120 EVOLUTION AND ANIMAL LIFE 



flow of life without materially changing its conditions. But 

 this is practically a universal rule: A barrier which prevents the 

 intermingling of members of a species will with time alter the 

 relative characters of the groups of individuals thus separated. 

 These groups of individuals are incipient species, and each 

 may become in time an entirely distinct species if the barrier 

 is really insurmountable. In the great water basin of the 

 Mississippi, many families of fish occur and very many spe- 

 cies are diffused throughout almost the whole area, occurring 

 in all suitable waters. Once admitted to the water basin, each 

 one ranges widely and each tributary brook has many species. 

 In the streams of California, mostly small and isolated, the 

 number of genera or families is much smaller. Each species, 

 unless running to the sea, has a narrow range, and closely re- 

 lated species are not found in the same river. The fact last 

 mentioned has a very broad application and may be raised to 

 the dignity of a general law of distribution. 



Given any species in any region, the nearest related species 

 is not likely to be found in the same region nor in a remote 

 region, but in a neighboring district separated from the first 

 by a barrier of some sort, or at least by a belt of country, the 

 breadth of which gives the effect of a barrier. 



Always the species nearest alike in structure are not 

 found together nor yet far apart, and always a check to 

 interbreeding lies between. Where two closely allied forms 

 are not found to intergrade, they are called distinct species. 

 If we find actual intergradation, the occurrence of specimens 

 intermediate in structure, the term subspecies is commonly 

 used for each of the recognizable groups thus connected. 



Widelv distributed across the United States and from 



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southern Canada to Arizona, we have the yellow warbler, 

 Dcndroica ccstiva. This bird is chieflv yellow, olive on the 



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back with chestnut streaks on the sides, the tail feathers colored 

 like the body, and without the white spot on the outer feathers 

 shown in most of the other wood warblers composing the genus 

 Dendroica. 



The yellow warbler throughout its range is very uniform 

 in size and color. Its nearest relative differs in having a 

 shade less olive on the back and the brown streaks on the sides 

 narrower. This form is found in the Sonoran region, and, as 

 along the Rio Grande it intergrades with the first, it is called 



