394 



EVOLUTION 



had even become established. A similar instance is furnished 

 by the occurrence of the same Arctic species on the tops of many 

 high mountains, although the present-day climate of the plains 

 offers an insurmountable barrier to their dispersal from one 

 chain to the other. These plants may, however, well represent 

 the last remnants of a widespread flora of glacial times which, 

 when the ice-sheet melted, found suitable conditions only on 

 the mountain summits. 



The conception of evolution regards the organic world, as we 

 find it to-day, as consisting not of a number of immutable forms, 

 but as presenting one phase in an everchanging series. The 



Fig. 228. — Outline map of the world, showing the distribution of Caly- 

 canthaceae (indicated by shading), Proteaces (black), and Resedacea; 

 (dotted). 



organisms of the present are the offspring of those of the past, 

 and will themselves, in turn, give rise to the organisms of the 

 future. Those animals or plants which have become extinct 

 must be supposed to have failed to " make good " in the com- 

 petitive struggle. From the fossil records it is known that 

 whole floras and faunas have thus perished, leaving no living 

 representatives or only much modified descendants (cf. Chapters 

 XXI, XXII). Such disappearance may well be an outcome of 

 the secular, but none the less profound, changes that have 

 marked the history of the earth's surface since life first appeared. 

 Organisms, unsuited to the new conditions, would inevitably 



