THE ORIGIN OF THE MARINE FAUNA. 173 



point of water, and we meet with Insect larvse 

 swimming freely in the water of the hot springs. 

 Some sea-water animals can only be induced to 

 live in the aquarium when the water is kept as 

 pure as it is in the open sea, and languish and 

 die as soon as any impurity occurs; on the other 

 hand, several of the Crustaceans seem to flourish 

 best in stinking and putrescent pools. The 

 desert, the forest, the swamp, the lake, the 

 river, as well as the surface and the bottom 

 of the sea have each their characteristic set of 

 animals and plants modified in structure and 

 form to support life in their natural habitats. 



There can be no doubt that at the time when 

 animals and plants first made their appearance 

 upon the earth, their distribution was far more 

 limited than it is now, and that all the adapta- 

 tions to life in special and extraordinary con- 

 ditions have been acquired in the course of 

 evolution by organisms which originally existed 

 in one particular zone of the earth. 



The reasons which have led scientific men to 

 this opinion are manifold, but not the least im- 

 portant of them are those based upon the pres- 

 ence of organs or rudiments of organs of animals 

 of the present time, which could only have been 

 called into existence at a period when their 

 ancestors had an altogether different habit of 

 life. 



For example, in the Birds and Eeptiles, as 

 well as in the Mammals, the presence of openings 

 in the throat during the early stages of develop- 

 ment, similar in their position, in their blood- 

 vessels and in other respects to the openings of 



