296 A NATURALIST IN THE GREAT LAKES REGION 



bog society. This and the pine association, found in our region 

 as more or less isolated islands in the midst of the prevalent 

 oak-hickory society, are, farther north, the prevalent thing. 

 The sphagnum bogs cover wide areas and have their maximum 

 development in a region extending from north of the Gulf of 

 St. Lawrence to northern New England in the East, thence 

 west, spreading from mid-Lake Michigan shores to Hudson 

 Bay and then northwest into Saskatchewan. There can be no 

 doubt that this bog society like the pine association invaded 

 our region in advance of the glacier and followed it back in its 

 retreat, leaving the isolated patches in favorable localities to 

 continue to the present, defying the encroachments of the plants 

 and animals that later came to occupy most of the region. There 

 is therefore no gradual transition from the sphagnum-tamarack 

 bog society to the surrounding societies, but an abrupt break 

 between the two, for this northern invader is surrounded by 

 still later arrivals with which it has no genetic continuity. In 

 the north it leads to the coniferous forest, but here the conif- 

 erous forest itself is at best struggling to maintain a none too 

 secure footing. The coniferous forest farther north leads on to 

 the hardwood climax forest, so that the transition from the pine 

 association here to the black oak association is a relationship 

 that is not strained. There seems to be some evidence, as at 

 Cedar Lake, that the tamarack bog does still develop in this 

 region. Young tamaracks are appearing in the sphagnum bog 

 there and are pushing farther out into it. 



How these northern elements came into our fauna and flora 

 is quite evident. The glacier forced them south and the rising 

 temperature in the south as the glacier retreated forced them to 

 migrate north again or suffer extinction, except as relics survive 

 in limited favorable localities. But what has forced the invasion 

 into our region, since the glacier left it largely bare, of forms 

 from the deciduous forest center in the southeastern part of the 

 continent, the prairie and desert regions of the Southwest, or 

 the Atlantic Seaboard ? 



