362 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [March 8, 



of species are both regulated by determinate laws, so are, doubt- 

 less, the apparently inexplicable associations of persistent, variable, 

 nascent, and moribund species. 



It would appear that every instance of wide dispersion of species 

 in former ages is equalled, both as regards the longitude and the 

 latitude, at the present day. From the Lower Silurian to Tertiary 

 times, identical species have existed over vast areas of every formal 

 formation — formal ; for although the succession of forms has never 

 ceased, still the science of geology requires some starting-points for 

 its nomenclature, and they are either determined by apparent breaks* 

 or more artificially by the culmination of the abundance of certain 

 living forms. The diffusion of Arctic and Alpine species during the 

 glacial period is considered in one of the most philosophical chapters 

 of Darwin's work ; and it testifies to the identity of species over vast 

 areas of latitude and longitude with or without gaps. And modern 

 investigation, the careful collection and examination of specimens, has 

 determined that whilst some species are restricted in their areas, 

 others have an enormous range ; and these belong to many famihes. 

 By considering the enormous range, with or without intermediate 

 gaps, of many freshwater forms of Arctic and Alpine species of 

 plants ; of temperate ferns ; of corals — from the Eed Sea to the re- 

 motest islands of Oceania, or from the Levant to Zealand ; of Bryozoa 

 and Algae — species being common to the European and Australian 

 seas ; and, lastly, of Echinodermata — some having a habitat of 50° of 

 latitude t, of 30° of latitude, and 80° of longitude f, and others being 

 found on the eastern coast of Africa and the GallapagosJ, at Gua- 

 daloupe and Port Essington§, at Labrador and Kamtschatka|| respec- 

 tively, — by these considerations it may be decided reasonably that 

 there is no restriction to the area of the difiiision of persistent or 

 identical species. The idea of time can only be brought to bear by 

 noticing the rapid diffusion of species artificially introduced into 

 oceanic islands or remote continents ; but it is certain that some 

 widely ranging species have had the opportunity for diffusion and 

 variation from the Midtertiary age to the present day. 



Amongst the identical species from South-eastern Arabia there is 

 one, the Pygaster truncatus, Agass., which is a very persistent and 

 non-varying form. It is interesting that this species should be the 

 last known of the important Oolitic genus Pygaster, that it should de- 

 part somewhat from the generic attributes %, and that, being a rare 

 fossil in the French Cenomanien, it should not have appeared to vary 

 in the distant Arabian Chalk. Another species, the Holectypus Ceno- 



* See Prof. Eamsay's Pres. Add. Geol. Soc. 1863-64. 



t Echinodiscus inaurita, Van Phelsara, sp., Eed Sea, Zanzibar, Mauritius; 

 Echinodiscus bifora, Lamk., sp., Madagascar, Red Sea, Indian Ocean ; Bendraster 

 excenfricus, Eschscholtz, sp., Kamtschatka, California. 



I Encope tetrapora. 



§ Echinoneus cyclostomus, Leske. 



II Echinarachnius parma, Gray. For these localities, see Synopsis of Echin. 

 Irreg. Brit. Mus. 



^ MM. Cotteau et Triger {op. cit.) have some excellent remarks on this species, 

 p. 117. 



