6] FOUNDATIONS OF MODERN DISTRIBUTIONS 177 



parental lines. Disturbance of the habitat is known to favour 

 hybridization, and when such disturbance is so common and wide- 

 spread as in the Arctic, with the frequent frost-heaving of open 

 habitats where there is little competition, we may expect ready gene- 

 flow — hence perhaps in part the notorious ' plasticity ' of arctic 

 plants. For related populations can be lastinglv sympatric (that is, 

 coexist in the same territory) only if they are reproductively isolated. 

 Length of life may be an important factor controlling hybridization 

 and introgression, especially where uniformity is maintained by a 

 preponderance of vegetative propagation, with consequent restriction 

 of gene-flow and limitation of genetical recombination. In such 

 circumstances, the ill-adapted oflFspring tend to be easily eliminated 

 by competition. On the other hand, with free gene-flow and a fair 

 amount of mutation in rapidly succeeding generations, much new 

 ' raw material ' is provided for natural selection to work upon, and 

 evolution may be expected to proceed with some dispatch. 



The distribution patterns of organisms, like the external appear- 

 ance and genetic constitution of the component individuals them- 

 selves, are the end result of the interaction of evolutionary processes 

 and climatic, pedological, and other changes over long periods of 

 time. Now similarity of distribution patterns suggests similarity of 

 general background including evolutionary history, and this may 

 give some possibility of divining the history of an organism well 

 represented in the fossil record but unknown genetically, provided 

 we have others of comparable distribution that are well known 

 in this last respect. Such elucidations, and indeed the broader 

 ones of plant geography, must, however, be indulged in only 

 on the basis of all known facts, and then only with the utmost 

 caution. 



In the light of modern knowledge some old assumptions should 

 be discarded or at least greatly modified — for example, that the 

 diversity of a group is dependent upon its age, which may be deter- 

 mined, at all events relatively, by counting the number of members 

 now living, and that the age of a species or other taxon is directly 

 related to the size of its area of distribution. Such are the main 

 tenets of the hvpothesis of ' Age and Area', discussed further on 

 pp. 182-3, 209. And although ideally there may be some basic truth 

 in these assumptions, some aggregate responsible effect in that diver- 

 sity may come with time and increase colonization potentialities, even 

 as time itself may increase the chances of dispersal, in actual fact 

 evolution and migration have proceeded at very different rates in 



