A. E. Ortmann — Separation, etc. 63 



Art. IX. — On Sejparation, cmd its hearing on Geology and 

 Zoogeograjphy ; bj Arnold E. Ortmann. 



In a recent publication* I pointed out the importance of 

 the principle of separation or isolation as regards the distribu- 

 tion of animals as well as the origin of species. The present 

 article intends to give a short review of the views set forth in 

 the book referred to, especially regarding the geological and 

 palseontological aspect of the matter. 



While some authors believe that natural selection is sufficient 

 for the explanation of the origin of species, and that " the mode 

 of formation of the living world as a whole " (Weismann) can be 

 understood by his principle, 1 maintain, on the contrary, that nat- 

 ural selection is only one of the factors cooperating in the forma- 

 tion of species, and especially, that it acts only in a particular 

 line, namely effecting a slow transformation of existing species. 

 In the book referred to (p. 31) I have shown that by natural 

 selection only a transformation of one already existing species 

 into one other species is possible, but that it is impossible to 

 think that two or more different species originate from one 

 original species by selection. The transformation of species in 

 time is entirely different from the differentiation into several 

 contemporaneous species. The former process, well known to 

 palaeontologists under the term " mutation " (Waagen, Xeumayr, 

 Scott) has been generally not properly understood by biolo- 

 gists, and I cannot enough emphasize, that mutation of one 

 species in time should not be confounded with differentiation 

 into coexisting species. 



As I have demonstrated (1. c, p. 31), the latter process is due 

 to the factor called by M. Wagner separation or isolation, and 

 although this principle has been discussed by many subsequent 

 authors, it has been — with few exceptions (G-. Baur) — generally 

 misunderstood or not accorded its full value, since most authors 

 have conceived it only in a purely geographical sense. Yet in 

 nature we have not only a purely geographical separation, but 

 many other causes may have the same effect. Generally we 

 may say, that any causes which induce certain groups of ani- 

 mals to live under particular conditions of existence, while 

 other related groups are not subject to the same conditions, in 

 other words, that any causes effecting a permanent interruption 

 of the bionomical continuity between certain groups, come 

 under the head of the term separation. Separation keeps par- 

 ticular groups permanently under particular conditions, and 



* Grundziige der marinen Tiergeographie, Jena, 1896. 



