Means of Dispersal. 25 



single seed accidentally transported from some distant 

 region. The British flora is full of anomalies of this sort. 

 I may also point out as a geologist that sufficient time 

 cannot be allowed for this method of spreading, even on 

 the unwarrantable supposition that our plants could find a 

 continuous belt of suitable country all the way from 

 Central Europe, or whatever country they were obliged to 

 take refuge in during the Glacial Epoch, to the furthest 

 point they have now reached. Though the Postglacial 

 period counts its thousands of years, it was not indefinitely 

 long, and few plants that merely scatter their seed could 

 advance more than a yard in a year ; for, though the seed 

 might be thrown further, it would be several seasons before 

 an oak, for instance, would be sufficiently grown to form a 

 fresh starting point. The oak, to gain its present most 

 northerly position in North Britain after being driven out 

 by the cold, probably had to travel fully six hundred miles, 

 and this without external aid would take something like a 

 million years. I doubt whether anything like this time 

 has elapsed since the Arctic flora occupied the lowlands of 

 the south of England and the reindeer inhabited Central 

 France. 



Most of our plants have special adaptations for dispersal 

 over long distances, and, as the different modes of trans- 

 portation must necessarily lead to different geographical 

 distributions in different orders, a classification of plants 

 and animals founded solely on method of migration ought 

 to throw much light on some obscure problem in geo- 

 graphical distribution. I am afraid, however, that at 

 present we have not sufficient direct evidence and can only 

 speak in a general way of these facilities; though new 

 observations are made from day to day, and Darwin 

 collected a large body of evidence on this subject.* The 

 * Orison of Species^ 6th edition, pp. 323-330. 



