A TANGLED SKEIN 179 
bygone land surfaces in distant times and successfully 
survived the terrors of the Glacial Period; others 
claiming a much less remote period for their immigra- 
tion. Indeed, one eminent recent writer on the sub- 
ject, the late Clement Reid, considered that the Lusi- 
tanian plants are among the most recent arrivals in 
the country, their introduction being due mainly to 
birds driven by exceptional gales. 
The question of the Lusitanian and American ele- 
ments in our flora has been treated at some length 
both because it offers one of the most interesting 
problems in British botany, and because it affords a 
good illustration of the far-reaching nature of the 
questions which may lie behind the occurrence on 
our hills or in our valleys of even the humblest plant 
or animal. Each organism has a long record behind 
it, stretching far beyond the earliest periods of human 
history; and it is only by wide and patient study that 
we can hope to trace any portion of its story. 
