160 Trotter, Relation of Genera to Faunal Areas. [April 



the common type thus produced would tend to become more 

 firmly set in the new form through geographical or habitat isolation 

 in the breeding season. During an extended period of time this 

 differentiation would tend toward an equilibrium as the inherited 

 characters strike a balance with the environment. Time, and the 

 segregating influence of wide geographical areas, with their oppor- 

 tunity for varied climatic and vegetation habitats, appear as the 

 responsible causes of the phenomenon of species and of species 

 distribution. 



This is so well-worn a theme that I must ask your pardon for 

 bringing it forward. It seems to me worth while, however, to 

 recall these underlying principles. It is a habit of mind to regard 

 a species as very definitely related to its range. It is much more 

 definitely related to its habitat. By this I mean that a species is 

 much more likely to extend its range than to alter its habitat. 

 And further, I feel convinced that temperature per se does not effect 

 the sexual tissues of a species so profoundly as to set barriers to its 

 breeding area. Temperature, rainfall, soil conditions, and topog- 

 raphy affect the character of vegetation and this is apparently 

 the most direct and dominant factor in the distribution of species. 



In a paper read before the Delaware Valley Ornithological Club 

 and published in 'The Auk' for July, 1909^ I have stated my belief 

 that the present geographical groups of species which we recognize 

 as faunas are more or less temporary phases in a general northward 

 spread of species during post-glacial times, and that the true 

 interpretation of faunas is not to be found in any single condition, 

 such as temperature, but is related to the geological history of a 

 land. In the present paper I wish to bring forward the thought 

 that in this very advance or northward spread we have the con- 

 ditions which have broken an original common type into several 

 varietal and specific forms. 



Let us take, for example, the genus Hylocichla, a group of wood- 

 land Thrushes quite similar to one another, presenting, one might 

 say, a minimum of departure from an ideal type, both in adult 

 sexual and juvenal phases of plumage. It seems quite possible 

 that a generalized ancestral form common to all may have existed 



» The Geological and Geographical Relations of the Land-Bird Famia of North- 

 Eastern America, pp. 221-233. 



