ECOLOGY OF NORTHERN MICHIGAN. 27 



Another change is the occurrence, on the upper part of the slope, of the 

 Red-bellied Snake, Storeria occipitomaculata, which prefers dry, rather open 

 woods. With the coming in of the deciduous forest, the humus becomes 

 composed of leaves, and with this change in the nature of the humus is 

 associated an increase, at least in the number of individuals, of molluscs. 

 When the fallen leaves are examined, they are found to be connected by a 

 film of water. Among these leaves seemed to be the favorite habitat of all 

 the snails collected in these woods. The forms collected are Zonitoides exigua 

 Zonitoides arborea, Vitrea ferrea, Euconulus chersinus polygyratus, Helico- 

 discus lineatus, Punctum pygmaeum, Cochlicopa lubrica morseana, Sphyra- 

 dium edentulum, Pyramidula striatella catskillensis, Strobilops virgo, Polygyra 

 albolabris, Vitrea indentata and multidentata. 



It has been suggested (Whitford, '01, p. 301) that the presence of isolated 

 W'hite Pines in the mesophytic forest indicates a former dominance of xero- 

 phytic forms. This at once suggests an explanation for the origin of the 

 present biota that is in accordance with the observed facts. When the 

 waters of the glacial lakes receded and left the mountains exposed to denuda- 

 tion, the wash on this slope must have been considerable. The vegetation 

 that first gained a foothold was probably a society analogous to that on 

 the Middle Beach at the present time. These forms would be followed, 

 as the conditions became favorable, by the heath society of the Upper Beach. 

 At the bottom of the slope where the soil was thicker and the superficial 

 wash less, the heaths were probably soon succeeded by the conifers that 

 usually follow the heaths in this region. With the accumulation of humus 

 in the coniferous forest, the superficial wash would become less, while the 

 soil formed on the higher parts of the ridge would be held in front of the 

 forest, permitting the coniferous types to ascend the slope and restrict the 

 area occupied by the heaths. 



As the coniferous forest became well developed at the bottom of the 

 slope, owing to the more favorable edaphic conditions brought about by the 

 accumulation of humus, the pine seedlings which require considerable light 

 would no longer be able to develop. The seedlings of the deciduous trees, 

 Maple, Bass wood, Hemlock, etc., which would find a favorable habitat 

 in the increased shade and humus of this forest, would then invade the 

 coniferous forest as underbrush, so that, as the pines died off, they would 

 be replaced by the forms of the present forest. 



The fauna of this slope would have a history similar in many respects to 

 that of the flora. If the slope was first invaded by the heath plants, it was 

 probably at the same time invaded by the fauna that is associated with this 

 society; a relation which would also hold in the case of the succeeding societies. 



The conditions to which the forms in the different societies are adapted, 

 or the habitats, would thus tend to migrate up the slope from the point 

 of invasion, and the mountain top would be the scene of the extinction of 

 the pioneer societies, the last place where they would be foufid. Since the 

 mountains were islands for some time after the retreat of the ice sheet, the 

 level at which this invasion of life took place presents an interesting question. 

 Evidently it must either have taken place above one of the old beach lines 

 while the mountains were yet islands, or after they had been joined to the 

 main land by the subsidence of the lake. In the latter case, the invasion 

 would have come in near the base of the mountains, as they are entirely 

 ., surrounded by a low plain, but in either case the succession of societies 

 would have been much the same, with the exception that in the former 



