44 



ORTHOGENETIC EVOLUTION IN PIGEONS. 



is broken into stages that represent results in the progress of differential develop- 

 ment which are unfolded at successive epochs. The intervals between the stages 

 give no outward indications of steps connecting them. Are we to conclude that 

 this sequence is one of discontinuities? 



Although the transitional series linking two characters in one and the same 

 plumage is the main theme in this paper, it will not be without interest to make 

 clear that mutation has just as little to do with the second sequence as with the 

 first. But first let us finish the examination of the stages individually, as this is 

 necessary to complete the story of the transitions actually realized in each of them. 



The juvenal stage of the male, as we have seen, presents three transitional steps 

 or intergrades between the two characters. I have not thus far examined the 

 corresponding stage of the female, but it is safe to assume that the contrast between 

 the last band and the first bar is less than in the male. 



The third stage in the order above named is that of the adult female (pi. 17, 

 third row). In the uppermost of the long coverts we find a weak transverse band 

 on the lower web. In the second feather we see just a shadow of a band on the upper 

 web, and on the lower web a rather dim spot, in a half-way condition between the 



Jv.<? 



i 

 i a — 



Ad. 

 u 



— >-• 



Ad.c? 

 ra 



2»- 



I 

 i 

 l 

 I 



i 

 V 



I 

 I 

 I 

 i 

 i 

 I 

 i 



y 



i 

 v 



Text-figure 12. 



Diagram to illustrate pattern sequence in 

 the anterior wing-bar of male Ocyphaps. 



1, 2, 3 represent 3 feathers of the an- 

 terior liar. I, II, III represent 3 plumage 

 stages of the bar in the male — juvenal, 

 adolescent, and adult. 1-3 show the first 

 or feather sequence of transitions. I 1 1 1 

 show the second or plumage sequence of 

 the transition s-ries. 



two characters. In the third feather the longitudinal spot comes to fuller develop- 

 ment on the lower web. If the feather be turned just enough to cause the irides- 

 cence to vanish, we see more plainly the shape of the spot and notice in its slanting 

 proximal border a first step in the transformation to the band condition. In the 

 next three feathers the spot elongates somewhat, but shows no sign of transforma- 

 tion. The bronzy iridescence is strongest in these mid-feathers of the row and 

 vanishes wholly on the tenth feather. 



In the fourth stage, seen in the adolescent male (pi. 17, first row), the transi- 

 tional steps are extended through the whole row of spots, if we accept the last 

 trace of a spot seen in the ninth feather. The series then consists of no less than 

 eight steps in a single row of nine spots. No one can look at this series and fail to 

 see that each step is a demonstration of transition, and that the series, taken as a 

 whole, puts the genetic continuity of the two characters beyond a doubt. Here 

 nature, as if to forestall the indirections of biometric gymnastics, draws a straight 

 line between her two extremes, thus disclosing in a small number of steps a path- 

 way of variation essentially different from anything within the purview of the 

 large-number curve. 



