THE HUMAN OR PRESENT PERIOD. 533 



to have been quite successful in pushing the more boreal and arboreous 

 forms to the northward, or in forcing them to ascend the mountains; 

 but the movement was less sweeping and more complicated than that 

 of the east, because of topographic interference and the restraints of 

 the lingering mountain glaciation. 



In this re-dispersion of the North American faunas and floras there 

 is a world of suggestive detail of which only a small part has been 

 worked out into clear definition. From the viewpoint of investiga- 

 tion, it is a rich and almost virgin soil, forming the turn-row, as it 

 were, between the more cultivated fields of the geologic and biologic 

 sciences. 



The rate of re-distribution. — Most of the plants were so well pro- 

 vided with means of dispersion by winds, birds, or other agencies, 

 that they doubtless followed the retreat of the ice nearly as fast as 

 climatic conditions permitted, and the abandoned ground was thus 

 promptly clothed with such vegetation. But certain forms were not 

 provided with these devices, and their relatively slow rates of migra- 

 tion furnish an independent mode of estimating the time since the 

 ice began to retreat. That which we have really to estimate is not 

 the least time in which given plants could migrate the required dis- 

 tances, but the time normally occupied in the migration of an asso- 

 ciated group of plants, or a plant-society, some of which were slow 

 migrants; for the plants are now grouped according to what seem to 

 be their natural relations. They are not sporadically mixed as if 

 they were in process of individual migration independently, each at 

 its own pace. This group-migration is, however, difficult to deal 

 with, and cannot here be discussed. To illustrate studies on indi- 

 vidual migration, the walnut family is one of the most suitable, for 

 walnuts and butternuts are so unwieldy as to be habitually carried 

 but limited distances and buried by nut-eating animals, while the 

 bitter hickory-nuts (pig-nuts) can scarcely be presumed to have been 

 purposely transported and planted by the aborigines. There is little 

 reason in any case to think that transplantation was practiced by the 

 pre-Caucasian peoples of the eastern wooded regions, or that acci- 

 dental transportation by them was an appreciable factor in the dis- 

 persal of the plants, for if it had been, the plant-grouping should betray 

 it. But the distribution of the edible hickory-nuts is not, so far as 

 we can learn, more extensive than that of the inedible species, and 



