376 PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 



viduals of the same species inhabiting distant portions of a common 

 habitat, been duly recorded. In the work of registering these instruc- 

 tive data, it has fallen to Americans to take a leading part, large 

 credit in the matter being due not only to the activity of our profes- 

 sional biologists, but to the liberality of the General Government in 

 attaching competent natural history observers and collectors to the 

 numerous surveying parties it has sent out during the last twenty 

 years to explore the till then practically unknown geography and 

 productions of our w^estern Territories. 



The combined fruits of their labors, together with those of the 

 agents and correspondents of the Smithsonian Institution, have re- 

 sulted in the accumulation of an amount of material far exceeding 

 that elsewhere accessible to single investigators, representing, as it 

 does, at least two of the vertebrate classes of animals from the whole 

 North American continent so fully that generalizations may be made 

 from their study which could not otherwise have been reached for 

 many years and for which no similar facilities for any other equal 

 area as yet exist. The recent investigations of American mam- 

 malogists and ornithologists have been in consequence largely di- 

 rected to the subject of geographical variation, and their publications 

 teem with tabulated measurements and records of variations in form 

 and color that accompany differences in the climatic or geographical 

 conditions of habitat. Among the results that have followed are the 

 discovery of numerous interesting geographical varieties or subspecies, 

 iind the demonstration of the complete intergradation of many forms, 

 often quite widely diverse in color, size, and proportion of parts, for- 

 merly regarded (and properly so as then known) as unquestionably 

 distinct species, which discoveries have of course necessitated a large 

 reduction in the number of recognized " specific " or nonintergrading 

 forms. But most important of all has been the correlation of local 

 variations with the conditions of environment, and the deduction 

 therefrom of certain laws of geographical variation. Upon these 

 have been based hypotheses that go far toward explaining many of 

 the phenomena of intergradation and differentiation observed among 

 existing animals. In the present paper will be given not only a 

 summary of the results thus far attained, but enough of the details 

 of the subject to show the nature of the evidence on which rest the 

 conclusions already reached. These results, it is claimed, show that 

 other influences than natural selection operate poAverfully in the 

 differentiation of .specific forms, and that geographical causes share 

 more largely in the work than naturalists have heretofore been pre- 

 pared to admit — at least to consider as proven. 



As is well known, animals vary greatly in respect to the extent of 

 the areas they inhabit. "V^^iile a few species are nearly or quite cos- 

 mopolitan, many others are restricted to single small islands or to 



