AMERTCAN WARBLEHS. 87 



Wings tViroughout their entire range? I do not now i-ecall anj- examples of local di- 

 chi'omatisni among birds which are so aftected. 



The theory of evolution now remains, and it must be confessed in the beginn- 

 ing that there are at first sight apparently some serious difficulties in regntd to the 

 adoption of this theory- , In the beginning, why is it (hat two such diversely cohered 

 forms as Brewster's and Lawrence's Warblers are derived from the Golden-wing ? 

 The reasons for the seeming inconsistencies of evolution are difficult to explain. 

 While no one who has given the subject the proper amount ofattentioa will lor an instant 

 doubt that all of the Yellow-throated warblers of the genus GcotklyJ>ii were deri\ed 

 from a common ancestor in the not very distant past, or that the Golden Wai biers :il^() 

 had a similar origin, for all of the members of both of these genera are quite al ke in 

 general coloration and form, diiiering only in minor points, a careful study of all our 

 American warblers will also show us that some species even though they are quiie diff- 

 erently colored were veryeviiiently derived from a common ancestor. In the genus Dcu- 

 •droica we have a number of instances of this, and singularly most of the species that 

 show more or less close relationship appear to be two in number. The Blnck-poll- 

 ed and Bay-bieasted Warblers go together, the BIack-throa;ed Green and Townsend's, 

 nnd the Yellow-runiped and Audbuon's, the latter named having given rise to two 

 known siib-species. Coming nearer to the subject in hand we have the GoUien-win- 

 ged and Blue-winged, the latter showing its relationship to the former in the varying 

 color and width of the wing hands. Aided by these suggestions it becomes natuial 

 ("or us to look for the evolution of other species in pairs as we find l-awreiice's and lirew- 

 sters. One of the first things we learn in the study of the e\olution of species is that 

 newly separated forms are apt to be \ariable, and, singularly, they not only show vari- 

 ations that indicate their direct parentage but those characters which weie possessed 

 by ancestors more remote. Hence it is that Brewster's Warbler shows a tendency to 

 assume the yellow i4nder coloration of an ancestor which was yellow beneath, an ances- 

 tor from which both the Golden-wing and Blue-wing may have been derived. That 

 both the Golden-winged and Blue-winged had an ancestor either near ov remote with 

 yellow under parts and black markings on both throat and breast is highly probable. 

 Hence it is that Lawrence's Warbler, even though it be derived from the white breasted 

 Golden-wing, has through reversion yellow under parts, and a more yellow suft'usion 

 thrciughout, as indicated by the greenish back. 



The limited range of the two incipient species can be readily explained under 

 the theory of evolution by supposing that only a few of the new form apiH'ared, poss- 

 ibly only a single brood, but with each individual possessing sufficient potency to 

 transmit the changed chara:lers to its oli'spring, thus the species gradually spread 

 from one center, and later even becnme stragglers to more remote regions. It must 

 be borne in iriind that it is somewhat difficult for us to estimate the numbers of a giv- 

 en bird; we say a species is very rare because ornithologists have seeti a few speci- 

 mens, but trained observers cannot be everywhere and thus inany individual birds es- 

 cape notice. Hence both of these warblers niay be more common than we suppose, 

 and both may have been in existence long before the first specimens were obtained. 

 1 believe, however, for a number of reasons, that species often comeinto existence 

 very quickly, comparatively speaking. 



Just why these forms were evolved in a section where two closely allied species 

 (Overlapped, or, for that matter, why they were evolved at all, must probably forever 



