RELIC COLONIES 



55 



it seems that the temperature at the time of maximum ice advance 

 was high and probably not unlike that of the present time. The imma- 

 ture development of the drainage systems over the Wisconsin drift, 

 particularly toward the west, may also be due not merely to youth 

 but also to an early postglacial climate drier than that of the pres- 

 ent day. 



If a considerable number of species are now confined to an area 

 immediately beyond a moraine and seldom or never occur within it, 

 their distribution may be explained in several ways. First and most 

 obviously, present conditions of soil and climate may exclude them. 

 But if the general vegetation and climate of the two regions are similar 

 and if habitats apparently identical, as judged by the vegetation, exist 

 on both sides of the moraine, this explanation is less plausible. Sec- 

 ondly, their migration up to the present time may have extended 

 merely to the moraine, which they will later cross. But it is difficult 

 to understand why the boundary of the range of numerous species 

 should be so nicely adjusted to the moraine. The third explanation, 

 which is here accepted, is that they occupied this area before or during 

 the formation of the moraine, have occupied it ever since, and are not 

 at present extending their range. 



Relic Colonies. — Lastly, and most important of all, past migra- 

 tions may be judged by relic colonies. Unfortunately, they can be used 

 only for the more recent movements, since the relics of earlier migra- 

 tions have been completely destroyed. Many such colonies still exist, 

 isolated by recent migrations, or did exist long enough to be recorded 

 before being sacrificed to agriculture, and afford the most valuable 

 evidence of the past range of the floras which they represent. They 

 even give valuable clues to migrations of still earlier date, during 

 which the flora of the colonies entered the region, and therefore show 

 at once an early advance followed by a later retreat. 



Although the probability of extreme migrations, except in the case 

 of very mobile species, is very slight, the time available is very long 

 and has certainly been sufficient for the improbable to happen many 

 times. Therefore, in general, isolated stations of a single mobile species 

 shed little light on its past history. But isolated colonies of several 

 or many species, provided with diverse methods of dispersal, represent 

 a different condition. It is entirely beyond the limits of probability 

 or coincidence to presume that a dozen or more species, normally grow- 

 ing together in one section of the country, should also be found together 

 in another remote section beyond the normal limits of dispersal of any 

 of them. Isolated colonies of this character are common phenomena 

 in the Middle West and can have but one significance, that at some time 

 in the past these colonies were more numerous and separated by narrow 



