138 AGE AND AREA FROM A [pt. ii 



of plants at different periods, not only so as to gain a knoAvledge 

 of the species occupying the country at successive times in the 

 Pleistocene, but so as to gain also some knowledge of the pro- 

 portion in which those species flourished (60). 



Or, again, we may take in our own country the succession 

 seen in our eastern counties. In the Cromerian (83) at the close 

 of the Pliocene period, Ave find a temperate flora almost identical 

 with that now inhabiting East Anglia. At a later period we find 

 a flora composed of plants now inhabiting colder regions — sub- 

 arctic, alpine, or cold temperate (15, 72, 82). Yet again, in the 

 present day, after a further interval, when the climate has once 

 more become temperate, we find the old temperate flora of the 

 Cromerian back in its former locality, shorn only of a few of its 

 elements. 



The instances of such successions could be multiplied, but the 

 above are sufficient to show that we haAc definite evidence of a 

 continual swaying to and fro of plant-life. 



Evidence of this kind can scarcely be interpreted otherwise 

 than as indicating the movement of plant assemblages, imder 

 the influence of climatic change ; in other words, migration. 



But if migration has occurred, how has it been brought about? 

 The answer is suggested by Dr Willis' theory of Age and Area, 

 though the idea of plant movement embodied in it would seem 

 to need some modification. Dr Willis suggests, as a result of his 

 work, that newly arrived, or newly formed, species tend to 

 spread outwards in all directions from their point of arrival, or 

 point of origin, like rings formed by casting a stone into a pool. 

 In such a tendency Ave see a motive force; but migration is a 

 directed movement, and the combined CA'idence of geology and 

 palaeobotany indicates that the directing force is change of 

 climate. Each species flourishes best under definite climatic 

 conditions, Avithin limits appropriate to itself. Change of climate, 

 acting ecologically, Avorks as a Aveeding process, so that move- 

 ment, instead of being general all round, becomes a moA^ement in 

 one definite direction — migration. 



From A'arious considerations of geology, palaeontology, fossil 

 and recent botany, the conclusion has been reached that if 

 change of climate has been from cold to heat, in a flat country 

 migration has been poleAvards, in a mountain country upAvards. 

 If the change has been from heat to cold, then in a flat country 

 migration has been equatorAvards, in a mountain country down- 

 wards. 



