W. K. BROOKS ON THE GENUS SALPA. 167 



In the formations which follow the lower Cambrian, species grad- 

 ually become more numerous, but this is due to divergent specialization, 

 and Walcott says that if a comparison be made between the Olenellus 

 Zone (lower Cambrian) and the Silurian fauna, the superiority of the 

 latter in number of species, genera and families is at once apparent. 



"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 exceptions, 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 various 

 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 organ- 

 ized, as compared with the higher representatives of the group to which 

 they belong, like appendicularia, or else, like the siphonophores, they 

 have been perfected 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 

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

 other influence which might have replaced these. We are, therefore, 

 forced to believe that the differentiation and improvement of the primi- 

 tive flora and fauna was slow, and that for a vast period of time life 

 consisted of an innumerable multitude of pelagic organisms made up of 

 a few forms. During the time which it took to form the thick beds of 

 older sedimentary rocks the physical conditions 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 



