42 
equisetifolius, Pterosiphoma thuyoides and several species 
of Ectocarpus. The influx of these algae does not appear 
to be merely a phase of frequency-variation, controlled 
by physical conditions in any one season, whereby the 
plants may be common in one year and only of occasional 
occurrence in the next ; rather does it represent a steady 
forward movement continued over at least ten years, by 
which the area of distribution is widened and at the 
same time the frequency of occurrence within the area is 
raised. 
The question naturally arises as to why floristic 
variations of this type should occur. Scrutiny of the 
newly-added names shows that the majority of the larger 
forms are plants occurring commonly at points widely 
distributed on the British coasts. The Isle of Man is 
therefore well within their area of distribution. Its insular 
position may have delayed the appearance of these species 
on its coasts ; but as soon as chance immigration occurs, 
it is only a matter of time before the species in question 
become well-established components of the Manx flora. 
Standing as it does midway between north and south 
of the British Isles, the Isle of Man occupies the border-line 
between northern and southern floras, with the result 
that such northern plants as Odonthalia dentata and 
Euthora cristata exist side by side with typical southern 
forms such as Spondylothamnion multifidum. Many of the 
newcomers such as Asferococcus bullosus, Ectocarpus 
luteolus, Nitophyllum uncinatum and Tratllella intricata 
have a definite maximum occurrence on southern coasts. 
These species are therefore on the northern fringe of their 
area of distribution. -Similarly, species such as Sézctyo- 
siphon subarticulatus and S. tortilis belong to higher 
latitudes and have moved southwards. The Isle of Man 
therefore is a meeting ground for migrants from both 
northern and southern floras. The greatest influx of 
