182 IOWA ACADEMY OF SCIENCES. 



ance should be made for a probable slight advantage for Fay- 

 ette in number of granitic rocks, though Minneapolis has the 

 advantage in the per cent of species on these rocks. Of the 

 five species gain for Fayette in the figures given above, three 

 or four, about 75 per cent, could be expected to occur at Minne- 

 apolis if the substrata were present. We could add as many 

 more species for the possible advantage of Fayette in granitic 

 rocks as substrata and still only have a total difference of seven 

 species resulting from difference in substrata. This would 

 reduce the advantage of Fayette to be accounted for by differ- 

 ence in atmospheric conditions to thirty-seven species or 24 

 per cent. Subtracting this from the total difference of 28 per 

 cent, leaves a doubtful 4 per cent to be accounted for by lack of 

 substrata at Minneapolis. 



It may also be thought that I have not taken into account the 

 usual decrease in number of species in passing from warmer to 

 colder regions. The distance of about 150 miles from south to 

 north between the two localities compared is so small that lit- 

 tle difference in number of species could result, the difference 

 in mean -annual temperature being between 2*^ F. and 3^ F. 

 The smaller number of individuals at Minneapolis also tends to 

 prove that the difference in latitude has not helped to produce 

 the difference in number of species, as the decrease in number 

 of species, caused by colder climate, usually gives place to an 

 increase in number of individuals. If the difference in lichen- 

 floras were due to the above cause, northern species should 

 come in, to some extent, at Minneapolis, to take the place of 

 those found at Fayette, and not at the former place. Parmelia 

 olivacea (L) Ach., Evernia prunastri (L) Ach., Cetraria ciliaris 

 (Ach.) Tuck., and possibly Alectoriajubaia (L) Tuck var. Ghalybe- 

 iformis Ach. are more numerous, and occur on more substrata 

 at Minneapolis as a result of more northern location, but not a 

 species has come in. 



A comparison of the amount of precipitation of moisture at 

 St. Paul and Fayette for the eight years since the record has 

 been kept for the latter place, shows a difference in annual pre- 

 cipitation of 6,77 inches in favor of Fayette. Comparison of 

 St. Paul and Dubuque for twenty- three years shows a differ- 

 ence of 9.31 inches per annum in favor > f Dubuque. Now, com- 

 parison of Fayette and Dubuque for the eight years shows a 

 difference of .50 inches per annum in favor of Fayette. Thus 

 these last two places, only about fifty-five miles apart, show so 



