DISTRIBUTION OF MARINE ALG.E. 177 



partly from Agardh's more just appreciation of the value 

 of specific forms, notably in the Helminthocladiaceas which 

 contribute six of the excess of ten to be found in Sender. 

 Under Chlorophyceae and Protophycese there are in the 

 present estimate excesses of three and two respectively 

 arising from my own judgment in framing the former 

 lower estimate. It was necessary in constructing the 

 former tables to use one plan, one estimate of specific 

 value (the Agardhian) for the whole world flora and so 

 Sonder's catalogue was not then taken bodily. However 

 with these differences, amounting to nineteen only in the 

 aggregate of the entire flora in question, we need not con- 

 cern ourselves farther than to explain their origin. 



Let us first look at the aggregate totals that we may 

 form some idea of the extent of the floras we are dealing 

 with. The arctic marine flora consists of 259 species 

 contained in 111 genera ; the West Indian of 788 species 

 in 150 genera ; and the Australian of 1132 species in 255 

 genera, the last owing its preponderance to its embracing 

 subtropical and south temperate types. The most note- 

 worthy observation on these totals is the proportions of 

 genera to species. In the arctic regions the genus 

 averages slightly more than two species only, while in the 

 West Indies it averages rather more than five, and in 

 Australia rather less than five. These proportions be- 

 come more instructive when we trace them through the 

 tables showing the numbers which the different regions 

 have in common. The Arctic Sea and the West Indies 

 have 42 genera and 30 species in common ; the West 

 Indies and Australia have 109 genera and 135 species 

 in common ; the Arctic Sea and Australia have 42 

 genera and 21 species in common ; while 32 genera occur 

 in all three regions and only 12 species. It will be seen 

 that exactly the same number of genera are in common 



