558 



SCIENCE. 



[N. S. Vol. XXII. No. 566. 



In the animal kingdom generally we may- 

 say: whenever a barrier is to some extent 

 traversable, the forms separated by it are 

 liable to cross from one side to the other, 

 thus producing intergradations, or forms 

 more or less intermediate between the one 

 and the other. For every subspecies, 

 where the nature of the variation has been 

 carefully studied, there is always a geo- 

 graphical basis. This basis is defined by 

 the presence of some sort of a physical bar- 

 rier. It is extremely rare to find two sub- 

 species inhabiting or breeding in exactly 

 the same region. "When such appears to 

 be the case, there is really some difference 

 in habit or' in habitat; the one form lives 

 on the hills, the other in the valleys; the 

 one feeds on one plant, the other on an- 

 other; the one lives in deep water, the 

 other along the shore. There can be no 

 possible doubt that subspecies are nascent 

 species, and that the accident of intergrada- 

 tion in the one case and not in the other 

 implies no real difference in origins. 



Of all branches of science, we may say 

 that the one most advanced in its develop- 

 ment, most nearly complete in its conclu- 

 sions, is that of the systematic study of 

 American birds. No other group of nat- 

 uralists has made such extensive studies of 

 individual or of group variations as the 

 ornithologists who have dealt with Amer- 

 ican birds. And for this reason, and on 

 account of the excellence of the preserved 

 material with which they have to deal, the 

 students of our birds have thoroughly 

 understood the relation between species- 

 making and geographical distribution, the 

 persistence of group variation and the 

 origin of species. 



While the processes of natural selection 

 may be always at work destroying those 

 individuals not fitted to their surroundings, 

 and accentuating the adaptation of the spe- 

 cies through those which survive, and while 



sometimes natural selection may work with 

 great acceleration where conditions are 

 widely changed, yet it is clear that the 

 characters by which one species is actually 

 known from the next are rarely traits of 

 utility. Such traits bear no visible relation 

 to the process of natural selection, using 

 that term in a strict sense. 



For example, we may compare the spe- 

 cies of American orioles constituting the 

 genus Icterus. We may omit from consid- 

 eration the various subspecies, set off by 

 the mountain chains, and the usual assem- 

 blage of insular forms, one in each of the 

 West Indies, and confine our attention to 

 the leading species as represented in the 

 United States. 



The orchard oriole, Icterus spurius, has 

 the tail all black, the head all black, the 

 lower parts chestnut, and the body rela- 

 tively small, as shown by the average 

 measurements of different parts. In the 

 hooded oriole. Icterus cucullatus, the head 

 is orange, the throat black, and the wings 

 are black and white. This species, with its 

 subspecies, ranges from Southern Cali- 

 fornia over much of Mexico. Our other 

 orioles have the ta,il black and orange. In 

 the common Baltimore oriole. Icterus gal- 

 hula, of the east, the head is all black, and 

 the outer webs of the wing coverts are black 

 and white. In the equally common bullock 

 oriole. Icterus hulloc'ki, of the California 

 region, the head is partly yellow, and the 

 greater wing coverts are mostly black. The 

 females of all the species are plain oliva- 

 ceous, the color and proportions of parts 

 varying with the different species, while in 

 the males of each of the many species black, 

 white, 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 



