SALPA IN RELATION TO EVOLUTION OF LIFE. 159 



" If the comparison be extended to class characters, the disparity 

 between the two is very much reduced, and it is made evident that 

 the evolution of life between the epoch of the Olenellus fauna and 

 the epoch of the Ordvician fauna has been, with one or two excep- 

 tions, in the direction of differentiating the class types that existed 

 in the earlier fauna." 



The ground which we have covered in our review of these vari- 

 ous broad aspects of the animal kingdom brings us, then, to the 

 following point of view : 



There are no highly organized animals which have been pelagic 

 through all the stages of their evolution. The metazoa, which 

 have been pelagic through their whole history, are either small 

 and simply organized, as compared with the higher representa- 

 tives of the group to which they belong, like appendicularia, or 

 else, like the siphonophores, they have been jjerfected through 

 competition with higher types. 



Marine life is older than terrestrial life, and as all marine life 

 has shaped itself in relation to the pelagic food-supply, this itself 

 is the only form of life which is independent, and it must therefore 

 be the oldest. There must have been a long period in primeval 

 times during which there was a pelagic flora and fauna, rich 

 beyond limit in individuals, but made up of only a few small 

 simple types. During this time the pelagic ancestors of all the 

 great groups of metazoa were slowly evolved, as well as others 

 which have no living descendants. So long as life was restricted 

 to the surface, no great or rapid advancement through the influ- 

 ences which now modify species was possible, and we know of no 

 other influence which might have replaced these. We are, there- 

 fore, forced to believe that the differentiation and improvement of 

 the primitive flora and fauna was slow, and that for a vast period 

 of time life consisted of an innumerable multitude of pelagic organ- 

 isms made up of a few forms. During the time which it took to 

 form the thick beds of older sedimentary rocks the physical condi- 

 tions of the ocean gradually took their present form, and during a 

 part, at least, of this period, the total amount of life in the ocean 

 may have been about as great as it is now without leaving any 

 permanent record of its existence, for no rapid advancement took 

 place until the advantages of a life on the bottom were discovered. 



