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