SUGGESTIONS FOR RELIABLE HYPOTHESES 177 



time were connected across the Behring Straits the 

 animal world is very similar, since fresh blood was 

 constantly coming in which by continued crossing 

 rendered an alteration in a definite direction impossible. 

 The objection might yet be raised that the various 

 continents could certainly have had their own fauna 

 and flora from the beginning. But then it is entirely 

 forgotten that the present continents did not always 

 exist : how otherwise could marine animals be found 

 in the alpine strata ? Furthermore it is certain that 

 in many geological epochs the animal and plant worlds 

 in Europe and America were almost entirely the same 

 for instance, in the coal period ; that at times, over the 

 entire northern hemisphere as far as Greenland, a warm 

 climate prevailed in our regions a tropical one, without 

 marked seasons ; and that even in the Ice Age with us 

 the Siberian plants and those of the high Alps still 

 lived in the valleys. There is thus proved by evidence 

 that there has been a continuous changing and constant 

 wandering from place to place : therefore one cannot 

 speak of the countries which at present chance to project 

 above the level of the sea as having their own organisms 

 from the beginning. 



2. Suggestive points in the domain of Parasitism and 



Symbiosis. 



A very fruitful domain for historical evolutionary 

 hypotheses is presented by the manifold relations of 

 various species of animals and plants to each other or 

 of animals to plants. When individuals of different 



