14 PROCEEDINGS OF THE 



behaviour, but a combination o£ causes may be acting, though in 

 that case each cause must act mechanically on all alike. 



The cause which seems the determining factor in dispersal is 

 (Kje witiiin the country concerned. Tais opinion was based upon 

 the results of his work on the Ceylon flora, which showed that 

 the endemic species (presumably the youngest) occupied much the 

 smallest areas, those common to Ceylon and South India (next 

 youngest) areas considerably larger, and those of wider dispersal 

 than this (the oldest) are as yet larger again, while at the same 

 time each group showed a graduated series of plants occupying 

 each size of area, the endemics varying down the scale from 

 90 to 233, the " wides " up from 144 to 402. 



Not only do the grand totals show this distribution according 

 to age, but also family by family, and genus by genus (of 

 I'easonable size) do so. The rarity of all the endemics (in tigures 

 from 1 to 6) is 4-3, and when taken in groups of not fewer than 

 14 it only varies from 3"9 to 4-9, while that of the other two 

 groups varies similarly about 3"5 and 3-0. 



Some having objected that Ceylon is a special case, he obtained 

 confirmation of his views by working out the flora of New 

 Zealand. To test his hypothesis, he wrote the paper first pre- 

 dicting what, under that hypothesis, must be expected, and as 

 all his predictions were confirmed by the .facts, the result has 

 given him considerable confidence in the truth of the hypothesis. 



One does not often come across cases like Ceylon, where the 

 local species can be divided into groups according to age, and 

 confirmation of his hypothesis must rest on finding cases to 

 parallel one or more of the features which showed so con- 

 spicuously in the Ceylon flora. New Zealand parallels it in 

 several respects, and other cases are quoted in which similar 

 parallelism is exhibited. 



Some of the objections to these views have been considered, 

 e. <j. the hackneyed argument that introdticed species spread 

 rapidly over islands at the expense of the indigenous flora ; this 

 is shown by the cases of Ceylon and Rio de Janeiro to be an 

 unsound position. The objection that the endemic species are 

 the oldest in a country is also dealt with, likewise that which 

 asks why one does not see the spreading going on, if it depeud 

 upon age. 



A discussion followed, Dr. A. B. Eendle, Dr. D. II. Scott, 

 Dr. 0. Stapf, and the President taking part, the author replying. 



The following papers were, in the absence of their authors, 

 read in title : — 



Mr. R. J. TiLLYAUD, M.A., B.Sc, F.L.S.— A Study of the 

 Rectal Breathing Apparatus in the Larva) of Aniso- 

 pterid Dragonflies. 



]\Ir. AValteii E. Collixge, M.Sc, F.L.S. — Description of a 

 new species of Idol^a (Isopoda) from the Sea of Marmora. 



