EVOLUTION OP THE COLORS OF BIRDS. 171 



becoming obliterated. This objection wholly misses the 

 point, however, which is, not why does this little dot 

 which can be of no utility exist but why does it occur as 

 anticipating a larger spot? This is only one instance out 

 of an innumerable number of cases where a well marked 

 character in one feather is led up to by anticipatory 

 stages. I would suggest that the term Repetitive 

 Marks be used to designate these characters. These rep- 

 etitive marks, from their very nature, cannot be useful 

 characters nor can they be rudimentary. The most 

 probable explanation of them appears to be that they 

 have been developed in accordance with the law of repe- 

 tition. This is a form of correlative development, but 

 it does not appear by any means adequate to completely 

 explain the case. In a general way it might be under- 

 stood that if a definite manifestation of growth force 

 were located in one feather, there would a tendency to 

 repeat this less and less completely in succeeding feath- 

 ers. It might seem, however, that this repetition pre- 

 cedes and leads up to the complete form rather than fol- 

 lows it. Of course it is difficult to say which end is the 

 beginning of the series, but it might seem at first glance 

 as if it should begin with the first primaries and end 

 with the tertiaries, these latter being less specialized. 

 In reality, however, it seems more rational to suppose 

 that the tendency works from the simple to the complex. 

 Going from the secondaries toward the primaries it is 

 found that while certain characters are becoming more 

 developed others are fading out. Thus in wing of the 

 sparrow-hawk (Plate III), proceeding from the tertiaries 

 towards the primaries the black bars are found to be 

 progressing in complexity, while at the same time the 

 black subterminal spot of fig. 39 diminishes and lingers 

 on as a repetitive spot to fig. 30. All that can be said 

 of this successional taxology in marks as exhibited on 



