64 CHAS. C. ADAMS. 



made a detailed comparison of the arid types of the two conti- 

 nents. 



The distinctness of the southeastern and the southwestern cen- 

 ters is frequently overlooked or confused. And this is especially 

 liable to be the case when allowance is not made for the influence 

 of local conditions upon the occurrence of certain southwestern 

 types which have overflowed into the eastern center. This brings 

 up the following question, which as will be seen later on, clearly 

 emphasizes the importance of habitat study in geographical dis- 

 tribution. In estimating biotic areas, how much weight should 

 be given to the occurrence of forms dependent upon limited local 

 conditions ? A bare census gives no idea of the relative weight 

 of the units recorded or the degree of representativeness of them. 

 The importance of such a study in a proper estimate of local 

 conditions has been suggested repeatedly in attempting to deter- 

 mine the relations of these two centers. These relations have 

 suggested that perhaps biotic affinities can be more easily and 

 safely determined by habitat and biotic associations than pri- 

 marily upon a faunistic or floristic basis. This would mean that 

 the ecological relations rather than the taxonomic affinities should 

 receive greater attention than is customary. It should be noted, 

 however, that this view does not in any way belittle the impor- 

 tance of taxonomic work in distributional studies. But it is suffi- 

 cient, at this place, simply to call attention to the ecological 

 aspect of the subject. 



But to return to the consideration of the southwest, the vege- 

 tation of this arid region is composed of grasses on the plains, 

 and cacti, agave, yucca, and many other types of desert vegetation 

 in the more arid places. Reference need only be made to the 

 recent paper of Coville and MacDougal ('03) for the character- 

 istic features and the literature on this flora. The fauna is equally 

 peculiar and interesting. This is the region where prairie dogs, 

 spermophiles, pocket gophers, pocket mice, wood rats, kangaroo 

 rats are so characteristic, and the horned toads, rattlesnakes and 

 many other reptiles reach their greatest variety and center of abun- 

 dance. This has been the center for many other forms as well. 

 Certain crawfishes (Ortmann, '02) have originated here. Many 

 groups of insects are also characteristic. The bees of the genus 



