagreement for the other, the way is clear. 



As an experiment, we may take one or more pairs of pure-bred, 

 typically barred pigeons, and keep them isolated from checkered 

 birds for several years, in order to see if the young ever advance 

 toward the checkered type. 



Another experiment should be tried for the purpose of seeing what 

 can be done by working in just the opposite direction. In this case 

 we take checkered birds, selecting in each generation birds with 

 the fewer and smaller checkers, and rejecting the others, in order to 

 see if the process of reduction can be carried to the condition of three, 

 two, and one bar, and finally, to complete obliteration of both 

 checkers and bars, leaving the wing a tabula rasa of uniform gray 

 color. 



If these experiments are continued sufficiently far, it will be found 

 from the second experiment that a gradual reduction of pigment 

 to the extreme conditions named can be comparatively easily 

 effected, and that the direction of reduction will always be the same, 

 from before backward; while, from the first experiment, it will be 



