174 Mr. J. G. Jeffreys on the Marine Testacea 



that a uniform temperature once prevailed, accompanied by a 

 general diffusion of all animals over the whole world ; and that 

 owing to successive changes of temperature, induced by the 

 formation or elevation of land in some parts and its submersion 

 under the sea or depression in others, certain species became 

 extinct, or survived, as the climates became colder or warmer 

 in different parts of the globe. Many species (as Dentalium 

 dentalis) occur in the tertiary strata of Great Britain, which, 

 apparently, have ceased to live in our seas, although they 

 still inhabit the Mediterranean; and others (as Mya trun- 

 cata) are recorded by Philippi as occurring in the same form- 

 ation in Sicily, which in like manner have apparently ceased 

 to exist in the Mediterranean, although they still inhabit the 

 British and North Atlantic seas. In each of these cases, the 

 species are conspicuous and abundant in their respective locali- 

 ties. The upper and lower tertiary formations, both in Great 

 Britain and Sicily, as well as in the Subapennine district, con- 

 tain also many species v/hich now inhabit only arctic or tropical 

 climates; and they are found associated with other species 

 which now live in the British and Mediterranean seas. Besides 

 the vast extent of coast and ground in these seas, which has 

 never yet been explored by naturalists, as well as of the tertiary 

 strata in Italy and the North of Europe (where a few scratch- 

 ings here and there, rather than a systematic examination of 

 their contents, are all that appears to have been hitherto at- 

 tempted), it must not be forgotten that the whole of the North 

 Atlantic and Mediterranean seas, together with the area of dry 

 land in which the tertiary formations in Europe have been 

 traced, form after all but a small portion of our globe. 



The above remarks must be understood to apply rather to 

 the extent and mode of geographical distribution than to the 

 existence of special faunas. I do not deny that certain species 

 may, and probably do, occur only in limited areas. The ques- 

 tion as to the distribution of land animals and plants seems to 

 me to involve other considerations than those which refer to 

 marine animals, namely the influence of climate and winds, as 

 well as (in the case of plants) the dormant vitality of seeds. 



Too much care cannot be taken in the formation of materials 

 for extending our knowledge of geographical distribution. The 

 most accurate discrimination of species and varieties is indis- 

 pensable; as well as the collection of specimens ah ovo, and 

 from as many localities as possible. The young of the acepha- 

 lous Mollusks, or Bivalves, always exhibit, after undergoing their 

 metamorphosis, the peculiar character of the species to which 

 they belong ; but in the Gasteropoda, or Univalves, the case is 

 often different, owing to the first whorls of their shell being 



