486 GEOLOGY. 



were at least as extensively peopled by plants and animals as they 

 arc to-day. This carries the conclusion that the migratory swing in 

 these more pronounced cases was at least 2000 miles in North America, 

 and more than 1000 miles in Europe. As indicated in the physical 

 description, the geological evidences drawn from erosion, weathering, 

 and organic accumulation warrant the belief that the interglacial 

 intervals were long enough to permit a complete northern return, 

 and the fossil evidence supports the conclusion that the climates were 

 congenial enough to invite it. 



The forced migrations must, in their nature, have been peculiarly 

 effective in bringing to bear a severe struggle for existence, and in call- 

 ing into play the full resources of the plastic adaptation of the life. 

 Forms previously specialized to meet local conditions were put to a 

 most adverse test, for the invading ice forced every form within the 

 glaciated area to move on, while the fringing zones of depressed tem- 

 perature encircling each ice-sheet, forced plant and animal life, even 

 beyond the ice border, to seek new fields and new relations, both phys- 

 ical and organic. An incidental result of this wholesale migration 

 was an unwonted commingling of plants and animals, for every aggres- 

 sive form pushed forward in the van of the advancing zone, and hence 

 came into new organic environment, while every laggard form fell 

 behind, and was overtaken by the less reluctant migrants. 



Definite climatic zones. — From the nature of the case, and from 

 the evidence, it appears that not only must sharply defined climatic 

 zones have surrounded the invading ice-sheets, but that these must 

 have been much more strongly distinguished from one another in 

 temperature than had previously been the case since the Permian times. 

 As these diverse zones were alternately pushed forward and withdrawn 

 by the advances and retreats of the ice, every organism was forced 

 by a special stress, either to adapt itself to a new zone, to migrate, 

 or to suffer extinction. 



Climatic adaptations. — Two or three notable results appear to 

 have followed. Certain forms became more highly adapted to special 

 climatic zones than they had been previously. It has been remarked 

 before that the floras of the middle Tertiary were highly mixed, judged 

 by the present climatic adaptations of the species. Types which we 

 now regard as tropical were living in high latitudes, commingled with 

 forms which are now boreal. So also forms that are now boreal were 



