hearing on Geology and Zodgeograj^Jiy . 67 



II. Pelagic life-district. 



1. Arctic regioD. 2. Indo-Pacific region. 3. Atlantic 

 region. 4. Antarctic region. 



III. Abyssal life-district. 



ISTo regions distinguishable. 



It is very remarkable that it is impossible to distinguish any 

 regions in the abyssal district. As this whole district is char- 

 acterized by a low and very nearly constant temperature, no 

 climatic zones can be distinguished, and further, as this whole 

 district is continuous throughout its extension over the bottom 

 of the oceans, no topographical divisions are present. There- 

 fore we cannot establish any zoogeographical regions within 

 this district, which agrees with the well-known fact that most 

 of the true abyssal animals have a world-wide distribution. 



But even in the Tertiary period the limits of the zoogeo- 

 graphical regions of the littoral and the pelagic districts were 

 often very different from those of the recent time, because the 

 topographical featui-es of the seas were different. As the most 

 important difference should be regarded the continuity of the 

 circumtropical belt maintained during a great part of the 

 Tertiary. This continuity was due to the well-known fact, 

 that the connection of North and South America was not yet 

 formed, and, perhaps, that there was present a communication 

 of the southern European seas with the Indian Ocean. The 

 separation of the Mediterranean Sea from the Red Sea and 

 the Indian Ocean, and its connection with the Atlantic Ocean, 

 and especially the separation of the Pacific Ocean from the 

 West Indian seas by the rising of the Isthmus of Panama, 

 were the last steps accomplishing the development of the 

 recent conditions, and the latter are, geologically speaking, 

 of a very recent date : they were not formed until the second 

 half of the Tertiary period. 



It is very interesting, that the present geographical distribu- 

 tion of many animals points often and clearly to the former 

 conditions existing a comparatively short time ago, and the geo- 

 graphical distribution, if regarded under the principle, that 

 species are formed by separation, may give us some hints as 

 respects the former geological and geographical conditions 

 It may be well here to indicate some principles worthy of 

 notice.* 



* I have myself endeavored to apply the following principles in a series of 

 monographs discussing the distribution of smaller groups of Decapod- Crusta- 

 ceans. As regards the influence of external conditions, climate, facies, etc., upon 

 the distribution of species in a particular locality, I should like to refer to a most 

 valuable work published by Petersen (Det Yidenskabelige Udbytte af Kanon- 

 baaden " Hauchs " Togter, 1893, p. 433-464). It would be very advantageous for 

 science if investigations of this kind were made more extensively. 



