INTRODUCTION 13 
That there is great diversity in the marine coast 
floras of different regions is well known, and a 
comparison of three remote and dissimilar regions 
furnishes an extreme case. It would be difficult to 
select three instances of less geographical relation- 
ship than the Arctic Sea, the West Indies, and 
Australia. The first has 259 species in 111 genera, 
the second 788 species in 150 genera, and the third 
has 1,132 species in 255 genera. The Arctic Sea 
has 42 genera and 30 species in common with the 
West Indies, the same number of genera and 21 
species in common with Australia, while out of the 
two larger totals from the West Indies and Australia 
there are 109 genera and 135 species in common. 
If we take the forms common to all three there are 
32 generic types, but only 12 species out of these 
large totals sufficiently cosmopolitan to occur so 
widely. An analysis of the totals shows that in the 
Arctic regions the genus averages slightly more than 
2 species only, while in the West Indies it is rather 
more than 5,and on Australian coasts rather less. The 
north temperate Atlantic yields an average of about 
44 species to the genus, while the South African 
coast gives us only 3, a result which may be attributed 
to its small coast-line. There is still a great lack of 
material on which to found such comparisons with 
many regions of the ocean, and in the absence of full 
records the making of contrasts is only misleading. 
Enough has been said to show the diversity of such 
floras, the means of distribution, and the principal 
natural barriers that delimit the boundaries of areas. 
A comparison of the Arctic and the Antarctic 
