172 AMERICAN JOURNAL 



immense; to the middle, on the contrary, it is excessively cir- 

 cumscribed. The differential proportions of the middle and 

 northern areas is as 1 to 200. 



" 7. To the north of the centres the acclimatation or radia- 

 tion of the species is effective to the middle of the region. 



"8. The radiation from the Alpine centre embraces nearly 

 all of Europe ; that of the Spanish centre is very feeble ; that 

 of the Taurian centre is nearly none. 



"9. To the middle of the centres, on the contrary, the spe- 

 cies is always localized ; generally it is special. 



" 10. A species is so far special that it has a centre where 

 it inhabits solely, without intermixture with another, when 

 not submitted to maritime influence or accidental acclimata- 

 tion. 



"11. The maritime influence is powerless in the regions to 

 the north of the centres. 



"12. To the middle of the centres, on the contrary, the 

 maritime influence is asserted with such energy with certain 

 species, that these shells have become, for malacologists, the 

 positive evidence of a sea-coast. 



" 13. The species accidentally transported are an exception, 

 nevertheless, to those subject to maritime influence, being 

 acclimated, 1. Those of the north to the middle, and not those 

 of the middle to the north; 2. Those of the Orient to the Oc- 

 cident, and never those of the Occident to the Orient. 



" 14. The conchological population of the great centres of 

 creation is always less numerous (comparatively) than that of 

 insular centres. 



" 15. Every island, however inconsiderable, separated in 

 origin from the Continent, possesses its peculiar species. 



" 16. Every island, anciently united with a Continent, and 

 separated by the sea at a more recent epoch, naturally con- 

 tains the fauna which inhabited it prior to the separation. 



" 17. In the same island are found certain types of form, and 

 a multitude of modifications of these types. 



" 18. The area of insular species is much more restricted 

 than the area of Continental species placed to the middle of 

 the grand creative centres." 



The above interesting extracts, necessarily presented here 

 in a condensed form, present to American Conchologists seve- 

 ral facts (those relating to insular faunas) which are by no 

 means new, Prof. C. B. Adams having discussed them at length 

 in his "Contributions to Conchology." 



The final conclusions of M. Bourguignat, in applying these 

 facts to the Algerian fauna, are that — 



1. At the commencement of the modern creative pericd the 

 north of Africa was nearly an island, joined to Spain. 



