88 LAWREXCE S WARHLEE. 



remain a mystery. To mv mind, however, taking into consideration the exti-cttie? 

 of individual variation, the wonder is, not that few species are evolved but that more 

 do not appear. It is to the overbalancing of these extremes of individual variation 

 that we must look for the sudden appearance of new forms, as in Brewster's and 

 Lawrence's Warblers. Species appearing through more gradual changes brought 

 about by environment, as, for example, in the Grackles of Eastern North America, 

 and in many other species and sub-species, assume recognizable characters only aftef 

 the lapse of many years. 



I have here discussed this matter at some length as it involves one of the 

 burning questions of the day in oniitho'.ogical circles, and have made a number of 

 statements which from lack of space I. have not offered proof; yet I have not made a 

 single assertion which has not been based upon faets that I have traced out in some 

 department of animal life. In fact, all I have said is covered by well known laws of 

 evolution, which are familiar to all students who have given the matter proper at' 

 tention. In closing I give a brief summary of the whole matter. 



SUMMARY. 



Two forms of birds appearing somewhat suddenly and increasing considerably 

 within a few years have offered problems which have greatly puzzled ornithologists. 

 These forms are known as Biewster's and Lawrence's Warblers. In some wa^ys they 

 appear to intergrade not only witii each other but with two other species, namel}-, 

 the Golden-winged and the Blue-winged Warblers. These peculiarities have been 

 considered by those who have written about them to be due to three causes : i, Hy- 

 bridization of the Golden-winged and Blue-winged Warblers; 2, Dichromatism in 

 the same species; 2, Evolution of Brewster's and Lawrence's Warblers from the Gol- 

 den-winged. 



1. Hybriilization is improliable, for, even allhough the so-called hybrids may 

 present all intergradiiig stages of plumage between the sujiposed parent species, they 

 do not occur all over the region oxerlapped by the species of whicli they are thought 

 to be the offspring: but chiefly because known hyljrids among wild birds ai:; raielv' 

 fertile, and the liybrids tliemselves very rare]\. if ever fertile whin mated togtther. 



2. Dichromatism is also improbable, because ihe dii. hromaiic phasis octur 

 only over a limited area, whereas they should be Icund over all of the section occu- 

 pied by both Golden-winged and Bhie-winged, local dichromatism being imknown 

 among birds. 



3. Evolution appears to be the most natuial explanation of the matter, both 

 of the forms in cpiestion having been derived from the Golden-winged Warbler; be- 

 cause they both present at least one character in cottim.on with the supposed parent: 

 that of the peculiar form of the white spot on the outer web of the second tail tlallier. 

 That Brewster s and Lawrence's Warblers are unlike is explained by the law cf re- 

 version, each form assuming the characters in part of its iiiimeiiiati' pai'onts, and it> 

 part the characters of some remote ancestor. Limited distribution is in favor of this 

 theory. The appearance of new forms on ground overlapped b\' the two old species 

 is most likely coincidental. There is nothing in the colorancn and other facts regard- 

 ing the two species in question but what can be accounted for by the action of well^ 

 known laws of evolution. 



