64 A. E. Ortmann — Separation^ and its 



thus they are prevented from migrating from one station of 

 definite conditions of life into others with other conditions. 



Separation has acted since the beginning of the geological 

 history of organic beings upon their differentiation into species. 

 In the most remote geological periods we can distinguish par- 

 ticular parts having each different conditions of existence. 

 The most general differentiation is created by the existence of 

 particular life-districts :* districts of different primary condi- 

 tions of life. The chief differences (1. c, p. 18) are made up 

 of the conditions of the light, medium, and substratum, and 

 according to the different features of these three conditions I 

 have distinguished as follows : 



1. Light. The medium is air. Substratum present. — Ter- 

 restrial district. 



2. Light. The medium is fresh water. Substratum present. 

 — Fluvial district. 



3. Light. The medium is salt water. Substratum present. 

 — Littoral district. 



4. Light. The medium is salt water. Substratum wanting. 

 — Pelagic district. 



5. Dark. The medium is salt water. Substratum present. — 

 Abyssal district. 



These are practically the most important life-districts, and, 

 as a rule, every form of animal life is restricted to one of them. 

 But there are a few exceptions, that is to say, sometimes cer- 

 tain animals migrate from one district into another : such 

 changes, however, are regular ones, and are governed by strict 

 laws. Any artificial or unnatural transplantation of a species 

 from the proper life-district into another involves the impossi- 

 bility of its farther existence. 



Life-districts have existed since dry land rose above the sur- 

 face of the ocean. It is generally accepted, that first an ocean 

 of an equal (but considerable) depth covered the whole surface 

 of the lithosphere, and subsequently, by the subsidence of cer- 

 tain parts of the latter, and by the compensating upheavalf of 

 other parts of the earth's crust, greater depths of the ocean 

 were formed, and parts of the lithosphere rose above the sea- 

 level. As soon as this was accomplished, the ^yq different 

 life- districts were present. I have shown further (1. c, p. 39), 

 that the topographical continuity of each of these life-districts 

 is different, and especially, that during the geological develop- 

 ment of the earth the external limits of each changed in a con- 

 siderable degree (1. c, p. 64) : continuous parts were separated, 

 and separated parts connected. These changes are most con- 

 spicuous and important in the terrestrial, fluvial, and littoral 



* " Lebensbezirke," see 1. c, p. 15. 



f I differ in this respect from Siiss' opinion. 



