THE STRUCTURE AND HABITS OF ANIMALS 31 



in so many points of structure that the two forms 

 have always been unhesitatingly ranked as separate 

 species. Of these differences one will suffice for 

 the present purpose. In Artemia salina the tail 

 is split at the end into two long pointed lobes, 

 each of which bears a large number of bristles and 

 hairs of a peculiar structure ; the tail in the other 

 species, Artemia Milhausenii } is abruptly terminated, 

 and presents only a very slight separation into two 

 blunt rounded lobes, which lobes are completely 

 devoid of hairs or bristles. There are besides this 

 many other points of difference between the two 

 forms which need not here be noticed in detail. 



Schmankewitsch has recently made the very in- 

 teresting discovery that the anatomical differences 

 between the two species depend directly and solely 

 on the different percentages of salt in the water 

 they inhabit. He finds if he takes Artemia salina, 

 living in water containing 4 per cent, of salt, and 

 gradually increases the percentage of salt, that the 

 structure of the Artemia becomes gradually modi- 

 fied ; the lobes of the tail become shorter and 

 shorter, and the hairs diminish in size and number. 

 Ultimately, by increasing the strength of the salt 

 up to 25 per cent., he succeeded in completely 

 transforming Artemia salina into the distinct species 

 Artemia Milhausenii. Not only can the trans- 

 formation of one species into another be effected 

 in the laboratory, but it can be demonstrated 

 to actually occur in Nature. Of this fact 

 Schmankewitsch has recorded the following very 

 striking instance : Two lakes in the neighbour- 



