1907] Beebe: Geographic Variation in Birds. 31 



8. When the concentration of the melanin has reached a 

 certain stage, a change in color occurs, from dull dark brown or 

 black to a brilliant iridescent bronze or green. 



9. This iridescence reaches its highest development on the 

 wing coverts and inner secondaries, where, in many genera of 

 tropical and sub-tropical doves, iridescence most often occurs. 



10. All these changes take place by continuous variations, 

 and there is no change of color without moult. 



D — Significance in Respect to Direction of Evolution. 



The increase of pigment in a single individual under hu- 

 midity during a comparatively short period of time, and the 

 subsequent correlated development of metallic tints, assume a 

 new importance when we consider that, in these experiments at 

 least, mutation and natural selection have no place. 



In a recent paper embodying some interesting comparisons 

 of many wild species of doves and pigeons. Prof. C. O. Whit- 

 man advances the theory that the various gradations of color 

 patterns, such as those on the wing coverts, are hints of closer 

 or more remote biological relationship, and the apparent direc- 

 tive progress of the character he explains by orthogenetic varia- 

 tion, affected to a certain extent by natural selection, but not by 

 mutation. He says : "If a designer sets limits to variation in order 

 to reach a definite end, the direction of events is teleological ; but 

 if organization and the laws of development exclude some lines 

 of variation and favor others, there is certainly nothing super- 

 natural in this and nothing which is incompatible with natural 

 selection. Natural selection may enter at any stage of ortho- 

 genetic variation, preserve and modify in various directions the 

 result over which it may have had no previous control." 



Setting aside discussion of the orthogenetic theory itself, 

 we find that Prof. Whitman is able to offer in regard to the 

 actual transition of one stage of coloring to another, evidence 

 only from comparison of wild species, or from young and adults 

 of some one species. He says : "But is there any direct proof that 

 the transformation is actually making progress to-day? May 

 not these transitional steps go on appearing generation after 

 generation, without ever making any permanent progress? We 

 have to concede that we can not follow the processes that reveal 

 themselves in steps. We can at most only see what is done — 

 not the doing. We are entirely in the dark as to the time re- 

 quired to carry the change through a single row of feathers." 



