Apeil 3, 1896.] 



SCIENCE. 



515 



whom he obtained information.* This is tlie 

 more regrettable because the similarity between 

 Dr. Ortmann's conclusions and my own is more 

 manifest than that between his and any other 

 investigator's. 



The differences between Dr. Ortmann and 

 myself chiefly result from our different modes 

 ofapproaching the subject. Dr. Ortmann prefers 

 the deductive mode and teaches that ' ' we are to 

 disregard each definite group of animals, and 

 to investigate only the physical conditions in- 

 fluencing the distribution."! I prefer the in- 

 ductive mode and have been influenced mainly 

 by the consideration of the assemblage of the 

 several groups of animals. 



Dr. Ortmann, in accordance with his views, 

 recognizes five 'life-districts,' distinguished as 

 follows:,]; 



' ' 1. Light. The medium is air. Substratum 

 present. Terrestrial district. 



" 2. Light. The medium is fresh water. Sub- 

 stratum present. Fluvial district. 



"3. Light. The medium is salt water. Sub- 

 stratum present. Littoral district. 



' ' 4. Light. The medium is salt water. Sub- 

 stratum wanting. Pelagic district. 



"5. Dark. The medium is saltwater. Sub- 

 stratum present. Abyssal district." 



While there is a symmetry in these definitions 

 that may be attractive, analysis will demon- 

 strate that the 'districts' themselves are of 

 very unequal value. In fact, they are framed 

 in contravention of another principle enunciated 

 by Dr. Ortmann: "The topographical contin- 

 uity of the range is a fundamental principle 

 influencing the dispersal of animals. "§ 



Now, there is no greater interrupter of topo- 

 graphical continuity for land or fresh water ani- 

 mals than wide intervening oceans, and inas- 

 much as such land areas, with varying limits, 

 have existed for long geological periods, they 

 have been more effective barriers to extension 

 of inland life than the differences connected 

 ■with the several districts whose 'medium is salt 

 water.' The land and fluvial faunas have con- 

 sequently been long differentiated and, although 



* Grundziige, p. 59. 

 tPr. Coll. Bull., VII., 103. 

 X Pr. Col. Bull., VIL, 101. 

 § Pr. Col. Bull., VIL, 102. 



in every age there has been doubtless an invasion 

 from the sea into the rivers, the bulk of the 

 fresh water forms in most regions .has been 

 long settled and specially developed as such. 

 The districts in question must therefore be 

 segregated under two primary categories, ma- 

 rine and INLAND. 



But the marine districts still left are likewise 

 of very unequal value. They are distributed 

 by Dr. Ortmann as follows: 



"I. Littoral life-district. 1. Arctic region. 2. 

 ludo-Pacific region. .3. West American region. 

 4. East American region. 5. West African re- 

 gion. 6. Antarctic region." 



"II. Pelagic life-district. 1. Arctic region, 

 2. Indo- Pacific region. 3. Atlantic region. 4. 

 Antarctic region." 



"III. Abyssal life-district. No regions dis- 

 tinguishable." 



These ' districts ' and ' regions ' would answer 

 well to divisions which I have established as 

 follows: 



I. Arctalian realm (1875) = I, 1. 



II. Tropicalian realm = Tropical zone (O.),^ 

 I, 24-3 + 4+5. 



III. Notalian realm (1875) = I, 6. 



IV. Pelagalian realm = II. 



V. Bassalian realm ^ III. 



These combinations appear to me to better rep- 

 resent the facts known respecting the distribu- 

 tion of marine vertebrates as well as inverte- 

 brates. The first three were distinguished as 

 early as 1875,* but not named till 1877. f Later 

 I deemed it advisable to subdivide the Arcta- 

 lian into Arctalian (restricted) and Pararctalian 

 and the Notalian into Antarctalian and Notalian 

 (restricted). I also added the Bassalian and 

 still later the Pelagalian. The Pararctalian and 

 Notalian proper have less value than the others, 

 except the Pelagalian, which is the least special- 

 ized of all. 



I have thus pointed out the chief differences 

 between Dr. Ortmann's views and my own. 

 Naturally, from the difference in our starting 

 points, ensuing differences are great. Dr. Ort- 

 mann's method leads to a consideration of ' life 



*0n the geographical distribution of fishes, in An. 

 Mag. Nat. Hist. (4), xv., 251-255, Apr., 1875. 



t "Wallace's Geographical Distribution of Animala 

 in Nation, xxiv., 27, 28, 42, 43, July 12, 19, 1877. 



