256 CALIFORNIA ACADEMY OF SCIENCES. 



be in general climatic, the southern form being darker 

 than the northern, but the ranges of- the two species 

 overlap. The distinct white edge to the outer tail feather 

 is an excellent example of a discriminative mark for 

 distinguishing the two very similar races. 



In the preceding discussion forms have been consi- 

 dered which are on the very threshold of divergence — 

 species which are either still forming or barely formed. 

 It is seen that in a large number of instances the cause 

 of the first divergence has been the direct action of the 

 environment. In some cases, however, it was impossible 

 to see how the environment could have produced the 

 change, and in these instances the new character was 

 ascribed to spatial segregation, or some other form of 

 geographical isolation. In still other species the geo- 

 graphical environment seems to have little or nothing 

 to do with the variation, since the diverging forms oc- 

 cupy the same territory. Here the factor of sexual isola- 

 tion and discriminative marks may be called into play, 

 or some other mode of segregation too intricate to be 

 unraveled may be at the bottom of it. 



Before taking leave of this branch of our inquiry, one 

 interesting case of direct enviromental influence must 

 be noted. It is of special interest because the direct 

 climatic influence, instead of merely producing varieties, 

 seems to have pushed on, and been the principal, if not 

 the sole, factor in the establishment of species. I refer 

 to the three North American species of crested flycatchers 

 (Myiarchus), the colors of which are shown on Plate 

 VI. The darkest species is found in the Eastern States 

 (21. crinitus), while the palest species, which is nearly 

 identical with it except that the colors are almost com- 

 pletely bleached out (j\I. cincrascenN), inhabits the arid 

 regions of the West. Exactly intermediate in color be- 

 tween tliese two species is M. me x i canus oi eastevn Mexico. 



