BOTANY. 551 



dykes and bosses of analcite-nepheline-basalt, which are beUeved 

 to be due to a reawakening of the focus in Late Carboniferous or 

 Permian times. 



The classification and nomenclature of igneous rocks has 

 been recently discussed in the following papers : 



L. V. PiRSSON, The Classification of Igneous Rocks — A Study for Students, 



Anter. Journ. Sci. (v), 2, 1921, 265-84. 

 DixEY, F., The Magnesian Group of Igneous Rocks, Geo!. Mag., 1921, 485-93. 

 Tyrrell, G. W., Some Points in Petrographic Nomenclature, ibid., 494-502. 

 Hackmann, v.. Some Criticisms of Idding's Classification of Igneous Rocks, 



Bull. Comm. Geol. Finlande, 53, 1920. 



BOTANY. ByE. J. Salisbury, D.Sc, F.L.S., University College, London. 



Plant Distribution. — Willis, in 19 19 {Ann. Bot., July), restated 

 his " age and area " hypothesis in the following terms : " The 

 area occupied at any given time, in any given country, by any 

 group of allied species, at least ten in number, depends chiefly, 

 so long as conditions remain reasonably constant, upon the 

 ages of the species of that group in that country. ..." The 

 positive action of age and the profoundly modifying action of 

 geographical, climatic, and ecological barriers, which exert a 

 negative influence, were at the same time emphasised. 



Later, Willis dealt with the plant invasions of New Zealand 

 from this point of view {Ann. Bot., October 1920), and it was 

 shown that the present distribution of the genera, when 

 classified according to the density of species in the different 

 parts of New Zealand, is indicative of several invasions by 

 land, viz. a western, probably by the Lord Howe Island 

 ridge ; a Kermadec ; a northern and a southern. The northern 

 invaders are represented by genera of Proteacece, Lauracece, 

 Myrtacece, etc. ; the southern by Cardamine, Veronica ; and 

 the western by Clematis, Senecio, etc. The Kermadecs, Norfolk, 

 and Lord Howe Islands contain a number of endemics which 

 would naturally be expected to be more numerous in the early 

 arrivals than in genera whose arrival was comparatively recent. 

 It is, therefore, very significant from the standpoint of " age 

 and area " that most of these endemic species belong to genera 

 which have reached New Zealand. 



Quite recently Willis has given a summary of his views 

 {Ann. Bot., October), and incidentally offers several warnings 

 with respect to their applications. He regards plant distri- 

 bution in the present era, with respect to its broad outline, as 

 governed mainly by the time factor, whilst the detailed dis- 

 tribution is chiefly dependent on ecological factors. 



The subject of endemics is here dealt with, and these are 

 shown for the Pacific Islands to be most numerous in the 

 larger, and hence presumably older, genera ; also in islands 



