GEOGRAPHIC ISOLATION AND SPECIES-FORMING 129 



The orchard oriole, Icterus spurius, has the head, back, and 

 tail all black, the lower parts chestnut, and the body relatively 

 small, as shown by the average measurements of different 

 parts. In the hooded oriole, Icterus cucullatus, the head is all 

 golden orange except the throat, which is black, the tail is 

 black, and the wings are black and white. This species, with 

 its subspecies, ranges through southern California and Arizona, 

 and over much of Mexico. Our other orioles have the tail black 

 and orange. In the common Baltimore oriole, Icterus galbida } 

 of the east, the head is all black and the under parts orange. 

 In the equally common Bullock oriole, Icterus bullocki, of the 

 California region, the head is yellow on each side, the belly 

 rather yellow than orange. The females of all the species are 

 plain olivaceous, the color and proportions of parts varying 

 with the different species, while in the males of each of the 

 many species black, white, yellow, orange, and chestnut are 

 variously and tastefully arranged. Each species again has a 

 song of its own, and each its own way of weaving its hanging 

 nest. 



That which interests us now is that not one of these varied 

 traits is clearly related to any principle of utility. Adaptation 

 is evident enough, but each species is as well fitted for its life 

 as any other, and no transposition or change of the distinctive 

 specific characters or any set of them would in any conceivable 

 degree reduce this adaptation. No one can say that any one 

 of the actual distinctive characters or any combination of 

 them enables their possessors to survive in larger numbers 

 than would otherwise be the case. One or two of these traits, 

 as objects of sexual selection or as recognition marks, have a 

 hypothetical value, but their utility in these regards is slight 

 or uncertain. Naturalists now look with doubt on sexual 

 selection as a factor in the evolution of ornamental structures, 

 arid the psychological reality of recognition marks is yet un- 

 proved, though not impossible. It may be noted in passing, 

 that the prevalent dull yellowish and olivaceous hues of the 

 female orioles of all species seem to be clearly of the nature 

 of protective coloration. 



It has been shown statistically that certain specific charac- 

 ters among insects have no relation to the process of selection. 

 Among honey bees the variation in venation of the wings 

 and in the number and character of the wing hooks is just as 



