178 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



different groups and at different periods, both because of inherent 

 differences and of being rarely unimpeded. 



Again, such generalizations as those which seek to give directions 

 for determining the place of origin of a particular plant group are 

 apt to be dangerous, as for example the supposition that the original 

 home of a group is the place in which the largest number of its 

 representatives exist. Thus old groups have frequently survived 

 great alterations of climate and have died out in major regions where 

 they formerly flourished — sometimes, with little doubt, including 

 those in which they originated — and have apparently found secondary 

 centres of diversity in favourable areas where, it may be expected, 

 conditions for evolution are different. And it should be noted that 

 for genera and higher taxa of Mammals, where the fossil record is 

 far more nearly complete than with plants, the evidence is largely 

 contradictory to this hypothesis of diversity indicating the centre 

 of origin. Nor is the newer generalization, that the centre of origin 

 is the area in which the most advanced species are found, any less 

 dangerous — especially with its stated corollary that the most primi- 

 tive species will be those remote from this centre. Indeed, in many 

 groups of plants, the more advanced members differ from the 

 primitive ones in being more effectively specialized for dispersal and 

 more genetically ' open ' for migration, so that they may be expected 

 to overtake their ancestors in colonizing the earth. 



Finally, even the common assumptions of a single (poar or 

 tropical) region of origin and differentiation of the groups of Ihigher 

 plants, and of a simple basis for their migration from one continent 

 to another, are presumptuous in view of the present meagre state 

 of our knowledge and the assertion that the main centres of mam- 

 malian differentiation have been at middle latitudes in the interiors 

 of the large land-masses. For, as we have seen, evolutionary change 

 and migration are fundamental activities that seem to go on practically 

 all the time everywhere, though at very different rates in different 

 instances and places. 



Polyploids and Their Areas 



The plant geographical implications of polyploids (organisms 

 whose body-cell nuclei contain more than two haploid or single 

 chromosome sets) have been so much discussed in recent years that, 

 although most conclusions still remain tentative, the subject needs 

 to be mentioned here. Polyploids are found in most of the major 



