FpcIv, The Affinities of Mniotiltid^e. 39 



highly specialized of the southern genera, still stand considerably below these 

 t'oar. Aa^ain, all those numerous forms showing Flycatcher affinities, with the 

 exception of three species of Sylvania and one of Setophaga, are confined to the 

 southern region. 



Now it will be remembered that the order of Passeres is divided into two 

 sab-orders, Clamatores and Oscines. The former of these embraces but two fam- 

 ilies, the Tyrannidae and Ootingidae the Tyrannidse may be taken as the type 

 of the sub-order, as they show, in general, less affiliation with the Oscines. 



Finally, as regards the origin of the Mniotiltidse, the consideration of these 

 facts would lead us to adopt one of two theries. Either they originated some- 

 where in tlie southern region and at a period not much subsequent to the separ- 

 ation of the order of Passeres into the two sub-orders, considering the close 

 affinities of some genera with the TyrannidiB, from which starting point the 

 more highly specialized genera moved northward into temperate America, 

 while the more generalized forms, those retaining the tyrannidine characters, 

 remained in the southern region ; or on the other hand they originated in the 

 manner in the northern region and the tyrannidine types, with other less spe- 

 cialized genera, moved southward. From the fact that the whole sub-order 

 Clamatores, with the exception of a comparatively few species of TyrannidaB, 

 are confined to the southern region, we would at first be led to conclude that the 

 birth place of the family must have been there, but a closer consideration of the 

 facts go to prove the contrary. It is generally believed, from a broad study of 

 distribution, that the Passeres originated in the north temperate zone, where 

 nearly all of the most highly organized forms of animals took their rise. The 

 Passeres are the most perfectly organized and specialized of all birds and there 

 is little reason to doubt that they follow the general rule. Again, let it be re- 

 membered that the Mniotiltidye originated not very long subsequent, in all 

 probability, to the seperation of the order into the two sub-orders, therefore be- 

 fore-a general movement southward is likely to have taken place. Further, it 

 is reasonable to suppose that where great physical changes would not necessitate 

 a universal migration, the most highly specialized forms would be found where 

 the family originated, for as species and individuals became numerous and the 

 straggle of life mf)re severe, those adapting themseh^es most readily to condi- 

 tions, that is, tliose becoming most .specialized would drive out the less adapt- 

 able, that is, the more generalized, which would be compelled to migrate into 

 localities not yet occupied by species having a similar mode of life. It is prob- 

 able that at the time the MniotiltidcP became a seperate family, a much warmer 

 climate prevailed over the northern hemisphere than at present, bvit as the 

 clinuite became more rigorous, the less hardy forms, which are of course the 

 more generalized, would be driven southward, along with most of the Tyran- 

 jiid*, into those tropical regions where we now find them. In this movement 

 would also be included most of those showing close relationship to the 

 Tangaridae, wliich are very susceptible to cold. 



But, it may be asked, how are we to account for a number of highly special- 

 ized forms in the southern region and of generalized in the northern 'i It seems 

 surprising rather that there are not more of these exceptional cases than really 

 exist, when we come to cnisider the immence migratory range of the family. 



