180 S. IT. Scuddcr — Effect of Glaciation and of the 



many genera and species were common to both, and how 

 many peculiar to each, of the two areas ; and finally, to com- 

 pare these results with those reached by a similar study of the 

 existing fauna of somewhat equivalent areas upon the Pacific 

 coast where, it is claimed, no continental ice-sheet covered the 

 face of the country. Here one would have to choose some- 

 what arbitrarily a delimitation between the two areas, north 

 and south, which should tolerabty correspond climatically with 

 that of the terminal moraine of eastern America. Accord- 

 ingly, before beginning any tabulation, I selected the follow- 

 ing areas for comparison : East of the "Rocky Mountains, the 

 "barren ground" of the high north, the immediate vicinity of 

 the Rocky Mts. and the extreme south of Florida and Texas 

 were left out of consideration, and the two areas were separated 

 by the line of the great terminal moraine as mapped by 

 Chamberlin. West of the Sierra Nevadas, the two areas were 

 separated by the northern border of California, and the 

 southern district was limited southwardly by the omission of 

 the region south of Los Angeles ; while the northern district 

 was made to include the northern part of Washington and the 

 southern portion of British Columbia, though drift covered, 

 so as to embrace in the lists the numerous collections made on 

 Vancouver Island and the adjoining main land. 



Of course it is well understood that the fauna of eastern is 

 far better known than that of western America ; but this is no 

 real obstacle, since the comparison is to be made in either sec- 

 tion between the northern and the southern districts of that 

 section only, and there no great inequality exists ; the special 

 point of enquiry is as to the relative faunal weight of the 

 north, if the expression may be allowed, in the east, where 

 the ground required complete reoccupancy, and in the west, 

 where occupancy had not been interrupted. 



This would furnish a gauge, as it were, of the effect of the 

 G-lacial Period upon the present faunal distribution of life. 

 For if it should be found that the relative proportion of 

 endemic northern genera and species was distinctly less in the 

 east than in the west, and the relative number of genera and 

 species common to north and south also less ; then surely such 

 relative poverty might properly be taken as a gauge of the 

 insufficiency of the time that has elapsed since the glacial 

 period for the fauna to have recovered its ancient territory. 

 If, on the other hand, no sign of such poverty can be dis- 

 covered, then we may fairly assume that the east has fully 

 recovered from the shock of the Glacial Period, and that, 

 excepting in minor points which special investigation would 

 point out, the present distribution of life is much what it 

 would have been had there been no Glacial Period. I may 



