EVENING DISCOURSES. 647 



Such a contrivance prevents exposure to cold during, or risk of storm preventing, 

 gradual decompression in the ordinary way by the diver climbing the shotted ropo. 

 In the carrying out of these researches, and in the demonstrations given this 

 evening, I am greatly indebted to Messrs. Siebe, Gorman & Co. for their help. 



MONDAY, SEPT EM BEE 4. 



Links with the Past in the Plant World. 

 By Professor A. C. Seward, F.R.S. 



The primary object of this discourse was to call attention to the existence in 

 the Moras of different parts of the world of survivals from the past; incident- 

 ally, other topics were referred to which were suggested by a consideration of the 

 records of the rocks. 



1. The British Flora. — There are many problems connected with the origin 

 and manner of introduction of British flowering plants, which afford scope for 

 speculation and research. The occurrence of Mediterranean plants in Ireland 

 and in the smith-west of England is a case in point. Are such genera as Arbutus 

 and Dabcecia survivals from the Tertiary period, or have they been introduced by 

 natural agency since the Glacial period'; 



2. Ferns. — Recent ferns supply interesting examples of species with a wide 

 geographical distribution, and of species or genera confined to a small area. The 

 study of fossil ferns enables us to demonstrate in certain cases the great 

 antiquity of some of those genera which are now characterised by a very restricted 

 geographical range. The Osmundaeeae, the genera Matonia, Dipteris, and 

 Gleichehenia are selected as illustrations of the bearing of pakeobotanical data 

 on the history of existing ferns. 



3. Conifers. — The relative antiquity and past distribution of the Conifers 

 are questions beset with numerous difficulties, but we have satisfactory evidence 

 in regard to the former abundance and wide distribution of such genera as 

 Sequoia and Araucaria. The mammoth trees and redwoods of California and 

 the two members of the Araucaria family, Araucaria and Agathis, represent 

 survivals from the past which were formerly abundant in many parts of the 

 northern hemisphere where they have long ceased to exist. 



4. The Maidenhair-tree.- — One of the most striking examples of a link with 

 the past is afforded by the Maidenhair-tree of the Far East, Ginkgo biloba (L.). 

 This tree is no longer known in an absolutely wild state ; it is frequently found 

 in China and Japan in the groves of temples, and is now common in cultivation 

 in Europe and America. Ginkgo and allied genera are recorded from Mesozoic 

 and Tertiary strata in nearly all parts of the world, and fossils usually assigned 

 to the Ginkgoales are not uncommon in Palaeozoic rocks. 



5. Geographical Distribution, &c. — Researches into the floras of the past, 

 more particularly those of the Mesozoic period, have thrown considerable light 

 on changes which have taken place in the geographical distribution of certain 

 families and genera of seed-bearing plants. 



The Jurassic floras of the world exhibit a remarkable uniformity in their 

 general composition as contrasted with the diversity of existing floras. The 

 study of the plant-geography of former ages is well worthy of attention, not only 

 from the point of view of the relative antiquity of plants but in connection 

 with the capabilities of plants as colonisers. The late appearance of the flower- 

 ing plants, the dominant group in the plant-kingdom, is a fact of great import- 

 ance in relation to comparisons between the plant-geography of the Jurassic era 

 and the present distribution of vegetation. 



Questions connected with the study of plants as ' thermometers of the ages,' 

 though presenting many difficulties, are of considerable interest and importance, 

 and deserve closer investigation than they have hitherto received. 



